The Chrysler Slant Six Engine (170-198-225)
The Slant Six became a legend for its endurance, but for a few brief, shining moments, it was a true performance engine as well.
Nicknamed for its 30° slant when installed, the slant six had a better mixture of power and economy than contemporary GM and Ford straight-sixes. The 30° slant came about becuase it had to fit under the compact, low-slung Valiant’s hood; it made room for efficient manifolds, which may be why they kept the slant when it was used in bigger cars and trucks.
The slant six manifolds’ bends had large radii; intake tubes and exhaust tubes were nearly equal in length. The design assured that each cylinder got the same amount of air and fuel at the proper time, and that exhaust gases were fully, equally vented. Competitive designs made some cylinders “double up,” or had sharp bends or tubes with different lengths.
The 1960 HyperPak option used a four-barrel carburetor and numerous other performance parts to push out a V8-like 196 horsepower. Modern racers have used turbochargers and different carburetion and manifolding to produce far more.
The 1977 “Super Six” with a two-barrel Carter carb compensated for smog controls and increased responsiveness without loss of economy, and with the Feather Duster package, the slant six was capable of 30 mpg on the highway. An export version for Australia pushed the engine to 160 bhp, again using a two-barrel carburetor.
|1960 Slant Six
vs GM and Ford
|Ford 223||Slant Six 225||Chevrolet HyThrift|
|145 hp||145 hp||135 hp|
|206 lb-ft||215 lb-ft||207 lb-ft|
|Choke||Manual||Fast-reaction automatic||Tube-fed automatic|
Heavy-duty slant six engines used in industrial and some truck/bus (perhaps fleet as well) applications used a double-row roller timing chain, chrome-plated upper piston rings, and other features depending on service duty (polyacrylic valve stem seals, positive valve rotators, stellite-faced exhaust valves, and/or a high-volume oil pump). You can retrofit the double roller chain and gears into car slant sixes, according to some sources; the double row timing chain is NAPA part 9168, used from 1962 to 1975 on the slant six; the sprockets are part S338 and S339 (thanks, Bob Rodger).
To simplify maintenance and increase reliability, the plugs, distributor, and coil were grouped closely together on one side. Most replaceable parts, including the filters, air cleaner, fuel and water pumps, coil, and dipstick, were relatively easy to reach. In 1960, the pistons were a tin-plated aluminum alloy, with cast-in steel struts for thermal control; the top ring was chrome plated, the second ring was tin-plated, and a sectional oil control ring had chrome-plated steel rails.
The slant six replaced an old flat-head design starting in 1960 models, and lasted in US-built cars through 1983, in US-built trucks until 1987, as a marine engine until 1991. After production in Trenton, Michigan, ended, it was produced at Chrysler's Toluca, Mexico engine plant, until the Magnum 3.9 liter V-6 MPI engine started up. The slant six would be the last in-line Chrysler engine with more than four cylinders.
Charlie S. noted that you can tell the difference between the 170 and 198/225 engines by looking at the rubber hose that goes from the water pump to the underside of the head: if it is about 1 1/2 to 2 inches long, it's a 170 block (G engine). If it's about 3 inches long, it's a 198/225 (LG engine).
Why the six is slanted
By slanting the engine in the compartment, four distinct advantages were achieved, according to a 1960 press release:
- The center of gravity is kept much lower than in upright engines.
- A low, stylish hood is made possible (in other words, the engine fit into the Valiant).
- Engine accessories are more accessible.
- The water pump can be moved to the side of the engine to reduce the engine's overall length.
While the press release did not mention it, the most important advantage might have been making room for an unusually efficient intake and exhaust manifold. This gave room for wide bends and nearly equal-length tubes going to each individual cylinder, preventing airflow restriction and uneven airflow. But the lower center of gravity would also help the Valiants — the early, more nimble Valiants and Barracudas in particular — to be in the top of their class for handling.
Facts for owners
The slant six gained electronic ignition in 1973 (many earlier engines were retrofitted). The only component likely to break down was the ballast resistor, which costs about $3 and two minutes, but many mechanics replaced the entire electronic ignition system instead.
Valves needed to be adjusted manually until hydraulic lifters were used, starting in 1981 (a small number of earlier fleet vehicles had hydraulic lifters).
Once a year or so, a couple of drops of light oil need to be placed onto the felt pad under the rotor in the distributor. The factory recommended engine oil for this.
