Cam retarding?

[asshanson]

How about on a stroker, does it hurt top end as much, seeing as there isn't much above 6500rpm as it is? Maybe I won't install those gears...

[Pushit2.0]

I would degree the cams.

[wheelhop]

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)].

[Swifty1638]

so that's why i was thinking 3 deg advance on the intake, and 1 retarded on the exhuast..happy medium?

-A. Swift

[Shane@DBPerformance]

There is no happy medium. Every setup is different. How milled your head and block are will effect it. Exactly how your exact set of cams are ground will effect it. The backpressure in your exhaust housing will effect it. Most of these cam companies are not perfect at setting up their cams. Some might have some advance/retard built into them on purpose or on accident. If you do play with your intake cam gear, it will also change your ignition timing.

[wheelhop]

I was editing my post while you posted :) ^^^^^YEP! You have to test and see. It depends on the cam duration, initial cam centerline angles, turbine, intake manifold design, even exhaust manifold design, displacement. There's SO many variables. The factory cams have a certain cam timing for the stock setup and intented results with such a setup. The same goes with aftermarket cams. They are setup for the most common range of upgrades. I would use a dyno or trapspeeds to dial them in.

[niterydr]

I would degree the cams.

Could you maybe check that when you are assembling his wiring/piping?
:KANE:

[Swifty1638]

he is ;)

-A. Swift

[Cammaxon]

im not retardingggg

[turbotalon1g]

I'm a little retarded?
I tried this on my car and I didn't like the top end drop off i got. Mess with the tune for spool if you are still not happy do one of the many things shane said.

I know a guy selling a dp nitrous kit for dirt.