Mopar taxis, 1935-2003

The reliable 3.5 V6

1997 Plymouth Pronto
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Performance Upgrades for the Chrysler, Dodge, and Plymouth 318, 273, and 360 engines

See Rick Ehrenberg’s bolt-on performance tips

The 318 is a good, solid and dependable engine. I will be the first to admit and try to sell that. Secondly, the 360 is an outrageously great engine because it has so much potential, greater than a Chevy 350 or Ford 302/5.0. The SBChevy guys have a little advantage because all the heads and intakes all bolt on without major changes other than combustion chamber and valve sizes. MOPARs have two sizes of runners and various valve sizes.

What you have to do is make the determination of what you really want to do. A 318 can be built rather easily using stock components and be very reliable and strong, the 360 you can do the same thing with a lot more aftermarket options. About the only things that are different between the two other than the heads/intake, are motor mounts (right side, actually), harmonic balancer and torque converter (being external balance on the 360) and rear main being smaller on the 360 means oil pan will not interchange. The 318 does have the advantage of a steel crank stock, so power and rpm limitations are minimal.

If you are on a 'budget', your best choice is to 'freshen' the 318 and only replace or machine items as necessary.

There are some differences between the 318 and the 360, but many parts interchange. It is easier to get raised or decent compression pistons for a 360, not as easy for the 318. Another common thing is there is no replacement for displacement. 4 inch bore is always good as opposed to the 3.91 inch bore for the 318, but you can still get whatever valve you want into the head so that is not an issue either.

There is no better source than either the MOPAR Engines or MOPAR Chassis 'Speed Secrets' books available through your nearest dealer.

I always advocate that anyone who lacks experience and/or technical training and wishes to 'build-up' or 'modify' an engine, should talk to engine builders, ask lots of questions and read and learn. Read and talk to a reputable machine shop, talk to MOPAR people at car cruises and swap meets in your area and find out who they recommend in the way of a machine shop and go talk to them about a hi-po 318 build.

First performance enhancement

The first performance enhancement you do on a stock 318 is to install the (factory) 340/360 H.P. cam (or equivalent). With stock heads, 2Bbl carb and even 2.76 gears this is already a 'fun-package'. Even with 4.11 gears (and street tires), the FIRST 5 feet that the car travels is the most critical. Over carburetion or a non-functioning accelerator pump will pretty much have the same effect, the car may indeed 'fall on its face'. You will benefit from a quicker advance curve in the distributor and enough initial lead to give about 36 degrees max. A dist with 13 inside will double to 26 degrees on the crank then you can set the 'static' to 10.


The 360 heads and Intake swap will fit on the 318, but the compression will be around 7.2:1, which is not very good for performance, which is not to say that the heads cannot be shaved .050 and intake be rematched to fit, adjustable rockers added to offset the rocker geometry, and away you go.

The book says the 360 to 318 works well, but, take the 318 heads, shave .020 off the head, install 1.88 intakes vice 1.78, port the heads and you would have a better combination, especially on the bottom end response and will still rev as high as you want and float the valves before the power falls off (how can you beat that?). It can be done, but, if you have a 360, which is physically the same dimensions externally as the 318 and 273, why not go the least expensive route?

Keep in mind that the 318 runs about 8.5:1 compression and will be about 8:1 or as low as 7.5:1 with the J heads of the 360. This is going to drop the compression pretty low, which is great for cheap gas, but bad for the performance side, thus most 318s are rated at the 185hp, which is a shame. If the engine is out and heads are off, do some measuring to determine what the compression is going to be. Some of the pistons can be as much as .080 below deck just to start with. I am not trying to squelch your combination, but, as many others have done with similar combinations, if it is a dog, don't be surprised.

As stated, the piston sits between .025 and .080 down in the bore. The area of the chamber of a 318 stock head and a 360 J head is greater. Unless the block was 0 decked to the pistons, aftermarket pistons were limiting the distance below the deck, or the J heads are shaved, measure the compression or it will be lower than stock.

Now, there is a solution to this problem. Last month's Hotrod mag had an excellent article similar to yours, but the heads were with smaller chambers. They shaved theirs .050 to raise the compression, whereas you may have to do the same. When removing that much metal from the heads, the cork gaskets front and rear of intake may be too thick, so be cautious during installation of the intake.

Here is the thing about the 318 heads compared to the 360 heads. The intake valves are smaller on the 318, but the stock 360 valves put into a 318 head (1.88 vice 1.78) and then the bowl tuliped, vice opened up all the way, really makes the head flow in the chamber side. The pushrod pinch on the intake port is the same for the 318 and 360 head, so there is no gain there, but the height of the port is taller than the 318, and the port mismatch can have detrimental effects. Port matching can be done on most castings; some are close at the top of the gasket.

Here is the hard thing to deal with on the 318 head of 1969: The exhaust valves will be sunk within a couple years because the seats are not hardened and need the leaded gas to survive. Figure you can get approximately 25,000 miles out of the heads before they are affected completely by sunk valves. Have the 360 heads shaved .020 or get new pistons is the best solution because the cost of the work on the 318 heads will be a couple hundred dollars and the porting time, if you do the porting yourself.

Another solution is to hit the junkyards. I had several sets of 318 heads that were closed chamber that I have had in 73-74 Plymouth engines. Don't know the casting number but they do exist. Then put the 360 valves in the heads and port them. These heads already have the hardened exhaust seats.

Intake valves are not affected. Did it on two sets of heads and have had wonderful results in the power increase world. I originally ordered stock 318 valves and ended up with stock 360 valves. Cut the seats, ported the heads, and really noticed the breathing and torque increase. It was fun!

