I have a sbf, 13.8:1 cr .080" quench, was wondering what is a good cranking compression should be, and whats the theory behind it.
165-180 psi with a N20 Cam Anything over 200psi the tunning window gets smaller
Sorry, but I don't agree with what you said. In your example the cams can't be identical, the events are different when you move the LSA numbers, plus, the 110 cam installed on 104, still has the same overlap and LSA, you just advanced the events to happen earlier, so of course it pumps the same. Overlap, means both valves are open at the same time. This was done in years past, because the heads sucked and needed help pulling the charge in. If both valves are open longer at the same time, some charge is pulled through. There is a lot to cam profiles, I agree, but a big motor generally uses a wider LSA for nitrous use, than it does for N/A. All about event timing and cylinder pressureHow can overlap have any bearing on cranking pressure? , take two identical camshafts, one on a 104 LSA the other on a 110 , install the 110 on a 104 intake centreline & cranking pressure will be pretty much identical .............as far as nitrous being shot out the tail pipes on overlap, this has more to do with exhaust scavenging rather than lobe sep/overlap#s.
Big inch motors use wide lobe seps for a reason , nitrous not being one of them......just correcting you Monte as i know you do the same for me.
Are you saying the "5th cycle" is in play during a cranking compression test? Or, is this just a rule of thumb based on the fact most tight lobe center (high overlap) cams close the intake valve earlier, thus trapping more swept volume.Another thing to consider, is that if an engine has a lot of cranking compression, that translates into a lot of overlap.
Overlap has no bearing whatsoever on cranking PSI , you've said this before & i just thought it was a typo error .....an engine with high cranking pressure does'nt necesarily mean it's ground on a tight lobe sep , could be of a short duration with a wide lobe sep , many variables to consider , a 220/220 110lsa installed straight up in a 600" motor will net some good cranking psi , notice overlap is @ zero.if it has a lot of cranking psi, that generally means it has a shorter LSA angle and generally high amount of overlap. .