Valve Overlap

[John Andrianakis]

A system like this is used in the Ford Escort Cosworth rally cars to reduce 
spool up time of the turbo. When the throttle is shut the ignition advance 
is retarded by about 30 deg and the idle control motor opens all the way to 
maintain a rather high idle(1500 rpm?). This leads to late combustion, 
mostly igniting in the exhaust manifold without power to rev the engine. 
The exhaust gases keep the turbo revs high and when you depress the 
throttle you get almost instant boost. The system is very demanding on the 
exhaust valves and turbo turbine. For that reason the turbine is made from 
inconel- high temp metal(the same as used for the McLarren F1 exhaust 
system tubing). Several people have tried to use the system on the street 
but the high noise, backfiring and flame spiting exhaust along with the 
cost of the damaged turbos makes it not such a good option. The special 
turbine wheel  I ve seen costing around $1300.
Mitsubishi rally cars have employed a different system. They have a blow 
off valve that directs some of the air through a one way valve directly 
into the exhaust manifold. To make use of the extra air in the exhaust 
manifold to spool up the turbo the ecu adds fuel when the throttle is 
shut(above certain rpm, maybe sensing intercooler plumbing pressure). Along 
with retarded timing this creates exhaust gases with no additional power. 
The problem is that the system is only effective for a second or two and 
mostly between shifts.

On Thursday, March 25, 1999 8:26 PM, Bruce Plecan [SMTP:nacelp at bright.net] 
wrote:
>
> -----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
>
> >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.
>