Auto Racing: racing engine, camshaft profile, lightning bolt

Question
hello,
I have a 4-cylinder mini stock 2.5 ford.  ITs punched out to a 2750, well my question is the car comes out of the turn like a lightning bolt, but by the time it hits the end of the straight away it flattens out, first we thought the head was to small so we went to a motor man and head machine work done to it different cam (518) and it didnt help at all if anything its slower. some one say to us it could be to tight that the motor is not breathing, what do u think we should do ?

Answer
Hi Matthew:

I'd say go back to your old cylinder head configuration for starters, There are quite a few things to try, but you must try ONLY ONE at a time.  

The head you are now using was ported AND a new camshaft profile used. Now you don't know which one is helping and which is hurting.  You could go back to the old cam , see what that does for you.  It sounds like you have too much port and valve area. You have reduced the velocity of the mixture going to the cylinders. BUT

It could also be your gearing.  What RPM do you see at the end of the run-up ?   With the 518 lift you should have horsepower at 7500 or beyond. If you are hitting 8000 or beyond you have too much gear, you need to go a step taller. But if you are hitting your braking point at 7500 and the motor is flat, it's more likely a problelm with the head work and cam. You've got more camshaft and port volume than the carburetor can keep up with.

Ideally your straightaway should start at about 5500 RPM and you should arrive at the far end turning about 7500 to 7800.  Gear for that, and your times will improve.

The most important thing though , is to remember to try ONLY ONE thing at a time.

Good luck

Dan Liddy
Sarasota Florida