Slant Six Performance

Slant Six Performance

Improving the Slant Six

• 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. On a 225, you can replace the rods with 198 rods and run Keith Black KB268 pistons. This combination gives you a much better piston and ring combination than stock, and boost compression ratio to about 9.2:1. The caveat is that you have to find a set of rods from a 198 Slant Six. For a more detailed description of this conversion click here.
• 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 come 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