custom exhausts

[StereoInstaller1]

I am doing dual 2" on the S50 with a goal of street driveability.

E36 M3s make about 210 RWHP with dual 1.8". I don't mind losing a little on the bottom if I get a HUGE rush on top, as long as mileage does not suffer too bad.

[nando]

Nope, not true.

Larger tubing has less scavenging at lower RPM, THAT is where the torque loss happens. Larger tubing has less exhaust velocity; how could anyone see it differently? Until the RPMs go up to the point where the tube(s) can properly scavenge, larger tubing WILL decrease torque due to less frequent gas pulses causing slower exhaust velocity. This is not debatable, it is fact.

It is all about the resonant frequency. Smaller tubing has higher Fs, even if it is dual, with or without Xs or Ys.

Smaller tubing also means less cabin drone (that higher Fs again), which is why BMW did it on their higher performance models, not to mention better ground clearance.

IF your goal is street driveability and torque, do dual tubing. If your goal is high RPM performance, go large single tube. Either way, match your exhaust to your useage.

yes, TOTALLY true.

THERE IS NO SCAVENGING IN THE EXHAUST PIPES. It all happens inside the headers. if you have stock shitty cast manifolds, then yes, a too-big exhaust might lose you some torque. But if you have the proper size headers for your desired powerband, after the final merge your exhaust can be too small, but never too big (with clearance and fitment in mind).

The only thing the exhaust does past the merge is get it away from the car, clean it up, and make it quiet.

repeat after me.. there is no scavenging effect in the exhaust pipes. The only place velocity matters is in the header.

http://www.superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/index.html

I have a very quiet 3" exhaust on my stroker. Is it too big? No, because all the heavy lifting is done by the RD headers. is it loud and droney? nope, been there, done that, and hated it. :p

but it is light, with lots of breathing room for the future, and extremely easy to take apart.

[ForcedFirebird]

I put a single 2.5" on my car with a high flow cat and DTM muffler. Looks great, sounds like shit (needs at least resonator, maybe baffles), don't expect to hear your stereo, phone, or even conversation with a passenger - ears will ring when you get out of the car after a long drive.

Oh, and it ran more lean than it was before, especially in the 4k RPM range (still working the tune, 2.7i, Motronic 1.3).

[StereoInstaller1]

yes, TOTALLY true.

THERE IS NO SCAVENGING IN THE EXHAUST PIPES.

repeat after me.. there is no scavenging effect in the exhaust pipes. The only place velocity matters is in the header.

http://www.superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/index.html

Nope, wrong. That article does not refute the concept that scavenging is done in the exhaust. I have no clue why you would think scavenging ensd at the collector, but you are absolutely wrong.

in fact, anywhere that there are exhaust pulses there is scavenging: it is all about the waveform and pressure front leading edge/trailing edge.

[StereoInstaller1]

More, from http://forum.concours.org/index.php?topic=45090

Exhaust theory 101.


One of the biggest issues with exhaust systems, is the relationship between gas flow volume and gas flow velocity (which also applies to the intake track). An engine needs the highest flow velocity possible for quick throttle response and torque throughout the low-to-mid range portion of the power band. The same engine also needs the highest flow volume possible throughout the mid-to-high range portion of the powerband for maximum performance. This is where a fundamental conflict arises. For "X" amount of exhaust pressure at an exhaust valve, a smaller diameter exhaust pipe will provide higher flow velocity than a larger diameter pipe. Unfortunately, the laws of physics will not allow that same small diameter pipe to flow sufficient volume to realize maximum possible power at higher RPM. If we install a larger diameter pipe, we will have enough flow volume for maximum power at mid-to-high RPM, but the flow velocity will decrease and low-to-mid range throttle response and torque will suffer. This is the primary paradox of exhaust flow dynamics and the solution is usually a design compromise that produces an acceptable amount of throttle response, torque and horsepower across the entire powerband.

A very common mistake made by some performance people is the selection of an exhaust system with pipes that are too large in diameter for their engine's state of tune. Bigger is not necessarily better and is often worse.

And seriously, don't you think I know a wee bit more than you about acoustics? C'mon, man, that shit you posted refers to flat-plane crank V8 motors, not I6. It is also all about full-on high performance high RPM exhaust, which IS NOT my interest.

I think the guys at Eberspacher (that is who designs BMWs exhaust systems: the highest-tech of any manufacturer anywhere) know just a wee bit more than both of us, and their goals are much the same as mine... and they ALWAYS do a dual exhaust on street-driven I6.

Doesn't that tell you something?

[ForcedFirebird]

More, from http://forum.concours.org/index.php?topic=45090



And seriously, don't you think I know a wee bit more than you about acoustics? C'mon, man, that shit you posted refers to flat-plane crank V8 motors, not I6. It is also all about full-on high performance high RPM exhaust, which IS NOT my interest.

I think the guys at Eberspacher (that is who designs BMWs exhaust systems: the highest-tech of any manufacturer anywhere) know just a wee bit more than both of us, and their goals are much the same as mine... and they ALWAYS do a dual exhaust on street-driven I6.

Doesn't that tell you something?

