Where can I get an exhaust system and what size should it be
Posted: Fri Apr 05, 2002 11:42 pm
I am happy to see the discussion turning to exhaust sizing and the responses
that are coming in. It gives me an excuse to stir the pot with one of my
long winded discussions, which is intended to promote some overlooked
considerations and provoke additional attention and response. With a
potential group buy at stake, I would like to present information which
could be useful in the decision of whether to go group or solo. I have acquired
some serious reservations about a 3 inch system that I need to resolve.
Reading time is about 5 minutes, so hang with me
A very good question was asked; "Where can I get an exhaust system and what
size should it be?"
And we got some interesting responses; "3 inch", "Anything bigger than
stock", 2-1/2 to 3 inch", nothing smaller than 2-1/2 inch, the bigger the
better for a turbo motor.
I have been investigating the configuration of available systems on the
market, and studying reference material to try and determine what would be
best for me. In doing so, I believe it is important to pass along the
information for your consideration. Plus, I'm still looking for the answer.
There is no - " BEST " - size.
3 inch is popular, I think, because it is used a lot in competition motors,
so it must be good. Also, if one were to choose a most correct answer size,
3 inch is a safe bet to be LARGE enough for most any 2 liter application.
I believe those reasons are only partially correct, much too big is better
than much too small, but a little too small can be better than a little too
big, depending on where you're looking for a performance improvement.
It may well be true that all of the answers we have seen so far would
provide better engine performance than the stock system for most people with
enhanced engine configurations.
*B*U*T*, what's best for a competition engine, with a hundred horsepower
more than I expect to develop, is not necessarily the BEST thing for me.
Better than stock does not mean BEST.
>From Hugh Macinnes' "Turbochargers", Corky Bell's "Maximum Boost", Headers
by Ed's reference material, communication with MRT, calculations, charts,
everything I can find... 3 inch is not an optimum size for my expected
application, which is in the 300 horsepower ballpark.
There is a tendency to look at exhaust system size in half inch increments.
When you really start looking closely at available information, you will
find that size differentials, up or down, of one-eighth of an inch (1/8" - 3
mm) in pipe diameter can result in SIGNIFICANT power gain or loss. What's
significant? I think in science class 40 years ago it was 2%. I am
concerned with numbers of twice that magnitude, which in the 300 HP range
represent 10 or 12 HP.
This is because the change in cross sectional area is proportional to the square
of the change in radius. So a 3 inch system is 9.2% larger than a 2-7/8 system,
and 19.7% larger than a 2-3/4 inch system. So if your optimum system size is 2-3/4,
and you're off by a quarter of an inch, you're off by about 20%! Significant.
If one examines engine parameters such as displacement, compression, speed,
volumetric efficiency, valve duration and lift, desired peak power and/or
torque location, and on, and on...to determine OPTIMUM exhaust size, choice
answers for a particular engine will likely be something like 2-1/2" vs
2-5/8", or 2-3/4" vs 2-7/8", not a spread like 2-1/2" vs 3". So if I'm
going to change out my exhaust system, I am hoping to zero in on a size that
will provide a 4 or 5 percent gain of 10 or 12 horsepower, IN THE POWER BAND
WHERE I EXPECT TO OPERATE rather than a 2 percent gain of 5 or 6 horsepower
...maybe...hopefully, or a gain at the top and a loss at the bottom.
But peak horsepower is only part of the story and not even the most
important part. Proper exhaust sizing also determines where in the engine
RPM range the peak horsepower is generated, and where the peak torque is
generated, and more importantly for my street driven application, how flat
the torque curve remains or becomes. A 10 horsepower gain at 6000 RPM is no
good to me if I sacrifice 8 to 10 from 2500 to 5000, and end up going slower
most of the time because my system is too large. A system which is too
large can also permit exhaust gasses to expand so quickly that at low engine
speeds where you may need it most, turbo response suffers. And whatever you
do, a restrictive muffler or cat, can nullify what you spent on your fancy system.
So I submit to all, there is more out there to what size exhaust should I
get than first meets the nose. The answer is like the answer for a lot of
other hot rod stuff. What is your goal? Circle track? Drag strip? World
Rally Championship? Daily driver? Stock or near stock power levels? 50%
power gain? 100% power gain? The answer is different for all those combos.
The stock exhaust system is more restrictive than it needs to be. Almost
any aftermarket system will be less restrictive, but depending on where you
are now, and where you want to go, it is possible to wind up with a system
designed for a racing application that is too much less restrictive for a street
driven application, and performance could remain unimproved or even degrade,
especially at low RPM, where you're likely to notice it most.
That's all for now. I think it's enough. Now I need feedback.
