Bill's Isuzuperformance; Dyno Testing and Comparison of Cat Back Exhaust
Systems for 90-93 Isuzu and Geo Storm Cars
For quite some time, there has been a great deal of discussion and debate concerning
what particular type and which brand cat back exhaust system works best on the 1990-93
Geo Storm, Isuzu Impulse, Isuzu Stylus, and Asuna Sunfire. This discussion has been
rather heated at times, and there have been, in most cases, convincing points presented
by all involved, to support their own beliefs in areas of pipe diameter, pipe thickness,
muffler type, tip type, price, and availability.
After a great deal of time and consideration, we decided that we would go to the expense
of testing the available cat back exhaust systems, side by side, on a dynamometer, and
establish which system worked better.
To accomplish this, we came to an agreement with Mustang Muscle Performance and Dyno
Service in High Ridge, Missouri, to use their DynoJet Dynamometer to measure performance.
The exhaust systems to be tested were the stock cat back exhaust system, the Paceseter
exhaust system, and the Stan's Headers brand cat back exhaust system.
The vehicle used in the testing was a 1991 Isuzu Stylus XS 1.6 liter DOHC, with a fresh
engine rebuilt to stock specification with all OEM Isuzu parts ordered directly from
Isuzu, a Mueller Fabricators aluminum flywheel, a Unorthodox Racing underdrive pulley,
and a Isuzuperformance cold air intake system.
Dyno testing involves parking the vehicle with the drive wheels and tires on the
rollers of the dynamometer and then straping the car down to the ground so that it
will stay on those rollers while the drive wheels and tires spin them.
An inductive electrical current sensor is placed around one of the ignition wires
so that the machine can measure information on power produced at each specific engine
The settings of the machine are adjusted to match the gear ratios of transmission of
the vehicle by correcting engine speed and roller speed to match.
Once the machinery is ready, the vehicle is accelerated through first, second, and
third gears, to fourth gear, and the engine speed is held at 2,000 RPM by the opperator.
The opperator then hits a button to begin the measurement and presses the accelerator
pedal to the floor. Engine power is measured by the machinery at wide open throttle
from or just above 2,000 RPM, all the way just short of the rev limiter, or 7,600 RPM
in this case.
The machinery measures the air temperature, humidity, and barometric pressure, so that
it can correct the results for the atmospheric conditions. This correects the results
to SAE, which is what we have used in all of our comparisons.
In order to avoid any possible impropriety, as we are the exclusive distributor of one
of the products being tested, all of the testing was conducted by Daryl, the owner of
Mustang Muscle, or Matt, his son. They both have many years experience running
dynamometers and had no intrest in what the results of the testing were.
For our comparison, we sellected the best results obtained from each session
with the exhaust in each configuration. We must stress that we are comparing the best
results obtained from each product to the best results obtained from the other products
in the comparison. Results for comparison were chosen specifically so that each
product's best performance was being compared to every other product's best performance.
We do have copies of the results we did not use if anyone wishes to verify that we did
in fact compare the best results to the best results in all cases.
Dyno Chart Showing All Three Exhaust Performance Results
As mentioned before, our baseline numbers were taken of the 1991 Isuzu Stylus XS with
stock exhaust manifold, stock down pipe, stock catalytic converter, and stock exhaust
system. The pipe diameter of the OEM exhaust system on the DOHC cars is 2 inch from the
catalytic converter to the muffler with two 1 1/2 inch diameter tail pipes exiting the
muffler to carry the exhaust gasses out through the notch in the rear bumper.
For the sake of detail, this particular vehicle had the factory OEM forward
pipe with resonator and a Bosal brand rear section and muffler. The resonator and front
section are in good condition, and the rear section and muffler are less than a year
old with less than 7,000 miles on it.
Performance On The Dyno
Baseline performance was measured at a peak horsepower of 101.0 and a peak torque
of 97.4 ft-lb.
Notice on the chart below that the engine performance follows what
appears to be two, overlapping curves which meet at 5,000 RPM. This is due to the dual
intake runner system of the 1.6 liter DOHC Isuzu engine. The engine opperates with
only four intake runners open at low RPM for improved low end power and torque response,
and then opens up the other four intake runners to allow each cylinder to be fed by
two side by side intake runners, which improves high RPM engine performance.
The Pacesetter Cat Back Exhaust
Our first problem with the Pacesetter product was obtaining one for testing. On
April 6, 2001, we ordered a Pacesetter PN 88-1217 exhaust system from Pacesetter's
Midwest wholesaler, Reliable Automotive, to be drop shipped directly from Pacesetter
to Isuzuperformance. We made no attempt to hide who we were or that our intentions
were to dyno test the product against the Stan's Headers brand cat back exhaust system.
