(I think I have a good understanding of exhaustion, however...)
ok, I don't know squat about exhausts, tuning exhausts, etc. etc. I have had stock exhausts on every car I've owned with the exception of a custom exhaust system that was recently on my M42 (2.5" pipe, magnaflow muffler, magnaflow cat with no resonator).
I *hated* the setup. As you have probably guessed, it was way too loud for my fuddy-duddy tastes (it drowned out NPR on the radio) but more importantly, I'm not sure exactly what it was doing for the car's overall performance.
At WOT, I'm reasonably sure the setup boosted performance; I'm know any car engine is basically like an air pump, and (I suppose) the more air that can move out of the engine, the better... at the same time, I felt like I was driving a bi-plane.
It was really really loud, which did have it's thrilling moments... mothers pulled their children away from the curb when I drove by. Guys on motorcycles would turn and point... When I'd cruise Rice Vegas (I live near chinatown) I had a few envious looks from the dudes in their honduhs.
But... at PART THROTTLE, the system was wildly inconsistent... sometimes it sounded quiet, sometimes sounded good, and sometimes... sounded like... well, like ass.
(please excused the following wildly, probably inaccurate subjective observation) It sorta sounded like... sometimes air would flow very smoothly through the thing, and other times, there was just a ton of air hanging out in the system.
for example... say I'm doing around 75 mph at around uh 3600 rpm... the exhaust sounds terrible (un-natural gurgling, etc. etc.) ...I downshift to 4th, speed up to say, 85 (4000rpm-ish), there's a rasp, a loud long farting or whatever (not me, the exhaust), and then put it back into fifth, back to 75mph, and... the exhaust sounds great! kinda nice actually! What gives?
sooo... if anyone with an engineering background or a lot of experience with aftermarket exhausts (eg, everyone else on this board except me) can chime in, I await yer knowledge and insight.
thanks!
I just found this thread (missed it during my move before), and I'll try to throw in my .02 on this subject.
First, as has been said, BMW engineered the origional exhaust system quite well, ballancing all of their requirements nicely. It's quiet, doesn't take up more space than necessary, and doesn't rob too much power.
Most people will replace their exhaust for one of three reasons: 1) it's worn out (rusted, etc...) 2) they're looking for a performance improvement or 3) they're looking for a different sound. As far as those reasons go, #1 is totally legitimate. #2 may be legitimate, but you are really wasting your money when there are much bigger gains to be found elswhere on anything close to a stock motor. #3, well, I guess if you have a couple hundred extra dollars to throw away, whatever.
OK, now that that's out of the way, let's get into how the exhaust works. Lots of times ( and in this thread as well) people will tell you that you need "Backpressure" that can be gained by using a stock (relatively small diameter) pipe to help with low end torque. They are getting to the correct answer, but their science is flawed. You will absolutely hurt your driveability and performance if you go larger than 2.25 on the street, but the reason is due to scavenging effects. When you use the correct diameter pipe, the exhaust must increase velocity as it travels through the exhaust system. As it speeds up, it will help create a slight vacum on the ehaust port at the cyllinder head. When the next exhaust valve opens, the vacum "sucks" the exhaust out, allowing more air to enter the cyllinder during the next intake stroke. When you increase your pipe diameter, the exhaust velocity slows, you don't get the vacum, and you actually will lose power, compared with the smaller pipe.
As a side note, this is the exact reason that V type engines with dual exhaust incorporate a ballance "H" or "X" pipe. The crossover enables better scavenging.
So, what is the best type of exhaust for any particular car? That's a question better suited for an engineer, not a criminal justice major. All I know is that it depends on a lot of different factors, including the size and type of engine, and the desired rpm range of your powerband.
Febi, to answer the part of your post regarding the strange sounds coming from your exhaust, the main reason is that you have an exhaust that does not match up to your engine at all. When you get systems that don't work together, you can get strange harmonic frequencies at different rpm ranges, and that's what you're encountering.
Bottom line: Those German engineers knew what the heck they were doing. If free power was as easy as slapping on a bigger pipe, our cars would come from the factory with huge ricer exhaust systems.
Here's my recommendation for a (fairly) cheap exhaust that will not sound bad, and will give you as good power as anything: 2.25 inch pipe, with either a Dynomax Super Turbo muffler, or a Summitt Turbo muffler. any exhaust shop can hook you up with the pipes for cheap. I have experience with both of the aforementioned mufflers, and would recommend either. Both are slightly louder than stock, but not so much that small children will cower in fear. More importantly, both work well and are inexpensive.
OK. Long post. Hopefully I've answered some of your exhaust questions and cleared things up a little bit.
Brian
