2G Cam overlap?

[2gtalonman]

ok so heres the thing, im looking at my head I just got rebuilt today by a shop, it has cams in it I was told 264 intake 272 exhaust, heres my question, are my cams over lapping? I can turn the cams and watch the valves but some seem to stay open a bit longer.. is this over lap? can you put the cam gear on wronge so this might happen?? if you need more de-tails let me know.

 

[Defiant]

No, you can't put the cams in "wrong". You can eff-up the timing, but that goes on when it's on the block. "Overlap" refers to the degrees of rotation that the exhaust valve's open while the intake is.

 

[2gtalonman]

Thank you sir, head go's on tomarrow woo

 

[Clutch GSX]

Go here and mess with the last diagrham. Switch it to performance cams to see what you have compared to stock cams. Its pretty helpful.

Howstuffworks "How Camshafts Work"

 

[Bender]

The overlap is what you want for higher rpms. With the intake and the exhaust open at the same time, cold intake air can clear all the hot air out of the combustion chamber, then go out the exhaust before it closes. At higher rpms you have fresh air, not the stagnant hot stuff that didn't have time to be released from the last cycle. Another plus about them both being open, is that the pressurized intake air (depending on how much boost you're running) is flowing into the compressor wheel of your turbo. The downside of this, is at lower rpms, the exhaust is flowing back into the cylinders. This causes turbulence, and fresh intake air cannot enter the chamber. That is why the idle gets rougher and the toque decreases as you go up higher in the cam grind and duration.

 

[dsm-onster]

The overlap is what you want for higher rpms. With the intake and the exhaust open at the same time, cold intake air can clear all the hot air out of the combustion chamber, then go out the exhaust before it closes. At higher rpms you have fresh air, not the stagnant hot stuff that didn't have time to be released from the last cycle. Another plus about them both being open, is that the pressurized intake air (depending on how much boost you're running) is flowing into the compressor wheel of your turbo. The downside of this, is at lower rpms, the exhaust is flowing back into the cylinders. This causes turbulence, and fresh intake air cannot enter the chamber. That is why the idle gets rougher and the toque decreases as you go up higher in the cam grind and duration.

Not for turbocharged applications.

Overlap improves spool, but loses breath up top a bit. That doesn't matter too much considering running stock valves, stock cams,stock intake manifold, and stock turbo. But it does SIGNIFICANTLY die once you get flow to the head at the end of the rev range.

Very little cold air ever rushes into the cylinder where a turbo is present as most of the time there is more exhaust gas pressure than intake manifold pressure. If there is more intake pressure than exhaust pressure, this happens at the opposite time as mentioned. Exhaust gas pressure is always lower at lower rpms. Whether or not your turbo is fully spooled and at what boost determines if there is a pressure differential favoring the intake. 99.999999999% of the time at high rpms, the exhaust gas pressure (back pressure) is over double the intake pressure (boost).

When you're off boost, there is also very much more back pressure than intake pressure as you're running in vacuum in the intake manifold and intake valves.

Cam Basics

 

[Bender]

Is this why, on turbocharged cars, they sometimes go with a bigger intake cam than exhaust cam?

 

[dsm-onster]

Is this why, on turbocharged cars, they sometimes go with a bigger intake cam than exhaust cam?

Depending on cam lobe centerlines, yes.