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Old 12-01-2016, 03:07 PM   #1
e30strokr
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e30 exhaust pipe diameter chart

Hi all

I been looking for some information for the correct exhaust diameter for my stroker and everybody just goes 2.5 inch diameter pipes and looking thru I didn't find any good information of why, I found this chart in my search for anybody to check it out and see if it works for you.. and analyze... I myself went with catback 2.25 diameter single pipe with a borla pro xs dual tip and it sounds just the way I want .. its not stock sound and its not loud and obnoxious....

basically 2.25 in dia is good for 185HP
2.50 in dia is good for 232hp
2.75 in dia is good for 283hp
3.00 in dia is good for 339hp

http://www.exhaustvideos.com/faq/how...92871093750000
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Old 12-01-2016, 09:51 PM   #2
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Interesting.
Does that mean a 300hp 1.6L turbocharged 4 cylinder engine should have the same exhaust pipe size as a 5L naturally aspirated v8 with 300 hp? As well as a 300hp 8.4l naturally aspirated v12?

IMO, this is like websites telling you that ANY engine with 6 cylinders should have (x)-pound/hr injectors for 200hp and (2x)-pound/hr injectors for 400hp. Not taking into consideration that a 1977 Porsche 911s 2.7L engine might have a different volumetric efficiency as a 1987 BMW 325E 2.7L engine.

Maybe a good rough guess, but I wouldn't take this info very seriously. Unless I'm mistaken?


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Old 12-01-2016, 10:50 PM   #3
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The stock E30 exhaust is about equivalent to a 2.5" single pipe - so yeah, dont pay much attention to that.

The most important part of an exhaust is between the ports on the head and the last merge - everything beyond that is just dead weight or a potential restriction.

Basically, x" per 100hp is mostly nonsense.
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Old 12-02-2016, 02:07 AM   #4
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It's only meant to be a rule of thumb. Using area and equating to a HP is reasonable concept for sizing tailpipe of full length exhaust but single will have more potential than dual for same area. Obviously it's more complicated but as a quick guide.....the actual numbers are a bit low and there are better guides out there that use a simlar approach. You can get somewhat more than what those numbers suggest if you do it properly
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Old 12-02-2016, 08:25 AM   #5
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Name:  insane_exhaust.jpg
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Put this in and call it a day
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Old 12-02-2016, 08:54 AM   #6
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Quote:
Originally Posted by dnguyen1963 View Post
Attachment 111978



Put this in and call it a day


This is the answer to all of our problems!


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Old 12-02-2016, 10:17 AM   #7
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Old 12-02-2016, 11:06 AM   #8
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Old 12-09-2016, 03:47 PM   #9
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Quote:
Originally Posted by berlow94 View Post
Interesting.
Does that mean a 300hp 1.6L turbocharged 4 cylinder engine should have the same exhaust pipe size as a 5L naturally aspirated v8 with 300 hp? As well as a 300hp 8.4l naturally aspirated v12?
You do realize that capability to support power output basically comes down to mass flow? More air more fuel more power, less power less air less fuel. Generally speaking, a 5L gasoline V8 making 300hp isn't producing a significant amount more mass exhaust flow at peak output than a 1.6L turbocharged gasoline I4 making 300hp, unless the V8 grossly is inefficient from a thermodynamic point of view and is wasting so much power as heat that it must take in more air and fuel to make the same power output. That turbocharged 1.6L engine has the capability to flow as just as much air as the V8 with 3.125 the displacement and the same peak power output because the turbocharger is increasing mass flow by way of increasing charge density, this is in turn increasing the volumetric efficiency of the engine. The 8.4L V12 isn't moving much (if any) more air than the 5.0L V8 at peak output if they're both making 300 peak horsepower, but it is demonstrating a much lower volumetric efficiency at peak power. There are many variables in these kinds of things, but you can't make 300hp without moving enough air to support 300hp, and you won't make 100hp flowing 300hp worth of air unless your engine has a serious thermodynamic efficiency problem.

Back-of-the-envelope rule of thumb calculations are not meant to be taken as holy gospel, and while I can't speak for the accuracy of the numbers in that article since I can't be bothered to research it and do the math, the methodology of these estimates is sound and back to back testing such as the below video, as well as the comparing the sizes of the factory exhaust pipes on cars with varying amounts of power output confirms that the figures are at least in the ballpark.

You are indeed mistaken about fuel injectors. Brake Specific Fuel Consumption is the variable that matters to fuel injector sizing, not volumetric efficiency, and choosing the appropriate size fuel injector is indeed as simple as plugging the correct values into the calcluator and using the ballpark figure it gives you. Garbage in, garbage out, though. If you plug in the same BSFC figure for two very different engines, the accuracy of the result will suffer.


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Last edited by varg; 12-09-2016 at 04:18 PM.
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Old 12-09-2016, 10:13 PM   #10
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the main difference between number of cylinders and tailpipe size for a given hp level is the more cylinders the more "uniform" the flow is in the taiplipe. take a 2 cyl 100hp engine all the blowdown action happens in two short windows. on a 8cyl there are 8 smaller blowdowns spread out more uniformly so the average flow is the same (100hp worth) but peak demand on 2cyl is much higher so you'd probably make the 2 cyl slightly bigger so your back pressure doesnt get too high. this is ignoring scavenging etc etc

for the above reason on a 6cyl with proper header grouping the pulse are spread evenly with basically no interfence (or very little) such that you can actually get away very small secondary tubes from a flow resistance perspective. normally you would add a small choke there with a merge collector (tapered conical piece) which doesnt restrict flow but gives the good anti-revserion characteristic that high velocity pipes do
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