You can indeed 'shave' the (360) heads 20 or 30 to raise the CR but that should still keep it under 9.0 to 1 and allow the use of (cheaper) 87 octane. The power potential is already there, but everything has to work together; the timing, the intake and exhaust flow, the distributor curve, the plugs and wires, the cam specs, the carb itself, and even the type/grade of engine oil to make any 360 dependable, efficient and a pleasure to drive.

The 340/360 intake is a larger runner than the 318/273 heads. This is like taking a 3/4 inch pipe and running it into a 1/2 inch pipe. They have to be port matched, or, if that cannot be done, find a 318 specific Streetmaster type dual plane intake.

Smog is a concern for some of us, not for others, but there are some things that can be done without too much worry. For starters, we have to remember that the 34/360 have better breathing heads with larger ports AND larger combustion chambers. The amount of shaving to get them down to the 318 size, so as not to lose compression (takes it down to about 7:1 vice 8.5:1). An old favorite, and extremely helpful thing to do is have 1.88 valves from a stock 360 installed. There is a requirement to tulip the intake port to do this, but the small amount of work is well worth the effort and money. This also prevents having to change out the intake manifold to fit. If you do go with the stock 360 heads, you have to stick with a 360 intake, for the 340/360 heads and 318 intake do not match, and vice versa.

Edelbrock makes a really nice aluminum head, with the smaller chamber size, if they are anything like the big block aluminum heads, would be fantastic, along with headers/dual exhaust and a 4 bbl and 600cfm carb (or police 4bbl), would add in the neighborhood of 100 hp/ftlb torque.

Go out to the salvage yard and find a set of closed chamber heads. Anything past 1973 will have hardened exhaust seats for added durability. The closed chamber heads add compression and simplifies the need to shave so much off to raise compression. Get a set of 360 valves (1.88inch) and get the intake seats cut for them. Port the heads to gasket size and tulip the bowl to the valve size, but don't just bore the bowl out to the bigger valve. The less you shave the heads, the less likely you will need adjustable rockers. (big bucks to add).

If you can't get a set of heads that are closed chamber, only shave the heads .020 so as not to throw the geometry for the rockers off any more than necessary. Get a set from the salvage yard and get them prepped and ready instead of waiting a week with your car down. Port match and bowl work is the best. Do not switch everything over to 340/360 heads and intake. It really kills your compression and stock ported heads run higher velocity for the 318, which you need.

You suggested replace the stock 1.75 (or 1.78) /1.50 valves with the 1.88/1.60 360size. Other people recommend this as well. Now I know that the 318 heads are better that 360 for compression. My question is, would the 0.10’ increase in valve area make that much difference on a slightly modified 318 (4bbl, duals, & quick bolt-ons)? Wouldn’t it be easier & less expensive to increase the intake flow with a 252 (stk 360) or 260 (Crane) cam? Or 1.6 rockers?

I—d imagine for competition you—d want larger valves, but for a daily driver, would the effort & machine shop fees be worthwhile? And what about increased fuel consumption? I—m thinking of installing late 80s #302 swirl port heads on a 318. (about 500cfm & duals, no headers) Has anyone done just this upgrade alone, that could verify performance gain & MPG difference?

I did just this to my 74 Barracuda 318 when the wrong valves were sent to me and I was not in a position to wait for them to be replaced. The difference between 1.78 and 1.88 is 100 thousandths of an inch. That's one tenth of an inch.

Mileage did not decrease, it actually increased because the added efficiency with the small runners breathed better, not worse. I was using the Crane .444 cam and it was superb. I could feel the difference from stock and it worked well. It was coupled to a Streetmaster dual plane and Hedman headers, ported heads, balanced stock rods and pistons, and Holley 650 double pumper, 2.71 rear 8 3/4 rear. Oh, exhaust valve change is not necessary.

All the machine shop has to do is cut the valve seats, which I simply ground with the standard 45 degree stone cutter, then tuliped the cut to the edge of the seat diameter (i Marked the edge of the seat with the new valve, properly ground at 45 degrees, by doing a quick lap job on they dyed valve seat so I could tell how much material needed to be removed). Do not hog out the whole pocket because that just defeats the purpose of the tulip design. The charge enters the pocket, expands to the outward direction, as the valve is opened and then closed, the velocity in the runner remains packed and at a higher velocity, but the valve closing compacts a larger charge in the tulip area for higher charge to wait and enter for the next valve open motion. The idea of a higher lift cam will drop the vacuum and velocity of the charge and then you have no bottom end power.

Going with the 360 heads, unless you are running high rpm and higher compression domed pistons, are pretty dead on the bottom end because of the size of the intake runners and drop of compression to low to mid 7:1. The bore is too small and the stroke is too short to get good bottom end draw on the 318.

Magnum heads, although good, have poor pushrod geometry, so I don't like that idea too much. They work, but I fear bent pushrods because of the poor angles. 1.6 rockers, other than not being able to work stock on the early heads, and are around $450 with pushrods in the aftermarket. (extra cost). If you can go with a complete Magnum block to go with the heads, that would be the best thing.

I could routinely wind this motor up to 7500 rpm without it dropping out, and one time slipped a gear on the highway to move quickly to avoid an accident and hit first doing 55(7500 rpm) and shifted at 75 before I really realized what had happened (panic mode to avoid the accident). I figure I hit 8400 rpm and not only did it not float or bend a valve, but was still picking up rpm and power before I could correct my mistake. A stock 1.78 definitely would not have been able to do that.

It's your decision, just some good solid personal experience with this set-up. Need more info or help, just ask.

