mark kibort

Yeah, the intake closes and there is a full revolution before the exhaust opens and closes again. Is the overlap at the point where the exhaust closes and the intake starts to open again? If so, it looks like the exhaust is close for a few degrees before the intake opens.

AO

Pulling this thread back up again, but I'm in the process of installing Devek Level 2 headers, and re-found this thread.

I've a smaller pulley to keep the boost up, but was curious if we ever found out if the back pressure helped the SC'd cars. Also, was there ever any conclusion on valve overlep? I'm with Bill... how can we pressurize the intake if there is valve overlap?

Cosmo Kramer

The valve overlap if any is very short in duration. This microsecond only helps chase out the remaining exhaust gases, the exhaust valve closes and the cylinder pressurizes. If you were testing the intake for leaks and one of the cylinders was at just the right position I could see there being a possible pressure leak.

Imo000

The way I know it is that boosted cars don't need any backpressure.

smith 928

You need to get a flow map of that S/C first to make sure you will not be off the map before changing things...

smith 928

You are wrong ever car has some back pressure

toofast928

Imre is correct as under boost the exhaust system should have 0 back pressure. just watch tractor pulls or funny cars and see zoomie headers.

Under none boost conditions the 928 exhaust system should have 2-4 psi of back pressure. Back pressure is created by the cats, mufflers, or exhaust pipe size. With no back pressure the 928 is a dog until 3500 RPM. Ask me how I know all this..............

AO

Mark,

You've got about as much ground to stand on as a double amputee in a high jump contest.