Presented by Nine Lives Racing
Stefan (Forum Supporter)
Stefan (Forum Supporter) GRM+ Memberand MegaDork
5/21/20 5:02 p.m.

A buddy made this for their Endurance car and while it looks great, there has been some debate on its actual effectiveness.  I thought I'd show some pics and see what other, more smart people, might think of it?

One of the first comments from someone that knows and has tried a thing or two when it comes to aero on cars was;

"So... good profile, but the stepped design won’t work without exhaust blown on it. - too many angles for the air to stick.
The air will detach itself without more strakes and the profile needs to be curved up to no more than 15 degrees.

At best, it’ll create super turbulent air for someone behind you and ruin mpg.

There was more to the discussion, but from the light reading I've done so far, he's probaby not wrong.

SVreX (Forum Supporter)
SVreX (Forum Supporter) MegaDork
5/21/20 6:19 p.m.

It looks nice installed, but I agree with him.

I'd also try to taper the width so it gets wider as it exits the car, and longer to the rear (if your classing allows)

NOT A TA SuperDork
5/21/20 8:50 p.m.

Cool looking decoration.

I was taught a long time ago:  Don't call anyone's baby ugly.

It is a start.  My experience is with flat bottom cars that are less than an inch off the ground, where good diffusers produce inexpensive hundreds of pounds of downforce above 100 mph.

I fear that there is too much dirty air under the car ahead of the diffuser, and some of it actually getting on top of the unit, to make it produce much force. If the side plates extended down to almost the track surface it might pump better.   If it did work and produced a bunch of downforce i think the attachments are not strong enough, and possibly the material itself would distort.  Our diffusers and attachments are very robust units, usually out of CF.  My fear as a tech inspector is that the object could break loose and be on the track in front of another car.

All that said.... Spray paint it flat black, put a GRM sticker on each side, and go have fun!

collinskl1 GRM+ Memberand Reader
5/22/20 6:34 a.m.

The sudden angle changes in both directions make me wonder if that diffuser will be effective.

Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
5/22/20 11:13 a.m.

The center portion might work pretty well.

Has he done any testing? "Conventional wisdom" can often be wrong, especially with things like aerodynamics. I'm not sayings it's good as is, or that it couldn't probably be improved.

Also, what is he trying to achieve with the diffuser? Is the car lacking in high speed stability beforehand? Is he also planning to add front downforce to match?

But, plain and simple, if lap times decrease, it's working!

Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
5/22/20 10:35 p.m.

Here's a video for your buddy. Great book too, BTW.


Stefan (Forum Supporter)
Stefan (Forum Supporter) GRM+ Memberand MegaDork
5/22/20 10:48 p.m.

The car was tested yesterday with the diffuser and it was a second faster per lap using last years tires and settings.

boxedfox (Forum Supporter)
boxedfox (Forum Supporter) Reader
5/23/20 12:27 a.m.

I actually think it will work, just maybe not as well as it ideally could. Undercar flow is naturally quite turbulent, which is good for keeping the flow attached. I think the middle and the back portion of your diffuser are working exactly as you intended them to. You're probably just getting a big boundary layer around the sharp angles and losing efficiency in those areas.

Now, if you eliminated the sharp angles and replaced them with gentle curves, I think you'd probably get less drag and more downforce. And of course, lowering the strakes so the whole thing basically scrapes on the ground always helps. But that's subject to sensibility.

I donno. I like it.

EDIT: Forgot to rate the diffuser.


Honsch New Reader
5/23/20 2:11 a.m.

I'm trying to figure out what their goal is.  They already sandbag pretty hard not to break into superdog class.

NOT A TA SuperDork
5/23/20 10:11 a.m.
Stefan (Forum Supporter) said:

The car was tested yesterday with the diffuser and it was a second faster per lap using last years tires and settings.

I'd think to get good information they'd need to compare with/without diffuser same day, same driver, etc.

Our Preferred Partners