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Old 04-12-2008, 12:26 AM   #11 (permalink)
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What if one were to go at it in the opposite direction--meaning, use the area of low pressure behind the car to help suck the exhaust out the pipe? Is there a potential for increasing the exhaust flow velocity this way?

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Old 04-12-2008, 02:14 AM   #12 (permalink)
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I would think that you would want to point it up into the wake zone, letting the exhaust gas 'fill in' the volume evacuated by the car. I don't know if there's enough flow to make much of a difference.

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Old 04-12-2008, 03:07 AM   #13 (permalink)
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You're talking about a tiny FE gain. Look at the size of the air intake on a 4 cylinder Honda. That's the amount of air stream you're affecting, if you're putting the petal to the metal. After making other more cost and time effective mods, you're going to be using so little engine power and hence air that I doubt you could really see an effect.

However for something like a satellite rocket, this plays a big role, and is the reason that the bottom of rockets can flare outwards with little effect on their drag.

It's also worth noting that the exhaust gas isn't constant, and so it comes out turbulant, which doesn't usually bode well for aerodynamics.
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Old 04-12-2008, 04:03 AM   #14 (permalink)
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FWIW, at 50mpg, 600F exhaust temp, and half an atm where the exhaust exits the vehicle, the Carbon Dioxide should occupy a ~1'x1'x3" volume per second, so not a whole lot. It seems it would be better for pushing the separation point a few inches farther down than actually filling in anything.
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Old 04-12-2008, 06:11 AM   #15 (permalink)
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Originally Posted by .Cd View Post
On a lot of race cars I have seen the exaust routed in such a way that it can be used to energize ( reattach ) any stagnant air behind the car.

Can we do the same sort of thing with our cars ?

I would think there would not be enough force behind our puny exaust pipes to do any good.

Also I'm confused about whether a large exaust pipe and free flowing muffler or a tiny exaust pipe with a stock one is better for fuel economy.
I have seen arguments for both.
The manufacturer of Aptera has a vent at the very tip of the end of the car from which interior air is exhausted. They placed it there, according to them, for the reason you mention above.
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Old 04-12-2008, 09:33 PM   #16 (permalink)
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Originally Posted by Figjam74 View Post
IMO, backpressure IS always bad. Backpressure increases your pumping losses. The trick however, is minimize backpressure without sacrificing exhaust velocity. It become trickier yet when you realize that the amount of exhaust gas you're tuning for varies with engine speed.

Absolutely correct.
At full noise (19 000 RPM) a current F1 engine (2.4 litres) puts out about 11 400 litres/minute of exhaust gas most of which is put to use aerodynamically to fine tune the rear wing and other bits and pieces the wind tunnels tell the designers can be used for downforce such as when exiting a slower corner.

I would have assumed that F1 cars were using short exhaust pipes due to the wide range of RPM's that they need to run at, while the Nascar teams can tune for specific RPM ranges that the cars typically run at. I'd also imagine that weight is a bigger factor in F1, and that NASCAR is supposed to be 'Stock' car racing, and rules would require a full exhaust system.

Also correct on both points.
NASCAR is slightly different since the engines run at fairly much constant speeds over a narrow rev band and virtually all of the action takes places at the top end.

As far as using the exhaust as an aerodynamic aid, my guess would be placing the exhaust somewhere in the middle (taking a hint from Lamborghini here)
The Lambo is using a set of venturi ducts either side of the engine and inboard of the rear wheels so the exhaust pipe is in the middle as a result.
I suggest the volume of air travelling through the venturi ducts is the dominant factor and it may even help the exhaust by creating a relative vacuum either side of the pipe.


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