Exhaust in sedan rear diffuser

[Fifty]

I'm trying to sort how i will have the exhaust exit the rear diffuser in my track car. It's production car, with a flat bottom and side skirts (not made yet).

Do I have the exhaust exit the diffuser face horizontally, pushing air straight back?
Or
Do I have the exhaust pipe made so theair exits the diffuser in a manner where it tries to follow the shape of the diffuser and head up wards?

If I can create enough front down force I will add a wing to the rear of the car. It would sit about 15 inches behind the bumper.

All of the "research blown diffuser" help I have had so far, has led me to open wheel cars where the exhaust exits either on top of the floor ahead of the wheels and blows on top of the diffuser or it exits where it creates a vortices to block air from getting tangled in the rear wheels. Nothing about assisting flow on the bottom side of the center of the diffuser.
Closest thing I can find is the current exhaust affecting the monkey seat, but regs say it has to go straight back. Would the teams point it up to the wing if allowed?
There is no rule book for this car. It's a hobby and for fun so I can do what ever makes me smile!

[Paul]

The question you have to ask yourself first is even if you make it actually produce downforce will the fluctuations in car handling depending on throttle make it more fun or just plain dangerous?

[Fifty]

Already been asked and answered. So now that we have that covered. Anyone have any insight as to the question asked?

[Jolle]

Just thinking our loud, as I believe the diffuser itself doesn't create the df, but the creation of a low pressure at the floor in front does. That would mean that adding air in the diffuser would make it counterproductive. The speed of the air coming from the exhaust might pull some extra air with it, but that and tuning it correctly (the exhaust diameter comes into play as well), is Newey territory I guess.

I would google my ass off to find out how other designers have done it, with similar engines/rpm and try to work out their thoughts about their solution.

[Fifty]

Thanks Joelle.Yea. I have been searching and searching. So far the majority of cars that have horizontal pointing exhaust, I've talked to their builders and they have stated that they wanted the extra gas and velocity from the exhaust. But the reason they want horizontal was purely a matter of it was easier...

The fastest and unlimited time attack car in the us just built theirs. Changed from side exit to diffuser exit. When I asked him as he was building thediffuser and the car was on the rack, he also said the "it's the easiest" way...

So I guess my question is, will directing the exhaust gasses have enough of an "every little bit helps" effect. If it's a matter of increasing 1-2% then no. But 5% and up then yes.

My specific car, the greater the possibility I can run with out a wing the better.

[mrluke]

Thinking back to the "coanda" exhausts we had a few years ago I don't think the angle of the exhaust gas really matters as the under car air flow will be far far stronger.

[mep]

Hello,
Did you see this page:
http://www.formula1-dictionary.net/diffuser_blown.html

There is a quite good picture of how you can do it:


Basically, I say it is ok when you make it point rearwards. The exhaust does not need to follow tangential into the diffuser surface. In a diffuser with high expansion angle, the flow might not perfectly follow the surface of the diffuser anyways. There can be big vortices spinning up. The exhaust should bring additional energy into the flow field. However, what you see from the picture is that it seems to be important that the exhaust exits close to the kick line of the diffuser. I think you should copy that.

To get a perfect answer you should do some CFD iterations. If you don’t have these opportunities then just build it and test it. If it can work in principle than you should be able to get it working without being to scientifically. However, the question is if your engine is able to generate enough flow to get a high exit speed. Something you could calculate by hand with your displacement, rpm`s and exit diameter.

[Fifty]

Thank you very much for the photo and the opinion!

One of the goals of this car is a land speed type run. Not super fast in the ways of the world.

I was warned (by a bunch of scta guys) not to do a side exit, as was the original plan, as the car at the speed it will be going, will be pushed sideways by the exhaust.

Close to a thousand hp, so 250 per puff (lol, I know that's not how it works but it sounds funny)
I just used the Donaldson formula and did some guestimation... assuming that I'm running a wee bit hot egt, and get max chp at high rpm at high gear toward the end of the run, 7800 cfm coming out one pipe. So 130 cubic feet a second...
That makes me think that the amount of air coming out of the exhaust should do something and I'd like it to do something for me!

[Jolle]

I assume this car you're building isn't restricted too much, couldn't you use the exhaust somewhere to improve drag? Like on the trailing edge of the side of the car or the top of the rear hood? A blown diffuser works best when you don't need it in most cases, this way you can use the energy as efficient as possible

[Fifty]

Any where. But if the exhaust is in front of driver I will need an O2 machine.

But yeah. Really anything I want to come up with as long as it passes safety.

[mzivtins]

Hey Fifty, is having the exhaust in/at the diffuser the only way the setup of the car will allow?

I agree with the picture above, good and relatively the most simple way about Could you please post some pictures of your setup/car because im a nosey git and it sounds awesome!

I noticed you mentioned side skirts, moving the exhaust further towards the centre of the car would increase the exhaust blown effects, because you're running skirts they can 'seal' the floor gases and pressures away from the outside
It is kind of the same as those winglets we see on aircraft, they reduce spill-over on the wingtips, reducing drag as keeping airflow going the right way over the wing:


If you cant move the exhaust, your skirts are going to be a great way of maintaining good quality air under the car without introducing much drag

[Fifty]

Mzivtins-

If you click the link on my signature, it will take you to the Facebook page for this car. Feel free to message me there as to what you want to see and I'll post it.
Posting crap on Photobucket and then linking it is a pain in the butt.
Sometimes photobucket freezes up.

Ritaedine- as expensive as that sounds. I'm not running a hemi. I'm sticking with an EJ for this build...

Too all the others that chipped in, I think I'm going to build the flatbottom as low as I can, so that 1) it will give me extra space to hide the suspension inside of the floor...2) the extra space allowing for a longer more gentler curve of the diffuser. I'll be able to start it "hopefully" in front of the rear diff.
This will help
Bring the suck closer to the middle of the car, and I'll have the exhaust exit at the bend, so I won't have to bend it around the rear diff causing head aches and heat.

It's a few weeks to a month and a half out until I'll start building the floor but the exhaust should be started in 2 weeks or so.

[flynfrog]

why are you trying to add DF to a landspeed car in the first place. I would be more worried about your CP and CG to start with. Aero drag is you biggest enemy. That is why you see ballast and water tanks in LS cars not wings and diffusers. Are you planning to run on the salt? I try to make it but missed the last few years.

[Fifty]

No, air strip attack. Standing mile and then hopefully the magnum. No salt though.

Lots and lots of thought are put into the center of pressure and rotation.

I need that rear lower drag aero for a few reasons. One is I don't have 5 miles to accelerate and a lot of spins at the 110 mph shift and then also on the braking point due to lack of rear grip.

Weight is a double edged sword on airstrip attack. Yes you want weight for grip and stability, but you need lightness for acceleration. It's a double edged sword.
I've got a lot of weight in my over built cage. It's a NASA rally cage with some scta added to it. (It's about 90% done, but it ain't light.)

Also for DF, really what I'm doing is subtracting lift. So the flat bottom will be instituted and for a bit of free DF with out a bunch of drag and to assist moving the air spill behind the rear window as far away as possible, the rear diffuser can only add help. Not subtract.

For the top of the car, im doing nothing more than VG's pushing to a gurney flap on the factory "wing" to also extend the airs idea of the end of the car as much as possible to reduce that parachute drag effect.

There is tons to it. I just need to gather as much info I can on the rear diffuser to make it as efficient as my poor fab skills can.

[roon]

Can you post a copy of the ruleset you need to conform to? Might help with suggestions.