986 aero & cooling mods

[woodsman]

Originally Posted by j.fro

I have no quantitative data, but I believe that my spoiler configuration creates a low pressure area that helps pull air/heat through the vents in the bumper cover. The back end of the car certainly gets dirty quickly!

I think you have created a parachute with a slow vent at top and a huge back draft- none of which you want. It won't take effect until around 60 mph though. Be careful cornering at the limit above that.

[woodsman]

Originally Posted by The Radium King

I've thought about the 'parachute' effect when the oem muffler was replaced with a smaller aftermarket muffler, and how you could vent the rear bumper cover to avoid it. also thought about adding something like this that would run from the transmission pan to the rear bumper cover (trim the cover so that the diffuser came just under the licence plate) - one on each side of the exhaust outlet:

Universal Rear Diffuser Kit Under Body Spoiler Increased Down Force Reduce Drag | eBay

That looks like a good price for what it is but I doubt it will have enough positive effect to offset it's weight (if you care). Seems perfect for a civic or BRZ. I've thought about splitting the exhaust as well and building a deep diffuser up to the height of the bumper but really don't understand how they work- it's got to earn it's keep on my car. I've wondered if I could trade a diffuser for a wing on a track car to decrease overall drag while sucking the rear down...Porsche did real aero and anytime we let airflow in somewhere
we risk adding drag and/or increasing lift.

[jaykay]

Originally Posted by Dlirium

I really want to hate this, but it is WAY too cool!

Yep gotcha....I am a bit of purist when it comes body kit mods .....but this is even has me wavering a bit

[njbray]

I figure that Porsche must have considered the design of the rear bumper and found any alterations unnecessary - modern cars have to be designed with max mpg in mind so if there were any 'parachute' effects from the rear, they would have addressed them.
At speed I expect the air just flows by - kind of like an upside down pick up - where it is much better to leave the tailgate up!

[The Radium King]

Originally Posted by woodsman

Seems perfect for a civic or BRZ.

or a boxster spyder:



or a 918:

[woodsman]

With respect to the rear diffuser, Porsche forgot the 'form follows function' rule on the Spyder- but not the 918.

[JAAY]

I will say that I noticed something a couple years ago at brbs. I was parked next to a new spyder and my aerokit 1 rear spoiler was pretty much exactly the same slope and shape as a new spyder. Quite interesting when I noticed it. Not a great comparison but it was cool to see in person.

[tonichristi]

Originally Posted by woodsman

Porsche forgot the 'form follows function' rule on the Spyder- but not the 918.

Are you some kind of expert in aerodynamics, rear diffusing and aesthetic design? More so than Porsche? I'm sure Porsche would love to hear from you. Just shut up, already.

[j.fro]

Alright, moving to the front of the car I've replaced the destroyed front wheel air dams with ones similar to those in the rear. As well, I've added a small air dam to the middle of the underside. Both sets direct air towards the GT3 brake ducts.


Another bit, while we are under the nose, is that I've closed off the factory air exits for the center radiator. Air is now forced to exit through the vents cut in front of the hood. Theoretically, this should produce some downforce. The less air there is moving under the car, the better off it is, right?

[jaykay]

You might consider front fender liner vents:

Flow Through Front Fender Liners

[j.fro]

Originally Posted by jaykay

You might consider front fender liner vents:

Flow Through Front Fender Liners

Got 'em

[JayG]

Originally Posted by j.fro

Another bit, while we are under the nose, is that I've closed off the factory air exits for the center radiator. Air is now forced to exit through the vents cut in front of the hood. Theoretically, this should produce some downforce. The less air there is moving under the car, the better off it is, right?

are you an Aeronautical Engineer?

I guess theoretically, you could produce some down-force, but without putting it in a wind tunnel there is no way to actually know what is happening. you are likely to cause other things like drag.

Yes, the GT3 has them. Porsche did aerodynamic design and testing to calculate and design exactly what was needed.

Without that design and testing, who knows what is happening. You could actually cause more lift on the from end by disrupting the aerodynamics.

its kind of like using a butt dyno to calculate HP increase

I'm not trying to pick on you. IIRC, you were asking about making modifications to the spoiler as well to increase MPG. Its just without advanced computer modeling and actual wind tunnel testing ANY modification to the aerodynamics of the car as designed could cause unwanted properties and have a negative effect on handling and efficiency

[The Radium King]

well, I went to school for such engineering, but even an application of "where does the air go?" works here. quickly, where does the air go from the side radiators? down. where does the air go from the centre radiator? down. we are taking a whole mess of air hitting the front of our car at, say, 100 mph, and deflecting it downwards. think harrier jet. what effect does this have on the driving experience? understeer. from a litigation point of view (ie, after their experience with the 930 and having to remove it from the us market) would Porsche rather have understeer or oversteer? honestly, you want as little air under your car as possible - this is why cars have front spoilers and side skirts. rear diffusers are there to turn what little air does get under the car into downforce.

now then, what does Porsche to with the gt3, gt3rs, gt2? side radiators vent into the wheel wells or out the side of the bumper cover. centre radiator vents upwards. why on these porsches and not all porsches? one is an entry level street car for the unwashed masses, the other is a dedicated track weapon.

