Aerochargers are better than a twin screw!! hahaha ;-)

[Bernd Felsche]

Jeff Meager tapped away at the keyboard with:

> You cannot get something for nothing in any situation.  Take a
> turbo. This means exhaust backpressure.  If exhaust BP wasn't a
> bad thing, you wouldn't put a big cam onto an Normally aspirated
> car would you?  Right, so a big cam for peak power is best to get
> all the crap out....

Most of the power a turbocharger gets to compress the intake charge
is from energy that otherwise gets lost down the exhaust.

> Now we put on a turbo and reduce to buggery the overlap etc.  The
> turbo makes up for the lack of power due to the cam, but now you
> have a large amount of residual exhaust in the cylinder due to
> high BP before the turbo....

The turbos allow the reduction of overlap because they actively
 pressurise the inlet.

> SEE, it wasn't FREE!  You've cost yourself a clean burn by putting
> the turbo on.....

Thing is, the work you can actually get (near BDC) that greater
residual pressure to do before the exhaust valve opens is minimal.

Besides, some EGR is desirable to reduce the tendency to knock. :-)

> Yes, on nearly every factory turbo setup, that will make usable
> boost at 2500rpm, you will by peak power have twice the
> backpressure vs boost....measured it, seen it....

Look at variable vane setups.

-- 
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