Buy a set of 1.88, stock valves for a 360. I have personally found PAW to be an excellent mail order company. They have never steered me wrong and they have all the good stuff. You can find ads for them in all the car mags. At the machine shop, ask that the intake seats be ground to the size of the 1.88s, have the valves (new) redressed (just lightly ground to check them; little or no material is removed to verify factory tolerance and angle), and have the machine shop let you have the heads to do the seat position and port work yourself, then, when you have ported the heads as described, porting up to about 20 thousandths to the inner edge of the valve seat you have verified the valve will sit. Return the heads for assembly. If you have a friend, this goes a lot smoother, and can be accomplished in one day. Explain to the machinist what you want to do, and he will tell you how much it will cost. Valve grinds are usually around the $100-150 price range. Enlarging the seat may be slightly more.

45-degree angle, by the way, is the stock angle for about 95 percent of factory heads. For the three angle valve grind, there is a 60 degree angle cut on the inside edge of the 45 degree seat that is about 30 thousandths inch, and then a 70 degree angle cut into the chamber. This isn't necessary if you are porting and especially with this tulip design porting.

One reason I never like doing the 360 heads on the 318 is because the chambers are so large that you have to shave the heads .060 to get the compression up, have to use the 360 intake so there are not any vacuum leaks, and there is usually a flat spot on the bottom end because the intake runners are too large for the size of the engine. A good port job works good for the 318 heads, headers work well, and a dual plane intake with a 600 cfm Holley, or Carter, work well. Really want to make it a little better, get the closed chamber 318 heads, get a set of 360 intake valves 1.88 vice 1.78 (one tenth of an inch is a lot for a small engine, but they work really well), and port the intake runner in a tulip fashion from the floor to the valve edge. The small runners keep good throttle response, the tulip design packs the charge really good and adds a good amount of torque on the mid to upper rpm range. Cam selection is really good in the .444 range. Not too big to kill the bottom end, not so small that it won't rev to 7000rpm, either, and you won't need a stall converter with an automatic. There is a cam that was for the 340 in about this size, very nice. Remember, this is a small bore/short stroke engine, so they rev well, but too much lift and too large a runner (as in the 360 heads) does not work unless you are running above the 4000 rpm range, which is not feasible on the street, only the strip.

Factory 360 heads are indeed larger inside than 273-318's. However, they are not nearly like Ford's (351) cleveland. Around 1978, MOPAR first sold the 'LA' version of the 318-4Bbl. They simply bolted on a 360 intake/thermo-quad. Shortly after, a (so-called) H.P. version was 'unleashed' on the motoring public. This version used the (current) 360 heads with the 1.88/1.60 valves. In fact, this H.P. 318 was available through 1984. Hey, some of you must have owned one.

By 1977, 318 heads were only 1.0 c.c. smaller than the 360's. From 1968 through at least 1984, all 'LA' heads, either 318, 340 or 360 were of the 'open' chamber design. The 318 chambers were in the 61-62 c.c. range to 1975 then they slightly increased to about 63 and then to 65 a couple of years later. The 360 heads at that time were mostly 65's.

If you mill either the 318 or 360 heads by about .005'', the chamber volume will be reduced by 1.0 c.c. Therefore, a cut of .050'' will reduce the 'c.v.' by 10 c.c.'s. This will in turn raise the C.R. by 3/4's of a point, on average; depending on the piston diameter.

Finally, as taken right out of the MOPAR (factory) Engines manual, edited by Larry Shepard; "In general, the 318 lends itself to the 340 type conversion better than the 273, because of the larger bore, more cubic inches and basic low level of performance in standard 2-Bbl. trim. - - - the first conversion to be made will be assumed as the installation of the 340 heads (or 360). To install the 340 cylinder head (360 or '72-'73 340 is recommended on the 318) - - - The 318 bore does not have to be notched. - - - With the 340 heads installed, the next most likely change is the cam. - - - A good compromise choice for an automatic is the 'standard' 340 cam (1968-1971), - - - Now that the heads and cam installation have been discussed, the next step is the intake system. - - - etc., etc."

Next find a set of late model 318 heads I believe the are 302 castings, circa 86-91 pre-magnum, they flow better than the early 360 heads and have swirl port heart shaped combustion chambers. Have the valve unshrouded, the ports mildly cleaned up and clean out the bowl area under the valves, for an intake use Edelbrocks dual plane intake set up very good for building power out of small cubes like the 318, buy there complete kit, intake carb, cam and lifters package which can be purchased thru Jegs or Summit for a small amount of money.

Volunteer, one small correction to your open/closed chamber dates. I had a 74 Barracuda 318 with closed chamber heads in that car and floating piston pins, vice pressed pistons. I had to replace the motor after freezing and cracking a head (thanks mom and dad), and I found a 74 Road Runner engine of the same configuration. Might have been a fluke, but after about 1973, closed chamber heads have the hardened seats and closed chambers on occasion. Those really help to keep the compression with the rest of the smog pistons in around 8.5 - 9.0, depending. There is additionally, when you can find them, a tarantula 2 bbl intake manifold that is good for about 8-10 hp over the stock 2 bbl H pattern. I have seen them on cars, but usually on pickups and vans.

I assumed that all '68 - '74 318 heads (with the '675' casting) were of the 'open chamber' design because that's what all the manuals say.

When I rebuilt the original 318 from my brother's '70 Satellite back in '86, I remember that the chambers were 'round' and it did indeed have 'floating' pins, (like all 340's).

I guess nothing is 'etched in stone'. My '74 Challenger should have a 'blacked-out' rear-end panel according to Paul Herd's resto guide but it definitely has a 'charcoal-gray bum' and I proved it. Thanks.

