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3-Series (E21, E30) Chat relating to the BMW 3-Series from 1975-1983 and 1984-1991 line. Specific models: BMW 315, BMW 316, BMW 318, BMW 318i, BMW 320/4, BMW 320i, BMW 320/6, BMW 323i, BMW 320i. E30 Family models include: BMW 325e, BMW 325i, BMW 325is, BMW 325ix.

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Old 07-06-2005, 11:48 AM   #1 (permalink)
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hi guys, new to the forum and i just got a 1991 325i. I really like this car and have plans on making it as bad as my other rides, so first is first....i neeed more power.

I am going to remove ( cut ) the resonator and put strait pipes from after the cat, to all the way back to a new muffler. Anyone do this? The only thing holding me back is that i do not know what size pipe to use. I was thinking in using 2.25in, only beacuse i think 2.5 in would be to much for a 2.5L Naturaly Asperated Car. ANy one have experience in this kind of setup? Any info on it would be great.

Thanks guys
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Old 07-06-2005, 02:25 PM   #2 (permalink)
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Just so you know, if you're looking for more power, pretty much the only way you're going to get it is forced induction.
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Old 07-08-2005, 01:14 AM   #3 (permalink)
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the stock exhaust is dual 1.75", which is roughly the same flow area as a single 2.5", which is what most n/a guys go with.
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Old 07-08-2005, 02:18 AM   #4 (permalink)
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Quote:
Originally posted by draculia@Jul 8 2005, 06:14 AM
the stock exhaust is dual 1.75", which is roughly the same flow area as a single 2.5", which is what most n/a guys go with.
[snapback]360628[/snapback]

We're going with 3" back here in Iceland, but i wouldnt do that conversion unless first assuring yourself that you've got a good cone filter

dual 2" might do the job too
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Old 07-08-2005, 12:07 PM   #5 (permalink)
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thanks for the info guys, i wanted to keep the dual system on, but isnt 2 in to small. Would the torque curve chane that much with 2 1/4 in pipe? thanks, any info would be great

i do have a cone intake, and im thinking to get a chip.
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Old 07-08-2005, 12:46 PM   #6 (permalink)
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But dual 2" combined make 4" you odd' to to the math?
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Old 07-08-2005, 03:43 PM   #7 (permalink)
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Actually that is not right. Dual 2" pipes are not equal to a single 4" pipe. I am too lazy to do the math right now but simple addition does not work. Try working it out using pie R squared and you will see that the 4" is going to flow a hell of alot more volume than the two's ever will.
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Old 07-09-2005, 01:04 AM   #8 (permalink)
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from a performance standpoint, switching to a single is a much better idea. its also much easier and cheaper to fab.

if you want to keep the dual pipes for some reason, then i wouldn't go bigger than 2" on an n/a car. personally i would stick with the 1.75s and get the better flow from a better cat, muffler, and pipes that weren't flat in areas. magnaflow has dual inlet/outlet mufflers and cats
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Old 07-09-2005, 01:11 AM   #9 (permalink)
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and since rookie is too lazy here are some rough flow areas, note that all figures are using the OD of the tubing:

stock dual 1.75 = 15.10"
dual 2 = 19.72"
dual 2.25 = 24.96"
single 2.5 = 15.41"
single 3 = 22.18"
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Old 07-09-2005, 08:34 PM   #10 (permalink)
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Straight pipes = straight back from down pipe with nothing inline.

I have a straight pipe on my sunfire(yes i own a sunfire, its my dd.) Well its actually side mounted as well. My experience is this, if you run straights youll drop back pressure which on a NA car is BAD. As corky bell says somewhere in maximum boost, on a NA car you need back pressure to suck your pistons down. This is why boosted cars handle just downpipes perfectly fine(especially the way DSM's breath.) You need to remain some sort of pressure to keep your engines duty cycle efficient. I will say every engine is different, so what I said might not even affect you. Just something to ponder. If you run 2 straight pipes at a smaller diameter, check into running a h or x pipe to link out the two pipes. This is what a lot of v8's do to keep flow and volume. This will also keep back pressure, as will anything else inline with the flow.

...maybe none of this makes sense, Im to tired to read over it...i think im rambling on now..

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Old 07-10-2005, 02:29 AM   #11 (permalink)
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no dude ur not rambling..ur absolutely correct. I think i said straight pipes but i meant to say that from the cat i am going to do pipes to a muffler, without any resonator. The magnaflow muffler i got has a perferated x core, its a dual in dual out muffler i intend to use. X pipes are way better than H pipes, i have a x pipe system on my trans am. Any ways thanks for all the info guys, its really help full.
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Daily driving a 91 325i [ 2.5L inline 6 ]

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Racing in a 77 Pontiac Trans am [ 400 poncho ]

Well.... allways racing :-)
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Old 07-10-2005, 03:30 AM   #12 (permalink)
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No.1 REMOVE THE CATALYC CONVERTOR !

I'm not sure how it is where u are, but in Iceland Everything prior to 1996 you can run Cat free.

When you have removed the Cat, you will both sustain lower gas spendings and higher performance!

I've had this both with my BMW, my Nissan, and my Dad's Dodge Dakota V8 Magnum ! (Currently stated @ 373,4hp )
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Old 07-10-2005, 01:59 PM   #13 (permalink)
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Cant remove cat.. Live in Cali ghead. Heck i even require a cat to pass emissions on my trans am, to bad That was the first thing i removed when i bought it . besides, dont want to risk having no cat in my daily driver, i think the ticket fine is 1500 now, or thats what my friend had to pay when he got pulled over in his 79 trans am, of course he was going 70 on a 25 though
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Daily driving a 91 325i [ 2.5L inline 6 ]

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Well.... allways racing :-)
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