>
>Subject: Re: Re: Any advantages in removing the cat
>
>I believe that this idea of backpressure being good for low end power is
>an unfortunate myth. How can something that restricts the flow of
>exhaust gases be good?
Isn't that what a turbocharger does? sorry, couldn't resist ;-p
> I think what we should really be talking about
>is exhaust gas velocity. Long, narrow pipes are critical for obtaining
>the exhaust gas velocity necessary for low rpm combustion chamber
>scavenging (drawing exhaust gases out to make room for intake charge).
>The increased backpressure due to a smaller pipe, at low rpm is
>outweighed by the advantage of higher exhaust gas velocity.
>Unfortunately, it seems that people have associated this increased
>backpressure as being responsible for for better low end power, instead
>of increased gas velocity.
Very true, for N/A engines!
>Now, with a turbo engine, scavenging is not an issue because you have a
>compressor to force the intake charge into the chamber and push the
>exhaust gases out. Consequently, you want as little restriction as
>possible for the exhaust gases so the turbine can be spun as efficiently
>as possible. The best exhaust for a turbocharged engine is no exhaust!
>
>Nigel
Again, very true. This holds true for ANY forced induction engine, turbo or
supercharged because, as you stated, the compressed intake charge is forcing
the burnt combusted mixture out the exhaust whereas with a N/A motor the
intake charge is being drawn into the combustion chamber by vacuum from the
piston moving down the cylinder.
Look at any high-performance forced induction engine (Top Fuel dragster, F1
or Indy car) the exhaust AFTER the turbocharger is almost nonexsistant, just
long enough to direct it out from under the bodywork. On the supercharged
Top Fuel dragster the very short (non-merged) exhaust is nothing more than a
turnout which they aim up for increased downforce (yes,increased downforce
!)
Take a look at the elaborate exhaust headers of any high hp race-prepped N/A
engine and you will see to what pains they go to increase the exhaust gas
velocity, using the alternate cylinders exhaust pulses to help scavange the
next. It is a science unto it's own !
As to ever being "too large" for a forced induction engine? As far as noise
control, yes, a larger dia system will be harder to keep quiet. Cost and
ground clearance? Yup, these are real issues. Reduced low-end torque?
Sorry, does not apply to a turbocharged engine, as long as we are talking
AFTER the turbocharger.
By reducing the backpressure after the turbo we are also removing a large
percentage of heat from the turbo itself, therefore increasing its life! Of
course running higher boost will put it right back
SImple stated, forget all you have heard about backpressure being a 'good
thing' as it simply does not apply to a forced induction engine!
PeteH
'92 turbo Legacy with a 3" exhaust
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