Larry Witherspoon
ssspoon@aol.com
To unsubscribe from this group, please send an email to:
BC-BFLegacyWorks-unsubscribe@egroups.com
zZz <http://docs.yahoo.com/info/terms/> .
that are coming in. It gives me an excuse to stir the pot with one of my
long winded discussions, which is intended to promote some overlooked
considerations and provoke additional attention and response. With a
potential group buy at stake, I would like to present information which
could be useful in the decision of whether to go group or solo. I have acquired
some serious reservations about a 3 inch system that I need to resolve.
Reading time is about 5 minutes, so hang with me
A very good question was asked; "Where can I get an exhaust system and what
size should it be?"
And we got some interesting responses; "3 inch", "Anything bigger than
stock", 2-1/2 to 3 inch", nothing smaller than 2-1/2 inch, the bigger the
better for a turbo motor.
I have been investigating the configuration of available systems on the
market, and studying reference material to try and determine what would be
best for me. In doing so, I believe it is important to pass along the
information for your consideration. Plus, I'm still looking for the answer.
There is no - " BEST " - size.
3 inch is popular, I think, because it is used a lot in competition motors,
so it must be good. Also, if one were to choose a most correct answer size,
3 inch is a safe bet to be LARGE enough for most any 2 liter application.
I believe those reasons are only partially correct, much too big is better
than much too small, but a little too small can be better than a little too
big, depending on where you're looking for a performance improvement.
It may well be true that all of the answers we have seen so far would
provide better engine performance than the stock system for most people with
enhanced engine configurations.
*B*U*T*, what's best for a competition engine, with a hundred horsepower
more than I expect to develop, is not necessarily the BEST thing for me.
Better than stock does not mean BEST.
>From Hugh Macinnes' "Turbochargers", Corky Bell's "Maximum Boost", Headers
by Ed's reference material, communication with MRT, calculations, charts,
everything I can find... 3 inch is not an optimum size for my expected
application, which is in the 300 horsepower ballpark.
There is a tendency to look at exhaust system size in half inch increments.
When you really start looking closely at available information, you will
find that size differentials, up or down, of one-eighth of an inch (1/8" - 3
mm) in pipe diameter can result in SIGNIFICANT power gain or loss. What's
significant? I think in science class 40 years ago it was 2%. I am
concerned with numbers of twice that magnitude, which in the 300 HP range
represent 10 or 12 HP.
This is because the change in cross sectional area is proportional to the square
of the change in radius. So a 3 inch system is 9.2% larger than a 2-7/8 system,
and 19.7% larger than a 2-3/4 inch system. So if your optimum system size is 2-3/4,
and you're off by a quarter of an inch, you're off by about 20%! Significant.
If one examines engine parameters such as displacement, compression, speed,
volumetric efficiency, valve duration and lift, desired peak power and/or
torque location, and on, and on...to determine OPTIMUM exhaust size, choice
answers for a particular engine will likely be something like 2-1/2" vs
2-5/8", or 2-3/4" vs 2-7/8", not a spread like 2-1/2" vs 3". So if I'm
going to change out my exhaust system, I am hoping to zero in on a size that
will provide a 4 or 5 percent gain of 10 or 12 horsepower, IN THE POWER BAND
WHERE I EXPECT TO OPERATE rather than a 2 percent gain of 5 or 6 horsepower
...maybe...hopefully, or a gain at the top and a loss at the bottom.
But peak horsepower is only part of the story and not even the most
important part. Proper exhaust sizing also determines where in the engine
RPM range the peak horsepower is generated, and where the peak torque is
generated, and more importantly for my street driven application, how flat
the torque curve remains or becomes. A 10 horsepower gain at 6000 RPM is no
good to me if I sacrifice 8 to 10 from 2500 to 5000, and end up going slower
most of the time because my system is too large. A system which is too
large can also permit exhaust gasses to expand so quickly that at low engine
speeds where you may need it most, turbo response suffers. And whatever you
do, a restrictive muffler or cat, can nullify what you spent on your fancy system.
So I submit to all, there is more out there to what size exhaust should I
get than first meets the nose. The answer is like the answer for a lot of
other hot rod stuff. What is your goal? Circle track? Drag strip? World
Rally Championship? Daily driver? Stock or near stock power levels? 50%
power gain? 100% power gain? The answer is different for all those combos.
The stock exhaust system is more restrictive than it needs to be. Almost
any aftermarket system will be less restrictive, but depending on where you
are now, and where you want to go, it is possible to wind up with a system
designed for a racing application that is too much less restrictive for a street
driven application, and performance could remain unimproved or even degrade,
especially at low RPM, where you're likely to notice it most.
That's all for now. I think it's enough. Now I need feedback.
Larry Witherspoon
ssspoon@aol.com
To unsubscribe from this group, please send an email to:
BC-BFLegacyWorks-unsubscribe@egroups.com
zZz <http://docs.yahoo.com/info/terms/> .
Exhaust Close Degrees ATDC