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Discussion Starter #1 (Edited)
Why is choosing a performance exhaust so confusing?

Most believe that "bigger is better". Very large diameter pipes are not the way to go for optimum car performance. For street use, where we rely on low-end torque, big pipes just take energy out of the exhaust, both by slowing down the exhaust gas velocity and by cooling off the gases. Exhaust may leave the port at 1500° or more, but at the tailpipe it may be down to 150° - a sign that energy has been lost. Velocity and uninterrupted flow are the key performance-making factors.


What factors influence the design of an exhaust system?


  • Lengths and diameters of all pipes in the various sections of the exhaust – this includes the primaries, the collectors, the midpipes, the catback and the muffler (if used).
  • Exhaust port temperature – it will vary by engine, how the car is tuned, by what fuel is used, the A/F ratio, and by the engine speed.
  • Predominant engine rpm – will the exhaust be used primarily for the street, the strip, circle track racing, road course racing, or a combination? There is no such thing as a universal performance exhaust.

Why are the lengths and diameters of all pipes in the various sections of the exhaust so important?

The exhaust engineers at most car companies will design an exhaust system generally specific to the car and for its general use. Nissan touted the Spec V as a car with high-torque in the entire rpm band. The engineers at successful aftermarket exhaust manufacturing companies might offer products for more specific use. Companies such as Hooker, Edelbrock, Hedman and Borla have been around for decades. They thrive because they can produce superior exhaust systems based on how much time they devote to R&D. They will optimize the lengths and diameters of all piping based on particular applications, by the use of extensive temperature and pressure readings along the entire system. Their goal is to maintain exhaust velocity and uninterrupted flow.

Unfortunately not a lot of R&D has gone into the total Spec V exhaust system. We have a lot of choices for headers, midpipes, catbacks, axlebacks and mufflers. Keeping in mind, that "bigger is not always better"; it is up to us to choose a combination that works best for our individual cars.


  • Header – the proven performers are Stillen, NISMO, HotShot and AEBS, providing 4-10 whp more than the competition, but they are no longer available. 2JR and Trav are making a long-tube header for a high hp NA QR. Time will tell whether this header will perform as well in bolted and mildly built QR’s.
  • Midpipe – a 2½” pipe is acceptable for fully bolted-only, and a 3” pipe for all others, because the temperatures and pressures at the entrance of the pipe are sufficiently high to maintain gas velocity.
  • Catback – for a fully bolted QR the pipe diameter should not exceed 2½ to maintain design exhaust velocities. A 3” pipe is required for bolted and built QR’s with 200-260 whp. A 2½” pipe can be used for FI QR motors with 300 whp or less, a 3” pipe for 450 whp or less, and a 3½” pipe for 450-650 whp. A smaller pipe may be used if the length of this piping is shortened (side exhaust, cut-out, split exhaust).
  • Mufflers, resonators and catalysts – ???????????????
There are many factors that go into assembling an optimum exhaust package for each individual car and its intended use. Anyone telling you that he knows how to do it is either a genius or a liar. We can research, learn from others or experiment on our own (a dyno would shorten the process but most of us don’t have one in our garage).

As you can see, I have been doing some research in this area because this is the last mod I intend to do. I have installed a VQ TB and will be working on a one-piece intake/MAF housing with some air straightening built in. I have decided on a 3" exhaust to handle the additional flow. The final choice of header will come after some dyno-tuning. I want to be able to compare header performance differences.

You are welcome to chime in if you find flaws in my conclusions, offer suggestions from your experience, or just to start a discussion on the subject.
 

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Discussion Starter #5
You'll lose power if you don't have a header with a collector. An open exhaust has almost no back pressure.

A must read for all the YO BRO 3 INCH EXHAUST UP ON THIS 1.8S.

But still....the best exhaust is no exhaust.
 

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Good read man.

I think that what you are talking about are more in terms of a race setup rather than a daily driver.

I say this because only a racer will want every ounce of HP they can get out of the engine and the every day person wants more HP, but at the same time, don't want to get pulled over for being too loud.

Then you have to take in consideration the inspection laws, blah blah blah....

