exhaust size

pdx-tony

Member
:
03 P5
Looking at having a custom exhaust made, it seems I remember reading that the stock one is 2.5 til the axel then 2"
would 3" cat back be to big I looked at the Megan racing one and is a 2.5" anyone running a 3" and intake as the only mods?
p.s. please don't send me to the exhaust post to look threw 30+ pages to find the answer
 
I remember reading that the stock exhaust starts as a 2.5" but it shrinks to 2" at several points, and finishes at 2", so you're getting the performance of a 2". Can't say that 3" is too big because that's preference. But you're not getting much performance increase over a full 2.5" system, not if you're tuned for street driving. 3" is worth it if you're building a race spec car
 
I remember reading that the stock exhaust starts as a 2.5" but it shrinks to 2" at several points, and finishes at 2", so you're getting the performance of a 2". Can't say that 3" is too big because that's preference. But you're not getting much performance increase over a full 2.5" system, not if you're tuned for street driving. 3" is worth it if you're building a race spec car

There are benefits to having expanding and contracting exhaust diameters.

Choosing exhaust length and diameter is always a compromise between high and low end power, for the most part. You need to read some theory on how different sections of the exhaust system are affected by changing diameters and lengths. But its not only limited to those 2 characteristics.

2.5" is a good balance, if you go with 3" on a naturally aspirated, your exhaust velocity will be rather low at low RPM and you will be losing power.
 
There are benefits to having expanding and contracting exhaust diameters.

Choosing exhaust length and diameter is always a compromise between high and low end power, for the most part. You need to read some theory on how different sections of the exhaust system are affected by changing diameters and lengths. But its not only limited to those 2 characteristics.

Interesting.. I had no idea, maybe I will. If you have any good informational pages or articles you can direct me to, that would be great
 
So, I asked this on the MSP forums (was searching Google and came across a post there) but no one answered, maybe cuz it was an old post.

I found that the Msp midpipe has 2 cats on it whereas the P5 has one on the midpipe and one right off the header. I wanted to get a 4-2-1 header for my P5 but the shop I went to get an installation quote from said it couldn't be done because of the cat that runs directly off of the header. You would have to delete that cat and it would no longer pass emissions test. But if I were to put a header and swap the mid pipe with a Msp one, which has both cats on the mid pipe, would that work the same way as the P5's system, with one cat off the header and one on the mid pipe?
 
You can delete the pre-cat and still pass emissions. You have to do some work to fool the 02 but one working cat is usually enough.
 
You can delete the pre-cat and still pass emissions. You have to do some work to fool the 02 but one working cat is usually enough.

I'm no mechanic, you'll have to explain the "work" lol
Is there any way to put a header and make the system work without fooling it? Like the Msp mid pipe, if both cats and the O2 are on it, would it still throw a code?
 
I'm no mechanic, you'll have to explain the "work" lol
Is there any way to put a header and make the system work without fooling it? Like the Msp mid pipe, if both cats and the O2 are on it, would it still throw a code?

You could use a non-fouler which you'd install into the new header in place of the first O2 sensor, this would prevent you from getting a CEL.

At one point in time I almost installed a 4-1 header along with the MSP midpipe. If you go this route invest in the stock wiring extension for the O2 sensor that the MSP uses. Only way I could see this setup failing an inspection is if you have a visual inspection and how picky they'd be upon seeing the setup. More than likely they wouldn't even know.
 
Let me see if I'm understanding this correctly. If I use a header with the MSP midpipe, the non-fouler for the first sensor is not necessary?
 
4-1 header eliminates the first cat. You run the non-foulers in the 2nd o2 bung with the sensor inside them. The first o2 sensor is just screwed into header. No need to elongate any wiring. The 4-1 bolts up to rest of exhaust with no mods necessary. I have passed emissions in TX visually and on computer for last 11 yrs with no hassle ever. My midpipe is 2.25 flex fitted with magnaflow hi-flo 2.25 cat. The catback is 2.25 with magnaflow 5x8 oval race series muffler.
 
Wait there's two cats on the p5? If you need two cats can't you just buy two and make a custom exhaust then just buy the o2 sensor extensions?
Just saying that's what i would do, or going to have to do later down the road.
 
Let me see if I'm understanding this correctly. If I use a header with the MSP midpipe, the non-fouler for the first sensor is not necessary?

Yes as the MSP midpipe contains both O2 sensors but will require you to lengthen the wire for the first O2 sensor since you are moving it from the exhaust manifold to the midpipe. As I mentioned earlier you can buy this harness extension right from Mazda for easy pnp.

Part # for the longer harness - BPY1-67-020

Wait there's two cats on the p5? If you need two cats can't you just buy two and make a custom exhaust then just buy the o2 sensor extensions?
Just saying that's what i would do, or going to have to do later down the road.

Yup, the first is part of the stock exhaust manifold and the second is found in the midpipe.
 
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thanks guys, I think I will just run with a Megan racing or ebay cat back for now then later on when I can turbo it I will get the 3" custom one made
 
I talked to corksport they are local but they don't make p5 parts anymore but they said 2.5 unless turbocharged. Stock is 2.25 into 2
 
I'm running a 4-2-1 header with a single minicat installed just behind. The cat included a bung at the back for the second factory O2 sensor; the first (which went into the OEM garbage can precat on the factory header) plugs directly into my 4-2-1. Behind the minicat is a Magnaflow resonator into a short length of pipe. Behind this is a Magnaflow catback with twin tips. All pipe is 2.5". When dynoed, I gained 8+ WHP and a corresponding increase in torque.

This mod passes very stringent Colorado emissions testing with flying colors--the numbers in all categories are extremely low. The system has been in the car for seven years w/o problems of any kind. The sound is unlike any other IL4 I've heard, low and very civilized--no fart can rap or other "look at me" sounds.

Best of all, it's legal (at least here), absolutely a big notch up in performance for an NA churn, and it's been dead reliable. Overall cost, including custom exhaust work and parts, around $600. Worth every cent.

Hope this helps.
 
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