Blow off valve

[jengel at FastLane.NET]

> From:          einarp at ade.no (einarp)
> Subject:       Blow off valve

> >From what I read, the blowoff valve is there primarily to
> relieve the compressor impeller from the shockwaves it generates
> when it does not "move air" because the throttle is suddenly shut.
> It "stalls out" as you would say if it was an airplane wing.
> In other words it will give the turbo a longer life.
> My source in fact showed a curve where the turbo speed decreased
> _faster_ with a blowoff valve (on a Porsche). Probably because it
> pumps more efficiently when not stalled, and the backpressure is
> still there.
> 
> Blowoff to atmosphere is better than recirculation because the
> heat rise is lower. But a problem if there is an airflow sensor
> before it.
> 
> If the blowoff valve were controlled by the engine management unit,
> the blowoff valve could be opened _fully_ when going off throttle.
> (The EFI unit already have the data to detect this condition.)
> Then the turbo should have less resistance and keep the speeed.
> Right or wrong?
> 
> 
> --
> einarp at ade.no  ( Maserati Biturbo Spyder )
> 
>
A variation I've seen on the Porsche 924 Turbo had the blowoff valve 
feeding back into the turbo inlet in order to keep the air flow 
through the metering plate correct (the 924 had mechanical 
injection).

When I bought my SVO intercooler from Spearco (plug) they offered a 
device similar to the 924's.  It uses vacuum from the other side of 
the throttle plate to operate the valve.  Can't say how well it works 
right now because other problems have kept me from putting it on.

je