Exhaust Diameter Question

[IROC]

The outlet of my SSIs seem to measure about right at 2" in diameter. Hypothetically, would bolting a 2.25" exhaust to the SSIs be a bad idea? Is this one of those situations where "close" counts?

Mike

[Bill Verburg]

It's not so much the size differential as the sudden transition. Much better to have a smooth transition.

That said w/ a muffled system it doesn't really matter all that much.

[Randy Webb]

Beyond what BV siad, you could call up John Daniels. He created the system and has done dyno testing on various pipe dias. I expect he will tell you to avoid the larger pipe.

[IROC]

Hmmm..decisions.... Of course, the larger muffler inlet diameter will cause the exhaust gases to slow slightly, but I bet the answer to this question is not really simple. I might be able to smooth the transition somewhat...

Randy - for sure the larger pipe diameter would be detrimental, but hopefully the SSIs will take care of the vast majority of the exhaust system flow. Truth is, I found a decent deal on some stuff on ebay, but the exhaust is 2.25" in lieu of 2"....

Mike

[Randy Webb]

The muffler will sig'ly affect the restrictions in the system and hence total pressure. It will also damp pulsations ("scavenging"). So it's not that big a deal. Buy it. Or call SSI and then buy it.

Definitely smooth the transition...

If you search for that marathon exhaust thread you can find the title of a book I posted on this topic. If you want to get into it that far...

[350HP930]

There is nothing wrong with going to a slightly larger pipe transtion, its only a problem when you go the other way.

Just imagine what transition the gas sees when it leaves the exit of the exhaust pipe.

[Randy Webb]

Ah, but the end of the pipe is not the middle or end.

There can indeed be problems - they have been demonstrated empirically and shown theoretically and with numerical methods. Remember you are disrupting the flow.

Do this: whenever you think about flow in a tube, think about flow in a river or canal. That will get you part way to understanding potential problems. Here, there will surely be turbulent eddies induced - and that will mean higher resistance.

[RarlyL8]

Slide your 2.25" ID exhaust over a 2" ID pipe that is welded to a flange that will bolt onto the SSIs. Good enough for a street motor.

[IROC]

Originally posted by RarlyL8
Slide your 2.25" ID exhaust over a 2" ID pipe that is welded to a flange that will bolt onto the SSIs. Good enough for a street motor.

Exactly what I was thinking....

Thanks for all the replies.

Mike