Effects of camshaft timing on turbo spool-up

[Greg Hermann]

I think you have it reversed--higher temp exhaust gas will drive a
conventional turbo turbine more effectively, and an earlier exhaust opening
event will definitely yield hotter (because of less expansion in the
cylinder) exhaust gasses.

Greg

At 11:37 AM 10/27/01, Camden Lindsay wrote:
>Hello,
>I'm sorry if this is not EFI enough to be considered on this list.  If you
>feel it is not, please email to me personally....
>
>I was wondering if anyone could tell me why camshaft timing effects turbo
>spool-up--
>
>On my audi (83 turbo motor)  peak boost stock occured at appx 4500
>rpm-6000rpm (8-9 psi, manual wastegate... that is, just a spring, not
>electronics..)... I had read on audifans that retarding the cam (maybe i
>have this reversed-- maybe it was crank retard-- but don't remember) would
>cause spool up to be earlier, which i investigated (purchased an adjustable
>cam gear).  Indeed it did-  by retarding the cam timing 16 degrees or so,
>(which is what they said to try it at first) the peak boost dropped to about
>3000 rpm and the high rpm boost dropped significantly (so it was about even
>with the  3000 rpm boost, which was 5-6 psi)
>
>The main thing i noticed was the wastegate popping ALOT earlier (3k rpm)...
>so what'd i do...?  Put a stronger spring in!  After that it ran 8-9 psi
>from 3k to 5.8k or so, with a small taper above that.
>
>The only thing i could figure was that, with cam retard, the exhaust gasses
>are getting pressurized a small about by the piston before their release
>throught the valves, somhow increasing exhaust velocity.
>Is this phenomenon caused by the exhaust velocity, with the taper caused by
>turbine efficiency? Or is the taper caused by the VE of the motor dropping
>at the higher rpm because of the aformentioned (piston pressurizing exhaust
>more/not as mutch time for intake)phenomenon?
>If that is true, wouldn't this be a way to compensate (within reason) for
>using a larger turbo on the motor?
>
>Thanks for any insight,
>Camden
>
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