View Full Version : Performance questions...exhaust+cams

01-31-2002, 10:01 PM
Even though my car is automatic, i still want to push it as far as it can go for personal spite so hold back the comments =)

I am designing a new exhaust system and i need some second opinions on it.

i am thinking of designing a custom exhaust where two pipes come from the headers to two racing resonators, then straight from there to the back with sperate DTM style chrome tips on the end. No muffler, no cats. I've tried my car without a muffler before at the exhaust shop, (we put the ehxaust comming out of the side for fun...lots of fire! =)

Would the resonators be enough to tune the sound of the ehaust?
Question being, is i removed my cat the other day because it's slowly dying, and my car ran SOOOO much better. People always tell me about backpressure, but i just dont see how creating ANY pressure in an exhaust system would help at all. Isn't the point of an exhaust to get the ehxaust out of the system as quickly as possible?

Any comments, suggestions, educational facts, etc would be most helpfull. i designed a setup like this on my GM cars before and they ran great! (they were HO v6's and v8's)

Also, i have a good deal on M3 camshafts. How much of an improvement can i expect from these, and are they a DIY job?


01-31-2002, 10:07 PM
also what about an X pipe or H pipe design for the exhaust to balance the flow?
or is that just a Vette/Stang thing?

01-31-2002, 10:33 PM
dude, if you run just ressies (my short for resinators!), the exhaust noise will still be pretty load and hardly any back pressure.
Back pressure is important for low end power. Maybe try to even things out a bit, Ressies with smaller diameter pipes (not pea shooters though ). Also in some cases no back pressure damages the engine. Exhaust systems have to be built right to match you engine's needs depending on its driving style. I tell you, it ain't easy too figure out the best set-up. good luck buddy *th-up*

01-31-2002, 10:48 PM
yeah i was thinking of smaller diameter piping as well to create a backpressure.

does anyone else have any ideas?

01-31-2002, 11:00 PM
You definately need back pressure in your exhaust because of 2 things... the motor is only a 2.5L and the fact that it's an automatic. I definately recommend a combo of the three.... a cat, resonator, or muffler... but not just one. I've got basically what you're talking about. Dual 2.25" right from each header bank to a an H-pipe (yes, it's important) then into 2 seperate resonators and continuing back into a dual in- dual out Brullen muffler. I get pulled over enough for the noise... it's damn loud.... already got a ticket for "improper muffler" just idling in a parking lot.

M3 cams are definately a good idea. You'll see around a 15HP increase but be fore-warned that you'll need stiffer valve springs (nothing else) with these cams. If you're just swapping cams, then the job is easy, but the replacement on the springs makes it tricky not to drop the valves into the chamber (some mechanics can pressurize the chamber with an air nozzle through the spark plug opening to keep the valve from falling in).

01-31-2002, 11:11 PM
hmm interesting
your exhaust design sounds like how i want to try on my e36.
I would just need to find a dual inlet/outlet muffler of some kind.

I would love to hear your exhaust just do i can grasp an idea though before i spend any money time and effort.
i will take into account that mine will probably be no where as nice in comparison, but it will give me a ball park idea.

can you go into more detail about the pros and cons of backpressure though? Nobody i ask can ever seem to explain it to me in much detail.

also about the valve springs, how much do they cost for the upgrade? And is it something that if i dont change it until they break will ruin my valves or engine?

02-01-2002, 12:14 AM
do i need stiffer valve springs if i am putting in 95 M3 cams into my M52 2.8l engine? only M50TU engines need stiffer valve springs, am i right???

02-01-2002, 01:31 AM
Actually, the most important thing in an exhaust is exhoust gas velocity. Too big of piping, or too small, will slow down the flow. As I understand it you can never have too little backpressure, just too low of an exhaust gas velocity.

The topic of custom exhaust fabrication is discussed in depth in the November 2001 issue of Sport Compact Car (don't snicker, I buy it occasionally).

02-01-2002, 09:55 AM
Originally posted by M52_Alpina
do i need stiffer valve springs if i am putting in 95 M3 cams into my M52 2.8l engine? only M50TU engines need stiffer valve springs, am i right???

When my friend Mike swapped his stock 2.5L cams for the M3 ones, we made a trip to BMW and cross referenced part numbers off the M3 valvetrain vs. the 2.5L AND 2.8L heads. The only things different between the ///M and non ///M heads ('92 non-VANOS excluded) were the actual cam shafts and springs.... even the retainers were the same. The 2.8 and 2.5 have the same springs... so yes, you'll have to swap the springs as well for your motor.

After the install, we noticed most of the power difference after 5000rpm, under that it was pretty much the same. If you want some serious power upgrades (in a budget weary - natural aspirated sense) swap the hardware on your head to the OBD-1 setup along with M3 cams.... Man, what a difference! With that... a full exhaust, headers, intake, and diff... you'll seriously give E36 M3's a good run. There's good potential in your M52-B28 motor.

02-01-2002, 03:14 PM
thanks for your reply Randy!! :) but what do you mean by "hardware above the head", do you mean the OBDI intake manifold?? by the way, Dave@Motronix said i don't need stiffer valve springs for my M52B28 motor, i am now confused..

how much am i looking at if i want to get stiffer valve springs? where should i get it.. ?

02-03-2002, 11:51 PM
The motor will work fine with your stock valve springs, so Dave isn't really lying... but for safety measure (especially if you're gonna track the car and rev it past 7000rpm) it's always better to do things properly. The springs ensure the valves return to their original positions after the cam shaft pushes them down to open the chamber... if you're revving the motor fast, weaker springs won't retract the valves quick enough and there's a good possibility the piston might hit one. You can buy them at any BMW dealership for about $18.00/each (need 24!)... don't buy anything other than BMW OEM parts for your baby's internals.

02-04-2002, 09:35 AM
yeah i dont think i've ever revved my car past the beginning of the redline, so i may be in the clear. I will probably buy the valve springs a few at a time because $18 x 24 = more than i care to spend a month on my car =) I'll probably buy 10 at a time and then have them installed when i have 24 of them.
But i think i'm going to buy these camshafts i saw, they're under $100 so it's a sound deal in my eyes.

02-04-2002, 12:57 PM
yeah, i think i should start saving up for M3 S52B32 valve springs too.. :(