P.S. I remember seeing the infamous 'single-plane' 2 Bbl. intake on a '71 Challenger and it had a Rochester carb., p.n. 7041180. Seriously.

Hey Brad, I think the 318 heads would wheeze on a 360. I was debating myself on using my chiged' out 302' 318 heads cause I opened them up ALOT. But I'm thinking of staying with 360 heads instead. The intake runners are a lot bigger. Another reason why, is that I was told that going 2.02 intakes on the 302's was a bad idea, mainly because they would be too big for the 318. But looking at them more, (they are closed chamber) I think that putting in 2.02 valves would "shroud" and lose the effect anyway.

318 heads have 1.78/1.50 valves,<--that might be 1.74 intake///

most 360 heads have 1.88/1.60 valves, if you go with that, you can have the intakes enlarged to 2.02 and have the same thing as a 340 X head. Then just have it gasket matched and polished in the exhaust runners.

Dana was mentioning 73? 74? 360 heads that were closed chamber as well, but I don't think he ever mentioned a casting #? If he did I've forgotten already.

The closed chamber heads I was talking about were 318 heads. Boosting the compression by using 318 heads is one way to do it, and the smaller runners would work by opening them through porting. Thought on the subject is that the ports for a 360 can only be opened as wide as the pinch between the pushrods anyway, so gasket match and open up the runners wherever you can.

Other solution, as shown in the 400hp 318 is to use the 302 heads. Shrouding of the valve is not an issue if you simply port the chamber as well. I noted from the pictures that they never took the time to round the closed combustion chamber so, there was probably another 15hp easy out of the combination. I just hate it when they do that.

I do not know the casting number to the 318 closed chamber head. They were rather common early in the 318 history, but the 73-74 castings had hardened seats in them for unleaded gas, already.

Note, I thought the 302 casting was the 318/360 pre-magnum design and was on both 5.2 and 5.9 before the magnum engine redesign? Did I misread the description?

Also, dropping the 383 in the engine bay is quite a tight squeeze and a lot of work. I would not contemplate doing it without a full shop and lots of room, myself.

Were you talking about rounding on the valve side, or also rounding the v at the top of the heart? I thought that was the "swirl" in the swirl port design?

As far as I know, that's the same description on the 302's I've read, they still bolt up the same and all, the only real differences other then the chamber are the air holes, and the over sized 11/16 pushrod holes. Which is a bummer because i've had to be extra careful cutting in that area.

I heard that they were also a template for some sb aftermarket heads, MP?

After more thought, if any 318 heads were chopped and tricked they might be real monsters on a 360. you'd have close to similar sized runner to stock, but with all the obstructions gone, like a happy medium.

I think Brad's thinking about regular open chambered 318 heads, it still would knock the compression up though, they just need to be chig'd out, which btw, i have an extra set just sitting around.

That's about right. The 302 head with the small heart shaped chamber is fine to make the swirl, but after taking .050 off the top, it kind of destroys the purpose, thus, after the cut, the valves are shrouded, especially with the added valve size (which doesn't shroud the valve with the smaller valve). To make these heads work more efficiently, the edges of the machining process needs to be rounded, which will also allow more advance to be run, especially when they noted that they were getting the best results with less advance. SB always do best with about 38 degrees advance, brought in by 2400rpm, vacuum advance all in around 3500. They were talking about the best advance curve of a lot less, which confirms they had too much sharp edge, fuel not distributed properly. I would bet the piston top would show that there are quench areas at the spark plug top and the whole bottom of the piston. Poor flame travel because of the sharp edges.

Just to set the records straight, there was only one 273/318 casting, #2843675, from 1968 through 1974. It was of the open chamber design, 61-63 cc's, and the valves were 1.78" and 1.50". In 1975 and '76, the #3769973 casting was used but there were no other significant changes.

The next version was from '77-'84? but they were still of the open chamber config. After that came the 'infamous' and soon to be legendary - 302's.

Volunteer, not to doubt you or the books, but I have had two sets of closed chambered 318 heads with the 1.78 intakes. Both engines they came off of had floating pistons (beside the point) and may have been Plymouth (one a stock 74 Barracuda, the other a stock 74 Road Runner). I do not know what the casting number is or was, but they are out there. I wish I had more information to help than that, considering they are not in your books.

I rather liked the heads, simply punched to 1.88 intakes and ported because they would give the dropped deck flat top pistons a nice mellow 8.8:1 or so compression.


Why new pushrods? Are you getting adjustable rockers (would be a good idea with anything above stock camshafts) to go with them? Stocks are pretty durable in other words, cupped pushrods needed with the adjustable rockers.


Camshaft is the next best upgrade you can do for the 318.

They really like the 340 hipo cam, which is still very mild, but is just enough to wake a 318 up. It has a lift in the .440 range, so any aftermarket camshaft between .430 and .455 work really well and mileage will not suffer, won't have to be high compression and doesn't require any high torque converter. Any more than that and you will kill the bottom end and the 318 will run like junk. This is an excellent cam for the 318.

318s get a little radical with a cam larger than .450 lift on 270+ duration, 360s get radical when you get up to .485 and 290+ duration, so a .450 lift and 270 degree duration in a 360 screams, idles pretty smooth and will get a little better mileage without any sacrifices.


Headers are a nice addition. If you don't want to go that route, split the exhaust and do dual exhaust, and that is another big help.


As far as the ignition goes, stock MOPAR electronic ignition is one of the best factory designs out there, but an upgrade only gains with higher performance levels, not needed on the street, but it is your money.


