Rear Diffusor, Anyone ever think of doing this? Options

[TonyAKAVW]

While I was thinking about better ways to vent heated air from the radiator on a subaru conversion, I began thinking of adding a tray under the engine bay area to help isolate the bottom of the car from the top. This would be for the same purpose as the engine tin with a stock engine.

Anyway, when I got to thinking about having a tray under the engine bay I thought "why not extend it all the way back?" The rear valence on 914s traps air, why not put a rear diffuser there like the Lotus Elise has? If you have a continuous underbelly to the car that goes from front to back, it seems that this might work.

One potential source of problems is the exhaust heat from the headers, the pipes that go to the muffler, and the muffler itself. It seems like there might be a way to vent out that heat, maybe some grille material on the rear bumper? Bring in fresh air through the engine bay lid?

-Tony

[brant]

A few colorado 914 race cars have used diffusers at different times.

a diffuser is actually supposed to provide downforce by the effects of high/low pressure. I think your talking more about ducting heat than for aero advantage. Most of what is done in this area is actually for aero purposes more than heat ducting.

we are making a "tiny" diffuser right now for our car.... but its not really a vintage approved item so it may only be used at pca events.

Many 914 race cars also run venting on their rear bumpers. The most common style is to just cut away all of the fiberglass bumper and chassis, leaving only the fiberglass "top rubber" attatched to the chassis

but I know of one car with a real diffuser and real side skirts made out of landscape edging (black plastic)
testing on an oval track showed 10 or so MPH gain

rear bumper venting (or cutting away) is more done to just plain reduce aero drag.

the chassis pannel that the rear bumper mounts too is a huge source of drag. It is very flat up and down, and catches air like a huge sail.

Also... its not really a diffuser, but check out Patrick motorsports website for an aerodynamic flat bottom that they put onto a vintage car. They did it for aero advantage, but its similar to what your describing for heat purposes.

brant

[john rogers]

Check Jim Patrick's web site and check his silver/blue roadster and the full aluminum pan under and around the engine. If you can see the car in person from the side, you will notice it is shaped as a nozzle to lower pressure under the rear of the car to help with down force and as fast as that car was I imagine it works.

[TonyAKAVW]

Like this?

[Brando]

Tony, the diffuser idea you are having (similar to 964s and 993s, 944 S/S2s and 968s, 986s) will only help if it is ærodynamically correct. The effect you want is while driving the air is 'sucked' out from under the car and out the back. Since the subaru motor does not have a huge heat-pump pulling air from above the car, over the engine, and out under the car you may encounter some difficulty. The result you want is a low-pressure zone below the car and a high-pressure zone above the car. Airfolils and Airflow.

Take a look at the plastic/fiberglass piece underneath 964s and 993s. You'll see that it is convexed a bit with some holes and wierd angles in places. Granted it doesn't work in stop-and-go traffic but above 20mph it actually pulls air from the engine bay out and under the car -- due to the car's ærodynamics and the cooling fan forcing air out the bottom of the car.

[TonyAKAVW]

Brando: My intention for having this diffuser is primarily to pull heat out from the radiator. I am going to have my radiator in the engine bay and will be expelling heat out to the bottom of the car via a 20 by 5 inch wide opening. I really do want the bottom of the car to have lower pressure than the top to help in pulling air through. The secondary effect is the downforce added at high speed.

I am concerned about coming up with a design that really works, and since I don't have a wind tunnel or simulation software I'm going to have to hack it, hopefully with some insight...


So in the picture below I want the air to go into the radiator (blue lines) from the top (thats a whole other issue) under the car and out (red lines) and avoid collecting under the trunk and around the engine. It also keeps the hot air from being sucked back into the radiator. At speed, if this works as I hope, it will aid in increasing the pressure differential from the top to bottom of the car.

-Tony

[Aaron Cox]

uh... how are you getting air to your radiator tony????

i thought the engine bay radiator idea had a fatty scoop underneath?
-hard to scoop air from underneath, and return it under the car (IMG:http://www.914world.com/bbs2/html/emoticons/huh.gif)

[Crazyhippy]

The aero dynamics of the car are conspiring against you...