The crankcase inlet air cleaner was supposed to be cleaned annually, though often owners and mechanics simply ignored it, resulting in blue smoke from the exhaust as it clogged.
Wet weather stalling can be fixed by using a high quality distributor cap and rotor (e.g. Mopar or Bluestreak), high-quality wires, a distributor cap gasket (Standard PN AL-483G), and putting weatherstrip foam over the ignition and voltage regular modules. You can get wires and other parts cheaply on-line.
From birth through 1972, the oil filter standpipe was too long to allow for short filters, and long-canister filters, such as PureOne PL30001 and Fram PH8A, must be used. Scott S. recommended using Ford filter FL-30001 (made by Purolator) because its double seals, which prevent the engine from being starved for oil. Starting in 1973, shorter filters such as the STP S-16 become more advisable since the tall filters won't fill all the way, and the engine will get oil a little later on startup (thanks, Scott S.).
Owners would be well served by checking our page on slant six spark plugs and spark plug tubes.
Dan also advised owners to check for timing chain stretch.
Ben also suggested using a composite rubber/cork gasket: “I've found the rubber ones tend to harden up too quickly, and then leak, and pure cork isn't much better. In so far as additional sealants are concerned, I don't believe the rubber gasket is intended to have any sealer applied to it.”
Firing order is 1-5-3-6-2-4 (thanks, Ed Friedemann) - note the diagram in this section, sent by DJAlDogg.
A common problem is manifold cracking - see below for a discussion.
In most years, the slant six used a forged steel crankshaft, moving to cast nodular iron in 1976 as a running change.
The standard carb for many slant sixes was a single-barrel. You can increase performance and gas mileage by switching to a good dual-barrel carb, but that requires swapping the manifolds, too. Rodger also suggested an easier swap:
Go to NAPA and ask for a inside diameter 2 1/4 inch muffler with a 2 inch inside diameter outlet. The longer the body is, the better it will sound at the end of the summer. On the original engine, they used a smaller than needed diameter exhaust pipe to the muffler; this saved a production "nickel" every time a car was made. The high performance variations are different.
Dan Stern provided these standard carburetors used by Chrysler for North American applications (different setups were used outside North America):
Walt Ronk wrote about the Lean Burn (the world’s first such computer-controlled system, unless you count the electronic fuel injection used on some 1958 Chryslers). He said that, according to the Mitchell guide, there was no difference between 1979’s Electronic Spark Control ignition (ESC) and Electronic Lean-Burn ignition. The ignition box is mounted on the air cleaner. There was an early style with two pickups in the distributor and a later version with a single pickup.
Slant six performance
As for the Super Six, Pete Hagenbuch said:
Here I was [in 1975] the head of the valve group working in the road test garage with 2 road test mechanics that didn’t know much about engines. The three of us together managed to set up the first two barrel 225. I don’t know if we called it a power pack or not but that was one swell engine for performance anyway. That really turned out to be a winner, in fact my youngest son had one and he just loved it.
... The first and most important thing was the carburetor; it had 2 holes in it. Oh, there is so much you can do, Dave, that doesn’t even show. We messed around with the spark advance schedules and did a super calibration job on the carburetor. We had a low restriction air cleaner. Improved exhaust system but still single of course. I wanted in the worst kind of way to have a twin exhaust system because, man, will that do wonders for a six. You put one, two and three, and four, five and six together and you run ’em down about 6 or 8 feet and bring them together in one tailpipe and you’ve added great huge gobs of output.
Engines, especially engines that are low output, the things you can get for just a little bit of effort are just unbelievable sometimes. But to turn that 225 from what I always thought was a real slug, but a dependable, durable slug, into one hell of a nice engine, that was a fun program.
Like the standard slant six, the Super Six used a cast-iron crank, long-branch intake with curved pipes, hardened valve seats, a single exhaust with catalytic converter, aluminized steel exhaust, and, starting in 1978, steel-backed aluminum-alloy main bearings.
Bill Cawthon wrote: “The first Bricklin concept car, built by Bruce Meyers of Meyers Manx fame, had a Slant Six under the hood.” (The first production engines used 220HP AMC 360 V8 engines.)
All in the slant six engine family
Joseph Newhouse provided the following chart which applies to slant six powered cars from 1959 to 1978. Trucks could be rated somewhat lower than cars (140 hp vs 145 hp gross). These are Federal numbers and could vary in California by 6-15 hp. The slant six continued in use through to 1987 in Mexico, where, with a rejetted Carter YPF two-barrel carburetor and recurved distributor to reduce emissions by 30%, it was rated at 98 hp net.
