diffuse or not

[drunyon]

Hi Guys,

Diffuser?
As us minions don’t have access to a wind tunnel nor do we have
Adrian Newey on call I’d like the forums opinion.
I will be ordering my Can Am next week and I’m considering installation of
a rear diffuser. I contacted the UK diffuser manufacturer as to instructions (http://www.reverie.ltd.uk/universal_diffusers.php)
on attachment method, they had no specific instructions for a GTR (or Can Am). So my plan was to install a removable floor under the engine bay (Dzus fasteners), then attach
the diffuser to the aluminum floor.
Wait! Before anyone yells thermonuclear meltdown because the engine bay is enclosed here are my specs:
Ceramic coated headers
NACA ducts in place of the grills
The bonnet will be vented above the engine due to cutouts for velocity stacks
Electric water pump @ 55 gpm
Oversize aluminum radiator with dual fans
Dry sump
No heater or AC
Oil water heat exchanger
Rear screen wire mesh increase size

I guess in the final analysis, the floor will be easily removable if heat becomes an issue.

Upon further thought, I can be talked out of a floor very easily if someone knows the rear diffuser installation method.

I plan on some SCCA time, maybe 20%/80% race/street.

Thanks the voyeurism over the last year

don

[andygtt]

Rear diffuser won't work without a floor on engine bay.... I have a floor on my noble along with turbos so it is possible without overheating.
Don't forget to also add gearbox cooling.

[Steve_D]

This subject has been done here before so you may find it via Google.

Basically there is not enough room under the car.

For a diffuser to work, rather than look pretty, it has to have a specific depth to length ratio which is not possible with the design of the rear chassis members and rear clip.

Steve

[UltimaCH]

Indeed Google gives better results than the search function here...
The topic was discussed in 2010 with also some innovative solutions for front and rear body clips!
http://www.pistonheads.com/gassing/topic.asp?t=931...

[macgtech]

Steve_D said:

This subject has been done here before so you may find it via Google.

Basically there is not enough room under the car.

For a diffuser to work, rather than look pretty, it has to have a specific depth to length ratio which is not possible with the design of the rear chassis members and rear clip.

Steve

It has to have a flat floor to work - which it looks like you are addressing.

To the above point, there is enough space, you just end up with a small diffuser - you need a maximum angle of 15 degrees to maintain attached flow - which means that you end up without much 'height' at the rear. The answer is to extend the floor downwards (which is what we have done) but it is quite complicated. The other option is to have it sticking further out the back - but regulations normally prohibit this to 100mm. Having said that, due to the shape of the Ultima bodywork, this still gives you a pretty large diffuser.

You have to ask yourself what you are trying to achieve. Certainly our experience has shown that you would benefit from more front end downforce, and a diffuser would help to drive the front splitter, however there are easier methods, as this is quite labour intensive and reasonably complicated. Venting the arches would help a lot (both front and rear) as there is a high pressure build up in the arches.

Is your splitter attached to the clam or the chassis?

Jonny

Edited by Stig on Thursday 21st February 12:15

[anonymous-user]

Just going to a flat floor under the enginebay will effect a movement of the Centre of Lift rearwards, as the air flow velocity under that floor will be more constant (rather than getting slowed down by hitting the sump and other components). In conjunction with careful adjustments in vehicle rake (the nose down angle and ride height) you can get a significant reduction in rear end lift without a full blown difuser etc.

Regarding cooling, typically OEM's / race engineers use small NACA ducts let into the flat floor to peel off a small amount of the laminar airflow



and then duct it directly to exhaust manifolds, alternators, differentials, transmission (and coolers) etc. A good trick is to install exhaust manifold cladding and "chimneys" that exit on the top deck. This creates a strong thermal syphon to remove heat from the engine bay, even after shut down. Road cars have been doing it for ages:


Note the 4 chimneys exiting upwards to finish just below the bonnet lovrers on this Vanquish V12.

Another trick is to exit this "vent air" into the cars wake, a nice low pressure area and get some drag reduction for free also, and obviously easier to do with a rear/mid engined layout!

[macgtech]

As a visual representation of what can be achieved, I have included a picture below. Previously (2011) we ran our car in standard bodywork trim, so splitter attached to the clam, and no further aero aids other than the rear wing. In the wet, water sprayed EVERYWHERE, and it was very clear that there was no control of the flow.

