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-   -   Using exaust gas as an aero. aid (

Cd 03-17-2008 07:25 PM

Using exaust gas as an aero. aid
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.

cfg83 03-17-2008 07:40 PM

.Cd -

That's interesting. Our muffler-centric FE questions have always been focused at improving engine efficiency for MPG. No one has talked about the exhaust as an aerodynamic component.

Would this imply a wide/flat exhaust tip? Maybe splayed out like a river delta?!?!?!


Cd 03-17-2008 07:50 PM

That was my thought too, but again our puny exaust just will not 'cut it'.
I seem to remember this idea being used in tests on semis with very good results.

roflwaffle 03-17-2008 08:13 PM

IIRC we talked about it on the other forum.

trebuchet03 03-17-2008 09:54 PM

Check out basjoos' muffler guard thinger... It's pretty damn close to doing just that :p

In any case, a clean rear underside is necessary for this to work...

DifferentPointofView 03-17-2008 11:31 PM

Big exhaust will hurt fuel economy if you accelerate slow and drive slow, and MIGHT help if you drive normal.

You want a perfectly tuned exhaust. Here's an example :

Exhaust tuning

you want the puffs of exhaust to line up at a perfect distance, so that you get the most torque/power at low rpms, where you need it most in good FE. Too small will create too much back pressure, too big will create not enough. Back pressure isnt always bad, it just seems like it because when you talk about eliminating back pressure, your usually talking about racing and high rpms, where exhaust needs to escape fast and unrestricted. Read the link to get the just of it.


In the lower RPMs, pulses are smaller, and further apart. When you rev up into the high RPMs, pulses get bigger and closer together. So we want to keep the small, spread out exhaust pulses in line in the low RPMs
for low rpms, smaller exhaust is better for the smaller, faster exhaust pulses, they don't bounce around in a giant 3" exhaust, so they are in-line, and they can go faster cause the route is an easy one way, so no one (exhaust buddies) get confused :rolleyes:

LostCause 03-18-2008 01:05 AM


Originally Posted by .Cd (Post 14743)
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.

Exhaust flow would probably be much to low. You could argue that every bit helps, but that gas can be put to better use.

In my opinion, the most logical use of exhaust gas would be to improve the cooling system. Build a divergent/convergent plenum around the radiator and vent the exhaust gas tangentially to the convergent duct's walls. The increase in flow will help decrease cooling system pressure loss.

You could either the vent the radiator outlet on the hood to kill lift (best solution) or vent it to a diffuser on the underside of a skirted car to create down force (essentially an F1 technique).

Another cool, albeit impractical technology used during WWII was exhaust ejectors. I haven't been able to find out exactly how they work (I'm pretty sure they are more than rearward facing exhaust outlets). I've heard they had the equivalent effect of adding 100hp to the Hawker Hurricane's top speed, but take that statement lightly.

Hawker Hurricane Exhaust Ejectors


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 goal is to size the exhaust system to aid peak torque at the rpm you want. Exhaust systems are tuned by length and diameter for a certain rpm. Increasing diameter will probably raise peak torque. Apparently it is not easy to tune an exhaust (a lot of testing/experimentation) so it would probably be best to stay OEM.

- LostCause

donee 03-18-2008 08:14 PM

Hi Different...,

That was an interesting article, but its not about tuning an exhaust so much. Its about the aerodynamics of the fluid flow in the pipe, and how big an exhaust pipe can disrupt this flow, by allowing multiple flow modes that interefere.

Tuning an exhaust is about the reflections that the pulses in that article describes. These pulses and reflections add and subtract at the exhaust (or intake port). Lets just talk about th exhaust for simplicity. The exhaust valve opens and a pulse of waste gas leaves through the valve. This pulse will travel along within the exhaust system tubing until it hits something that has a different cross section shape. When that happens some of the pulse will reflect from where the cross sections change shape, or discontinuity. Now you have a dimished pressure pulse continuing out the exhaust system, and a smaller one headed back to the valve. As the valve is closed, the pulse reflects off the valve and starts to cause a vacuum at the valve opening. This is when the valve should be opening if the exhaust is tuned. The near vacuum at the valve then cause the exhaust gasses to be sucked out more completely and quicker.

In race cars there are no mufflers. The discontinuity is the change from the pipe to the open air. ISo the exhaust is just a pipe. The length of the pipe is set to the above condition described at the maximum power RPM ideal for the specific type of racing. In F1 the pipes are really short. In Nascar they are somewhat longer due to the pushrod nature of the engines. Take off the pipes from these engines and they would have considerably less power.

The speed of the pulses is and is not like waves on a string (an excellent way to envisions what is going on, btw). Waves on a string have a constant propagation speed set by the tension in the string. Exhaust pulses do not, as the exhaust waves are cooling as they travel and that changes their natural propagation velocity, since the gas density increases. So, this type of tuning is mostly done emperically, as computation is very difficult for such non-linear phenomena.

BTW, any shape pressure pulse can be described as a sum of sinusoidal waves. So, figuring out a solution for sinusoidal waves, and adding them together is the usual way engineers work on such problems.

aerohead 03-22-2008 03:27 PM

exhaust thrust
NASCAR teams would like to have the exhaust exit behind the racecar,however,are forbidden to do so by the rulebook.Of course,in a race situation,they're looking for any edge they can get.Drafting would place the following driver in a position to breathe concentrated exhaust,so perhaps its better the tailpipes remain where they are.In a commuting environment,and at the RPMs that engines typically develop,the volume of gases being expelled out the tailpipe,and the exit velocity at which they're being discharged,I believe would render a "thrust" of dubious value.As to the issue of tailpipe size,my opinion is,that for normally-aspirated Otto-cycle gasoline engines ( and no Nitrous oxide either!),that the inside diameter of the tailpipe should be about the same size as that of the throttle-body,for MFI/EFI,or the combined area of carburetor throttles.

Figjam74 04-09-2008 05:53 PM

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.

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.

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)

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