Confused about pipe size on exhausts...

[MP5Girl]

Ok, so I've been doing my research on catbacks and stuff and now I'm all confused. I read that for a NA P5 you shouldn't go any larger than a 2.25 pipe but every catback is 2.5 which I read is too big unless your going forced. So... am I reading the wrong s*** or what? Help!

Thanks people!

 

[1sty]

Ok, so I've been doing my research on catbacks and stuff and now I'm all confused. I read that for a NA P5 you shouldn't go any larger than a 2.25 pipe but every catback is 2.5 which I read is too big unless your going forced. So... am I reading the wrong s*** or what? Help!

Thanks people!

2.5?

basicly every NA exhuast I have seen for the P5 has been 2.25-2.4"
Bosal, Borla, thermal, racing beat, Jic,ect are all under 2.5"

 

[MP5Girl]

2.5, 2.4 whatever... "anything larger than 2.25 is too big."

 

[1sty]

2.5, 2.4 whatever... "anything larger than 2.25 is too big."

I wouldn't worry about it to much, so long as its around 2.25-2.5 you should be fine. Different manufacturers find advantages with their designs with different size pipes. Not to mention some sections of a 2.5" exhuast can dip down to 2.25 any ways.

 

[MP5Girl]

Well I'm probably going the custom exhaust route so I'm trying to figure out what size I should tell them. If "anything over 2.25" is in fact too big then yes it is reason to worry since then there would be a power loss thus defeating the point of exhaust to begin with. And if the piping is bent right... there shouldn't be any dips, right?

 

[shinzen]

The only problem with the custom route is usually they are crush bending the piping not mandrel bending it- and the claim that anything over 2.25" is too big really is incorrect. It depends on your modifications, both present and future. Jic racing claims one of the highest gains for an N/A protege at 12 hp, and the size they use is 60mm, which is 2.36".. Magnaflow, which is what I run is the 2.25" size, and it made a big difference for me, it sounds like their dyno graph on the website is about right 8 hp and 10lb/ft.. Hope that helps!

 

[MP5Girl]

That does help a bit. The only performance mod I have is a CAI... future... well anything that won't void the warranty so, not much! So it sounds like it depends on the bending... if it's crushed then shoot for a 2.5 if it's mandrel then go for the 2.25. Would this also depend on the muffler? silencer?

 

[1sty]

That does help a bit. The only performance mod I have is a CAI... future... well anything that won't void the warranty so, not much! So it sounds like it depends on the bending... if it's crushed then shoot for a 2.5 if it's mandrel then go for the 2.25. Would this also depend on the muffler? silencer?

If its crush bend then its already a pointless purchase, get a pebuilt aftermaket one in that case.

 

[shinzen]

^^
I completely agree- crush bent=garbage- you might save a few bucks, but really not worth the money- if you go with an aftermarket one, they will properly match the size of the muffler to the piping..
Crush bending is just as it sounds- it will really interfere with the airflow characteristics of the exhaust system- turbulence, less flow=less hp... to have one made locally you are looking at 200-300, including muffler anyway, and for that pricing you could pick up an inexspensive aftermarket that will do the job- protegegarage.com has a group buy going on the obx system, 2.5" piping, or the magnaflow can be had through group buy for around $300- if you are impatient, ptuning.com in an advertisement can get you the magnaflow shipped to your door for $330 in a few days- they were great to work with. Hope that helps
Eric

 

[NJP5Guy]

as far as i know racing beat is the only company that actually has a dyno graph. all the others just make claims and what is a claim w/o the proof to back it up.

 

[MP5Girl]

OK that helps alot but still the original question. Is this 2.5 too big? Every where I read said to only go to a 2.5 if you plan on forced. Otherwise to stay at 2.25. Altho you all are giving me great info here and I do greatly appreciate it...

 

[shinzen]

2.5" would be too big unless you are going to really start digging into the engine or going turbo. Are the differences going to be huge between a 2.25 and a 2.5? Probably not. As to the posting of dyno graphs, team moon also posted dyno's from an automatic at 5whp-
There was a dyno done by d's_mp3 on the boards about 2 years ago, unfortunately the links don't work anymore. You would be looking at 4whp and 7lb/ft gain with the magnaflow

 

[NJP5Guy]

a 2.5" won't really make a difference because the piping which leads up to the cat-back is much smaller(ie, the down pipe, cat and exhaust manifold). so what you have happening is the exhaust going through small piping into bigger piping halfway down the system. think of it kinda like traffic. if you go from a 3 lane highway to a 5 lane highway you will only speed up in the 5 laner. in order to get max speed you'd need to make the whole highway 5 lanes. so having the biggest diameter piping for the cat-back is not going to make that much of a difference because the system is all plugged up before that. hope that makes sense.

