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The Chrysler Slant Six Engine (170-198-225)

Thanks to Dan Stern! | Slant Six en español (traducción personalizada)

1961 aluminum slant six engineThe Slant Six, introduced in late 1959 for the Valiant, has a legendary reputation for endurance - not unlike the flat-head engines it replaced. It also has a hidden secret: good performance, given the right conditions. Unfortunately, for most of its life, Chrysler burdened it with a single-barrel carburetor that wasn't known for speed. According to one of its designers, and a developer of the Super Six project, the slant six responded quite well to better carburetion and exhaust. You can read a full interview with engineer Pete Hagenbuch here.

Even with the standard carb, the slant six (so named because it was a straight six - in other words, an in-line six - slanted over 30 degrees) had a nice mixture of torque and economy, with power better than similar, same-year GM and Ford offerings. The "Super Six" with a two-barrel Carter carb compensated for smog controls, and with the Feather Duster package it was capable of 30 mpg highway. An export version pushed the tilted engine to 160 bhp, and the fabled HyperPak (a Mopar aftermarket package with a four-barrel carb) raised the USA bar to 196 hp (on the 225).

The heavy-duty engines used in fleet and heavy duty truck/bus service used a double-row roller timing chain (but just one 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).

Hyper-Pak details including how to do it yourself

Slant six development story

slant six enginesThe slant six replaced a flat-head design in 1960 models and lasted in US-built cars through 1983, in US-built trucks until 1987, and survived as a marine engine until 1991. After USA production (in Trenton) ended, it was produced at Chrysler's Toluca, Mexico engine plant until the Magnum 3.9 liter V-6 MPI engine was introduced, continuing to serve in Mexico and possibly South America through (at least) 2003. The next new truck six would be a derivative of the 4.7 V8, and would be introduced in 2001; but the slant six would be the last in-line Chrysler engine with more than four cylinders.

Some major changes include a hydraulic cam (1978/1981, see below), and changes to the spark plug wells (their tubes were cast into the heads starting in 1975). Scott Fluke noted that with the later spark plug well design, you must replace lifters (if you plan to do that) before installing the head.

Charrlie 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).

This is hardly a complete list, and we would appreciate a chronology of the slant six.

Why it’s slanted (from a 1960 press release)

By slanting the engine in the compartment, four distinct advantages are achieved:

Development stories

slant sixGerard 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. 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! Anyway thanks for letting me share some of my memories at Trenton Engine, Home of the Fire- Eaters."

Dan Stern noted: "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, subsequently 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.

More development stories are told by Bob Scott.

Bob Scott wrote:

My brother George worked in the camshaft and intake research department, so there were lots of interesting things going on. He got to know Tom Hoover very well as the slant six was Tom's first big project.

Early on, Tom came into see George and showed him the flow chart results from the slant six heads. It was purely accidental, but the results were astonishing. The valves performed way better than anything that they had worked on, including the best of the Hemis. Tom remarked at the time that "it looked like we have a racing engine here."

Early on, they had many problems with the prototypes, one of which was excess oil consumption. It stymied all of the engineers working on it until someone discovered that the rods had all been installed backwards, thereby directing oil to the wrong area. Once that was discovered they had no problems with it from that standpoint.

Dan Stern noted: "In 1963 the conrod was turned 180 degrees (same rod, just with the oil hole pointing the other way) which was said to improve cylinder wall oiling."

slant sixSince the camshafts were part of their department, they were interested in experimenting with variations which might improve performance. Rather than do this hit and miss, they brought in high performance engines from other manufaturers and measured the cams. They were especially interested in the cams that were on the small block Chevys of that time, mainly the legendary Chevy 270 which was the scourge of Woodward Avenue. So they measured the dimensions of that cam and copied them for the slant six and it worked great. I believe that was the cam that was offered with the astonishingly effective hyper kit. Later on, they measured camshafts that were used by Porsche and discovered that Chevy had copied those. So, the engineering game is not always invention, but imitation.

He says that the only reason that the one barrel carburetors were used was cost. It was supposed to be an inexpensive engine in inexpensive cars, so they went on the cheap whenever possible. Everyone knew it would perform better with a good two barrel. As a matter of fact, they did make some marine engines with the two barrel [two barrel carburetors were also offered on the Volare/Aspen "super six."]

The more I learn about it, the more respect I have for that little 170 engine. Another reason that the 170 was/is great to "soup up" is that the lower end proved to be extremely strong and able to withstand just about anything. George proved that when he put the Paxton supercharger on his and got the boost up to a full atmosphere. Never fazed the lower end of the engine. [Several people have written to tell us that the 225 is most commonly souped up because of the larger chamber size.]

Later note: Clifford Performance sells an original specification long runner Hyper-Pak intake manifold for the 170 Slant Six and mentions a certain conventional bodied Six Pack car that turned in 12.34 seconds with their equipment.

For more from Bob Scott, click here!

