Originally Posted by HQHolden
Thanks for the detailed replies, keep them coming as I progress through this.
I don't think its valve train harmonics. The studs are girdled and pushrods are firm. I am almost convinced that the problem is a reversion. The intake has black soot inside and it goes all the way up to the bottom of the carb. I noticed the black before but never really took too much notice of it. Making changes to the intake system, i.e. bigger or smaller carb, air filter and no air filter, made changes to when the power dip occurres. Will connecting a MAP sensor and EGT confirm this 100%? Does the EGT sensor need to be right at the head to be a true reading, or can it be at the collector? Will also try a 2" spacer to see if the reversion will change significantly. What else do you guys think?
hcompton funny you mention the tyres. I am using 28x12.5 et street radials so yeah they have no problem putting down the power. Well on the track anyway, they do slip a little on the street. But the dyno guy mentioned that a semi racing tyre with a racing compound and side wall is the worse thing to use on the chassis dyno. He said standard street tyres are the best as they don't flex and twist or heat up under load.
So lets say it is plenum reversion, what to do from there? Is it a bad valve job or port job? Wrong cam timing?
Oh yeah drag radials cant be used on the dyno they are not stiff enough to take the wieght of the car once its properly strapped down. A 500 hp car will need a good amount of force to hold it on the dyno. I would think vette wide rear rims and tires made for good handling would be best. Also sounds like 500 bucks. So maybe some loaners or trades may be needed to find a proper setup for your car.
Large Tunnel ram will fix the reversion issues same as a 4-7 swap cam. But since the cam is installed and working i would think about changing the intake out for different versions. I would recommend a victor jr if you have one handy.
I would also like to add that edelbrock has some good experience with these issues i would think a quick call to them might present a simpler solution like larger 1050 race demon carb or large spacer. Would lower the vacum level at wot in the intake. But i would look for there recommendation since the victor series is designed to lower chances of reversion.
If you do take the intake off or have an inspection camera handy. You will want to take a good close look at the back side of the valves and see if they are leaking even a small amount will dirty up the intake somthing aweful. This may be the issues. A leak down test may show the problem.