We had intended to do our testing and have it completed by the first week of May, 2001,
in time to reveal the results at the Carlisle Custom Compact Power Jam car show in
Carlisle, Pennsylvania, on May 4-6, 2001. The product had not been received from
Pacesetter by June 9, 2001, two months after ordering it, and we began wondering if
the product had been discontinued or if the manufacturer sought not to allow us to
dyno test the product by refusing to sell it to us. The product distributor informed
us that the order had been turned over to the company and they had no explanation for
why the product had not been delivered or even shipped from the manufacturer at that
time. On June 26, 2001, UPS finally delivered the product to us, allowing us to go
forward in our testing and make the baseline tests described above followed by the
testing of the Pacesetter product.
This is being brought up specifically because many people have indicated that they
prefer the Pacesetter product based on availability and a perceived quick delivery
time from Pacesetter. In our case, nothing could be further from the truth, and
Pacesetter took twelve weeks (three months) to deliver their product to us, their
The Pacesetter cat back exhaust system arrived in one box. The bottom of the
box had been ripped open in transit due to poor packaging. The three main pieces
of the exhaust system, the two pipes leading from the catalytic converter back and
the muffler assembly, were in thin plastic sleeves. There was no wrapping or any
attempt to pad or keep the parts from hitting or beating against each other in
transit, as they were essentially loose in the box, a box that was a good bit
larger than needed for the parts, and as such, the parts rattled around against
each other and the box, tearing the bottom out of the box.
The Pacesetter cat back exhaust system consists of four pieces, a forward pipe
which bolts to the back of the catalytic converter, a center piece which goes over
the rear frame cross member, a rear assembly of muffler with inlet and outlet pipes
and hangers, and a clamp on resonated chrome tip. Hardware included is a fiber type
four bolt gasket, a fiber type two bolt gasket, two 2 1/4 inch U style pipe clamps,
two nuts and two bolts.
The pipe is 2 1/4 inch in diameter and has been painted black. The manufacturer
claims that the pipe is aluminized but has then painted over that aluminized pipe
with black paint, which draws the manufacturer's claim into question. In addition,
as there is no attempt to pad or protect the paint finish in shipment, the pipes
and muffler arrive chipped up, with surface rust showing through wherever the paint
finish has been scratched or marred.
The pipe itself measures .062 inches in thickness, which makes this 16 gauge pipe.
The muffler is a three chamber reflection design muffler, or a regular, conventional
style muffler. The sides of the muffler shows evidence that the interior walls of
the muffler, which form the chambers, are welded in place in four spots for each
wall, not a continuous weld around the circumfrence of the interior. This will
allow leaks around the walls as well as concern because those spot welds will
eventually fail allowing the walls to break free within the muffler and rattle.
The muffler's exterior is bent steel with a single edge weld bead holding the end
caps of the muffler to the body. This is of concern because there is no attempt
to lap weld or assemble the muffler body in a stronger fashion. Water will condense
and collect in these edge joints and the muffler ends will eventually rust out and
There are loose pieces of metal, probably globs of melted weld wire, within
the muffler and rattling around. They will not shake out of the muffler and will
likely cause some rather annoying rattling sounds when the system is on the car and
the engine is running.
The resonated exhaust tip, instead of having a perforated core, has a scalopped core,
with those scallops extending into and impeading the flow of air through the tip.
There are loose pieces of metal within the resonated tip which will not shake out and
will also likely cause some additional rattling sounds.
The parts are clearly marked as "Made In MEXICO".
There is a rather long winded warranty paper that came in the box with the hardware,
indicating a three year limited warranty, repair or replacement at Pacesetter's option
in the first year, and replaced upon payment of a prorated fee by the customer in the
second and third years (they will sell the customer another exhaust system to replace
the failed unit).
The warranty specifically excludes burnout, blow out, surface rust,
rust through, paint, plating, or any of the hardware. The most conspicuous part is that
they exclude rust, blow out, or burn out, the very reasons a exhaust system would fail,
therefore, they have eliminated any possible cause of exhaust system failure from their
There are no written instructions, but installing an exhaust system is fairly straight
forward. Remove the original exhaust system by unbolting each piece starting from the
tail pipe forward. Hang the new pieces in place from the catalytic converter to the
tail pipe, but do not tighten the bolts or clamps until all the pieces are in place.
There were several fitment problems with the Paceseter product.
- Poor packaging resulted in the ends of the pipe being somewhat less than round.
- The slip joint between the front and middle section
had not been expanded enough to allow the end of the front section to slide onto the
middle section. The pipe had to be expanded before the system could be assembled on
- The pipe had been bent
incorrectly such that the exhaust system was pressed against the botom of the car in
three places: the heat shield on the fuel tank, the front edge of the lower trailing
arm, and the side of the rear cross member. The connections will not allow any
adjustment, and short of bending the pipe, there is no way to solve this, short of having
the system bent by someone with pipe bending equipment, such as a muffler shop.
