Author Topic: Exhaust flow restriction- change in tube shape  (Read 3209 times)

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Offline aussievetteracer

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Exhaust flow restriction- change in tube shape
« on: November 07, 2012, 01:22:34 AM »
G'day from Oz- I am building a RHD C4 vette for the salt (and less ballast, for circut sprints). Have had to move the seat and steering column about 3" to the centre, to stay inside the inner plane of the roll cage. After the header collectors, the exhaust goes to the rear via 2 x 3" pipes- it will solve clearance problems, if I can use a short section of oval tube, 4" x 2" near the rear of the gearbox.
QUERY If I am careful to blend the change in cross section to minimise turbulence, can I tap into the collective pool of wisdom for advice on whether the change from 3" tube (7sq in.) to 4" x 2" oval (about 8sq in.) for about 12" in length, then back to 3" pipe, will have any substantial negative effect on exhaust gas flow?
                                        thanks, Denis
Denis

Offline Jon

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Re: Exhaust flow restriction- change in tube shape
« Reply #1 on: November 07, 2012, 02:08:04 AM »
An 4"x2" ellipse has pretty close to the same cross section as a 3" diameter circle.
If you blend smoothly there should be no significant loss in flow.

cheers
jon
Underhouse Engineering
Luck = Opportunity + Preparation^3

Online gray63

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Re: Exhaust flow restriction- change in tube shape
« Reply #2 on: November 07, 2012, 08:12:00 AM »
This should be similar to the exhausts used by NASCAR.
they have a very flattened exhaust coming out from under the rocker
panel to keep a low car profile.

Dave
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Offline hotrod

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Re: Exhaust flow restriction- change in tube shape
« Reply #3 on: November 07, 2012, 08:47:49 AM »
It is the exhaust pipe area that is important the change in size will create a slight reflection point for exhaust pulses. If the system is a tuned system that will put a small kink in the response. Since you are stepping up in area it won't be a flow restriction if done smoothly. Make sure your blends do not exceed 7 degrees angle change in the wall shape if you can, and you should be good. That is more important on the flare end than on the taper back to 3" round.

Larry

Offline aussievetteracer

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Re: Exhaust flow restriction- change in tube shape
« Reply #4 on: November 07, 2012, 03:39:57 PM »
Thanks for the responses; Larry- I have heaps of room on my "passenger" side, but would it be advisable to duplicate "the box" on that pipe also so that whatever the effect is, it will be the same for each bank? It will be a tuned length system.
                                                                                                    Denis
Denis

Offline hotrod

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Re: Exhaust flow restriction- change in tube shape
« Reply #5 on: November 07, 2012, 05:26:33 PM »
It would be nice to have a symmetrical system, so that both sides of the engine behave the same.
If the cross section change is before the end of the tuned length yes I would duplicate it if I could. If it is after the primary reflection point it would be less important as the engine is largely unaware of what exists beyond the reflection point if it is free flowing.

It depends on how radical the setup is, there is some advantage to "stagger tuning" a system where all the pipes do not come on the pipe at the same point in the rpm band, it will give a broader torque/hp peak but not quite as much max power and torque.
Sort of a choice of evils decision, perhaps best solved with the flip of a coin ;)

Larry