Exhaustive Research; Build Your Own Exhaust Part II.
Sport Compact Car Magazine; May, 2002
I really enjoyed the article entitled "Build Your Own Exhaust" by Mike Kojima in the
November 2001 issue of Sport Compact Car on pages 184 through 188. You might have widened
the title a bit from implying it was about only custom made exhaust systems, because the
information in the article would certainly be beneficial to anyone buying a pre-made
exhaust system and what those people should look for to find the best pre-made system.
I also admire the courage and honesty shown by Mr. Kojima in taking on what he calls the
"old hot-rodder's tale" that "engines need some backpressure to work properly and make
torque." I have been designing and dyno testing exhaust systems for several years and
find that I am confronted by believers of this myth on an almost daily basis. I appreciate
any help lent by the media to dispel this myth, because everyone benefits when accurate
information is provided to the enthusiast community.
However, I was a bit perplexed when I got to the portion of the article suggesting exhaust
pipe diameters. There seems to have been an editing mistake or a typographical error,
because the text reads that you are recommending 2 inch pipe for non-turbo engines from
1.5-2.0 liter displacement, 2.25 inch pipe for 2.1-2.5 liter, and 2.5 inch pipe for 2.6-3.0
liter. It appears that you are recommending the same overly small pipe diameters as the
group that espouses the "old hot-rodder's tale of backpressure" that you sought to dispel
only a few pages previous.
It might be helpful to point out that most cars with engines from 1.5 to 2.0 liter came
from the factory with 2 inch diameter exhaust pipe, and your recommendation would indicate
that the stock exhaust system is the optimum size for the performance of those vehicles.
I would have to disagree with that because of the dyno testing my company has done. We dyno
tested three different sized cat back exhaust systems for 1.6 liter DOHC Geo Storm and Isuzu
cars (the 2 inch diameter stock system, a 2.25 inch system, and a 2.5 inch system). With a
stock, non-turbo engine, the 2 inch diameter pipe resulted in performance numbers of 101.0
HP and 97.4 ft-lb., while the 2.25 inch diameter pipe resulted in 102.8 HP and 102.7 ft-lb.
(an improvement of 1.8 HP and 4.3 ft-lb.), and the 2.5 inch diameter pipe resulted in 105.2
HP and 102.0 ft-lb. (an improvement of 4.2 HP and 4.6 ft-lb.). That dyno test comparison
has been placed on the internet at:
I would also have to disagree with your recommendation of 2.25 inch diameter pipe for
engines from 2.1 to 2.5 liter displacement, because we also dyno tested three different
sized cat back exhaust systems for 2.4 liter DOHC Chevrolet Cavalier Z24 and Pontiac Sunfire
GT (the 2 inch diameter stock system, a 2.25 inch system, and a 2.5 inch system) and again
found your recommendation to be on the small side. With a stock, non-turbo engine, the 2
inch diameter pipe resulted in performance numbers of 127.3 HP and 136.7 ft-lb., while the
2.25 inch diameter pipe resulted in 132.3 HP and 142.6 ft-lb. (an improvement of 5.0 HP and
5.9 ft-lb.), and the 2.5 inch diameter pipe resulted in 134.9 HP and 146.1 ft-lb. (an
improvement of 7.6 HP and 9.4 ft-lb.). That dyno test comparison has been placed on the
It seems that your exhaust pipe diameter suggestions are between one quarter and one half
inch on the small side of resulting in the maximum performance power improvement, and that
might cost your readers, according to our testing, as much as four or five horsepower and
four or five foot pounds of torque. The chart might be most accurate if 2.5 inch were
recommended for 1.5-2.5 liter engines as the minimum size and larger pipe for 2.6 liter
and larger engines.
But, please, keep providing us with great articles like Mr. Kojima's, these are really the
core of a performance magazine and yours really goes above and beyond providing this type
of information to the enthusiast community.
Mr. Luton is the owner of Iperformance.