Aero...

Working on the aerodynamics for a project and thought I'd share with the 308/328 owners out there. The aero on a 3x8 is not very good, at high velocities the chassis gets a bit light and the rear is a jumbly mess of drag.

I built a quick mockup out of some most excellent used cardboard and masking tape, perfect if one is not going to drive around in the rain

It allows for a quick flesh out and then I can 'digitize' it into CAD run some CFD and optimize as needed.

the 308 and 328 while similar are different enough in the rear construction that the 308 actually lends itself to a rear diffuser with more ease then the later 328. The attached pic is the diffuser for the 328, the 308 version will be along in a couple months.

It may be odd to go thru this on a chassis of this vintage, however with the increase in requests for updating the chassis with double the power or more along with handling and looks this simply follows the natural progression. It's not for everyone and the goal as always with these projects is to make all updates and upgrades to look as though it would have left the factory this way if it was either an option at the time or they had the technology to do so.

A proper rear diffuser will increase down force and rejoin the air slipstream back together with less drag. There is a lot of technical details this entails and is not the purpose of this thread so I'll skip that at this time.

For the diffuser to work there will also be another under body panel that will cover the engine bay and meet up with the lower panel as it stops just aft of the seats.

Enjoy
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interesting question.. Off hand I don't think it'll cause problems as the air slip stream from under the vehicle will be directed and slowed to mach the air stream above and thus increase the efficiency of the air moving past the rear. It could actually help with extraction at speed, there may need to be some inlets for air exchange. As the model sits now the center section is fairly open, more so then needed but for a turbo engine it may be exactly what is needed.

 

I'm going to assume you have a late 328, '89? the end run 328's shared the mondial & TR style of suspension which really helped with stability and vehicle dynamics at speed, esp weight shift. 308's QV's and early 328's were softer and have a fair amount of 'body' roll. Front end lightness is really apparent in the 308's as the front end is very akin to the leading edge of a wing.

I'll be running CFD analysis in the coming weeks to see what can be gained with the design and full body under tray. As I mentioned, this will also include the addition of a full body under tray panel that picks up where the factory left off. There is a significant area that is open and does nothing more then disrupt the air stream under the chassis as it is now.

This project was also born out of the need to direct and control the air flow under the chassis as I'm adding two fan driven heat exchangers for the water to air laminova inter-cooler manifolds that will be behind the wheel wells in the rear.

Of all the things I find bothersome with the 3x8 chassis the fuel tanks are the worst and at the top of my list of needing to be tossed over board and placed anywhere else. The change of location when they moved to the 348 was best. The Mondial has the benefit of a longer chassis and rear seats to make the tank location work as well. A tank 5"x48"x24" is a hair over 20 gallons, something like that would fit right behind the seats in a GTB perfectly. Of course the chassis would need some 'modification' but the gains would be huge. However such things are extreme modifications and well outside the scope of this topic.

 

CF can be utilized however visually it would have to be painted or gel coated so that the weave was no longer visible or barely there. CF use in motor sports dates back to the 70's so it wouldn't really be an odd option, just a cost based one at the time.

 

They are essentially saddle tanks and require a balance tube and a pile of plumbing for venting etc.. to insure both drain reasonably at the same rate. Condensing two saddle tanks into a single tank that sits in the center of the chassis would improve roll and yaw control. No more saddle tanks would also free up a bunch of space that can be utilized for lots of things , if nothing more then working space in the engine bay.

Front header would not be an issue in terms of heat as such a modification would require modifying the chassis with a new wall and new tank. Not something that I would recommend for a vehicle in good condition or for the normal owner. This is more of a 'wish' list thing for a restoration type project or someone building a track car.

 

Update on the aero end..

Hoping to finish up the bluff body for the CFD model today, prelim CFD on just the diffuser shows promise but is really not complete without the full body effect.

 

Got the model done, having some PC issues with running such a large model thru CFD, just need a BIGGER BOX! well that's on the Christmas list this yr, a new souped up BOXX... yeah that's the ticket. Back to current reality..

Here some pretty pics of the lower panel and diffuser alone. Prelim on the full body model show the vortices to be absent. 308/328 profile is not the best though, low pressure zone above roof, that provides lift... updated model with the 328 rear wing and will see what changes.

Onto pics..
enjoy...
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