In 2012 we added a flat floor, side skirts, and attached the splitter to the floor, so the underside was flat right from the tip of the splitter through to the engine bay. We did not have a diffuser. You can see from the below that the flow was very well controlled, and we generated a nice low pressure zone under the car (see the spray being 'sucked' under the car, and stopped from doing so by the skirts, producing downforce). Even without a diffuser, we get a really god diffuser effect as the flow under the car was well controlled.

As I mentioned above, adding the diffuser would drive the splitter, but shift the centre of pressure backwards (as the apex of the splitter would be near the rear of the car). All food for thought. We may look at adding one if time permits this year.

[UltimaCH]

Max_Torque said:

Note the 4 chimneys exiting upwards to finish just below the bonnet lovrers on this Vanquish V12.

Nice pic of the engine but where are the louvres??

[F.C.]

UltimaCH said:

Nice pic of the engine but where are the louvres??

In the bonnet that would be above the "chimneys" .

[738 driver]

There's a few freebies to be had around the rear clip .......

[Ult-Jim]

Here is a photo of my Reverie diffuser:



I agree it is just for looks. However I was lying under the car this afternoon doing an oil change and started wondering if paneling the front section of the engine bay would assist in extending a smooth air flowing from underneath front end and middle section of the car? I also have a Carbon Bits front splitter which balances the aerodynamic / aesthetic look of the car. In the next 12 months I plan to cut open the vent hole behind the front wheels (Factory upgrade part).

[Storer]

If you open the rear clip and look at where the diffuser would end up (especially if you make your clip open wide like mine) you might decide it's not a good idea.

I suspect this is the reason Reverie didn't tell you how to fit it!!


Paul

[drunyon]

Lots of great input!

Like drinking from a fire hose

My initial thought was to install an engine bay floor to clean up the air flow. The diffuser was an after thought.

It sounds like the diffuse is a major project without any good substantiating data, so it’s adios for now. Plus as Storer has stated, it would be a pain opening the rear clip. I’d have to park by a ditch!

The front splitter mounted to the chassis to create a nice flat undercarriage, plus the engine bay floor seems like a good starting point.

Is the CF floor picture shown by Max Torque for an Ultima? If so, I want one!
And if it is, how is it attached? If I need tabs welded on, I want to do this prior to powder coating.

Thanks all for the great free advice

don

[Storer]

There is quite a lot of mods required to fit a chassis mounted splitter to the front of the car.
The section of the chassis under the radiator needs raising by the width of the splitter. This may require a different (thinner) radiator. In my case the splitter runs down each side of the car to the back of the fuel tanks. This requires chassis mounting brackets welding to the chassis. The fuel tanks raising 0.5 inches. The bottom of the side pods cutting away and a return creating. The front clip needs mods to the hinge point and bottom edge as well as the radiator opening.
I also have lengthened front wishbones and steering arms to reduce the scrub radius.

Lots of work (and expense) so make sure you really want this revised front before you start.



Paul

[F.C.]

drunyon said:

Is the CF floor picture shown by Max Torque for an Ultima? If so, I want one!

Not 100% but looks like an Exige / Elise pattern floor to me.

[k wright]

Best to invest your time elsewhere on your car.

if you want to attach a floor to the engine bay I would suggest"P-clips" with the mounting tab portion riveted together and a threaded captive nut to attach alloy sheet.
This will allow avoidance of drilling or welding on the chassis.

[dandare]

Macgtech, did I understand correctly that the underside of the engine bay remains open on your car?

[drunyon]

Thanks Paul (still can't figure out the quote in a box thing),
I don't think I want to go to all that work for a flush mounted front splitter.

F.C.

I checked out Lotus's web site and that certainly looks like an Elise floor. I think I'll take some design cue's (NACA ducts)

I have some photo's of some CF fender louvers installed on a black GTR, look great. If I could figure out copy/paste, I'd post them!
don

[Life Saab Itch]

Is it advised to keep the bottom of the engine bay open because of air pressure build up or because.of heat dissipation issues?

Or is there something that I have missed?

[macgtech]

Life Saab Itch said:

Is it advised to keep the bottom of the engine bay open because of air pressure build up or because.of heat dissipation issues?

Or is there something that I have missed?

We have kept it open for heat dissipation.