(red = gross)
170 1959-1963 8.2 to 1 101 @ 4400 155 @ 2400 170 1964-1966 8.5 to 1 101 @ 4400 155 @ 2400 170 1967-1969 8.5 to 1 115 @ 4400 155 @ 2400 198 1970-1971 8.4 to 1 125 @ 4400 180 @ 2000 198 1972 8.4 to 1 100 @ 4400 160 @ 2400 198 1973 8.4 to 1 95 @ 4000 150 @ 1600 198 1974 8.4 to 1 95 @ 4000 145 @ 2000 225 1960-1962 8.5 to 1 145 @ 4000 215 @ 2800 225 1963 8.2 to 1 145 @ 4000 215 @ 2400 225 1964-1971 8.4 to 1 145 @ 4000 215 @ 2400 225 1972 8.4 to 1 110 @ 4000 185 @ 2000 225 1973 8.4 to 1 105 @ 4000 185 @ 1600 225 1974 8.4 to 1 105 @ 3600 180 @ 1600 225 1975 8.4 to 1 95 @ 3600 170 @ 1600 225 1976-78 8.4 to 1 100 @ 3600 170 @ 1600 225-2V 1977-78 8.4 to 1 110 180
Lost slant six performance potential
Auto Trader’s Neil Newman (as interviewed by John Gunnell) said there was a slant six in the Engine Development Lab with a special intake carrying dual four-barrel carburetors — not a Hyper-Pack, but a dual quad setup. “We got a tremendous amount of horsepower with it, but it didn’t idle well.”
Executives dropped the aluminum RG in 1963, and rejected an even larger (246) block with side-fed hydraulic lifters, and a high-swirl, fast-burn aluminum head (developed in 1978) that could have pushed the slant six on past 1989 by increasing power and cutting emissions. Other possibilities (noted by Daniel Stern), most of which were never produced:
- A908, 1958-60: sand-cast aluminum 170 LG engine
- A909, 1958-61: die-cast aluminum 170 LG engine
- A785, 1959-60: Hyper-Pak 170 LG (Sold 1960-61 as a parts package)
- A941, 1962-66: overhead cam 225 RG engine (in 1962!)
- A106, 1964-65: G-RG engines with 180 and 246 c.i.d.
- A227, 1967-68: 246 CID EG engine with hydraulic tappets using added oil gallery in cyl. block
- A294, 1969: 4-cylinder derivative of G-RG engine
- A396, 1973-74: 225 RG engine with 3rd-valve prechamber [á la Civic CVCC and Mitsubishi MCA-Jet?]
- A420, 1975-80: Diesel version of 225 engine
- A431: 1976-79: Aluminum fast-burn cylinder head for 225 engine
- A463, 1977-78 225 RG turbocharged engine*
- A473, 1979-80: 225 engine with Bendix multi-point EFI
- A489, 1980-83: 2.2 litre 4-cylinder derivative of A420 225 RG diesel engine.
- A497, 1980-83: 2.2 litre 4-cyl turbodiesel version of A489 engine
- A513, 1981-83: 225RG turbodiesel engine
* Chrysler engine tester Marc Rozman remembers seeing a butterscotch-color 1975/76 Monaco with the turbocharged slant six — it ran very well.
Jim Grundy wrote, “I was an engineer at Chrysler from 1967 – 1971. I designed an overhead cam slant six while I was at Chrysler, and built it in 1971 - 1972 (170 cubic inch). I installed the engine in a Dart. I wanted to use the engine as an example of OHC conversion so I could convince someone to help me create an OHC conversion for the 392 HEMI. I was unable to find interest for this project, so I sold the car in 1973. Then, in 1992, a friend of mine saw my engine at the MSU engine laboratory. I purchased the engine from MSU and it has been stored in garages ever since.”
Some that actually made the cut but are not well known include:
- A826, 1961: die-cast aluminum 225 RG engine (sold from 1961-early 1963; dropped because Chrysler could not achieve high production numbers)
- A436, 1976-80: Lightweight 225 RG engine (hydraulic tappets); weight was only cut by 12 pounds.
