Spark Retard vs overlap

[Robert Harris]

Its an aftermarket thing.  The Buick GN/T Type home pages/mailing lists can
point to more info.  For the quarter milers, the best I can understand it is a
two stage rev limiter.   Stage I - you bring the engine up to launch RPM - say
3800, kick the first stage in and nail it to the floor.  This "stutter fire"
rev limiter works by progressively severely retarding the spark in selected
cylinders.  

The result of limiting by spark retard is just prior to launch you have the
turbo spun up to heavy boost, fuel flowing accordingly, launch rpm set, and
making virtually no power.  The turbo does not need to spin up - its there and
the throttle is WOT.   All dressed up with all the air/fule flow possible and
no spark to make it go.

Dropping the rev limiter instantly restores the engine to full power at a high
boost  making for a very hard and consistent launch.  The rev limiter then
resets and behaves in a conventional manner.

Since the boost was already very high at this point, turbo lag on launch does
not enter the equation - no way to lag if you are already there and no way to
improve when you are there so anti-lag is meaningless for this app.

I don't suspect valve life is much concern vs winning the race.

P.S.  It works.

On Fri, 26 Mar 1999 05:00:02 -0500, you wrote:
>Date: Thu, 25 Mar 1999 13:25:36 -0500
>From: "Bruce Plecan" <nacelp at bright.net>
>Subject: Re: Valve Overlap
>
>- -----Original Message-----
>From: Robert Harris <bob at bobthecomputerguy.com>
>To: diy_efi at efi332.eng.ohio-state.edu <diy_efi at efi332.eng.ohio-state.edu>
>Date: Thursday, March 25, 1999 11:03 AM
>Subject: Re: Valve Overlap
>
>
><Snip>
>>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.
>
>Can I ask where in the program this is incorporated?.
>>
>>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 .....
>
>Wouldn't that be kinda hard on the exhaust valve?.
>Bruce
>


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