kos11-12

Hi guys ,

I know this subject is been up a few time....
Well I am looking at another car project at the moment , witch make me do some research on aero underneath the car, I realize how importante it was to cover the rear area of our cars, specialy when I understand that the back bumper become a parachute....
I am thinking making one up in aluminium ....My exhaust is cup and bypass with the right silencer,
I was wondering if anyone had done one , pictures will be great ,

Thanks

Konstantin

springer3

What is the problem with the factory plastic cover? Is this for weight reduction? Most everyone with a 964 at the track has the ultimate in light-weight, which is to remove the cover altogether.

There is also a common perception that the undertray traps hot air around the engine, and some are convinced that early valve-guide wear is caused by the undertray. Does your aero research have any chance of proving or disproving this perception?

The Porsche literature says the underdray it there to improve aerodynamics. The factory engineers are competent, and I am sure aerodynamics to them included improved air flow through the cooling circuit. Any thoughts?

Marc Shaw

I removed mine (and the whole silver box around the underside of the engine) for weight savings.

I have a Fabspeed exhaust that runs across under the rear bumper so no parachute effect of the lower edge of the rear bumper catching the air flow for me.

Marc

kos11-12

Hi guys ,

I know our cars are not exactly competition cars.... But considering what has been done, to increase down force from underneath cars, I was thinking at least covering that area,
If the engine overheats at track or hot climat, then maybe fitting a secondary oil cooler is a wise option,
Obviously it is irelevant at low speed in traffic but I would think at hight speed or fast bends it could make a difference on handling....
What do you think ?!

I both my car without the under tray , previous owner removed it !

Johnny G Pipe

I personally am pro-tray whilst respecting those who exercise their choice...and so a good advantage of aluminium over the GRP original, would be weight loss whilst maintaining the aero advantages (that Porsche engineers spent long hours wondering about and designing whilst conceiving typ 964..)
But really, lets not go there again! It would look really trick too. Perhaps you could drill it or louvre it...maybe Rennline could fab up a nice pretty alloy undertray, leaving a gap to jack up the car on the engine case...

Goughary

the rennline tray idea isnt a bad idea - they could build in perhaps some convection cooling...

ALEX P

Prepfab make an ally underbody guard for their rally cars, I'll try to dig out a photo of it. I guess it's the same principle.

http://www.ekmpowershop2.com/ekmps/s...uard-232-p.asp

I understand what you mean with the parachute affect & even if flat floor that you're thinking of didn't cover the entire engine then it would surely still be beneficial if it was just behind to aide the airflow back out & under rather the up (if that makes sense).

I always thought that the standard 964 undertray was only there to catch the oil leaking out of the engine - They should have fitted it with a drain plug!!

kos11-12

Did not see a picture on their web site, seams interesting, please let me know what they are saying,
That would be good to have one that covers properly the hole area, one witch is design to fit aftermarket exhaust systems, it could have a few Naca ducts for cooling , also a good idea would be to drill holes on the rear bumper to realise any traped hot air....like on the old Ruf ...

Wonder what those square holes are for on the original under tray....?

gazzy

Where will the dripping oil go?

kos11-12

where does it go with the original under tray ?

ZG862

It fills the tray in order to give you something to think about when you take the tray off.

Cheeksyboy

Is it worth considering incorporating a venturi into any redesigned undertray?

This doesn't have to mean an increase in downforce, so much as a smoothing of the airflow to reduce drag!?

Though I think this would need to be countered with an improvement in the front aero if downforce was the intention?

Now where's Dr Alex (alexjc4) when you need him?

alexjc4

They look like they are there to let the air, that has been blown through the cylinder cooling vanes by the engine fan, out. I'd think they are fairly important to the cooling system. Of course they just be there to allow the oil leaks to drain. . .

alexjc4

Present!


I'll need to check my aerodynamics chapter of my new vehicle dynamics text book but from memory what you really want is a diffuser, I think you get less drag and more downforce thatway. BUT basically the engine and rear silencer get right in the way of where you want to have the air expanding. If you were proper hardcore you could go with a "long tail" like a le mans car that way you'd have plenty of room, might make parking interesting though. Normal saloon cars sometimes have enough room under the boot to have a diffuser retro fitted sometimes you just cut the boot floor out, lift the fuel tank into the boot, I think nascars do this.

kos11-12

Well Yes a diffuser... I did not want to mention it , as we are are digging far there ....
It a science on his own....
Being reading lots of info about undeneath downforce, so I have a basic idea to what needs to be done ,I think there is space as it needs to be between 7 to 9 deg maybe someone could do a aero simulation.....!?