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Old 09-08-2010, 09:59 AM   #1
iRazor
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Default need help with exhaust size

ok, new here just bought a new 2010 impreza 4 door 2.5 im buying catback system for it soon, i plan on adding a turbo after my factory warranty kicks out so i was thinking about borla but for my car, they sell a 2.25 inlet and a 4.25 outlet kit, now magnaflow offers a 2.5 inlet and 4in outlet kit for a wrx model. im afraid of the 2.25 being too small when i get my turbo and would have to be replaced. my local exhaust dealer told me the 2.5 would be pushing my limits for the current n/a engine ..opinions?
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Old 09-08-2010, 10:29 AM   #2
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2.25" for now while N/A and 3" when u get the turbo. pipe size after that cats doesn't really matter too much.
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Old 09-08-2010, 11:42 AM   #3
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Anything over 2.5" and you will loose the nessecary back pressure to maintain the proper exhaust flow. I put a 3" Ingen SES CBE on my 2.5i (when i didn't know any better) and ended up having to put in a restrictor to make it work right. If I were to do it again, I'd get the magnaflow CBE.
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Old 09-08-2010, 12:00 PM   #4
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Quote:
Originally Posted by iRazor View Post
ok, new here just bought a new 2010 impreza 4 door 2.5 im buying catback system for it soon, i plan on adding a turbo after my factory warranty kicks out so i was thinking about borla but for my car, they sell a 2.25 inlet and a 4.25 outlet kit, now magnaflow offers a 2.5 inlet and 4in outlet kit for a wrx model. im afraid of the 2.25 being too small when i get my turbo and would have to be replaced. my local exhaust dealer told me the 2.5 would be pushing my limits for the current n/a engine ..opinions?
2.25" is generally the most common size for NA's. Also good luck with that turbo, I hope you'll be doing the engine work along with it or else your engine will go kaboom

Quote:
Originally Posted by treksofar View Post
Anything over 2.5" and you will loose the nessecary back pressure to maintain the proper exhaust flow. I put a 3" Ingen SES CBE on my 2.5i (when i didn't know any better) and ended up having to put in a restrictor to make it work right. If I were to do it again, I'd get the magnaflow CBE.
NO, backpressure = BAD. To say that backpressure is necessary to maintain flow is a MYTH. The word you're looking for is exhaust velocity
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Old 09-08-2010, 01:09 PM   #5
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Save your money. Buy the turbo kit and exhaust at the same time... it will minimize changing things down the road.

If you really just want some extra sound, lop off the stock muffler and have a local shop weld something on... at cat-back isn't going to gain you all that much.
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Old 09-08-2010, 01:15 PM   #6
iRazor
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nah sound isnt me i go for performance/looks :P and oh yeah i plan to do the works before turbo so i dont make it go kaboom.. prob only gonna run 8 lbs max for boost so the 2.5 magnaflow wouldnt be the right route? just get the 2.25 and when the time comes, possibly get a pipe fitted and keep the 2.25 cat
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Old 09-08-2010, 01:17 PM   #7
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also because the borla catback is gonna be $560 and i would really hate to spend around that much again when the time does come, the muffler place quoted me $624 for magnaflow tips and them making there own piping o_O
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Old 09-08-2010, 01:33 PM   #8
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It's been said time and time again, save your money and buy a wrx. It will cost aaalloottt to build the engine to even support the "kit" at a reasonable boost pressure and you'll still likely make less power than a wrx as well as not being as reliable. If you insist on that car specifically having a turbo, you're better off doing a full engine swap. I hope you have the big bucks $$$
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Old 09-08-2010, 01:46 PM   #9
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Performance gains naturally aspirated with a cat-back are going to be somewhere on the order of negligible. It'll look cool and it will be louder, but it's not going to gain you anything appreciable 0-60 or in the quarter.

Now if you want to do a full NA build... intake, cams, P&P the heads, headers, full exhaust and a premium tune you stand to pick up a respectable amount... but that is going to be sinking a lot of money into a car that you "plan" to turbo later.

If you wanted speed you should have saved for another year or two and bought a WRX. Heck, you could have had a used one for the price you paid on that new 2.5i. The base Impreza may be fun, but it isn't a fast car and trying to make it fast is going to eat up a lot of money.
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Old 09-08-2010, 01:56 PM   #10
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i also have a 2.5i and i bought it because it was a steal..some people have their own reasons for getting a 2.5i instead of a wrx..obviously if I had the money (insurance) I would of purchased a wrx..i say if you want to rebuild the engine for a turbo then go for it..and when its all done be sure to lets us know what you did and how it came out
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Old 09-08-2010, 03:49 PM   #11
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well most people just usually say the same thing.. engine swap or sell and buy a wrx but the dealership i went to they could only order me one from either miami or from japan because wrx is a low selling car in florida (not only that i also think the hatch is doofy looking) im not looking for a super car with 500+hp just looking for a fun car with 300-350 whp i could even stick to n/a tuning so i could just do n/a events instead of turbo im just looking at ideas but i know the first thing i would do if i were to get a turbo is replace the crap block it has
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Old 09-08-2010, 03:59 PM   #12
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Let's put it this way... it all depends on how much cash you want to lay out and how soon.

