[Tim Ayers]

Originally Posted by alexiskai

Another way to think about HC head performance at high RPM is that, when you say a given head provides 6:1 compression, you're assuming that the intake stroke is filling the cylinder with a fuel/air mixture at atmospheric pressure. But as the RPM increases, the time available to fill that cylinder decreases. 2000 rpm means the crankshaft turns once each 0.03 seconds, and an intake stroke is half a revolution, so the carb needs to fill that cylinder in about 15 milliseconds.

The stock carb flows about 63 cubic feet per minute at 2000 RPM, which means that in 15 milliseconds it should flow 0.01575 cf, or 27.2 cubic inches. A stock cylinder bore is about 50 cubic inches. So at 2000 rpm, the stock carb is only able to flow enough air to fill each cylinder about half full. Which means the compression is also halved. So at 2000 rpm, your 6:1 head is only compressing to 3 times atmospheric pressure. It's like the carb is swapping in weaker and weaker heads as you keep the throttle open.

Makes sense and supports my thoughts on a stock Model A engine is under carbed.