When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
You can argue with math, but math will win every time.
1st, a 3" (ID) pipe has a cross section area of approx 7.06 sq inches.
a 2 1/8" (ID) pipe (stock D2 A8/S8 after cats to rear muffler) has an area of approx 3.53 sq inches...x 2 pipes = 7.06 sq inches, netting the SAME capacity.
I just looked at this thread and wanted to throw in some more useless knowledge.
Flow is actually based on the radius of a tube taken to the 4th power according to poiseuille's law. so one 3 inch tube would mean r^4=1.5^4=5.0625.
for two 2.125 inch pipes would mean 1.0625^4=1.2744ish. Double that and you get 2.549ish.
So one 3"pipe technically would allow better flow. However, does this make a better exhaust? That, I don't know. I'm only a man.
I just looked at this thread and wanted to throw in some more useless knowledge.
Flow is actually based on the radius of a tube taken to the 4th power according to poiseuille's law. so one 3 inch tube would mean r^4=1.5^4=5.0625.
for two 2.125 inch pipes would mean 1.0625^4=1.2744ish. Double that and you get 2.549ish.
So one 3"pipe technically would allow better flow. However, does this make a better exhaust? That, I don't know. I'm only a man.
Cool...thanks. I couldn't find (through google) a link calculator/equation for that....with an equation name, I found (and bookmarked) a great link for that >>>
I'd love to see a page that covers the effect on moving gasses being forced to turn sharp corners, rather than low radius curves.
I do know, from accumulated industry & tuner knowledge and personal experience, that you can have too little and, lesser known, too much flow.
In the old days, too much exhaust flow with a carburetor still jetted for a stock exhaust system could lead to a valve-melting lean mixture condition. Plus, it was my introduction to another effect that carries on into modern engines: Increased exhaust flow, over the stock set-up, tends to move more power to the top few rpms (with only minimal hp gain, if this is the only mod) and noticeably robs the low-mid range of it's original torque/hp.
The only way I've seen to have it both ways are systems with waste (or diverter) gates which open when needed (high rpms, heavy pedal, etc..). I believe a few high end "super" cars come with this sport feature.
On one past car (86 Scirocco 16v), which I highly modified the engine on (bore/stroke/cams, etc..), after driving with headers and Supersprint exhaust system, I ended up reinstalling the stock manifold & exhaust (minus the cat) to find a much more drivable car with the same improved 0-60 time...only lost a small extra kick in the last couple hundred rpms before redline (probably less than 1% of driving time).
Full flow exhaust may be suited to track driving, where one may keep the engine just below redline much of the time...does little more than add a lotta noise to a street car, my single biggest problem with most "high performance" aftermarket exhausts: More noise than performance...nothing more irritating or embarrassing to me...I sure get a laugh out of cars I hear roaring, but them accelerating at the blinding rate of a gravel truck. Old Yugo joke: The car come with a volume control in place of the gas pedal...press the pedal and the car gets louder, but goes no faster
Again, thanks for the enlightening info...believe or not, I mainly frequent this forum to share what I've learned, but more importantly, to continue learning. It's just an added bonus to help separate the men from the boys, when a thin-skinned poster takes offense at criticism, rather than defending their views with documentation or even theory.
I just looked at this thread and wanted to throw in some more useless knowledge.
Flow is actually based on the radius of a tube taken to the 4th power according to poiseuille's law. so one 3 inch tube would mean r^4=1.5^4=5.0625.
for two 2.125 inch pipes would mean 1.0625^4=1.2744ish. Double that and you get 2.549ish.
So one 3"pipe technically would allow better flow. However, does this make a better exhaust? That, I don't know. I'm only a man.
On a second look at the link I provided, the formula only seems to be dealing with liquids flowing through pipes. Liquids don't tend to compress or behave the same as gasses, so it makes me wonder if this law applies in any way to flowing gasses.
Also (patting oneself on the back), language at that link points out that this formula is based on the absence of any turbulence, such as gasses forced around corners and such.
Would love to learn more if you can provide a formula or link that deals with flowing gasses i particular or if the same formula applies in any adjusted way to gasses....especially since the formula refers to different results netted in the cases of different viscosities of fluids...but what of gasses, I wonder?
When it comes to exhaust flow things can be counter-inuitive. Buddy of mine is a plant manager for Faurecia. Sometimes a hard 90 will outflow a radiused bend based on their flow testing.
02-10-2015, 03:36 PM