Notching a half inch deep slot from front to back in the intake plenum has a two-fold effect. It balances the idle signal side to side, which at low speeds/idle/cruising speed below 2000rps. If there is a slight difference between the two halves of the engine, it corrects this. On the top end, it helps a small amount by giving the assistance (very minor) as an open plenum manifold signal. It is kind of the best of two worlds without sacrificing much from either. This adds balance for the carb without sacrificing bottom end. Don't go larger than 650 cfm on the carb, as in Holley, and usually recommend 600 cfm vacuum secondary.

(dana44 added:)

Keith_Indy, a couple things to add about the three engines and a little info on the Magnum 302 heads. The 340 and the 318 share the same crank and rods, but there are beefier 340 rods around...the last couple years they were the same when the 340 started dropping compression and power. The bore for the 340 is larger than the 360 (4.040 and 4.00 respectively). The bearings are shared with the 318 and the 340, including oil pan and rear main dimension. Basically two rods available. All are stronger and heavier than small block Chevy and taking a belt sander and/or die grinder, not bench grinder, to remove the casting off the beams and balancing the rods is all that is necessary for anything you can build for the street.

The 340 and 360 share intakes and heads, being the larger ports and low performance 340 had the 1.88 valves.

There is, apparently, a misconception through an article between High Performance Mopar and the latest Mopar Muscle concerning the Magnum 302 casting and the Magnum block and heads. The Magnum heads with the swirl ports, closed chamber and rocker arms on bolt-on pivots, oiled through the pushrods, is different from the 5.2/5.9 Magnum engine and heads, and will work on the old 318/340/360 blocks with some minor mods. The lifters have to have an oil hole in them to push oil thru the pushrods to the rocker arms that match the heads. The 5.2/5.9 Magnum is a completely different animal, other than external dimensions and mounting points. The pinch area between the pushrods/ports is corrected by widening the lifter bore positioning and that is the geometry problem I was talking about concerning Magnum heads on an older engine. My mistake. On the other side of that, I still don't like to have to shave heads that much in order to raise compression because intake geometry gets tricky and not worth the real hassle of it, especially if it is done incorrectly. Deck height of the piston, if kept near zero or .010 is a better way to get the compression up. Maybe we can find a couple aftermarket pistons that can give us corrected deck height to list on here.

One last thing for the 360, and that is the W2 heads. Stock they are better than anything from the competition, ported flow better than any big block engine, upwards of 310-320cfm. Thing is, they require a matching intake and rocker arm assembly, but if you have about $1600-1800 laying around, big numbers on the dyno.

Good collection for reference. Thanks Keith_Indy for collecting it.

Carburetor upgrade: by Daniel C. Stickney, 1996

(Disclaimers and safety notice copyright © 1996 by Daniel C. Stickney)

Proceed at your own risk. Some changes may shorten engine life or the integrity or durability of other components. The author makes no claim or guarantee that information contained in this article will be suitable for your situation. The author accepts no liability for any problems that you may encounter while working on your own car. Modifying your car may violate Federal, State and local emissions laws and regulations. Emissions equipment does make the world a better place (ask any Californian.) It isn't that much harder to set up an emissions legal system, especially with junkyard parts.

All trademarks mentioned belong to their respective owners.

Again, if the car is going be used on the street, check your local laws regarding tampering with emissions equipment.


Be smart and work safely. Wear appropriate clothing and eye protection. Never, ever wear contact lenses in the shop. Don't assume that prescription glasses will provide adequate eye protection. (My glasses have ballistic quality polycarbonate lenses.) Wear goggles, a face shield, or glasses with side shields when using grinders or any tool that throws chips or sparks. Use gloves and eye protection when handling chemicals and wear ear protection if necessary. I shouldn't have to tell anyone about jackstands. If you work under a car without them you deserve to die. Think of it as evolution in action.

Never start a project on Sunday if you need the vehicle to get to work on Monday. Always give yourself at least a full weekend in case you find that you need a special tool or part. Most accidents occur when rushing the job or working late at night to get done for tomorrow. Line up alternate transportation in advance (carpool, bus, or beater) and remember that you can't commute in a project. I plan on at least a week for any job that requires major disassembly. This gives me a cushion when I need to order parts. Finally, don't get frustrated! This is supposed to be fun! Try to work with a friend, preferably a knowledgeable friend. Listen to the radio. If you get stuck on something, STOP AND THINK! Get some coffee, talk it out with your friend, or your dog, look it up in the service manual. I find I can usually solve any problem if I get away from it and don't fight it.


Beiseker Bob wrote that a 600-650 cfm carb is about right for a stock 318, and that a four barrel carb would add some fuel economy as well as power. He recommended a dual exhaust for some added pickup. With regard to cams, he wrote:

if you want low-end torque and hi-revs is not really an issue, go with a hi-lift truck cam or something similar. What you'll be after is low duration, high lift. If you want to rev like a blender, go with lower lift, longer duration. The valves still open for a long enough period, but don't move as much, meaning less force on the springs, etc.. For a slightly hairy car, I've gone with a 0.450 lift (approx.) with a 290 degree duration. Does well in the quarter, but not so good for a street driver. If you're running an automatic be careful with how far you go, she may stall if you go too big.

If you're running a stick you have a few more options. While you're surfing the net, check out the Crane Cams website...I think they have a 'cam selector' section to help you figure out what you need. They'll slide you a catalogue with all the good stuff listed and available packages.

Another thing to look at is your gearing. I'm running higher torque, but with 2.90 gears. That way it'll still walk nicely, but cruise at 2100 RPM right smack in the powerband for that cam. Very important! If you're doing highway speeds and such, you don't want the engine to 'lug'. By the same token, if you're city driving, a rough idle will get on your nerves in no time...