That quote is also the reason why in naturally aspirated head porting, it is at the utmost importance to keep the minimal cross sectional area vs port volume to a minumum. Bigger is not better, better is better, heads that flow slightly less in a well designed port will out perform a head that flows well in a poorly designed port.

Sound, heat and velocity values continue until the pulses reach atmosphere.

EDIT: There are a few NASCAR teams that are still running 1.5" primaries in 700+hp engines, get that one lol

[StereoInstaller1]

So while we are on this subject, we all know X placement is highly critical to where the powerband is.

I am thinking about having the cat (cheapass magnaflow dual 2" in/out) before the X, as this car will be dual 2" from front to back, after whatever crap collectors the ebay headers have. I think I am gonna do a pair of small resonators, as I want it kind of quiet, but aggressive.

I am also welding braces between the tubes on the cheap-o headers to try and control some resonances, thus reducing rasp, and re-weld the tubing to the flange.

I know it is a lot of hassle, but I wanna try. I know I can make good power while having what I want for sound (OE plus kind of thing).

I also am planning on a Spintech muffler, or should I say a pair of mufflers side by side.

Might even end up with some derivative of UUCs dual muffler setup, I have not decided at this point.

What I do know is that I am gonna chop out my spare tire well and make my car have a late model valence (thanks, Andy!) and center exit exhaust...all for looks!

[nando]

Nope, wrong. That article does not refute the concept that scavenging is done in the exhaust. I have no clue why you would think scavenging ensd at the collector, but you are absolutely wrong.

in fact, anywhere that there are exhaust pulses there is scavenging: it is all about the waveform and pressure front leading edge/trailing edge.

maybe you don't quite understand what scavenging is.

scavenging can ONLY happen where there is a merge between two previously separate pipes. How can you have that downstream in the exhaust where the exhaust has already been merged? You can't. it's done. at this point the only goal is to get the exhaust gasses away from the car as quickly as possible, and as quietly as possible.

and of course the header primaries are super important and you definitely don't want to go too big there, but that's not what we are talking about. what does the size of the primaries have to do with the size of the pipes in the rest of the exhaust? absolutely nothing!

you must not have read that whole article, it clearly states that the size of the exhaust pipes past the merges is not important, except that it can be too small, but not too big.

and sure, there may be merit to the resonance of a bigger vs larger pipe - but in reality, it doesn't make an enormous difference providing you have adequate muffling, and certainly not at 3".

[StereoInstaller1]

maybe you don't quite understand what scavenging is.

scavenging can ONLY happen where there is a merge between two previously separate pipes.

Nope, You are dead wrong in your understanding of the principle.

That is why you aren't understanding things here.

Scavenging happens as the rear (trailing) edge of each individual exhaust pulse travels in a pipe. The trailing edge of each individual waveform (pulse) has a low pressure zone behind it, the leading edge of each pulse has pressure. When the pressure closely follows the suction of the trailing edge (of the pulse in front of it), scavenging happens.

That is precisely why exhaust velocity is so important... and why ALL high-performance BMWs I-6 street cars use 2 smaller pipes rather than 1 big one.


Read up on that link I posted, it explains it nicely.

[StereoInstaller1]

Oh, again: highest possible peak performance is NOT my goal, maximizing torque and driveability is.

A big single pipe is slow and lazy at lower RPMs, thus less scavenging effect. Velocity is way more important than CFM for MY application.

[ak-]

I'm enjoying this thread, in all respectabilities.
Good stuff to learn on.

[StereoInstaller1]

Some people want maximum power, some want loud, some want cool, some want fuel economy...I want to be able to creep along at 2 MPH in heavy traffic until I see an opening, then be able to get to 40MPH in a zillionth of a second...but still enjoy the song of 24 valves singing at 7,000 RPM!

[Killacortes]

Some people want maximum power, some want loud, some want cool, some want fuel economy...I want to be able to creep along at 2 MPH in heavy traffic until I see an opening, then be able to get to 40MPH in a zillionth of a second...but still enjoy the song of 24 valves singing at 7,000 RPM!

your goal is what i want! good stuff! i enjoyed reading all this.

[nando]

Oh, again: highest possible peak performance is NOT my goal, maximizing torque and driveability is.

that's fine, those are fantastic goals. I too do not want to wake everyone up as I crawl into the driveway. :)

A big single pipe is slow and lazy at lower RPMs, thus less scavenging effect. Velocity is way more important than CFM for MY application.

you've missed it again. There is no scavenging after the headers, there are no merging pipes where that could happen. it's done. all you have afterwards is silencing, emissions and getting the gasses away from the car.

[nando]

Nope, You are dead wrong in your understanding of the principle.

That is why you aren't understanding things here.

Scavenging happens as the rear (trailing) edge of each individual exhaust pulse travels in a pipe. The trailing edge of each individual waveform (pulse) has a low pressure zone behind it, the leading edge of each pulse has pressure. When the pressure closely follows the suction of the trailing edge (of the pulse in front of it), scavenging happens.

That is precisely why exhaust velocity is so important... and why ALL high-performance BMWs I-6 street cars use 2 smaller pipes rather than 1 big one.


Read up on that link I posted, it explains it nicely.

yes, I know what it is. but it only happens where two pipes merge together, not in one continuous pipe. Otherwise you wouldn't have headers, you'd have individual pipes for each cylinder.