I noticed when I look at pipe diameter recommendations, whatever is recommended for a single exhaust is usually also recommended for a dual exhaust.

Doenst that defy logic? If a 2" pipe is ideal for a 300 ci engine, how can it be ideal for a 150ci engine, when you run duals??

All exhaust companies have the same recommendations, so what is it Im missing?

 

You really have to know what the airflow capacites are for the 2 motors, it's very possible the 150cu in motor could move as much or more air as the 300 if it revs to twice the rpms.
Also consider that aftermarket manufacturers are going to promote what they have to sell, and that may not necessarily be what you need. In general they are all selling grosly oversized exhaust systems that are more about being loud than actually helping performance.

 

Note that guys like me who only use their trucks for occasional heavy duty but mostly light duty work are more concerned about sound quality and loud than actual performance. It's all about using common sense and knowing what you need and knowing what you're getting.

 

Well, part of it is marketing. People who buy duals are usually very concerned with how it'll look, and small diameter pipe looks wimpy.

But also, exhaust pipe sizing is an inexact science at best. Some people are concerned with keeping velocity up for some kind of "scavenging" effect, and some are concerened about backpressure.

I'd guess marketing has much more to do with it than any flow caclulation.

 

Actually in my scenario 150ci was half 300ci, it wasnt a comparison one vs the other but a whole engine (300ci single exhaust) vs half (dual exhaust 150ci each).

What Im finding is many exhaust companies have a chart thats independant of their cat back offerings and lists cubic inches and horsepower and gives a recomended pipe diameter for each, while they vary slightly from company to company, whats common, is if they recommend X for Y engine, they usually recommend dual X for Y engine, and that makes no sense to me.

 

What he said, but also different engines have different efficiencies. No engine no matter how carefully designed can achieve 100% volumetric efficiency (the cylinder actually filling to its fullest capacity) and none can achieve high levels of efficiency under all RPM and load ranges. However, some engines are more efficient that others, as a general rule engines that output more peak HP are either more volumetrically efficient, or can rev at higher RPM. Not that I'm a Chevy fan, but one example is the good old 350 engine, which is about 5.7L. Chevies newer 5.3L engine has a higher peak HP output, but due to valve timing and a few other design changes is much more volumetrically efficient than the 5.7L, and as such requires larger pipes to meet its airflow demands, even though it has a smaller displacement. Such is the case with many other engines. As a general rule newer high HP output engines need more airflow, though there are exceptions.

Flow calculation is relatively easy, though this method is simple. either you can assume that the factory system was close to the ideal size for the widest variety of conditions, or using more complicated figures that I do not have, you can make more elaborate calculations based upon the HP output, desires power range, and displacement of the engine, and calculate how much cross section of pipe you need.

If the factory system was say 3" in diameter, it is general concensus that 2.25" is a close match as a dual setup, and dual 2.00" is fairly close to 2.5" in total cross section of the exhaust.

It is my opinion that unless you make major modifications to the engine, there is little to be gained from significant changes in pipe size from the factory, and one pipe size increase is the best overall compromise in most cases.

 

I got 2.5" piping on my dual exhaust. Ended it up with 4" chrome tips. Sounds awsome with the flowmaster original 40.