As for head work, a smooth finish on the intake ports and as mirror as possible on the exhaust ports. You want that fuel/air to spin on the way in, and get out after it's burned. If you can get some 'vortex' action happening in the intake, it'll work like making a whirlpool when you drain the dishwater out of your sink. When you create a vortex, or helix, you increase not only the velocity which slightly compresses the air, but also vacuum at the top of the vortex which sucks MORE air down with it. Think of it like a free turbo. Note that the intake ports curve into the cumbustion chamber and the exhaust just goes straight out. That's how you want to approach the porting. You want that exhaust gas to vacate ASAP after burning, so make sure it has NO resistance and not only will it exit faster, but the smooth walls will be less prone to trapping carbon particles and such. ...I'm thinking [this] would broaden the powerband all the way across, more poop per puff.

... Opening the ports would open your bottom end as well as lift the band a little higher. It would also raise the temp a little bit because of the extra burn. If you're just polishing, temp is not so much of a concern as you still have all that cast iron to help with the heat transfer. In any case, it wouldn't be a HUGE shift...for powerband, I'm thinkin' carb and bumpstick. Important to match these two so you've always got fuel where the cam turns on the most. Fuel specs for the carb should be available from the manufacturer.

"Neil" responded: "My Dad and I did similar things to a '81 B250 Van quite a while ago. We started by taking the stock 2 barrel intake off and got an old Edelbrock Sp2P intake. They were smaller runner dual plane intakes designed during the late 70s and early 80s for mileage and torque. It flat works! We then increased the carb from the stock 2 barrel BBD to a Holley 600 vac Sec. (Stay away from the double pumper in the street). Put an Orange box 505 in for the Electronic ECU and a good dual exhaust with 2.5 in pipe and some Dynomax Super Turbos.That combo won't win races but the performance you'll get will be GREAT for torque and mileage.

Richard Currit wrote about upgrading Diplomat 318s:

Normally the computer for the lean burn system is mounted on the air cleaner housing.  On a copcar it is located under the dash right above the drivers feet, so you can't see it under the hood.  But you will have a dual pickup distributer.  So check to see if this is what you have, if it does, yes it will be sluggish.  First check to see if you timing is advancing with RPM's (mine wasn't on mine, the lean burn was shot, very sluggish)  You can swap in a MP Electronic Ignition kit, very simple and only about $120 from Mancini Racing.   The other thing to check is the oxygen sensor (does it run really rich?) its located in the exhaust manifold on the drivers side close to the spark plugs.  If this is shot it will send a bad message to the computer and it will run very rich.  My CopCar could barely get out of its own way when I bought it.  MP Electronic Ignition, New Oxygen Sensor, and a rebuild kit in the carb (around $200 total) and it scoots pretty damn good.  Will bark the tires nicely.

(A.J. added: According to a friend who used to be a cop down in TX, the cop-version Diplomat/Fury didn't run too well-- that is, until they swapped-in an MSD ignition box. That simple switch, and it had all the power they needed.)

Steve Knickerbocker wrote:

Most people seem to think you need to scrounge for rare 340 A body manifolds in order to get some flow in the exhaust.  Not so.  My 72 Dart is running B body (79 Cordoba to be exact) exhaust manifolds on it's 360.  Coincidentally, the 360 is out of the same 79 Cordoba.  Yes it is close to the POWER steering gear box, but it doesn't touch or rub.  I,  too, thought I'd have to run some 318 manifolds to clear the steering but I had the B body manifolds and Y pipe on hand so I said, "I wonder?"  They fit.  I was a bit concerned about heat transference to the P/S gearbox so I added a P/S cooler and I have had no problems at all for the last three years.  That is the only close spot.

Steve also wrote:

The 318 is a good motor and easy enough to hop up. In fact I was just reading Mopar Muscle's build up of a later 318, a 1985 in this case. They got 215 REAR WHEEL HP out of it with a cam and intake swap, still using the stock 1.75" intake and 1.50" exhaust valve no porting. The cam was a Comp Cams Extreme Energy grind, forget the exact one but want to say the 260 grind. The intake was an Edelbrock Performer, to save cash you can run a stock iron 4bb intake, and a Thermoquad.

The next step was Edelbrock heads and a Performer RPM, which necessitated swapping on a 750 AFB. They got 273 REAR WHEEL HP then. That's like 340 crank HP. (Opus) wrote:

Find any Mopar from 73 to 75 with a 318 or 360 in a wrecking yard. Should be simple. Remove the distributor, ignition module, carburetor AND the ignition wiring harness. (it has a 4 prong connector) I am assuming you have a service manual so I'll dispense with details. Keep your old EPA approved parts. You may need them. If you choose to stick with a 2bbl (for economy reasons) then get an Edelbrock SP2P-2V-318 intake manifold. Yes it IS for 2 barrel carbs. Monstrous bottom end power but you sacrifice top end. Then replace the timing chain (a major weak point in 318's) with a double roller so you can advance the cam 4 degrees. This will increase your bottom end power. By this time you will have a sweet little engine that gets over 25 mpg and can easily bake your tires.

Using a four-barrel on a 273 V8

Bill Mertz wrote: "One source says all of the LA motors will interchange intake manifolds, the other source says, 64-65 V8s had a different angle on the intake manifold bolts, which are unique to those years."

He was answered by Sandy Colter, who wrote: "You are right about the different bolt angle. You can most likely redrill or hog out the bolt holes as they are in the same place different angle. If [the replacement intake manifold] is a [Edelbrock] Torker or 2psp stay away from it. An LD340 or [Edelbrock] Performer are OK."

