What compression can one get away with on pump gas?

Reply #15 – June 20, 2013, 11:44:57 AM

What is your cranking compression?

What compression can one get away with on pump gas?

Reply #16 – June 20, 2013, 12:09:18 PM

Ok if you have 100% VE and your have 10-1 CR your cranking compression pressure would be 147 LBS at sea level ( in Theory!!! so lets use this as an example even though it is more complicated than that). So with that if your engine has 12-1 CR and both or one valve never closed you would have ZERO compression pressure. So if you have an engine with 12-1 CR and the valves were open 50% of the time when the piston was coming up to the compression stroke you loose in theory half you compression pressure. Example the early eighties engines had app 8.5-1 CR but still achieved good compression pressure. Reason being the cam allowed more sweep compression pressure. So when installing a big cam in a high compression engine you loose compression pressure. Hence a lower octane fuel in theory can be used. Bottom line once again the cam dictates SWEEP COMPRESSION PRESSURE!!

In the early eighties the engines had very low Compression ratios and they still detonated on low octane fuels. They also had very little spark advance. Reason being the cams tricked them in to Compression pressure. Several cam companies at the time came out with performance cams for these engines by altering the cam profiles to achieve more sweep compression. Once again never use a CR for an engine till you decide on the cam you intend to use. It is that simple!!!

Calculating compression ratios is an exact science. If your calculated CR is 10-1 that never changes and is a FACT

Sweep compression is determined by the cam and it's profile.

Roller cams are a good example they come up to full lift much faster than flat tapped cams and stay at full lift longer. And that is why they make more HP. More compression sweep more HP.