Dynamic Compression and Aluminum Heads?

clinteg said
I’m on the other side of the static/dynamic debate. My personal opinion and from what I have learned from those who build engines, static doesn’t really mean much other than to calculate dynamic and dynamic means everything when trying to tune for pump fuel. When I had my cam ordered, it was custom ordered based on the dynamic compression ratio needed for the combo. After we ran the engine, it was dead nuts what they said it would do. Run on 91 octane with 11:1 old *** iron heads. Now if I were to put a different cam in and if it didn’t bring the DCR down enough, I don’t think there’s any way I’d be able to run pump gas safely, even with the same static CR as before. There are variables that aren’t accounted for in DCR, but i think it gives you a lot better and more information than static will ever tell you. That being said, I would NEVER build an engine to run on pump fuel based on the Static compression. I’d aim for the dynamic number. Just for the simple fact that my builder aimed for the dynamic number and it was perfect for my application. Just my 2 cents.

Sounds great except for the fact that it is impossible. The dynamic compression ratio is meaningless for a performance engine since it changes with RPM. Sure you can put a big cam in a high compression ratio engine and drive it around at slow speed but what is the point? As soon as the engine comes up on the cam you have a high compression ratio engine and you need race fuel. The only time dynamic compression ratio means anything is if you are building a very low rpm engine such as a tug boat. When you design a low rpm engine you have to be careful about cam timing since you don't have any inertia charging. For a performance engine the dynamic compression ratio is meaningless since you are aiming for inertia charging. If you get the engine design correct you end up with a little more than static compression ratio on a performance engine since you get a little bit of inertia ram effect at the peak. For a race engine you look at static compression ratio and then add a small fudge factor for inertia tuning. That is why all of the race fuel mfgs have charts which tell you which fuel to use based on static compression. They don't care about dynamic compression ratio. The dynamic compression ratio of a Pro Stock engine is probably less than 9:1 but who cares. It won't run on pump gas.