(Source: A History of Chrysler Corporation's Slant-Six Engine, Volume II)
Revell made a model of the slant six for a brief time; it included a book by Willem Weertman on engine design.
Slant six engine - Problems and repairs
David Wordinger wrote about changing the spark advance on a 1971-76 225 engine. Other years are often similar.
Set the idle speed and timing according to the specifications (often on a sticker under the hood but also available in reference books.)
[If you have problems setting the timing], remove the spark plugs so the engine rotates freely. Pull the cap off the distributor. Now rotate the crankshaft back and forth a few degrees in both directions. The easiest way to do this is probably by turning the fan. Watch the rotor in the distributor. If you can turn the crank without the distributor moving, you probably have a bad timing chain.
I think all slant six distributors have two places to adjust timing. Looking down from above, there is a slotted plate that bolts to the engine that you have been using to set the timing. The other end of this plate has a similar slot that bolts to the distributor housing from the bottom. You can't see it with the distributor installed, but this will give you more adjustment.
[Warning: this second bolt should only be used in extreme cases. As the Slant Six Club’s Ben Deutschman wrote, “That other bolt is set in the correct position by the factory, and should not be played with. Remove the distributor from the engine, making sure to note where the rotor is pointed, before touching the hold-down bolt. Once the distributor has been removed, the other bolt can be loosened with a 7/16 socket or wrench. Since the distributor is gear driven, great care needs to be taken when removing it, to not damage the nylon drive gear, which is no longer commonly found in auto parts stores.”]
Don't forget to plug the vacuum advance hose after disconnecting it from the distributor. Even when the vacuum advance hose is disconnected, the advance changes as the engine revs. It's normal. The weights for the mechanical advance are under a plate in the distributor. [That’s why you need to adjust the engine speed first, then the timing.]
Again, the felt pad under the rotor must be lubricated once a year or so.
Cold driveability problems tend to stem from poorly adjusted choke and choke pulloff, bad accelerator pump, and sloppy carb rebuilds (Carter BBS one barrel is better than Holley 1920.) Other big driveability problem source is the fact that the vibration damper outer ring tends to slip, which makes the timing mark WAY OFF. Which means timing would never be accurately set. Also check for timing chain stretch.
Check by putting the engine at #1 TDC - top of compression stroke (both valves closed) and see where the timing mark is. There are companies that re-bond dampers with new silicone material. I think one is called Damper Dudes, out of California. I don't know if this happens on other CC engines. Basically, if your damper has an inner hub and an outer ring sandwiching rubber bonding material, this can happen.
One of the more common slant six problems, decades after their creation, is cracking manifolds. "Dave B" wrote: "por15 makes manifold repair kits which would fill a hairline fracture if you don't feel like replacing the whole thing." (Keep on reading for more opinions on manifold cracks and exchanges.)
I believe that all the heads from all the slant sixes are interchangable. The carb and choke linkages need to be taken from the donor car and you still might be short some parts but the manafolds are bolt on. You might just have to manufacture the linkage [if using a van engine on a car or vice versa] because the vans used a different setup than the cars.
Be sure and put the manifold bolts and washers back in their proper position and follow the torque specs and sequence exactly else the manifolds crack.
Dan Stern added:
The single-piece sand-cast aluminum two-barrel intake sold by Mopar Performance is an excellent casting. The two-piece welded-together "ultra light weight" EB-welded intakes used between about 1979 and 1982 were the subject of a few different TSBs and recalls over the short period they were used in production vehicles. Most of them were replaced under warranty with cast iron manifolds.
You can greatly reduce the likelihood of porosity problems by having one of these manifolds powder coated to seal the casting. Or, after proper cleaning, you can use several coats of epoxy paint (tough to find in Chrysler red) to do the same thing.
Warpage at the head sealing surface and the exhaust manifold junction are still much bigger issues with this manifold than with any other made for the slant 6. Thick gaskets can help here. The issue with these manifolds doesn't mean every single one is a piece of junk. It's like the porous castings in the 1975-77 aluminum-case A833OD transmissions. Many of them were fine and never seeped a drop of fluid, but many of them were "not-fine" and did seep fluid.
Because the bad ones have mostly disappeared from the roads, the odds are higher of finding a usable 2-piece intake are higher now than they were 10 or 15 years ago.
While all exhaust manifolds had the same two inch outlet, the headpipes on the super six engines differed.