Nevertheless, a damn good read as I am in the same boat. Choosing an exhaust is tough.
 

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Good read man.

I think that what you are talking about are more in terms of a race setup rather than a daily driver.

I say this because only a racer will want every ounce of HP they can get out of the engine and the every day person wants more HP, but at the same time, don't want to get pulled over for being too loud.

Then you have to take in consideration the inspection laws, blah blah blah....

Nevertheless, a damn good read as I am in the same boat. Choosing an exhaust is tough.
This is awesome advice for any level of modifications, I remember something from personal experience, a car that was boosted with 2.5" exhaust @ 8psi, the day the car got the 3" exhaust system it was nailing 9.5-10psi!

Efficiency FTW!!!



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awesome post. maybe you can help me

i'm working on my full N/A build, my mods are cai, cams, neo, short throw, clutch, flywheel, cf hood

as far as exhaust goes, i'm running a no name header, down to a fattened homemade midpipe before the cat. (no resonator). the piping after the cat is the skinnier stock piping which i intended to fatten as well, then the stock spec v muffler.

do you think it would be better replace the rest of the stock piping with something fatter or am i opening up the ass of my car too much and losing backpressure? i wasn't planning on doing the muffler, just the midpiping. any suggestions are appreciated
 

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Discussion Starter #11
Good read man.

I think that what you are talking about are more in terms of a race setup rather than a daily driver.
Actually this is meant for anyone looking to upgrade their exhaust. It provides a sort
of upgrade road map. If you just want to pep up your DD and have no ambitions to
spray, boost or build, a better eBay header and a 2.5" catback, totaling about $250,
will save you a bunch over a NISMO header, a 3" Stromung catback, and a 3"
downpipe costing around $1400. Okay, you may not get the extra 4-10 whp the
latter system provides but you might spend the money saved on other goodies. For
the boosted, sprayed, and build guys, go straight to a 3" or a 3 1/2" exhaust. Even
for a DD, it doesn't make sense to make this middle step first.

Then you have to take in consideration the inspection laws, blah blah blah...
Choosing a muffler/resonator with acceptable sound levels and a high-flow cat for
your DD will keep you out of the courts and with valid inspection stickers.
 

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awesome post. maybe you can help me

i'm working on my full N/A build, my mods are cai, cams, neo, short throw, clutch, flywheel, cf hood

as far as exhaust goes, i'm running a no name header, down to a fattened homemade midpipe before the cat. (no resonator). the piping after the cat is the skinnier stock piping which i intended to fatten as well, then the stock spec v muffler.

do you think it would be better replace the rest of the stock piping with something fatter or am i opening up the ass of my car too much and losing backpressure? i wasn't planning on doing the muffler, just the midpiping. any suggestions are appreciated
If you are staying at the stock compression ratio, I would say 2.5" piping will be fine for your setup. Definitely get rid of the stock stuff.
 

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Discussion Starter #13
awesome post. maybe you can help me

i'm working on my full N/A build, my mods are cai, cams, neo, short throw, clutch, flywheel, cf hood

as far as exhaust goes, i'm running a no name header, down to a fattened homemade midpipe before the cat. (no resonator). the piping after the cat is the skinnier stock piping which i intended to fatten as well, then the stock spec v muffler.

do you think it would be better replace the rest of the stock piping with something fatter or am i opening up the ass of my car too much and losing backpressure? i wasn't planning on doing the muffler, just the midpiping. any suggestions are appreciated
Not enough information. I see you have cams. Do you also have springs? Are you
planning to do any engine work (pistons, rods, crank, head work)? If your answer is
"no", then you are not using your cams for what they were designed and you won't
need anything more than a 2 1/2" exhaust. You could get a used one for about
$200, or an eBay, a 2 1/2" downpipe and catback for about $200-$300.

How do you "fatten" pipes?
 

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sorry for the riddles...
by full n/a i meant that i intend to get engine and head work done.
by "fatten" i meant cutting up a wider pipe and replacing the thinner pipe. i did this to the pipe before the cat and was just wondering if doing the same thing to the pipe after the cat would reduce power..
 

