REAR DIFFUSER

[318isturbo]

HI ALL IVE POSTED THIS IN THE STYLING SECTION ASWELL
im looking at putting a rear diffuser on my e30 for the track use but cant find one no where like f1 or touring car style proper channeled air under the car can anyone help me out at all

[harry_p]

i think i've only ever seen a couple on extremely modified race cars.

as a minumum, to get any decent angle on it you'll have to chop out the spare wheel well and cut a section out of the valance which would need to be boxed, then you should be able to get a decent run from the rear beam back. but the exhaust will get in the way too, so you'll either need to accommodate it, or move it out of the way. side exit is neatest but you'll struggle to meet track noise regs.

if you havent already, take a look at some of the cars in the german berg cup series, there are some crazy modified old school cars there, and i'm sure quite a lot of the ideas can be adapted.

don't foget it's all about balance, if you make something which is going to be functional you'll have to do something at the front to add downforce and at the sides to stop outside air coming under and spoiling the ground effect.

[318isturbo]

yeah ive got a guy at work who is the nuts at cutting and welding so that aint a problem just finding a diffuser that looks wright for the car and the front air ducts on front bumper are getting made up
thanks harry_p

[cee]

A rear diffuser will do very little unless you modify your entire under body. The purpose of a diffuser is to allow air to escape from under the car faster, thus lowering pressure and creating ground effect (sucking the car into the tarmac). However, with the underside of the car still being rough (metal parts on show such as exhaust, suspension components etc.), air will get trapped and create high pressure zones, negating any benefit you've gained from the diffuser.

Another thing to consider would be lower side skirts to prevent leakage, and a splitter of sorts to channel the air under the car in an attempt to achieve laminar flow.

[e301988325i]

You can't run a road car low enough to make it work, if it's seriously a track only weapon then I'd suggest the motorsport/trackday forum

http://www.e30zone.net/modules.php?name ... wforum&f=4

[318isturbo]

thanks cee im getting a front bit of kit made up plus getting a undertray sump protector with air channels on it aint got side skirts yet but was looking to see if a rear diffuser is possible?

[cee]

This is the kinda thing you should be aiming for;

And as e301988325i said, height is an issue. I can't remember the exact figures from when we covered race car dynamics at uni, but the car has to be prety damn low to make any of this work.

[cee]

You're better off concentrating on downforce generating devices such as wings, splitters etc.

[podiumprep]

Over the winter i will be looking to fit a full flat floor and diffuser to our E30, it already runs a large splitter and front section. To make full use of a diffuser it does really need the flat floor and the skirts to feed it. You might feel some diffrence on its own but not anything to write home about (more of a mind thing).

What do you use the car for? Trackdays? Racing? Sprinting?

[cee]

Take a look at some of the old DTM e30s

[harry_p]

dtm e30s had to use road homologated bodywork. there were the odd leftover cars in thehands of smaller teams which carried on developement after the official teams had pulled out, going into the wild bodywork of the mid 90s dtm cars.

worth noting that the underside of the m3 rear bumper has a slight upwards slant, and there are plastic trim panels to box in and smooth out the spare wheel well area.

[318isturbo]

thanks cee i think what im going to do is alot of front end work and rear bootlid spoiler from dtm but the under diffuser was gonna be like a 5% help wasnt expecting loads of downforce aint comp raceing still a road car just the turbo in it makes it arse happy and want to try plant it in the floor been looking at some toyo r888 and coilover set up
polybushes and rollcage to stiffen it up more rigid

[cee]

For a road car a standard Tech2 spoiler should suffice surely? - Citation needed!

[318isturbo]

was thinking more angled to push the rear of the car down

[cee]

It's not just about angles etc, it's a very complex art to get it right to provide optimum force with minimum drag. You also want to be reducing turbulence as much as possible to keep the pressure behind the car as high as possible, to effectively 'push' the car forwards. Turbulant flow behind the car caused by poor wing design creates a low pressure zone which will 'suck' the car backwards (although obviously it wouldn't actually pull the car backwards, but create a backwards force that fights against the engine and slows you down)