If you are really wanting 300-350 to the wheels go buy an STI before you start on anything. 350 AWHP is a lot of power and you aren't going to get it without completely building the engine and transmission on the 2.5i. You have to remember, with the STI you get more than just power... you get driveline components that are meant to handle it.

If you want to run the car in NA events and will be happy running the car in NA form then go after that. But you need to decide on a course of action first.
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Old 09-08-2010, 04:05 PM   #13
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cash? prob around 6 grand is what im willing to invest in my car (currently) time isnt of essence considering its a 10' the only reason why i had turbo in my thought is because it would get more juice vs just n/a tuning, plus its not as easy for me to pay cash for an impreza and a month later sell it for maybe a quarter then what i paid special order a sti/wrx then would have to pay the difference with car payments and my insurance would sky rocket
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Old 09-08-2010, 09:24 PM   #14
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still never got a clear answer :P 2.5 inch or 2.25? would i really lose that much/if any low end torque for a quarter inch larger?
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Old 09-08-2010, 10:28 PM   #15
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unless its possible to get the exhaust place to fabricate a bigger pipe when the time comes? would like to keep the same cat/tips from borla since they sound beautiful
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Old 09-09-2010, 09:49 AM   #16
iRazor
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btw is there a difference between the wrx catback vs impreza catback? im getting a couple of mixed answers from google saying it would bolt directly up and the flanges would be different?
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Old 09-09-2010, 11:13 AM   #17
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2.25" will be fine for NA operation... any larger and you risk losing back pressure which helps build torque.
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Old 09-09-2010, 12:33 PM   #18
iRazor
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because i know the bigger you go the more low end power you lose and the bigger (not extravagant) the more high end power you have but i suppose ill go with 2.25 and hope i dont have to worry about replacing *if* turbo is in my near future
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Old 09-09-2010, 01:30 PM   #19
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Quote:
Originally Posted by RaineMan View Post
2.25" will be fine for NA operation... any larger and you risk losing back pressure which helps build torque.
NO, BACKPRESSURE = BAD.

Back pressure does NOT help you "build torque". Whoever told you that needs to be slapped upside the head.
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Old 09-09-2010, 01:53 PM   #20
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i know backpressure is bad :P i was just mainly worried about 2.5' being too big
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Old 09-09-2010, 02:04 PM   #21
RaineMan
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Quote:
Originally Posted by nairbmik View Post

Back pressure does NOT help you "build torque". Whoever told you that needs to be slapped upside the head.
You are speaking in terms of absolutes, which is not the case here. TOO MUCH back pressure = bad, period.

An exhaust system is only as effective as its most restrictive part. Wherever that point is airflow will be restricted and this will back up the rest of the system.

Turbo cars run with exhausts as large as possible or even open DPs because the turbo is a restrictive device. Exhaust gasses must run through the turbine before exiting the tailpipe... it's how the whole thing works. Naturally, pushing those turbine wheels is harder than flowing straight out the pipe, so there is going to be some back pressure built here.

When it comes to the naturally aspirated engine your real killer is that catalytic converter. That is where the primary restriction comes from. Which is why so many people run catless. Put a 6" cat-back on there... it won't matter a bit where performance is concerned.

The second factor to be considered, which I really didn't want to get into because it goes over so many people's heads, is exhaust gas velocity.

In an internal combustion engine you have exhaust pulses. 4 cylinders = four pulses. For an exhaust system to function optimally it needs to be of the correct diameter so that the overall velocity is maintained between these pulses, in order to maximize flow and scavenging. You want the exhaust traveling as fast as possible away from the engine to maximize performance.

Now that we've been through all that... there is absolutely no need to slap a 3" exhaust on your 2.5i. You are going to lose pressure and velocity, which will hamper your performance. 2.25" is just fine for the amount of exhaust a stock (ish) 2.5i is going to be flowing.

For that matter, replacing the stock setup with a fancy (and expensive) cat-back isn't going to gain you much at all by itself. If you aren't planning a major build (intake, cams, p&p heads, headers, then exhaust) you aren't going to be flowing a significantly different amount of air over what the stock production is. If you just want more sound coming out the back end it is advisable to just lop off the stock muffler (another majorly restrictive part in the system) and replace it with something that flows better. Spend $200 on that over $600 on a full exhaust job.
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Old 09-10-2010, 06:20 AM   #22
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:/ ive explained im not a ricer punk i do not go for just sound thats dumb im going for all out performance i sold my 2 project cars and now going to invest both of those into my impreza either it just be the n/a tune or not who knows i may like the n/a meets i know 3" is WAY too big 2.5" would be my max for either build i would go i was just thinking the 2.5" may suit my needs due to which way i may build my engine IF i decide to go turbo the 2.25" may be restricting too much which then basically i bought a $600 catback for no reason and would have a very hard time selling it for any decent amount
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