I prefer to think through all the problems and their solutions before I start to work cutting metal, it generally works out better that way.
One of the things that has been bothering me is the spread between the torque rating and the horsepower rating of the engine. The engine is rated 158 HP at 5000 RPM and 203 ft-lb of torque at 4000. Given that HP = Torque * RPM/5252 the power curve is basically flat from 4k to 5k RPM as it makes 154 HP at the torque peak. This means something is choking the engine off above the torque peak, and I’m betting it’s the exhaust. That’s the big reason why I’m planning tube headers to replace the crappy cast-iron manifolds on the engine as it is installed in the minivan. The manifold on the front bank is the likely culprit as it is just something to collect the exhaust and run it away from the head while tucking up as close to the head as physically possible, the one on the rear bank is much better except for the place where the front bank dumps into it. I’m sure that after I install the headers I will see pretty close to 180 HP at 5200 RPM. That torque curve makes a lot more sense than 203 at 4k dropping to 166 at 5k, and it might be even higher because the rest of the exhaust system will be less restrictive than the minivan as well since I’m going to dual exhaust with high flow quiet mufflers instead of the single exhaust and restrictive muffler on the minivan.
Also while I have the engine out of the car I’m going to clean up the inside of the intake manifold, because I can and because I want to make sure there are no horsepower killers in there. Nothing fancy, just cleaning up any port mismatches where things bolt together or to the heads and removing any rough spots or flash inside the manifold so that air moves smoothly inside the manifold. I’m not trying to add horsepower, just keep from losing the power that is already there. I’m just doing the stuff that Chrysler would have done if they had the time and money to hand fit and assemble the engine at the factory. Like I said I think this engine is capable of 180 HP without changing any internal parts or losing any power below the torque peak. I also think there is some power and fuel economy to be found in the fuel and spark maps in the computer which will happen when I hook the new controller up to the engine. I’m thinking that 200 HP is within reach without changing the cam or other internals as installed in the T-bucket. This will give me the same around-town and autocross performance as a 2017 Stingray base model, but better fuel economy than the 15/22 hwy estimate for the 1996 minivan, assuming I can keep my foot out of the gas driving around town. Changing the transmission controller to full manual will help by keeping it in as high a gear as possible to keep the RPMs down.
And those have been my thoughts on the subject for today. Keep riding your bikes, I’ll keep riding mine.