Thomas Gottberg wrote: "The early 273 have different angle than the later, however as cylinder heads from 318 is easier to come by than the 4 bbl intake and the cyl heads are interchangeable through every year you may want to change the heads as well. In addition with the 318 heads you get bigger valves that would do your bigger carb justice. Obviously this will have some effect on the fuel consumption but I guess since you are considering a 4 bbl in the first place that has less priority."

The following upgrades are copyright © 1996 by Daniel C. Stickney.

Kickdown linkage

Three years ago I installed a four barrel carburetor on my 1977 W150 Power Wagon and had a great deal of difficulty tracking down the parts for the transmission kickdown linkage. Many of these parts are obsolete or superseded. I couldn't find an easy source for information about these important but overlooked parts. So I put this list together to help others. However, my goal was to put together a working linkage, not an authentic one. I didn't pay any attention to whether or not these parts are appropriate for a restoration.

Kickdown linkages come in two basic styles: The single-rod linkage and the three-rod linkage. Single-rod linkages tend to be model specific. The three-rod linkages are more generic. The easiest way to get the proper kickdown linkage is to find one in a junkyard. Unfortunately, many 1970's era Chrysler vehicles (i.e. Dodge trucks) are getting hard to find in junkyards (junkyards are full of Chevrolets for some reason) especially if you live in a rural area. You are also out of luck if your car did not come with a four-barrel option. So I put together a (not comprehensive) listing of the parts needed for the three link style kickdown linkage. (Parts are from 1983 Car manual unless otherwise noted.)

top linkages

bottom linkages

transmission levers

parts needed

Notes and hints

Get some service manuals, and read up before you start. I have always gotten factory service manuals for my vehicles. The factory manual has information that you can't find anywhere else. Aftermarket manuals vary in quality. Most of them are simply factory manuals condensed for the home mechanic. If you have to use aftermarket manuals, get several of them from different publishers as each may have something that the others missed. I once had an aftermarket manual that listed oil capacities in imperial pints!

Use a torque wrench to install all torque-critical parts. Don't blame the gasket if you don't own a torque wrench. If you can't figure out how to use a torque wrench you shouldn't be working on a car anyway. You can rent torque wrenches but that may not be cost effective as good, hobbyist quality torque wrenches go for 70-80 bucks. Or you can borrow one from a friend. If you're a true car enthusiast I'm sure you have at least one friend who can loan you a torque wrench. Better yet, borrow the friend too. Manifolds can be heavy.

Removing the hood will make the job go much easier. This is another place where your friends can help you. If they won't help you than they aren't true friends. If your car club friends can't help you, ask your best friend, i.e. your wife, girl/boy friend, significant other or main squeeze as hoods are no fun to handle alone. My wife helps me with mine.

You have to remove the valve covers before you can remove the manifold. I recommend removing all parts from the top of the engine (Carburetor, coil, wiring harness, distributor) so you can get to the manifold bolts. You may need a socket and universal joint to get to the middle four bolts on the four-barrel manifold. You will need a universal joint if you are installing the relatively tall 1983-& later 318 "police car" manifold.

Clean out the exhaust crossover/EGR passages on any used manifold before you install it (hot-tanking is best). Also remove the manifold baffle and clean out the oil & carbon deposits on the bottom of the exhaust passage. Good instructions for removing the sheet metal baffle covering the crossover passage can be found in How to Rebuild Small Block Mopar Engines by Don Taylor and Larry Hofer (HP books). While the used manifold is at the machine shop have them magnaflux it and check the gasket surfaces A massive vacuum leak is the wrong way to discover a cracked or warped manifold.

While you have it apart, consider changing the oil pressure switch or adding an oil pressure gauge You can reach the oil pressure switch it easily once the manifold is off. A low oil pressure kill switch (Mopar Performance Part #P4120223) is also a good idea. This is also a good time to change the water temperature sensor, the thermostat, the choke mechanism, and the various pollution control sensors if you have them.

Don't use manifold gaskets that block the exhaust crossover passage in a street motor. This causes severe driveability problems, especially in cold weather, and the choke won't work. These parts are for race engines only. Also, plan on replacing all of water and the vacuum hoses. It much easier to cut them off and replace them, and they usually aren't worth saving.

Torque the manifold bolts carefully IN THE PROPER SEQUENCE and re-torque them with the engine hot. Mopar A engine intake manifolds are notorious for leaking near the exhaust crossover when improperly torqued. Use an anti-seizing lubricant on the intake bolts to ensure proper torque readings. Put the lubricant on the threads and the underside of the bolt head. Remember that you may need a universal joint to get the middle bolts, especially on taller manifolds.

Never use Teflon tape on fuel connections. Tightening the fitting will tear off little bits of Teflon that can clog jets and other small carburetor passages. If a fitting leaks, replace it. Always install a new fuel filter. I like to use a high quality replaceable element filter. Remember that glass filters can break in an accident. Summit Racing offers some really neat carburetor inlet fittings with replaceable fuel filters built right into them for many carburetor models

Make sure you have the right temperature range thermostat: Your engine will not run right without a thermostat. The temperature range you select will depend upon climate, intended use, and engine modifications. A 160 degree thermostat will not make up for a clogged radiator and a 195 degree thermostat will not fix a poor heater. Also make sure that you put the thermostat in the right way (point up, spring down!) A junkyard manifold may come with a different diameter thermostat neck then the one currently installed in your vehicle, make sure you use one for the right size hose.

The single link style kickdown linkage is simpler and would probably be a better choice for a four-wheel drive vehicle. It doesn't need the vulnerable lower bellcrank and is unlikely to be jammed by ice or mud. However, these parts are obsolete and would only be available in a junkyard. Also, the single link style linkage is usually model specific. A linkage from a Cordoba may not fit a truck, etc. Make sure that the linkage you use does not interfere with the shifter.

