Exhaust Flow and Aero

[MLSE]

I am looking for some help from someone who KNOWS what the effect of discharging an extreme volume of hot exhaust flow (1000 h.p. worth of exhaust flow) underneath the chassis of a production class sedan over 200 mph? I am not an engineer but it would seem to me that with the air already being packed under the car (due to the lack of a front air dam) that adding the exhaust volume has to have some sort of significant effect, either good or bad. Does this add to overall lift (and drag) on the chassis or would the exhaust flow displace cooler denser air beneath the car and reduce drag?

Also since "exhaust" in the rule book sates only that it "can be modified. must point away from fuel lines etc, away from the salt and past or away from the driver" (and could be discharged through the hood without changing the body contour) what would be the effect on aero over 200 mph - of the same high volume, high temperature flow passing over the sedan or coupe production body?  Many big time turbo drag race cars are now just shooting the turbo discharge straight up through the hood.

Sure hope somone out there knows something (or everything) about this subject .

Thanks in adcance for any help you can give.

[manifest]

I have been pondering this alot too.  Running the exhaust out of a two 4in. collectors through the car will be tough in the Datsun we are building (slowly).  Shooting it straight out of the lower fender will look cool but will it hurt the aero? Could always turn them up through the hood like a tractor pull truck! Anyone that can comment on this with some knowledge will help MLSE and I out tremendously.

[SPARKY]

The reason I pointed mine straight back on the RATICAL was because of thrust and aero!  If you have rake on the car the underside is going to be negative pressure.

[Dynoroom]

Some people have been known to go through a lot of trouble to get the exhaust out the back of the car.......

[superford317]

I am in the planning and building stage of a bellytank, so i have been studying a lot of different things with aerodynamics and reseraching a few ideas i have, this was one of the thoughts i had also.
a lot of late nights at the computer reading and all i can come up with that may be closely related to this matter are some research papers from (aeronautical research committee report and memoranda no. 1683) and a LOT of very usefull tech and information if you google (the meredith effect) i still havnt read or understood all of the information that i gatherd from the search.
maybe someone here can check it all out and see if it would be anything helpfull with using the exhaust flow to give a  boost to performance.
Most of the above information has to do with using the heated air from fighter aircraft engine cooling radiators to boost the airplanes speed.
the supermarine spitfire was the first to make use of this concept and the designers of the P-51 mustang said the meredith effect was a major factor for the P-51's high speed.
at 25,000ft altitude the hot air from the engine cooling radiators running through special ducting had 1/3 as much power as the proppeller had at full power.
you would think that the exhaust having a lot more heat than cooling radiators would contribute to a LSR cars speed.
i hope this is helpfull, i am new here, just mainly lurking and studying for my own build up. 

[836dstr]

A little off the subject, but still of intrerest (at least to me):

When Dan Hostetter built his ever evolving '32 Coupe he ran 8 individual pipes out the back of the car. He remembered a show car roadster from the late '50's or early '60's with that exhaust system (Green and white body, I can still remember the car on the cover of Hot Rod magazine). The only reason he did it was that he thought it looked COOL, and had always wanted to do in. His little Desoto Hemi was injected so it didn't really benefit from individual stacks.

He wanted more power so he went back to a conventional header setup. When he ran the car it was less stable that before. The 4 horizontal tubes on each side acted as sort of a bellypan and had cleaned up the air under the car.

So much for unintended concequences!

Tom

[Rex Schimmer]

Exhaust gases are two things, hot and high velocity, which makes them, once they have exited the pipe, a high energy, low pressure gas stream. They can provide thrust and also if exited in the right area they can assist air flow over or under the car. I would never have the exhaust just point out of the car at 90 degrees or down, maybe up if you wanted down force but to the rear is best especially if you can use it to assist your aero package. I think Blue's (Eric)  comment about, testing, testing, testing etc. fits here.

If you wonder how much force the exhaust can develop, you should see the vids of the fuel funny cars that lost the headers on one side, it nearly blows them over backwards!!

Rex

[manta22]

Rex;

It may be a B-29 that I'm thinking of, but somewhere I read about using the engine exhaust gasses for additional thrust. I think there was an "augmentor tube" involved that sucked in air to add to the thrust.

Regards, Neil  Tucson, AZ

[Stan Back]

Tom --

The green and white roadster had an Olds (Cad?).  The one you're thinking of was Scotty's Muffler Service from dear ol' Berdoo.  Eight pipes outta an Ardun.  They were machined from Ford torque tubes, I believe.

Would be good ballast, too, for an LSR car.

Best regards,
Stan

[38flattie]

Exhaust gases are two things, hot and high velocity, which makes them, once they have exited the pipe, a high energy, low pressure gas stream. They can provide thrust and also if exited in the right area they can assist air flow over or under the car. I would never have the exhaust just point out of the car at 90 degrees or down, maybe up if you wanted down force but to the rear is best especially if you can use it to assist your aero package. I think Blue's (Eric)  comment about, testing, testing, testing etc. fits here.

If you wonder how much force the exhaust can develop, you should see the vids of the fuel funny cars that lost the headers on one side, it nearly blows them over backwards!!

Rex

Rex, where would you exit the exhaust on my car? With a bellypan we'll have to remove to change classes, out the back would be a pain. Out the side is the easiest, but....

[grumm441]

We ran our out the back fro a whole bunch of reasons
The Reverend, who drew the car wanted it to have clean lines, and therefor clean aero
there were also thoughts about the benefits of hot exhaust filling the void behind the car
I wanted a bit of length in the exhaust so it would run well
It took a bit of effort to get it all to fit and to deal with the heat, and it can make changing the engine hard work
However, it seems to work well and the engine runs nicer with the whole exhaust than with just headers
G

[Jonny Hotnuts]

I purposely routed my exhaust to come out the rear deck lid. My car has a stark drop off behind the rear window area and generates low pressure. The low pressure makes lift, and while I dont know if it really helps piping the exhaust into the low pressure area makes me sleep better at night.

~JH

[interested bystander]

Don't ya think that al this topic is becoming a bit dramatic?

Wouldn't ya wanna select the best tuned length for the system and then exit it in such  a manner that a pressure zone  isn't balking it's release, and then do it as simple as possible.

Ya might find routing all the pipe to an optimum exit location might tend to turn all yer plumbing into a horsepwer robbing mess.

Sumtimes ya gotta compromise!

[SPARKY]

a lot more likely to dramaticaly increase drag from trying to pick up 5-10 hp with equal length headers

[Dr Goggles]

Don't ya think that al this topic is becoming a bit dramatic?

Wouldn't ya wanna select the best tuned length for the system and then exit it in such  a manner that a pressure zone  isn't balking it's release, and then do it as simple as possible.

Ya might find routing all the pipe to an optimum exit location might tend to turn all yer plumbing into a horsepwer robbing mess.

Sumtimes ya gotta compromise!

To followers of the Church of Horsepower that would read like gospel, If you were a disciple of the prophet Aerodynamics you may see that as heresy.....Thankfully this is an ecumenical message board where we all act like we get along.....The better your aero, the less power you need,the better your aero the faster you go for the same power......... powerful engines are expensive to build, the better your aero the cheaper your fun.......

That horsepower that is "robbed" may well be un-necessary in a car with a half decent shape. That increase in power you cite by "exiting in such a manner that a pressure zone isn't baulking it's release " may well be inconsequential.

For one, it's low pressure  that is causing drag even more than the wall of air you think you are pushing...... so dumping exhaust into a low pressure zone is going to help........

To a surprising number of people this discussion is interesting...and important, to some people it's what it's all about..

Sort of like the Hokey Pokey.