Valve Overlap

[Mark_Duewel at dca.ca.gov]

Bob-

I remember watching the Zakspeed Capris blowing 3 feet of flame out the pipe
right before they hit the apex.  Same reason?

-Mark





bob at bobthecomputerguy.com (Robert Harris) on 03/25/99 07:25:51 AM

Please respond to diy_efi at esl.eng.ohio-state.edu

To:   diy_efi at efi332.eng.ohio-state.edu
cc:    (bcc: Mark Duewel/Smog Check/DCANotes)
Subject:  Re: Valve Overlap




A little overlap on a turbo to blow some mixture into the exhaust where its
burning will spin the turbo up faster.  Might work - cept for the facts as
reported by Dave Vizard, Corky Bell and countless others.

The plain and simple fact is that the exhaust manifold back pressure is
significantly higher than the intake pressure thru much of the region
including almost all of the part throttle area.

All overlap does is blow exhaust up the intake and give us EGR

The better the design, the less this amount is - but for most of us it is a
fact to deal with.

Now if you want to spin the turbo up in a hurry, you might consider selective
spark RETARD.  Buick GN's do it all the time.

By momentarily retarding the spark, say to 30 degrees after TDC, combustion
starts way late and slow and the majority of the charge burns and expands in
the exhaust manifold greatly increasing the volume of gas to spin the turbo
which greatly increases the inlet charge .....

The result is very quick acceleration by exactly the effect that you want -
blowing fire into the exhaust - without being defeated by the physics.  And
its on list topic.

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