Jobber Wholesale Cost: $152.72
Manufacturer's Retail Price: $186.99
Manufacturer's List Price: $209.95
Driving Around With It On The Car
The exhaust sound is atrocious, and there is an intense fart sound from
2,000 RPM to 4,000 RPM.
The sound is about as melodious as someone shaking a metal can full of
nuts and bolts.
The sound inside the car is as if the car has a bad wheel bearing,
engine vibration at all speeds, even when coasting.
This exhaust system is quite a bit louder than was expected for a chamber style muffler
with small, 2 1/4 inch diameter pipe. The rattling and annoying resonant frequencies
of the system which cause the buzzy, sticatto, weed whacker sound seem to actually
amplify the most shrill and harsh sounds within the sound of the engine exhaust.
The black paint begins burning off of the side of the car almost immediately once the
engine is started after installing the exhaust system. The stench of the burning paint
is pretty strong and very persistent. There was smoke visible, wafting from beneath
the car the entire time the system was on our test car.
Performance on the Dyno
The Pacesetter cat back exhaust system was measured at a peak horsepower of 102.8
and a peak torque of 101.7 ft-lb. This is a peak power gain of of 1.8 HP and 4.3 ft-lb
The graphed performance curves on the chart show that the Pacesetter system perfromed
relatively well below 5,250 RPM, however, performance trailed off above 5,250, to
a level just above that of the stock exhaust system. This most likely has to do with
the limited ability of the 2 1/4 inch diameter pipe to flow as much exhaust gas as the
1.6 liter engine is cabale of pumping through it at higher engine speeds, and the
limited flow capability of the chambered style muffler.
The Stan's Headers Brand Cat Back Exhaust
This is the exhaust system we have manufactured for the Isuzu cars. The part numbers
for this cat back exhaust system are SHBILL01P (single muffler, plain aluminized finish),
SHBILL01C (single muffler, ceramic coated), SHBILL01TP (two muffler, plain aluminized),
and SHBILL01TC (two muffler, ceramic coated).
Although we did not ship this product to ourselves for testing, we do go to a great
deal more effort to keep the Stan's system from being damaged in shipment. The box is
sized to match the product. Each piece is wrapped in either bubble wrap or corrugated
card board. The insssides of the box walls and ends are lined with additional layers
of currugated cardboard as the product is being placed into the box, and any extra
space is taken up with more cardboard or packing material to keep the pieces packed
tightly and prevent any damage from pieces hitting each other or from the pieces
breaking out the sides of the box itself.
The Stan's Headers brand cat back exhaust system consists of five pieces, a forward
pipe, a center piece which goes over the rear cross member, a muffler assembly with
hangers, a tail pipe with hangers, and a clamp on chrome tip (which is not resonated).
Hardward included is a four bolt catalytic converter hasket, three U style 2 1/2
inch pipe clamps, and one low profile band style pipe clamp.
The pipe is 2 1/2 inch diameter aluminized steel with no paint over the aluminized
The pipe itself measures .082 inches thick, making it 14 gauge. This pipe is 33%, or
one third thicker than the pipe used on the Pacesetter exhaust system, adding greatly
to the durability and expected lifespan of the Stan's product over the Pacesetter
The Magnaflow brand muffler(s) is(are) a straight through design with sound deadenning
material wrapped around a perforated core. This design allows a great deal more air
flow than conventional reflection type mufflers.
The muffler body is heavy gauge steel with overlapping welded joints.
The tail pipe is designed to be trimmed to fit the car, as the Stan's system is designed
to fit the two door coupe, three door wagonback, and the four door sedan cars sharing
this platform. This allows for correct placement of the exhaust tip in the bumper notch
on the back of the car for all these cars without having to buy a extension pipe.
The unresonated tip, that is larger in diameter than the tail pipe, causes a shift in
frequency of the exhaust note down an octave to give the exhaust sound a deeper tone.
The Stan's Headers brand cat back exhaust system is available in single or two muffler
configuration. The two muffler configuration adds a second Magnaflow brand muffler to
the forward pipe, in the same position as the resonator of the original equipment
exhaust system. This second muffler reduces exhaust sound more than the single muffler
system, further deepens the tone of the exhaust note, and reduces any resonances in the
RPM band to nealy nonexistent.
We don't put any stickers on the Stan's Headers products, but they are made in Auburn,
The Stan's Headers brand cat back exhaust system is available in ceramic coated finish.
The ceramic coating is Techline brand Cermakrome, 1,400 degree, aluminum colored coating,
applied over a grit blasted preparation of the exhaust system to ensure good adhesion.
The pipe receives three exterior coats and two interior coats to protect the pipe against
oxidation and keep the exhaust heat from radiating off of the pipe.
The Magnaflow muffler carries a lifetime warranty.
