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How to Build a Slant Six for Performance

Glenbrook is an online mechanics writer, who enjoys rebuilding and modding classic Chrysler cars.

A clean-looking 1971 Slant Six Duster for sale in Stockton, CA.

A clean-looking 1971 Slant Six Duster for sale in Stockton, CA.

The Chrysler Slant Six is my favorite engine. It's one of the most bullet-proof engines ever designed, its reliability is unmatched, and it has great potential for power and economy. A Slant Six built for economy can get almost 30 miles per gallon. Build it for speed, and it can put a lightweight car into the 12-second quarter-mile range.

Whatever your goals, building and modifying a Slant Six is a lot of fun. It's the perfect engine for Mopar's early A-Body cars like a 1964 Valiant Station Wagon or a 1966 Barracuda. If you're tired of all the "belly button" V8 powered cars, give the Slant Six a try. You'll have a blast.

Why Build a Slant Six?

The Chrysler 225 Slant Six probably offers the most fun per dollar of any classic American engine ever made. If your car has a Slant Six, there's almost no reason to convert to a V8. It's easily the most versatile PentaStar in the Mopar constellation (with the possible exception of the 318). If you want fuel economy, the Slant Six has it. If you want to go fast, the /6 can do that too, especially if you slap a turbo on it... you can really embarrass a lot of V8s. Try one—I know you'll like it...

Blown engine: not your grandmother's Slant Six...

Blown engine: not your grandmother's Slant Six...

Improving the Mopar Slant Six Performance

As much as I like the Slant Six, the factory left lots of room for improvement. Actually, that's one of the reasons I like it so much. It would be pretty boring if it were perfect from the factory.

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Here are the areas (in no particular order) that will pay the most dividends in terms of reliability, mileage, and performance:

  • Pistons and Connecting Rods. The stock piston and rod assembly on a Slant 6 is heavy, offers a less than optimal ring package, and gives a low compression ratio. For serious performance builds, I recommend replacing the stock rods and pistons with K1K1 or Molnar connecting rods and Wiseco pistons. If you're on a budget you can use 198 rods and Keith Black KB268 pistons (read a more detailed description of this conversion), though it might be tough finding a set of 198 rods. Either combination gives you a much better piston and ring combination than stock and boosts compression ratio to about 9.2:1.
  • Exhaust System. The exhaust system on a stock Slant Six is a puny single pipe. It is adequate (barely) for a stock vehicle, but for any type of economy or performance application, it should be upgraded. Options include headers (available from Clifford Performance) or Dutra Duals.
  • Intake Manifold and Carburetor. The best Slant Six intake for a street driven car is the factory Super Six 2 barrel setup. These came with either aluminum or cast iron intake manifolds. Although I usually favor aluminum manifolds for the weight savings, I've read that the aluminum Super Six manifold casting have quality control problems. I haven't confirmed it, but until I know different I prefer the cast iron version. If you need more performance, 4-barrel manifolds are available from Clifford and others. Offenhauser had a good 4-barrel intake that is no longer in production but is easy to find on eBay. IMHO, the ultimate intake system is an Offy dual intake modified to hold a pair of Weber 2-barrel carburetors. Clifford also offers an EFI setup for the Slant Six, as well as a triple Weber side draft manifold.
  • Cylinder Head. The Slant 6 really likes a multi-angle valve job. My old Mopar Performance shows a 5-angle seat, a concept that recently came into vogue for high-performance V8 engines. The stock valves are downright puny and should be replaced with larger Clifford valves for any kind of performance use. The ports could also use some work, the idea being more to straighten them rather than enlarge them excessively.
  • Valve Cover. The stock valve cover is stamped steel. It is also long, therefore somewhat flexible and prone to leaking. This won't hurt your engine, just make it look grungy. If you don't want the grunge, you can replace the stock cover with a nice cast aluminum valve cover from Clifford or Offenhauser. The cast aluminum is much stiffer than stamped steel, eliminating oil leaks. They also look a lot nicer, especially the polished aluminum Offenhauser piece.
  • Distributor and Ignition.

A Practical Street Slant Six With Improved Power and Gas Mileage

A Slant Six may never be as efficient as a modern fuel-injected engine but it still makes a great engine for a daily driver. The long stroke means the Slant Six has plenty of low-end torque which is just what you want for a daily driven street car. The recipe for this mild Slant Six build is pretty basic as well as affordable. It should give you at least 23 mpg on the highway (depending on driving habits, gearing, etc.) along with respectable performance—as long as you're not expecting to win many drag races with it.

  • Short Block. At this performance level, the Slant Six will do very well using the stock short block components. You will want to base your build on a 225 though, as the 170 and 198 Slant Six engines lack torque compared to the 225. You should also invest in proper machine work, which includes boring and honing the block with torque plates. A lot of shops won't have a torque plate for the Slant Six, and it will add to the cost, but the extra hassle and expense are well worth it IMO. You should also have the block and/or heads milled to give you a compression ratio between 8.5 and 9:1. Any higher, and a mildly cammed Slant Six won't like running on pump gas.
  • Cylinder Head. Even at this output level, the Slant Six will benefit from a little valve and port work. A 5-angle valve job would just be a waste of money, so stick with a good 3-angle job. Also, larger valves probably aren't necessary though you might want to consider the stainless steel valves available from Clifford Performance. What you really want to do, though, is smooth out the various lumps and sharp edges in the intake and exhaust ports.

Slant Six Resources

  • slantsix.org
    slantsix.org is the best source for information on the Slant Six. If you want to build your /6 for any kind of performance you can get good advice here.
  • For A-Bodies Only
    For A Bodies Only isn't a /6 site, but their forum has a Slant Six area that has lots of friendly people and useful information. It's also a great site if you're into Mopar A-bodies (duh...)
  • allpar.com
    Excellent article on the history of the Slant Six.

Questions or Comments about the Slant Six Mopar? I value your feedback, so please leave some!

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

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