I think the fro's comment about the airflow path on his centre radiator is regarding the path the air follows from the outflow of the radiator to the top vent. the gt3rs and gt2 use a duct that requires a bunch of other changes to make it work (spacing out the front bumper, revised side rad ducts to accommodate the fact that the centre rad is moved forward, etc.); without this the flow is a bit more convoluted. however, this would only affect cooling and not aero; less flow through the centre rad is better than a lot of downward flow through the centre rad.

[jaykay]

Originally Posted by j.fro

Got 'em

Nice...less hovercraft action! That is a quick and effective way of doing it!

...eventually you could go to mesh and seal the under side...but there has to be the correct amount of the cooling airflow through the liner.

[j.fro]

I'm not an aeronautical engineer, but I was hoping to hear from some. (Thanks, TRK!) My purpose with this thread was to generate some discussion, share what I've done, and see what others are doing. Although I keep insurance, tags, and state inspection on my 986, it's not exactly a street car. It's built to compete in SCCA F Prepared and PCA Improved autocross classes. Winning and losing often comes down to hundredths of a second, so I'm game to try things out to see what works. In a way, it's my hobbyist racer test lab, so if someone can make suggestions, improvements, or point out fatal flaws, I want to know about it.

By the way, I have moved the center radiator out towards the front of the car, and I added ducting to channel air through it and upwards. I don't have data on the aero, but when the car gets moving, the temps drop fast. On hots days at autocross, that's important.

[KRAM36]

Originally Posted by j.fro

I'm not an aeronautical engineer, but I was hoping to hear from some. (Thanks, TRK!) My purpose with this thread was to generate some discussion, share what I've done, and see what others are doing. Although I keep insurance, tags, and state inspection on my 986, it's not exactly a street car. It's built to compete in SCCA F Prepared and PCA Improved autocross classes. Winning and losing often comes down to hundredths of a second, so I'm game to try things out to see what works. In a way, it's my hobbyist racer test lab, so if someone can make suggestions, improvements, or point out fatal flaws, I want to know about it.

By the way, I have moved the center radiator out towards the front of the car, and I added ducting to channel air through it and upwards. I don't have data on the aero, but when the car gets moving, the temps drop fast. On hots days at autocross, that's important.

You are hitting it hard! That sounds sweet. When I finally pick out a front bumper for my car, I'm going to look at doing this. I like the sound of a cooler running car and ducting the air. Very nice!

[woodsman]

Originally Posted by jaykay

Nice...less hovercraft action! That is a quick and effective way of doing it!

...eventually you could go to mesh and seal the under side...but there has to be the correct amount of the cooling airflow through the liner.

I agree with the theory about venting the center rad upwards but how does introducing 10X the air volume into the front wheel- well reduce lift? With aero less is more- I think but than it depends on intended velocity. Auto makers spend days in the wind tunnel to make our cars 'work' with as little drag as possible. For instance, why increase flow through the rad by 500% ( my guess on JFRO'S liner) AND THEREFOR DRAG when blocking the bottom and venting the liner with 2 4" .25" slits would probably suffice? The key concern is ability to cool while creating the least amount of drag-right? At 60 mph we're hitting a cement-like wall of air and aero is gently persuading. And ya let's discuss- it's a FORUM so there's no need to get pissy or all-knowing.

[The Radium King]

996 gt2, 996 tt and 997 all vent into the fender. if you look behind your rad and in front of your fender liner there isn't actually that much of an opening for the air to flow into the fender given the orientation of the rad.

[jaykay]

Originally Posted by The Radium King

996 gt2, 996 tt and 997 all vent into the fender. if you look behind your rad and in front of your fender liner there isn't actually that much of an opening for the air to flow into the fender given the orientation of the rad.

Yes...and remember in the thread I started on liner vents and how to exhaust into the wheel well and when I said okay how about slots/lovers facing outboard? Had a look at 981 the other day and guess what I saw?

[jaykay]

Originally Posted by woodsman

I agree with the theory about venting the center rad upwards but how does introducing 10X the air volume into the front wheel- well reduce lift? With aero less is more- I think but than it depends on intended velocity. Auto makers spend days in the wind tunnel to make our cars 'work' with as little drag as possible. For instance, why increase flow through the rad by 500% ( my guess on JFRO'S liner) AND THEREFOR DRAG when blocking the bottom and venting the liner with 2 4" .25" slits would probably suffice? The key concern is ability to cool while creating the least amount of drag-right? At 60 mph we're hitting a cement-like wall of air and aero is gently persuading. And ya let's discuss- it's a FORUM so there's no need to get pissy or all-knowing.

Any "intake" events interacting with the main airstream will almost certainly increase drag. Let's not get into exhaust vents and blown surfaces etc. The front rad vents we are talking about remain as they are; they will cause drag.

This drag will vary with airspeed and perhaps a little with exhaust/venting; airflow through the radiators. The flow path is so convoluted and restricted that would guess very little effect. You need a wind tunnel to know. As a side note I believe I recall a Mustang (aircraft not car) cooling vent that actually produced thrust and hearing about open car windows giving very little drag due "filling" of the cabin at speed...thereby giving the effect of a "closed" window....dunno without testing.

In this case there no aim to reduce drag. We are considering aero and cooling. Consider that improved cooling might achieved by venting the liner and improved aerodynamic performance by closing the downward rad vent exhaust. How so you might ask....well it is felt by some that this is done to reduce front end lift at speed (hence the hovercraft reference). And yes optimizing cooling may hurt aero.

Now does lift occur in the wheel well? Perhaps there is just enough escape path areas ...