Cat Back diameter question

[paco]

I have been reading the forum and everybody says that 2.25 " is the right diameter for our cat-back but, isn't this the stock size??? I read on Import Tuner an article on cat-backs, and it said that on 2.4 liter engines the perfect diamater would be 2.50". The article showed the dyno numbers with both 2.25" and 2.50" cat-backs and on all cars the 2.50" cat-back gave between 4.5 and 5.5 more horsepower than with 2.25". The did tests on a Ford Pobe, Honda Civic and Ford Focuz if I'm not mistaken. Anyway I'll get my cat-back made on wednesday and I'm going to get it 2.5 ".

[Prophet]

Most people when going with 2.5 they are using a forced induction like Turbo or NOS. On a N/A car you will want to go 2.25. Also yes our car does have 2.25 piping but it is not mandrel bent. It is pressed bent.

Jason

[pinoyesv6]

on the I4 i'd go with the 2.25" unless you have some big mods in the future like forced induction, NOS, maybe a good set of exhaust headers. 2.5" might give u more hp, but then your butt dyno will say your car is going slower than b4, also you want to think about your exhaust muffler. too big of a pipe on a muffler can cause raspyness, and thats not good

[paco]

raspyness????? what is that?? Thanks for the replies guys. I do plan to get Nos in the future, I would like a turbo but right now is a little out of my budget.

[Kingman]

The article in Import Tuner did a test on NA cars to go against what people think the norm's are.

They used 3 engine sizes for their test

1 - under 2.0
2 - 2.0 - 2.5
3 - 2.5 - 3.0

For under 2.0 people were stating a max of 2" diameter. In their tests they found that going to 2.25 yielded a little more power, without hurting the powerband. They recomended 2.5" as the maximum diameter but it will hurt your low end alot.

For 2.0 - 2.5 liter their tests concluded that the 2.5" was the best all around and that 2.75" was max size.

for 2.5 - 3.0 they recomended 3" diameter and maxed at 3.25"

All these tests were compared on a dyno according to the article originator and then track tested. Sounds about right to me though. According to my knowledge 1" per liter is about normal.

[toc8g]

Well I guess I will have to speak up,..I am kinda lazy about this but here it goes.

I read in a SCC magazine in a very informative article that was about building your own exhaust system. I lost the magazine so I am not gonna be exact so sorry. Basically what it said was that for our CC's and I forget what they are, that 2.25 is as round as we wanna go because going 2.5 will be tooo big and as it is since we have a/t we have sorry ass low end, so why lower it even more, granted of course you will have great high end with 2 1/2.

So,...now that being said, I am not just acting as though I know what I am talking about I actually do know what I am talking about. I have gone through 3 different mufflers and 3 set of custom cat back piping.
Since we are just talking about the actual diameter of the piping I will tell you about the actuall piping. OK, first pipe I had a 2.25 custom non mandrel bent pipe made, really not realizing the stock pipe was also 2.25
it actually helped a little even though it was the same size becuase it got rid of the resonators and extra junk. I was kinda bumbed because I thought it was gonna be mandrel bent so consumed with drive for the best pipe I got my friend who owns a motorsports shop to replace the non mandrel bends with mandrel bends, and being the cool guy he was he replaced the whole piping with 2.5 along with all bends mandrel bent.
As a side note thats when my last and final muffler was added on and I swear by it! Ok back to the pipe. OK, so I am stoked I have mandrel bent pipe and I am loving it, the sound is loud and forboding and menacing,..roar!!! I am driving slowly onto the street and damn the shit didn't even peel out I mean the low end seemed allot worse the pipe was to big,..it was horrible. The top end was great man I made a new 2001 V6 GT Eclipse Sporty play catch with me on the highway,..and I have a I-4, granted I was moded(jet chip,injen cai,exhaust,cam gear), so I felt a little robbed because most of the racing I do is on the street and I need low end hp and torque not top end. So here we go again i went back down to 2.25 replaced it again and their I stayed and whipped allot of ass with my non mandrel bent custom cat back piping. Don't get me wrong I would love to have mandrel bent but money was becomming a issue so I took it. That was all on my old 2000 I-4 Fed Spec Galant. Now I have my new 2002 Cali Spec Galant and I have learned my lesson, I threw my muffler on and replaced the stock resonators with some strait pipe and all is good have good pipe and a good muffler and for now its good enough. If a True mandrel bent cat back system comes out that is 2.25 i will buy it but until then I am perfectly fine. :wink:

[Kingman]

The other cars were modded I do beleive. So I would imagine with a header and cai on 2.5" would be fine for a 4 cylinder. I would go 2.5 especially if I had NOS on it.

For my 3.0 I may even go to 3.5" when I am done.

