Re: Cam retarding?
I had my exhaust cam retarded by 1 tooth on my stock cams a long time ago. The boost cam on all at once, it hit earlier and it fizzled out up top a little more. It spooled my small 16g a little faster.
There is no general sweet spot. Every build is different, with different demands and different backpressures.
Hotter gases definately increase spool speed. But in the case of exhaust cam retard, valve overlap is the cause for the increase in spool speed, not hotter exhaust gases. As a check you can advance the intake cam and see a similar increase in spool speed. At lower demands on the turbine, the compressed air actually helps drive the turbine as you blow aircharge right through the combustion chamber and into the exhaust manifold. The exhaust pulse ends for the cylinder but pressure directed to the turbine from that exhaust runner lingers.
This is why the cam timing "sweet spot" is different with different setups. When do the exhaust pulse pressures end? When does there develop more back pressure than manifold pressure, reversing the flow back to the intake valve? Etc.?
Cam duration and cam timing is velocity dependent not rpm dependent. To be exact, cam timing and duration takes advantage of momentum of the individual particles. Momentum = mass X velocity. A stroker has a higher intake air velocity at a lower rpm, hense why strokers and v8s get away with more duration and overlap with out idle issues. If you degrade topend VE with cam timing for a 2.0L stroke, then you will degrade the VE of a stroker at the same air velocity, which occurs at a lower RPM. The stroker motor piston speed at 7300rpms is as fast as the piston speed of a 2.0L at 8000rpms. Piston speed and air velocity are nearly directly proportional. So if you kill VE from 6500rpms to 8000rpms with cam timing or a particular duration on a 2.0L, then you will kill VE with a stroker from about 5900rpms to 7300rpms [(6500/8000) X 7300)].
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Matt
Bolton Holset H1C w/ external gate, fp2X cams, DSMLink, 3" exhaust, FMIC, water injection
"There is nothing better for man than to eat, drink, and see good for all his hard work." Solomon
Last edited by wheelhop; 02-11-2009 at 02:44 PM..
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