Note that there are several bottom rod/bellcrank transmission lever combinations for the three link style of linkage. These combinations depend upon the engine/transmission combination, the type of shifter and the vehicle. The wrong bottom rod may interfere with the shifter mechanism or mess up your shift points. You might have to experiment or blacksmith the rod a little, especially with junkyard parts.

I used a stainless steel screw for the bottom rod swivel because I may need to adjust it someday. Also, don't use grease or oil on the kickdown linkage. The grease will collect road dirt and act as an abrasive. The linkage was intended to work without lubrication. Dry lubricants (like graphite) are all right.

Finally, you may want to adjust the transmission shifting bands. This is easy to do if you have an inch-pound torque wrench , and eight point socket, and a factory service manual. One word of caution: After I got the tranny buttoned up I found the modulator spring in the drain pan, which meant that I had to take it apart again! This was not happy news at 10:00 on a Sunday night. (see shop safety, above)

Carburetor choices

There are too many carburetor choices available to discuss more than general principles here. A general statement for carburetors is don't over carburet! An oversized carburetor will produce economy and driveability problems. At worst, an oversize carburetor will provide more gas than your engine can burn, causing oil dilution and spun bearings. A massive tunnel ram may look trick, but it is hard to look cool when you car bogs at the starting line or stalls at the light.

Gary Lee wrote: Installing a carb that is too big for a given application does not bog the motor down with too much fuel. The problem is too little fuel and too much air.

Spread-bore carburetors and carburetors with vacuum secondaries, air valves or air doors are more tunable and forgiving. Later model spread bores like the Carter Thermoquad and the Rochester Quadrajet are "one size fits all" carburetors designed to work on a range of engines with jet changes. They can get away with this because vacuum secondaries and air doors won't give the motor more air than it needs.

Vacuum secondary carburetors provide better gas mileage, and may eliminate bogging while providing better pickup in heavy vehicles. Mixture rod carburetors are easier to fine tune than power valve carburetors because you can replace rods without disassembling the carburetor. (You still have to disassemble the carb to change jets, though.)

Which setup you choose depends upon what you do with your vehicle. I use my 1977 W150 Power Wagon as a farm truck and I pull a 12,000 lb horse trailer, so my main requirements are low rpm power, driveability and reliability. My truck usually stays parked all week and works hard on weekends. It has to start and idle in cold weather and it has to start and run for my non gearhead wife without goofy tricks or voodoo.. It can't be too loud. It has a stock 360 w/ headers and a cold air induction system. I don't have much money to spend on it. My setup:

Everything has tradeoffs. The simple square bore carburetor trades a little fuel economy, driveability, and top end power for low end power and reliability. Likewise, the iron manifold trades power for cold weather driveability. The Cordoba air cleaner is more restrictive than an open element air cleaner but it is quieter, improves driveability and emissions, and allows me to use a leaner mixture. Finally, the whole setup was cheap. This setup produces about 10 to 11 MPG in mixed driving, which is better than the two barrel 318 that it replaced.

Scott Preston wrote:

Here is the math to figure out the size of carb you need. Max rpms you will turn with the engine times its cubic inches (is it a stock bore or oversized, exact number is needed), divided by 3456, times 0.85 for street carb. So for example if you're working with a 273 and its max revs are 6,000 rpm:

273 x 6000rpm=1638000/3456=474 cfm x0.85=403cfm carb

If you get a 450 cfm carb you will be oOK.

Engine tuning

Once you have your system installed and running you need to tune it for best performance. The best thing to do is take the vehicle to a facility that has a chassis dynamometer. The next best thing is a an oscilloscope. I dialed in my truck the old fashioned way by keeping a log of appropriate parameters (Idle vacuum in neutral and drive, jet & mixture rod size, mixture screw settings, accelerator pump setting,) Under this I would write down the driveability and MPG that I observed. I also made up a matrix that cross tabulated jet and mixture rod sizes so I always knew how to achieve a relatively leaner or richer mixture.


Anthony Matlock wrote:

I had no need to alter or replace my kickdown on my upgrade to a 4bbl Holley.  The only major modification to my linkage was with the throttle cable.  I purchased a CSI Racing throttle bracket, which according to the manufacturer would not work for my purpose.  I ground down the protrusions on the cable that prevented the cable from going through the mount.  I used a couple of nuts and screws to provide as the mounting points for the cable and the kickdown.  The kickdown is fully adjustable now that I have a nut and bolt placed in the slot to provide as a stop.  I would advise that you would only need to replace the valve cover gaskets if they are old, leaking, or the new manifold is equipped with an EGR valve, otherwise the replacement is not needed.  I must note that the intake is an Edelbrock Performer series aluminum manifold which is taller than any of the stock intakes available.  This posed different challenges which I had to deal with.

Swapping a 360 for a 318

KOG wrote about replacing a 318 with a 360, in a 1970 A-body: "You need a left motor mount bracket for 360, balanced torque converter, 3 piece kickdown rod (A body - which you should have) and driveshaft front yoke. Driveshaft will have to be shortened and the 7-1/4" rear axle will have to go - which it will do shortly without any additional help. An 8-1/4" from a '74-'76 V8 will work (2.45 gear) or you can try to hunt one of the fairly rare 8-3/4" axles from a 340 car. You'll really want to get the K member and disc brakes from a '73-76 while you're at it - the 360 is MUCH quicker than the 318." ... " you'll have to change exhaust Y pipe or use 318 manifolds - which will choke the 360 somewhat. The 904 and the rear axle will not tolerate much full throttle use of a 360. The brakes on the car are flat out inadequate for the 318, never mind the 360."

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