The rear window being recessed creates a nice vortex (low pressure area) behind the drivers compartment.

with the top off, this low pressure area @ 155-160mph is so strong thet the back tires will start to spin if you try accelerating (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/dry.gif) Obviously Porsche hadn't planned on the 914 trying to accelerate @ 160MPH (IMG:http://www.914world.com/bbs2/html/emoticons/chairfall.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/happy11.gif)

Try pulling air from under the car and exhausting it out the top. Think of it as a hovercraft in reverse (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

BJH

[VegasRacer]

I think it will work - if you plan on only driving in reverse.

QUOTE

I want the air to go into the radiator (blue lines) from the top (thats a whole other issue)

Do you have some super secret plan on how to force the air in from the top?

[Aaron Cox]

QUOTE (VegasRacer @ Dec 10 2005, 11:13 PM)

I think it will work - if you plan on only driving in reverse. QUOTE I want the air to go into the radiator (blue lines) from the top (thats a whole other issue) Do you have some super secret plan on how to force the air in from the top?

(IMG:http://motorlegend.free.fr/fiatouphile/X19.JPG)

[TonyAKAVW]

There was a long thread discussing the issue of whether the top of the car is higher pressure than below in the case of a non-stock engine. No REAL conclusion came out of it, because there isn't a lot of evidence for which is actually the case. Granted it seems like there's no air moving downwards, it may be higiher than below... Especially when I put a front air dam on and lower it a bit.

The problem with the scoop is scraping speed bumps. I'm worried that a scoop below the car will give me issues, especially if I lower the car as I intend to.

Something I saw on a car at the German Autofest a couple years ago was a stainless steel air deflector bolted to the rear lip of the targa top. It was maybe 1/4 to 1/2 inch above the roof and then abruptly angled down. It was not obnoxious looking and I'm sure at high speed it provides a lot of pressure in the back-of-the-window area.

So that could work for driving with the roof on... With the roof off I'm not sure.

I guess the issue is whether a diffuser would provide much suction at high speeds. If it does, then maybe ites enough to pull air from the top.

-Tony

[Goge]

I just wanted to add one comment, I don't think the type of engine plays any role in pumping air from above to under. Granted, the TIV has a turbine-style air-pump that pulls in air above the motor, and happens to blow it out underneath. If you flipped it over and had exhaust pipes coming out the top of the trunk, sucking air from below... it would still cool itself I'm sure. I don't think the volume of air is really enough to effect the net pressure difference above/below the car. The key is to keep your cooling flow working WITH the the natural pressures created by the body design.

Just my $.02... the TIV is a heat pump, but not a jet engine.

Also, from what I remember... air at 0 velocity creates a higher pressure than air moving fast. Isn't that how pitot tubes work??? Bernoulli knows the answer, even if I can't remember all of his equation...

-Todd

[dmenche914]

Agree reverse the air flow,. At speed, the air should flow up. The radiator needs to be in the air streem, more or less perpendicular to the flow for best cooling. A powerful electric fan is needed also for slow driving.

Consider mounting radiator(s) on engine lid, make lid all screen rather than half and half. Two small radiators might fit best.

If in stop and go slow driving the car over heats from sucking the cooling air past the headers, then make the fans reversable, so when car is barely moving (thus car aerodynamics not important) the fan blows down, fed by cool topside air. when car starts to speed up, reverse fan to assist with cooling from the bottom up.
A short really wide scoop could be fabricated to help get intake air from infront of the engine and the hot headers. (useful at speed) Mount it under car infront of motor, extend it down to as low ground clearance you can. Could just be a metal sheet cut and bent. extend it up into the engine bay as much as possible infront of the motor. It would be more of a directional vane, than a real scoop.

I'd look at getting the radiators up top under the lid, on either side of the engine, in the air flow

[dmenche914]

PS there is some eviedence that the air does flow up naturally at speed in a modified 914(ie non-sealed enginebay). My V-6 non sealed 914 blows hot air into the cabin with the targatop off, never have that issue with the stock 914, I suspect its cuase lots of hot air from the headers and engine is being sucked up from below and out the engine bay top (and above the rollbar, warming the cabin) I guess a simple test with some strings and tape and a freeway drive could confirm the air flow.

