[Cliff R]

The "4% per one full point of compression" is old/outdated information taken from one of the HO books.

In static terms it's probably pretty close, but in the real World the difference between 9 and 10 to 1 would be considerably more.

Here's why. Build a 400 engine with 8 to 1 compression, it will like a stock 066 cam or even a 204/214/112 cam or something close to it. Idle nice, good street manners, about 300hp or so.

Take the same 400, 9 to 1 compression, move up to the 068 cam, or a 214/224/112 cam and it will idle equally as well, good street manners, and at least another 40 hp or so.

Bump the compression up to 10 to 1, 224/234/112 or even move up to a Crower 60243 (228/235/112) cam, add high ratio rockers, Rhoads lifters and guess what. It will idle with plenty of vacuum for power brakes, great street manners, but closer to 400-430hp. Ours actually made 424hp on the dyno and 465 ft lbs torque with umported heads. Vacuum at idle speed 12" at 750-800rpms.

So a 1 point bump in compression opens the door for a larger cam with less negatives and considerably more than a 4 percent power increase.......Cliff