The 1974 and later cars used a smaller one-barrel carb, but the trucks continued to use the original larger carb. The 1973 and earlier cars were all rated at 110 HP as were the 1977 and later 2 barrel “Super Sixes.” The trucks, with different emission standards, kept the larger single barrel and continued to make 110 HP. The 2 barrel carbs are the same type as 318, but are jetted differently and the stock ones are a smaller bore than 318. The 2 bbl also usually gets poorer mileage in my experience. (Dan Stern noted that other differences include a different choke mechanism, linkage, and power valve calibration.)
Zachary Good wrote about fixing exhaust manifold cracks:
I'd suggest going to a junkyard and pulling an exhaust/intake manifold off any slant 6 around the same year (Valiant, Dart, Aspen, etc.) and replace the whole assembly. It's easier than pulling the two apart on both cars. Just make sure the new intake manifold is set up for your carb. It'll be worth it, repair kits last at best a few months.
Todd Johnson added: "You can also find a competent welder...he can weld cast iron."
Wes Moeller wrote: “Dutra Duals are another option. Depending on where the crack is on the exhaust manifold, you can cut off the front ‘leg,’ and then cap off the hole. Then this modified exhaust manifold would be for the rear three cylinders, and the new casting from Doug Dutra would be for the front. That's what I have on my 1978 225 Super Six Volare, works great.”
Gerard Duchene noted: "I remember when we first started running hydraulic cams on the 225 and did we have problems in the hot test area! I am pretty sure it was in 1978 [though they were not released in that year]. We shipped the 225 to Mexico in 1979.
As you can imagine, going from a solid lifter to a hydraulic lifter is no small feat. At the beginning 50% would not start or had pounding lifters. I guess there were some bugs they didn’t see coming.
I also spent a few years on the Merry-Go -Round, a huge revolving 100 foot diameter wheel with stands where we hot tested the 400 and 440 CI engines, man ..talk about a loud place to work! Nothing like listening to 20 440s running at the same time at 5:00 am with almost no exhaust!"
Dan Stern wrote:
The hydraulic lifter setup was designed and developed at WEP (Windsor Engine Plant, Chrysler Canada) and a small number were produced in 1978, and placed in fleet service to monitor in-use durability. No other hydraulic-lifter slant-sixes were produced until the 1981 model year, when all were so equipped.
No oil galleys were added to the engine block. Instead, the rearmost camshaft bearing was fully grooved to feed high-volume oil up through the head to the rocker shaft, through the rocker arm bodies, and down through hollow pushrods to new top-feed hydraulic lifters. This unique and innovative method of feeding the lifters eliminated the need to rework the block.
Pete Hagenbuch wrote about an early development problem with oil passing across the rings:
Aside from the "slant," the only other unusual features were its stamped valve rockers and the steel-rail oil rings, the latter resulting from a thorough investigation into all the factors affecting oil usage past the rings. In six months I was the oil ring guru or expert (we always said an expert was anyone more than 25 miles from the home plant) or something.
We looked at the finish of the cylinder bores, twist properties of the compression rings, clearance between the ring lands and the bores, the results of using chromium plated ring faces and their effect on run-in oil economy and we evaluated oil ring designs from all out ring vendors. On the 170 we left everything alone. On the 225 we released the first steel rail oil ring in Chrysler history, the Sealed Power SS 50 with chrome. We eventually released a chromium plated top ring and a reverse twist second ring. If I remember correctly, one of the vendors came up with a process to give the chrome plating an initial surface with an abrasive treatment just like we'd want it after run-in.
The entire package didn't get into the 1960 model year but it did in 1961 and the problem went away. Once the SS50s got into production it was no longer a panic but we needed the chrome for long-term. The reverse twist ring was nearly immediate, all the vendors had to do was put their "top" mark on the other side. Why? Because most compression rings were made with positive twist, which was produced by cutting a chamfer on the top inside corner of the ring. Reverse twist merely moved the chamfer to the bottom.
- Interview with engineer Pete Hagenbuch
- Interview with architect Willem Weertman
- Other stories of the slant six's development (at Valiant Varieties)
- Spark plug replacement guide, with spark plug tube seal replacement
- Full 1968 Slant Six specifications with selected specifications from other years
- The Valiant, Duster, (etc) Page
- Dodge Dart
- Slant Six Performance Upgrades (at Valiant Varieties)
- Modifications - one person's experience
- Inliners International
- Slant six.org
- Slant Six Club of New York and New Jersey