Facts for owners

A common problem is manifold cracking - see below for a discussion.

slant six firing diagram The slant six gained electronic ignition in 1973 (earlier models could be retrofitted, and many were). The electronic ignition was extremely reliable, and the only component likely to break down was the ballast resistor, which costs about $3 and two minutes. However, most mechanics seem to replace the electronic ignition system instead.

Also unknown to most mechanics is the crankcase ventilation system and the crankcase inlet air cleaner, an important part which must be cleaned once a year (unless you like blue smoke, in which case I suggest you buy a used Chevy). This applies to V-8s as well as sixes.

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.

Dan Stern recommended Autolite 56 (nonresistor), 66 (resistor), or AP66 spark plugs for pre-1975 SL6. In the ten or so years since that first recommendation came down, he wrote:

But the Autolite 925 works very much better, I've learned in the meantime. Tough to get it off the shelf, usually has to be specially ordered, but its extra-long electrodes put the spark closer to the center of the combustion chamber and away from a quenchout area. These plugs were originally designed for engines with difficult combustion conditions (stratified charges, extra-lean mixtures, etc.) such as the emission-controlled AMC 232 and 258, and Chrysler's own 1981-83 318 TBI engine in the Imperial.

Dan also advised owners to check for timing chain stretch.

Ben Deutschman of the Slant Six Club of New York/New Jersey wrote that owners should be careful not to put on spark plugs with the "crush washer" (the loose metal washer at the end), but to take off the washer first, if they have a 1960-1974 engine. After 1974, the head was redesigned, the plug tubes were eliminated, and taper seat plugs were used. If you do have a 1974 or earlier slant six, be sure to replace the plug tube seals - they are cheap and easy to replace.

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.

The standard carb for many slant sixes was a single-barrel. You can probably increase performance and gas mileage by switching to a good dual-barrel carb. Dan Stern provided these standard carburetors used by Chrysler for North American applications (different setups were used outside North America)

1960-1971: Carter BBS one-barrel
1962-1972: Holley 1920 one-barrel
1963 only: Stromburg WA-3 one-barrel
1974-1980: Holley 1945 one-barrel
1981-1987: Holley 6145 feedback one-barrel
1976-1981: Carter BBD two-barrel

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):

[My 1979 Volare Duster slant-six] manufactured in early 1979 has the Electronic Spark Control ignition (ESC) on it. I researched this in the Mitchell guide to find out what the difference was between the Electronic Lean-Burn ignition and the Electronic Spark Control ignitions... there is no difference other than name. The ignition box is mounted on the air cleaner. While it didn't go into a reason for the name variation; it did state that there were 2 different variations of this ignition, an early style with 2 pickups in the distributor and a later version with a single pickup.

Slant six performance

We have moved the performance section to our Valiant site and have sections on slant six racing and the slant six Hyper-Pak.

All in the slant six engine family

Engine Year Official Horsepower Ratings Torque
170 1959 - 69 101 to 115 bhp (gross) after 1966 155 lb–ft
198 1970 - end 125 (gross) ?
225 1961 - 87
  • Gross: 145 hp (before 1971)*
  • Single-barrel: 90-100 hp
  • Dual-barrel: 100-110 hp
  • Final California single-barrel: 85 hp (net)
  • Final Mexico dual-barrel: 98 hp net
    (with rejetted Carter YPF carb, recurved distributor
    to reduce tailpipe emissions 30%) 

215 lb-ft
(1-bbl)
225 1961 - 63 Aluminum block with iron head and cylinder liners to save 80 lb 215 lb-ft
(1-bbl)

Another, more detailed (and probably more accurate) chart (courtesy of Joseph Newhouse)

Engine Year(s)
Available		 Compression
Ratio		 Horsepower @ RPM
(see note)		 Torque  (lb-ft) @ RPM
(see note)
170 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 8.4 to 1 100 @ 3600 170 @ 1600

Journal Number

1980 3.7-Liter (225 CID) Slant Six

1

2.75' x 0.877"

2

2.75" x 0.877"

3

2.75" x 1.107"

4

2.75" x 0.877"

The engine had four replaceable steel-backed (aluminum alloy on steel) babbitt crankshaft main bearings. Oil grooves were in the upper half of the bearings, and in the lower half of the #1 bearing. The connecting rod bearings were aluminum on steel. In most years, closed-circuit crankcase ventilation was used; crankcase vapors were routed to the combustion chambers through a PCV valve located in the valve cover. Vapors were burned before being expelled into the air through the exhaust system. Oil pressure ranged from 35-65 p.s.i. @ 2.000 rp.m.  Crankcase oil capacity was four quarts (one more when changing the filter).

Lost slant six performance potential

Slant Six line drawingThe beancounters nixed the aluminum RG in 1963, an even larger (246) block with side-fed hydraulic lifters, and, finally, a high-swirl, fast-burn aluminum head in 1978 that could have pushed the slant six on past 1989.

The Slant Six had a lot of unrealized potential.

(Dan provided the following list of slant six possibilities, most of which were never produced):

Some that actually made the cut but are not well known were:

.