Glenn Evans

I've stated in the Bonneville and Mythbusters threads that I consider that the step at the top of the hatch glass generates a small bound vortex immediately behind it, which causes the flow to reattach to the glass, and keep it attached when it would have separated were the glass flush. The S rear spoiler reduces drag (by providing a clean breakaway for the airflow, which the rounded butt does not), and the MY87 hinged rear wing will lock into place, if left unlatched, at speeds as low as 40mph. IMHO, these prove that the airflow remains attached pretty much to the rear of the hatch glass on the stock body.

Remember that the prototype 928 was tested in VW's full size wind tunnel. I don't believe that Porsche would have accepted separated flow over the hatch glass. It appreciated the importance of aerodynamics before most other manufacturers; it was performing aerodynamic testing on racecars in the early 1960s, before it had access to its first wind tunnel, that of the Stuttgart Technical High School in 1965 (IIRC).

My interpretation of what is happening in the smoke test is that the rear spoiler is creating a dead zone in front of it, and that this is disturbing the flow over the hatch to the extent that it cannot reattach behind the step. I expected to see a dead zone in front of the spoiler due to the sharp angle between the hatch glass and the spoiler plate but I didn't expect it to extend all the way forward to the step.

The flow along the side of the car is quite clean. It looks to want to flow upward and inward over the rear quarter window, but is being blocked by the spoiler side plate. On a conventionally-shaped coupe, the side plates would increase the effectiveness of the spoiler by preventing flow spilling to the side, but I suspect they are blocking air flowing inward over the rear of the 928's quarter windows. The potential exists for the inward-flowing air to create a vortex as it separates over the sharp edge of a side plate, perhaps further reducing the effectiveness of the outer sides of spoiler, but those side plates seem to be high enough to prevent that happening ie effectively redirecting the flow over the quarter windows rearward. I suspect that a professional race team could spend a lot of wind tunnel time optimising that interaction of flows.

Fastest928

The rear window lip keeps air attached.

Good luck Carl! Sounds like too much fun!

Tony

Your watching a different video than i am then.

jpitman2

Re air inlet sizes for cooling...optimum situation is for a small inlet area feeding into a large plenum space that allows the air to slow down, pressure rises, much heat is picked up , and as long as adequate exit area is provided, the hot air is expelled. I have seen pics of Sea Fury racer at Reno (P&W radial, ~2500hp), with a gap between spinner and cowl of 1.5".
jp 83 Euro S AT 54k

Glenn Evans

Carl's problem at Bonneville is traction. This year's salt was poor and this has been typical in recent years. The car would easily do 250mph on good salt even at 6000ft density altitude (which is about what it was for his best run).

heinrich

Very cool and thanks for sharing, Carl. My experience was that at 140 & above, the stock wing provided enough downforce to be able to snap the rear flip spoiler into place. Iirc I measured it at 32lbs.

Also, at that speed my stock vents were closed and I saw temperature very quickly rise. This disappeared after removal of the flap system.. I hope you're right about needing a small opening for intake & cooling.

Essentially your results show that Porsche were correct in creating the flaps on the early S4's.

heinrich

That has not been my experience in 12 years of ownership and 2 flip-wing 928's. Around 140 has consistently been where the wing locks itself.

Carl Fausett

Constantine - yes, filling the recess and flush mounting the rear hatch glass is exactly what I wish I could do. But the rules prohibit it. The only other option is to do it anyway and hope not to get caught or protested, a risk I am not willing to take.

Kibort - I disagree and I know without question that the vortex generators gave me 5 MPH at Road America.
They do more than mearly creating vortices. The design of the AirTabs that I use are meant to curl the air flow around the void/drop off, with the efffect of reducing the length of towed trubulant air behind the vihicle, and therby reducing drag.

On my truck, those same air tabs on my race trailer raised my gas mileage when towing from 6 MPG to nearly 8 MPG. On the race car, they added 5 MPH to the top end. In both cases, they reduced drag by shortening the towed draft air behind the vehicle.

You can see them (in black) mounted to the back of the trailer.

Carl Fausett

+1 Spot-on.

In fact, we see where the air from the side of the car is trying to climb over the hatch to re-join at the aft of the car, and the trip-fence on the hatch is disturbing it. That trip fence is now being removed. Only the trip fence on the roof will remain.

Black Sea RD

Hi Carl,

Understood about the rear hatch and not being able to modify it. I wouldn't do it either if it is agaisnt the rules.

Guessing modifying the rear wing like you said is next.

Best,

NoVector

Fascinating stuff! Is wind control under the car allowed, or is it restricted by class? I’m thinking back to the flat bottomed IMSA cars and diffusers like the back of this lambo. The underside of the 928 rear bumper seems like it would act like a parachute when wind gets to it.

Glenn Evans

You can see photos of the underside of Carl's car on the 928 Motorsports Facebook page here:http://www.facebook.com/media/set/?s...02377033156459. They are a little over half way down the page of thumbnails.

andy-gts

where does one get the vortex generators? I have some trailers and big trucks that could use some help with mpg?

heinrich

This is only of value when the entire belly is designed to create a vacuum under the car. These direct air evenly out from under a car, but not if there is a mess under there, as is likely the case with our cars. Full bellypan, low skirts and proper nose treatment are required.

heinrich

LoL Hilton. Interesting. That's right about in agreement with my findings.