why do some cams have a shorter exhaust duration and lift than the intake duration and lift? seems like more air would be coming in than could go out. is there an advantage to this, maybe more exhaust velocity?

 

For the same reason that intake valves are larger than exhaust valves. The air/fuel being drawing into a non-FI engine is at max of 14.7 PSI difference (a vacuum). In order to get more air in, you use larger valves, longer duration, etc. The exhaust stroke on the other hand is under pressure (piston pushes it out) and that can occur at pressures greater than 14.7 PSI so you can get more out even with smaller valves and shorter durations.

 

Exhaust velocity and to prevent reversion.

 

i did a quick search on da interwebz, and it seems like the shorter exhaust duration is what contributes to more reversion, because pressure cant escape so its forced the opposite direction. is this incorrect?

also, are you guys viewing reversion as a bad thing? i will be running nitrous, so it seems like a little reversion would be good to help keep engine temps down with the hot nitrous burn. thanks

 

Reversion basically dilutes your intake charge, costing you hp. It's a good thing for fuel economy purposes, but a bad thing for making hp.

 

overlap also contributes to reversion right?

so would a good na cam be one with decent lift, longer duration, and a very wide lsa to keep overlap to a minimum?

 

boy, this cam choosing stuff is complicated

 

Overlap is good for WOT NA performance, assuming you have a well designed header and exhaust system that aids in the exhaust scavenging of the cylinder. As the exhaust valve opens on the exhaust stroke and force out the spent gasses, having a little more overlap while the exhaust valve is still open will help pull in more of the air/fuel mixture.

 

i got it, thanks

 

ive been doing some reading and there seems to be a couple different schools of thought about overlap. one, that scavagening happens with overlap and in this case its a good thing to move air in and out of the cylinder quicker. two, that when both int and exh valves are open and the piston is on its way up, that the piston is pushing air out the exhaust valve and out the intake valve (reverse of the direction its supposed to be going), and this makes overlap a bad thing.

thoughts on which you guys think is correct, or what is actually the case?

 

no love?

 

cam secrets are well kept... I fixed cams are "tuned" for a powerband.... VTC cams are able to change overlap times allowing a wider powerband and VVEL can even change duration... soon VVEL will be on exhaust side too!!! exhaust reversion only happens under vacuum... and scavenging works at atmosphere... boost blows right though if you have too much overlap... that is why cams cost as much as they do R&D is pricy... there are companies that just copy cams and call them their own, do your research you can never know too much

 

ok, so if you are still getting vacuum in the upper rpms, then reversion is probably happening, but if no vacuum, then scavenging is taking place?

 

vacuum only occurs with the throttle plate closed or partialy open... if vacuum occurs at high RPM then there is a restriction in the intake tract somewhere