 

[Captain KRM P5]

i am of the mindset that having some backpressure is good, that losing too much of it will rob you in the end of some power, so in my opinion there is no need to go any bigger than 2.5 inch on a catback exhaust system, and then only if you have a turbo. i have seen a few proteges running around with a 3 inch which i think is a little extreme. on an "all motor" setup 2.25 would be great. what really hinders our exhaust systems is a pair of restrictive catalytic convertors and a weenie midpipe.

my advice on a setup like this is to scrap the stock exhaust manifold and first cat, replace with a header, slap on a decent 2.25 catback exhaust and find someone who can make a good midpipe for you and thats my recipe for better protege breathing.

 

[MP5Girl]

Fantabulous! Thanks for the help!

 

[sidpro5]

i am of the mindset that having some backpressure is good, that losing too much of it will rob you in the end of some power, so in my opinion there is no need to go any bigger than 2.5 inch on a catback exhaust system, and then only if you have a turbo. i have seen a few proteges running around with a 3 inch which i think is a little extreme. on an "all motor" setup 2.25 would be great. what really hinders our exhaust systems is a pair of restrictive catalytic convertors and a weenie midpipe.

my advice on a setup like this is to scrap the stock exhaust manifold and first cat, replace with a header, slap on a decent 2.25 catback exhaust and find someone who can make a good midpipe for you and thats my recipe for better protege breathing.

I was of the same belief. Then I was hit with this nugget from Installshield.....

<TABLE class=tborder cellSpacing=0 cellPadding=6 width="100%" align=center border=0><TBODY><TR vAlign=top><TD class=alt1>It is misleading to refer to backpressure as any type of benefit to any engine, and it is honestly incorrect...

Backpressure is any inversely directed pressure gradient within the exhaust system...and it usually forms around bottle-necks or restrictive kinks in the piping...Now it does nothing but hurts power and torque, and can even cause detonation problems as well as poor fuel efficiency...It is no ones friend...

Just as a little intro to exhaust theories, to anyone that is unfamiliar with it: It is said that exhaust gasses travel as pressurized pulses through the piping, and out of the car...The diameter of the said piping is directly correlated with the velocity the individual pulses travel through the system...The bigger the ID, the slower the velocity...and vice versa...at a given rpm...

Sadly high-end hp and low-end torque do not agree on a perfect internal diameter...At maximum volumetric efficiency, which is where an engine makes maximum torque for the most part, results in huge individual pulses...and these are usually occuring at a relatively low rpm compared to redline...at these engine speeds maximum torque is created when these individual pulses can travel extremely quickly out of the exhaust system, and literally make a vaccum pulling combusted gas out of the engine, and fresh air in (that is scavaging, and the complexities of that are beyond the scope of this)...So overall a smaller ID results in a stronger pull at low rpm...and a whole ton of people claim that this is caused by backpressure...I assure you it isn't...

but on the other hand, apply this to high rpm hp output..pulses at this engine speed are relatively smaller, but there is a lot more of them (Crazy car dorks refer to this as "pulse frequency")...and despite getting smaller, they end up bouncing off of each other and searching the exhaust system for any blemish, kink, bind, or restriction that they can...and they are good at it...the solution is to use a bigger ID for the piping...with a smaller ID, hp output usually suffers due to choking affects...

So you can see where this is going...Some dudes like to grab a 3" exhuast for a 2.0L engine, because it sounds cool when they tell there friends that...But it is way too big for lowend torque, and unless there 2.0L can rev to 11grand...it will never make enough pulses to get anywhere near hung up (honestly that is bad example because an exhuast of this size would never promote fast pulse velocity, and would overall kill output everywhere...not just lowend torque)...

But again backpressure is simply a pressure "wave" pushing towards the engine, rather than pulling exhaust pulses out...and it always makes for more strenuous exhuast strokes in the engine and very poor breathing, and overall hurts engine output...

So finding an exhuast with a good ID for both is the best bet...a little on the large side promotes better topend output...a little on the small side gives better lowend torqu, with a little less highend output...and the extremes are decent as long as you are not over them...For NA proteges trying to stay streetable, I would put a ceiling at 2.5"...and that would be reserved for some pretty highly modified FS's...
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