2.5" vs 3" exhaust video

[hybrid]

Interesting video of 2.5" vs 3" on a 600HP engine.
I'm not sure I would bother putting up with the extra noise for 14HP, but I'm sure others would prefer 14 horses over their sanity
Torque figures are weighed more heavily in favour of the 3" - at least at full throttle.

[boofhead]

Good one - very little to pick on. I agree on all points.
1) Less back pressure from the exhaust (after the headers) the better
2) Indeed larger pipes are harder to keep it quiet (but who cares - )
3) Magnaflow mufflers are excellent for flow with their straight through design.
4) Not using press bends in the pipe is important as it will kill flow. They showed pipes with no pressed bends which is the way to go if your looking for excellent flow.
5) Big street engines rule. They are not using that much cam for the size of the engine. Good setup is that engine.

Glad the boys are doing those videos. I should look at more of them. I just get frustrated with some of the conclusions when little information is provided. This one was a good one and explained well.

Lastly my comment; there does need to be a restriction on the intake and the exhaust. This is via the Valves in the heads - controlled by the Cam - so select the heads and ports sized correctly for the application then make sure the intake and exhaust does not introduced uncontrolled restriction. Mind you once flow reaches the speed of sound sonic choke occurs so I limit port speed to 300 fps while the rest should not restrict the speed further. This allows port flow to be close to desired speed of sound.

Note: There is a point where it makes no sense to go larger - so a 302 with similar rev range would see not advantage from the 3 inch exhaust over a 2.5 inch system. So it is all a factor of volume of air/gas mass as factored in relative to the RPM. Higher RPM means more air/exhaust to flow. Obvious I know but is often forgotten when selecting the parts package for your car.

[Ausjacko]

Boof, your last para is the key in my view. As they note in the vid, these tests are at WOT. I impose a rev limit of 5500rpm and if honest rarely go close to it. As much as I like the sound of the three inch exhaust, my driving style and vehicle use does not really warrant it. Given the engine in the grey car was jetted on a dyno at WOT and I use an exhaust now (2.5 I think), it probably means I am running rich as the engine cannot pass enough air to match the jet size.

As such, this is one of those occasions when 2 inches is better than three.

[Pinto-Pete]

Boof, re point 4 do you have evidence of this..? as this is not my understanding, so,......
to paraphrase Pauline please explain...

[Pinto-Pete]

jacko, am I right that your grey car was 2.25 originally..?

[boofhead]

Agreed - build to the operating speeds you intend to use. In your case Max HP would be best designed for 5000 RPM which leaves your limit of 5500 as maximum RPM. So depending on your engine size a 2 inch system might be right though I would be thinking 2 1/4 as a minimum. That is because I am keeping in mind, the losses from a restriction is greater than any affects from going to large. So always bias toward larger is not sure.

[Ausjacko]

Pete, yep

[boofhead]

Boof, re point 4 do you have evidence of this..? as this is not my understanding, so,......
to paraphrase Pauline please explain...

I am not sure what level of evidence you are after though I am glad you asked. I enjoy these kinds of discussions. Naturally I am open to your evidence that shows my statement is incorrect.

In any case, my position is based on that press bends cause a kink in the tube which decreases the internal diameter. Naturally flow potential is influenced by the diameter so really is a factor of cross section and length. For example, if you have a 2.5 pipe and to fit over the diff you insert a 2 inch pipe section. The flow is limited by the smallest cross section hence the pipe will flow only a little better than a full 2 inch system.

In addition, a bend in a pipe causes the greatest wall turbulence then you add into that the smaller diameter results in a higher restriction. At least a Mandrel Bent pipe does not narrow the cross section thus flows better than a press bent section. When thinking about these ideas or discussions - I like to take the premise further, for example, a very small decrease in diameter would not have a big influence which I accept though if you keep narrowing the area to be say a more extreme example (which you would not do) say 2.5 pipe to a 1/4 inch reduction - I would imaging everyone would agree this would be a restriction. So logic thus tells you any reduction in pipe diameter will restrict flow. It can be shown to be true mathematically.

Experimental evidence is harder - I would have to look around to find dyno evidence - hard to get as very few people do full exhaust testing - I will see what I can find. If any.

Edit: Try this link: http://www.bobsmuffler.com/muffler_myths.htm



Dyno results showing my statement is correct.