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Discussion Starter #15
sorry for the riddles...
by full n/a i meant that i intend to get engine and head work done.
by "fatten" i meant cutting up a wider pipe and replacing the thinner pipe. i did this to the pipe before the cat and was just wondering if doing the same thing to the pipe after the cat would reduce power..
Okay, this makes it a lot clearer. From what I've learned, a 2.5" exhaust is sufficient
for fully bolted mods up to about 180-190 whp. By getting engine and head work
done, you could be looking at 190-220 whp running NA. For that you will need a full
3" exhaust. This includes a better header, a 3" downpipe and a 3" catback. So stop
fattening your pipes and replace everything at once.

I'm sort of in the same situation you are in, except that I've already done all the
engine and head work. I had an eBay 4-2-1 header and an eBay 2.5" catback. My
last dyno showed that I pretty much stopped making any significant power after
6900 rpm. To me, that means I'm either starving the motor or choking it, or both.
So I'm installing a VQ throttle body with a custom intake and MAF housing. I just
bought 2 eBay headers with 2 3/4" collectors ($90 and $180, shipped) and will be
buying another one within the next 2 months. I will dyno-test all three and choose
the best one. I also picked up a Megan 3" downpipe for $170. I will be getting a full
3" catback soon. I'm guessing I should get about 225 whp and 208 ft∙lb torque.
 

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Megan makes a 3" downpipe for our cars? Could you please post a link? Would be very useful information seeing as how the Stromung 3" is discontinued.

Also looking forward to your dyno results. What are the brands of the headers you ordered/will be ordering?
 

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Discussion Starter #17
Megan makes a 3" downpipe for our cars? Could you please post a link? Would be very useful information seeing as how the Stromung 3" is discontinued.

Also looking forward to your dyno results. What are the brands of the headers you ordered/will be ordering?
They are all no-brand headers. I will probably end up using the header Jason will be
making for me when he has some time. BTW the downpipe was $170 shipped (best
offer).
 

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Just to give you a heads up, that Megan downpipe is not 3", it's actually 2.5". The ID and OD measurements they give are backwards; the downpipe is even shown backwards and upside down in the picture. The 3.5" is where the header attaches and is the width of where the donut gasket belongs. For some reason after the resonator on the Megan they flare it from 2.5" to 2.75" where it collects to the catback. I use to have the exact same midpipe/downpipe.
 

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Discussion Starter #19
Just to give you a heads up, that Megan downpipe is not 3", it's actually 2.5". The ID and OD measurements they give are backwards; the downpipe is even shown backwards and upside down in the picture. The 3.5" is where the header attaches and is the width of where the donut gasket belongs. For some reason after the resonator on the Megan they flare it from 2.5" to 2.75" where it collects to the catback. I use to have the exact same midpipe.
What you're saying is that the pipe where the resonator ends is only 2.5"? It may not
be too bad though because the residence time of the exhaust gases in that pipe is
very short. I can always replace it later to get a full 3" system. BTW that Stromung
catback is still available without the downpipe for about $650. What's your gut feeling
on the 2 headers? Both should be better than my 4-2-1 OBX knockoff though.
 

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What you're saying is that the pipe where the resonator ends is only 2.5"? It may not
be too bad though because the residence time of the exhaust gases in that pipe is
very short. I can always replace it later to get a full 3" system. BTW that Stromung
catback is still available without the downpipe for about $650. What's your gut feeling
on the 2 headers? Both should be better than my 4-2-1 OBX knockoff though.
The piping is 2.5" throughout, then after the resonator it flares out to 2.75".
I thought I remembered hearing the Stromung downpipe was discontinued, but I could be wrong. Their 3" catback is still available.
I'm not sure on which header would make more power. My theory is that all of the cheap ebay headers have terrible overlapping runners in the collector (just like smokinjoe showed in his header dyno comparison thread) which will limit flow regardless design. My OBX shorty had terrible overlaps in the collector and I had to spend a few hours porting it out by hand which ended up giving me an extra 1/2" total inner diameter at the final collector.
 
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