[99DE]

I'm personally into performance alone, some people just want the sound like the ol baseball card in the BMX spokes trick. I've heard plenty of people here say they lost power with just replacing the muffler alone. This leads me to believe that our cars need the backpressure which is the age old myth. I am confused too, I keep reading about straight through designs is the way to go, but our cars seem to suffer with this. I still have the stock exhaust, and I swear that after adding on the Injen intake I increased my torque and power through all power bands, now my car takes off the line faster with a little tire spin. I'm still waiting for a dyno proven specific galant cat-back.

[toc8g]

<div class='quotetop'>QUOTE(Kingman)</div><div class='quotemain'>The other cars were modded I do beleive. Â*So I would imagine with a header and cai on 2.5" would be fine for a 4 cylinder. Â*I would go 2.5 especially if I had NOS on it.

For my 3.0 I may even go to 3.5" when I am done.</div>

Well let me refraise this. 2.5 is too big for a n/a engine that has a automatic transmission. The a/t will eat up a tremendous amount of low end hp and tq. You would have to have a turbo or Supercharger, or even noz to support the car through the bottom end or hey even a torque converter. And a header my friend is not forced induction. as you said,
<div class='quotetop'>QUOTE</div><div class='quotemain'>So I would imagine with a header and cai on 2.5" would be fine for a 4 cylinder.</div>

I had the mods also except the header. The mods are not gonna help much especialy if you get a header, most header(s) are for mid to high end performance very few help the low end unless a large amount of R&D is put into them. So again you would only be robbing your self of ore low end hp.

Now their is always another side to th story so,....let me say this.
If a m/t n/a car with mods (cai/e/h) were to have a 2.5 diam.
then I would think it could be ok simply because you can minipulate the transmission inot giving you your low end,....anyway so you and the mag are sorta right, I guess, but defiantly wrong for 8G I-4 Galants because we are all 99.9% a/t. I have done the reasearch and I know for a fact even with mods 2.5 is too big unless your going with some sort of forced induction. Oh snap!! I Just had a thought, If you had a aftermarket torque converter on the 8G it could counter act the loss of low end to counter act
the loss of hp from the bigger pipe so you could hopefully see the benifits throught the power band. But you see for a n/a car with bolt ons 2.5 is still too big,...gotta have some sort of forced induction or tranny work, so for the normal guy in a a/t 8G 2.5 would be too big.

[toc8g]

<div class='quotetop'>QUOTE</div><div class='quotemain'>I've heard plenty of people here say they lost power with just replacing the muffler alone. This leads me to believe that our cars need the backpressure which is the age old myth.</div>


Of course our cars need back pressure that is why I am saying 2.5 is too big for a n/a vehicle. You need to stick with 2.25 but preferebly have mandrel bent corners. And who ever told you replacing your muffler alone will lose hp is so full of crap, I dunno I have my muffler basically just replace and the resonators replaced with straight pipe and my shit is better then before. And check this out I have just a injen cai and my shit is faster,...so don't be lfet behind :wink:

<div class='quotetop'>QUOTE</div><div class='quotemain'> I am confused too, I keep reading about straight through designs is the way to go, but our cars seem to suffer with this. </div>


Seem to suffer,...yeah if not done right.

[Kingman]

On another note, I have seen some very poorly designed pipes of any diameter that lose power because of either improper bends(not mandrel). Or just plain dumb.

But depending on what you want to do is what size you will need. If you are going to go with just the intake, I think I would stay smaller. If you are going to put an intake, headers, and fuel control I would go 2.5" because with the headers you not only are shooting for top end but a general increase.

If you are doing a turbo learn from the DSM crew. GO BIG

[toc8g]

<div class='quotetop'>QUOTE</div><div class='quotemain'> because with the headers you not only are shooting for top end but a general increase.</div>

You see,..you can't assume that we are going for Top end with a header.
I plan on the RPW header and they have done their R&D for low to mid
range hp because as you know the 8G a/t sucks on the low end. Now I say if all your interested in is top end and get a Header with shorter runners to help out on the mid to top end hp then yeah go with 2.5 but your I-4 will greatly suffer from the low end. Remember I am not just giving you my opnion from what I think I know I have actually learned from trial and error,... Remember I had the cai,the exhaust, the Jet ECU mod, and also the Cam Gear timing,...all that,..and in my opnion with a n/a I-4 8G 2.4 liter a/t Galant their is no reason to go 2.5 unless you are gonna be boosting in one form or another. I lost to much low end with the mandrel bent 2.5 to make it worth the top end. I went back down to 2.25 and it made a world of difference. Bottom line if your going n/a 2.25 mandrel bent is my recomendation at thats comming from some one who has invested the time and money to find out.

[Kingman]

They are the Race Design Headers, and from several reviews they gain in mid to top end. Thats what I was going buy.

I know low end sucks it does on almost anything unless you have a really large motor or NOS.

Get acouple together and do dyno runs.

