Effect of profile on exhaust flow

[bexupnorth]

Hi Gang,

I'm wanting to make a change in the Zephyr exhaust cross section from circular to rectangular and was wondering if anyone knows what dimensions to use.

Say I have a 75mm (3") diameter tube and want to run it down so that it is 20mm (3/4") high.
Should I use dimensions that will give the same area or is there more to it than that?

I know I'm being a bit anal, but it will affect some chassis parts for routing so I don't want to get it wrong.

thanks,

Ade.

[sporter]

I'd give Dr Gas/Spintech a call and ask. They make lots of oval and rectangular exhaust components and they've always been helpful when I've called them in the past.

[bexupnorth]

Thanks Sporter, seems I was trying too hard and so long as the cross sectional area is the same, there shouldn't be any significant effect on gas flow.

Ade.

[parsonsj]

You got it. Same area is the ticket.

I'm sure you understand, but remember that flattening a round tube will reduce its cross-sectional area, reducing flow.

jp

[bexupnorth]

That's the bit that was bothering me! Knew there was something that didn't feel right. Once I sat down with the calculator and some formulae, it was quite surprising how much longer the perimeter, circumference whatever increased the more you squash the pipe but the csa stays the same.

Thanks

Ade

[parsonsj]

Exactly. A circle has the largest ratio of area to perimeter. When you squash it, you reduce cross-sectional area. Oval shaped tube has more perimeter to achieve the same cross-sectional area as round tube.

jp

[Denvervet]

I'm not an engineer but rectangular tubing even of the same cross sectional area won't flow the same due to turbulence in the corners. Someone on here that's a fluid type engineer could spell how much difference there would be. If it's short distance probably not significant but if the entire length exhaust a different story.

[bexupnorth]

Thanks Denvervet

I'm only looking at the last foot or so, basically, I want the exhaust to come out of the sills (I think they're called rockers over here?) just in front of the rear wheels.

I was looking at bending up some sheet metal so the corners wouldn't be sharp and this won't be a race car so it's probably not that critical. However, I would be interested in the math if only as a technical exercise.

Ade.

[novaderrik]

you could put some NASCAR style dumps under the rocker panels- they are only a couple of inches tall, but the opening is fanned out to maintain flow on those 850+hp cars that are screaming at 9000+ rpm for hours on end. i think they even use oval tubing from the headers back for ground clearance.