We have no specified warranty on the exhaust system as a whole, as we have been selling
them for five or six years now and keep getting reports back from customers indicating
that the pipe still looks like new.
Again, there are no written instructions, but installing an exhaust system is fairly
straight forward. Remove the original exhaust system by unbolting each piece starting
from the tail pipe forward. Hang the new pieces in place from the catalytic converter
to the tail pipe, but do not tighten the bolts or clamps until all the pieces are in
The Stan's systems are bent extremely accurately and fit very well. We had no problem
installing one on the car for testing. It went on like original equipment with no rattles.
Single Muffler Aluminized: $339.00
Single Muffler Ceramic Coated: $439.00
Two Muffler Aluminized: $399.00
Two Muffler Ceramic Coated: $499.00
Driving Around With It On The Car
The Stan's Headers brand single muffler cat back exhaust system appeared to be
significantly quieter than the Pacesetter system, mostly because it did not make the
high pitched farty sound like the Pacesetter system did.
The exhaust note was noticably deeper in tone with only a hint of resonance around 4,000 RPM.
The Stan's Headers brand two muffler exhaust system was quieter still, and even deeper
in tone, without any resonance at all.
Both systems have a rich sound that more closely approximates a classic sports car
The Stan's systems create a burbling sound on acceleration, similar to that of a
large displacement muscle car.
None of this is to say that the Stan's systems make a Isuzu or Storm sound like it has
a V-8 engine in it, but they are noticably deeper in tone and sound than the sound import
cars are often made fun of for making. The sound remains that of a four cylinder engine,
but has a much more refined and dignified tone.
Performance on the Dyno
Performance between the Stan's Headers brand cat back ehxuast systems with the single
muffler and two muffler configuration are essentially identlcal. We decided to show only
one curve on the dyno chart, because the curves were essentially identical, which would
only confuse the chart. The exact curve we showed was from the two muffler system.
The Stan's system was measured at a peak horsepower of 105.2 and a peak torque of 102.0
ft-lb. This is a peak power gain of of 4.2 HP and 4.6 ft-lb of torque.
The Stan's cat back shows a maximum increase of 5.8 HP at 3,100 RPM and 10.0 ft-lbs of
torque at 2,700 RPM.
The graphed performance curves on the chart show that the Stan's Header brand cat back
exhaust system performed very well across the entire engine opperating range. The Stan's
system turned in better low end numbers than the Pacesetter system, and produced significant
gains over the Pacesetter system at higher RPMs at every point above 5,500 RPM.
The Stan's system produced more power and more even and consistent performance across
the entire RPM band in both horsepower and torque.
The Stan's system resulted in over twice the peak horsepower improvement as the
Pacesetter system (4.2 HP vs. 1.8 HP, 2.33 times more power).
- The Stan's Headers brand cat back exhaust system outperformed the Pacesetter cat back
exhaust system by a rather healthy margin.
- Comparing the two on the basis of dollars spent per peak horsepower gained, the
Pacesetter system costs $105 per horsepower, while the Stan's system costs $80 per
- Fit and quality between the Pacesetter and Stan's systems is drastically different,
illustrating the old addage, you get what you pay for.
- The quality difference will save the user even more over the long run because the
Stan's system will easily outlast the Pacesetter system.
- Any concerns about loosing torque or low and midrange power by using 2 1/2 inch pipe
as opposed to 2 1/4 inch pipe would seem to fall flat, as the system using 2 1/2 inch
pipe, as well as straight through style mufflers, gained both more horsepower and more
torque across the entire RPM band, and gained significantly at higher engine speeds
without sacrificing any mid or low range power.
The 2 1/4 inch exhaust system performed better than the 2 inch diameter system, with
most of the power increase below 5,250 RPM, but the power gained was still less than
that of the 2 1/2 inch diameter system, which gained and additional .5-2 HP over the
2 1/4 system. Above 5,250 RPM, that disparity widens, as the performance of the
2 1/4 inch system returns to a level of performance just a hair above that of the 2
inch system, while the 2 1/2 inch system gains almost three more horsepower at peak
widenning to almost five horsepower past peak horsepower.
- Similar concerns about using a straight through muffler and concerns that this will
cause a loss of mid and low range power are equally dispelled.
The Final Word
What is the difference between the Pacesetter exhaust system and the Stan's brand exhaust system?
2.4 Peak Horsepower and 4.2 Horsepower at 7,500 RPM.
Daryl and Matt at Mustang Muscle Performance and Dyno Service have stated that
they are more than happy to discuss their testing and equipment with anyone who
has any questions, and they are more than happy to help if you want to dyno test
your own car at their facility. They can be reached at:
Mustang Muscle Performance and Dyno Service
High Ridge, MO 63049
TEL: (636) 677-9987
FAX: (636) 677-3632
to return to Bill's Isuzuperformance Catalog Directory.
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