[dlo914]

hmm i got bored and brewed up 3 ideas for a cooling solution: (IMG:http://www.914world.com/bbs2/html/emoticons/smile.gif)

[TonyAKAVW]

QUOTE

I just wanted to add one comment, I don't think the type of engine plays any role in pumping air from above to under.

I agree that the volume of air moved by the motor under speed is negligible compared to the volume of air moving around the car due to simple aerodynamics. Any downforce created by a rear diffuser will be dominated by speed related airflow not engine generated airflow.

QUOTE

Also, from what I remember... air at 0 velocity creates a higher pressure than air moving fast. Isn't that how pitot tubes work??? Bernoulli knows the answer, even if I can't remember all of his equation...

This is exactly what I'm thinking of... If I can get the air under the car sped up by using a rear diffuser, I should be able to create a reasonable suction through the radiator exhaust opening.

QUOTE

Agree reverse the air flow,. At speed, the air should flow up. The radiator needs to be in the air streem, more or less perpendicular to the flow for best cooling. A powerful electric fan is needed also for slow driving.

Scott Thacher has his airflow reversed from what I'm discussing and has put thousands of miles on his car with no overheating problems. He uses the under-car scoop, and seems to have had good results. So I know that method works. I'm interested in finding out what can be achieved by going from the top down. The default design however would be to go from the bottom up with a scoop.

QUOTE

I guess a simple test with some strings and tape and a freeway drive could confirm the air flow.

I'm thinking a driving test with a fog machine in the engine bay would be interesting...

QUOTE

hmm i got bored and brewed up 3 ideas for a cooling solution:

I have thought about doing scoops in those places and I really can't think of a way to make a scoop look like it fits on a 914. The rocker panel scoops are interesting, but plumbing the air to the radiator becomes really difficult.

Lastly, as far as mounting the radiators on the engine grill, this is difficult for a few reasons, and cumbersome when you need to get into the engine. Also it moves the weight up higher, and I think strange things might happen to radiators when they are used in that orientation, but I don't know.


-Tony

[spare time toys]

QUOTE (TonyAKAVW @ Dec 11 2005, 04:19 AM)

I think strange things might happen to radiators when they are used in that orientation, but I don't know. -Tony

Some one here had the radiator mounted flat in the fron trunk so they didnt lose trunk space. I dont remember who it was but I dont remember troubles with it either.
-Larry

[jd74914]

QUOTE (spare time toys @ Dec 11 2005, 09:57 AM)

QUOTE (TonyAKAVW @ Dec 11 2005, 04:19 AM) I think strange things might happen to radiators when they are used in that orientation, but I don't know. -Tony Some one here had the radiator mounted flat in the fron trunk so they didnt lose trunk space. I dont remember who it was but I dont remember troubles with it either. -Larry

Sure its not the MSDS intercooler?

[brant]

Tony,

I could be wrong.. I'm certainly no aerodynamacist....
but I don't think a real diffuser will draw more air through anything.
A diffuser is supposed to apply some type of force (downforce) to the chassis.
by applying force I don't think it draws more air
just like a spoiler on top of your rear trunk, wouldn't necessarily draw more air over your trunklid.... it would just direct the air that is already there.

so a diffuser at the rear of the car may not draw more air under the car.
its a tunneled downforce device, such as the old IMSA GTP's and Indy cars used.

The factory figured out how to draw more air down through the engine compartment in the form of the 2 little flaps AHEAD of the engine compartment area. These create a wake or turbulance that creates a low pressure to help facilitate the pull of the air.

Unless you had the rear fenders and sides of the car completely sealed to the ground... I really don't think a rear diffuser is going to effect air flow as far forward as the engine compartment.

brant

[Trekkor]

Anybody got a windtunnel? (IMG:http://www.914world.com/bbs2/html/emoticons/rolleyes.gif)


KT