Home Built Exhaust

[midwestTI]

I got tired of looking at all the ridiculous prices for cat-backs, and the lack of choices, so when I replaced my clutch and flywheel (with the M3 units) I cut off my exhaust just past the O2 sensor. It's an M42/OBD 1, and I plan on putting in 2.25" tubing with a high flow magnaflow cat and high flow muffler, while also deleting the resonator.

As far as I know, there's not science to a cat-back, is there? I figure the science is in the headers, since a cat-back is basically straight piping.

This should be done by next week (a buddy of mine works at Meineke, and he's going to help me with the welding and mounting) and I'll let you guys know how it goes.

[nick_hegel]

My buddy did his own exhaust on a '97 M3 and its sounds just as good(if not better) as my brother's '98 M3 who has full Dinan exhaust. Good luck on it and hope it sounds good(and goes well)!!!!

[aceyx]

you cut it off or just unbolted the resonator and muffler?

the main "science" is just making sure that it bends at the right places and has braze-ons where the hangers are.

off the top of my head, i don't know what the stock diameter of the tubing is, but there is such a thing as "too big" a pipe. the hotter the gasses traveling through the pipe, the easier (faster) they will move. if you go too big, the gasses will cool, become dense, and be more restrictive (it's harder for the pistons to push the exhaust out).

you also want pipe diameter to be about the right size so that the exhaust exiting the manifold at any time to line up with the previous exhaust pulse (think of putting marbles into a tube, all of them touching). if the diameter is too large, the marbles become spread apart, and uh, this gets difficult to explain. it's a bad thing, because instead of an even stream of exhaust, it gets burped out, and this is not a good thing.

if the stocker is 2", going up to 2.25" presents no problem. remember, only inside diameter counts for flow, exterior counts if it'll fit. mandrel bending is key, especially if you're going around the stock diameter.


this is just my advice, but smooth out the welds that are mid-pipe. i chopped off the stocker and welded on a $34 can and the welds have started to rust a bit. just surface rust, but it can look bad.

it was supposed to be temporary (to pass inspection) but i've had no problems with it so i left it on. take your time, since you plan on this lasting longer.

[needle332rur]

I made my own exhaust, i used a dynomax muffler and kept stock cat (bmw cats flow very well) 2.25 all through. the front mount on the rear section i used a solid mount (bolted the rear section to the subframe) and put a flex pipe between the rear section and the front (because im planing on turbo charging it). i suggest using a lift. But the whole thing cost about 220 bucks not bad compared to the cheapest being 450 (stomung)

[midwestTI]

Day one with the new exhaust, my little brother got pulled over and recieved a noise violation fine......

wonderful

any suggestions on mufflers that can help this problem? I'm thinking either magnaflow, or a series with a silencer included. any suggestions would be great, thanks

[nick_hegel]

Cops are gay!!! was your bro driving completely without a muffler??? I would suggest Magnaflow, I've never heard complaints on their parts

[GDB]

dynomax. I personally think they sound better than anything on the market.

[IV318]

you can also cause valve damage if you do not have the proper amount of back pressure. in the high rpm range without proper back pressure the valve's can stick, carbon deposit's can build up, etc. the point is more often than not it is better to go stick with resonable mods to maintain the proper amount of pressure so you dont damage the engine. remember its a BMW not a honda.

[pdxmotorhead]

Hmm. Never seen this in 30 years of car tuning.....

Only effect I've seen from too big of pipe is poor performance.

Dave'

[DustenT]

you can also cause valve damage if you do not have the proper amount of back pressure. in the high rpm range without proper back pressure the valve's can stick, carbon deposit's can build up, etc. the point is more often than not it is better to go stick with resonable mods to maintain the proper amount of pressure so you dont damage the engine. remember its a BMW not a honda.

No. An internal combustion engine doesn't require ANY backpressure to run, valves don't need any "resistance" to work, in fact they work best without any pressure on them. The back pressure myth is an urban legend. Check out Corky Bell's book, "Supercharged" for more information on properly construncting and testing an exhaust system. There are several places in the exhaust that you want to test for pressure drops/increases. If you are going to build your own, do the research.

[IV318]

Perhaps back pressure was the wrong term to use. but yes you can do valve damage.(in a worst case scenario) running only headers (exhaust manifold) as the superheated gasses leave the manifold and mix with oxygen in the air you can often see flame (it looks cool but as the gas burns it can leave carbon deposits and scortch the valves) UNLESS you are using a forced induction of some form to increase the amount of air going into the cylinder their no point in trying to let rediculous amounts of hot gas out. but yes DustenT you are correct an internal combustion engine does not require back pressure.

[FATHER]

Hey i have 3" pipe with a AC muffler and the car run good and sound cool, i not have cat, only one resonator and stock header. i not think those damage the engine.