Compression Analaysis

Jack Roush said "the XR needed more compression. It is well known that we can run 20 psi boost (275 psi chamber). For the same 275 psi, we can run 9.0:1 and 16 psi boost. This will give more off boost power with the same top end power." The main problem will be getting gas that won't detonate.

Per Nick " I would not do that. I try to lower the combustion ratio a little when ever we do a 2.3 head. I have seen this done and the only thing it adds is detonation. If you have your head off, get the head and manifolds ported and polished. This will get you the best bang for your buck. Spend some money in better air flow in and out of the motor, again better bang for your buck."


If you are working on your head, use the charts below to determine te ompression ratio you have, then determine what you want. I started with a 59cc chamber (8.28:1) deshrouded the valves (64cc) then did a surface cut (0.070") to get 55cc (8.65:1).

Note the surface cut table may result in less reduction in chamber volume depending on chamber wall angle. You will want to slightly reshape the chamber after the surface cut to get the chamber close to the cylinder wall edge. Start with a smaller cut, measure volume and recut.


One further issue with cutting the head surface (decking) is the cam gets slightly retarded since the cam pullys move together. See the following chart.