BTW If I was going forced I would go larger than 2.5"

[toc8g]

These are quotes from RPW's Website and Club 3G but are from Mitsiman.


Our "True Race" Design goes one step further though by extending the primary pipes 150mm before merging into the collector. This has been done to help improve the torque at a lower rpm over the "Street Race" design and make them similar to an "Interference design" but retaining all the benefits of our four into one design.
Header Differences
There is actually a lot of difference between the two designs.

The street Design is a basic 4 - 1 tuned length extractors with the length of the pipes before they join the merge collector around 27" long. The merge collector is around 3" long and has a flange welded onto it. A second flange is supplied and gasket so you can weld the 2nd flange onto your flex pipe and then bolt them together. This extractor will not enhance bottom end torque, but will not lose any either. It will enhance mid range and top end performance.

The Race Design is different in the sense that the header pipes are longer at 32" to provide more torque before merging into a collector. Retaining the 4 - 1 design, this has all the benefits of a 4 - 2 - 1 design without the side effects. The merge collector is the same setup with a flange, but we also supply a flex pipe of a larger diameter - min of 2 1/4" diameter that will bolt up to the cat converter. This flex pipe is a lot shorter than your original to accommodate the longer design on the headers.

These headers will enhance torque as well as mid range and top end performance. They have been proven on a dyno in there design to out perform other headers because of there extended piping.

David Thomas
www.rpw.com.au
__________________
Mitsubishi Magna / Diamante with RPW 70mm TB, FSE Regulator, RPW Quick shifter, RPW Race Design V6 Extractors, 2 1/2" RPW Exhaust, K&N Pod Filter.

Soon to be released with Twin Garret Ball Bearing Turbo or Supercharger kit.
www.rpw.com.au


So I guess we are right and wrong,.. I am right becuase he says, "This has been done to help improve the torque at a lower rpm over the "Street Race" design and make them similar to an "Interference design" but retaining all the benefits of our four into one design.
Header Differences"

And you are right when he said this, "These headers will enhance torque as well as mid range and top end performance. "

But I must stick to my previous statement 2.25 mandrel bent is the way to go because as much as we wish it was a Header is not forced induction,
and for a n/a vehicle thats all you need. So the burdon of proof is on you,,,...I have been through mandrel 2.5 and 2.25 and 3 different mufflers and I say 2.25,...not that I don't respect your "opinion" its just that you have knowledge but I have knowledge and hands on experience.
So we will have to agree to disagree.

[Kingman]

We both have good points. And it could be interesting to throw your 2.5" back on if you still have it after you have a header. I think with your JET chip you may also have been running rich causing a lower egt. The lower exhaust gas temp will slow your exhaust down. (Just thought about that). I know most aftermarket chips run way rich normally.

Anyhow check out this link.
http://www.eclipseforums.org/showthread.ph...=&threadid=1059 ('http://www.eclipseforums.org/showthread.php?s=&threadid=1059')

We are both right to an extent.

BTW I don't think I said headers were FI, but if I did I screwed up.

[dicksteel]

well fellas, with all of this talk about I-4's and 2.25" exhaust, how about a V6? would 2.5" still be to big or would it work alright because it is a bigger engine? thanks

[Kingman]

Once I have headers and cams I am using 3"

But 2.5" is ok if you are not doing more than a cai

[paco]

Well, I just read all the replies and let me say this, I got an I4 and I just got a custom cat-back 2.50in wide. There is a stop sign right before I get to my apartment there is never traffic there so I always try to spin my tires there. And as you all know our cars don;t spin that much (when it's dry anyhow) but with the cat-back it spins more now. Corect me if I'm wrong, but isn't this more low end power??

[99DE]

If you gained more torque with 2.5" then I guess everyones gotta do what they think is right. What muffler did you put on?

[b-boy_sky]

hey paco i put a 2.5" mandrel bent cat back on my i4 and i notice a difference.....when i punch the gas on a take off my car has a lot more kick than it did before.....i do think that i lost some power in my upper range but when i get my headers that should fix that.....

[paco]

I have a Matrix universal muffler. I like the sound.

[Reelax]

<div class='quotetop'>QUOTE(dicksteel)</div><div class='quotemain'>well fellas, with all of this talk about I-4's and 2.25" exhaust, how about a V6? would 2.5" still be to big or would it work alright because it is a bigger engine? thanks</div>

umm i must be the freak of the group but i have mandrel bent 2.75" piping on my N/A V6. i have a THERMAL R&D straight through muffler, MPH downpipe w/ hi-flow cat (no pre cats) and an AEM CAI w/ bypass. we figure i am pushing about 225hp to the flywheel; my car is faster than stock as i run 3G GT's often and almost always walk away (unless of course if they're modded similarly). if u r not going forced induction or squeeze, i suggest doing 2.5" on the V6 to avoid scavenging. once i get my turbos, i will move to 3" piping and a new muffler.