Rear diffuser

freesaints · 10-31-2019, 11:26 AM

I was appreciating (once again) the Singer DLS, particularly the design of the rear bumper and rear diffuser. After doing a quite exhaustive search to see if anyone has attempted their own integrated design for a rear diffuser, I came up with nothing. No aftermarket solution, no homemade solution (other than a few bolt on solutions for the track, typically made from some aluminum-type metal), but nothing that seemed to attempt anything near the aesthetic level of Singer.

Has anyone done something similar?

Jack Olsen · 10-31-2019, 02:04 PM

Especially with an air-cooled engine, it's tricky to make a diffuser without some big compromises.

But then, diffusers on production cars -- or any car with a relatively soft suspension, which includes just about all cars modified for racing -- are going to be compromised by the changes in ride height from side to side when you want them to work, which is in corners.

I think most diffusers on contemporary production cars are there to make the cars more stable at high speeds in a straight line. This makes sense, since flooring it on an open stretch of pavement to see what the top speed is like is going to take place on a lot of these cars, whereas really testing the limits in corners will happen on relatively few new Porsches. 99% of Singer owners are going to break the limit on a straight stretch of freeway, for example, while maybe 1% will do any meaningful racing or time trialing with their cars.

So the demands of a splitter on modern production cars is going to be worlds apart from what is/was done with ground effects race cars. Those cars (as I understand it) needed extraordinary stiff suspensions to keep the whole of the car's underside at a specific height up off the driving surface, especially in fast corners.

I experimented with a rear diffuser on my air-cooled 911. It required openings for air to pass around the cylinders and heads, and I didn't have as much control over specific angles of the underside of the car as I would have liked. But I think primarily because of body roll (even with 600# springs) it made the car unpredictable in corners -- which is why it has a lot of dust on it in the picture.

More specifically:

More recently, I took a more modest approach. The changes in ride height are obviously greater the farther away you get from the center of the car. So I made a flat panel down the middle, with vertical pieces that bolt on at the track. These pictures were taken while I was making it. I've made two versions -- one drops down closer to the pavement than the other.

In fact, all my aero folds up and fits in the car for the drive home:

Sebscst · 10-31-2019, 02:45 PM

Really like all the bolt on aero very ingenious.

Jack Olsen · 10-31-2019, 02:49 PM

Thanks. It's a bunch of stuff that goes on:

And then back to something like normal, for street driving.

RSTarga · 10-31-2019, 03:55 PM

You need to plumb the exhaust into the diffuser.

freesaints · 10-31-2019, 05:10 PM

Jack, thanks for the write up. Love the thoughtfulness and approach to your modifications. I do agree with the straight line performance that a diffuser can help with, but I guess I thought that there would be linear improvements in turns as well. That being based on no scientific knowledge, just an assumption.

So, would someone need to design a diffuser more for handling around turns or more for straight line (with a different shape, angles, etc) but not both, or is there a "universal" diffuser design that would improve performance across the board?

Also, beyond the shape of the diffuser, what sort of considerations would there be in trying to make it fit with the existing car components (I have an '87 930 with Andial intercooler, modified turbo, B&B headers/muffler)? As in, would it work in addition to the rear bumper/lower valence, or would it replace such? We're getting into an area (aerodynamics) I know very little about and how one change affects something else.

ian c2 · 10-31-2019, 07:08 PM

As I (very little) understand it the diffuser is there to pull a bit of a disruption vacuum on the backside to help with the low pressure underneath the car helping the aero aids on the top of the car , and once you’re totally flat bottomed and have front and side skirt sending the flow to where it can be used on radiators etc and creating other low pressure points , the diffuser really helps with that low pressure underneath and is a must for the stuff on top to work at its best compromise ...
As with most aero , the car can be a pig if you’re not going fast enough .
It’s all down to air-flow , and to get more airflow you need more speed .
Unfortunately , downforce creates drag so MO POWAH BABY is needed to overcome the drag to get the aero working right .

I was entertained by an f1 transmission guys rant this last week , as he’s complaining true mechanical technology is playing second fiddle to aero and energy-regen etc.
he made a few very valid points .

They’re at the stage now , with full downforce aero if you where to turn off an f1 car on a straight and not touch the brakes it would slow quicker than the average streetcar on its brakes !!

ian c2 · 10-31-2019, 07:09 PM

I think it was the bbc website that had “the secret aerodynamicist” .
I’ll go take a look and post it if I find it .

Edit :
From March this years f1 season

https://www.bbc.com/sport/formula1/47527705

.

ian c2 · 07:28 PM

Also , what I find more interesting on that singer prototype is the roof .
Check out the channel in the roof that goes all the way through the rear window !!
I read the bbc article about it weeks ago and kinda skimmed the bit about the Perspex “hidden” spoiler , but a couple of weeks ago I was looking over a nascar Toyota and talking to it’s driver and asked about the North Carolina race I’d watched on tv that weekend (my first) where somebody span dropping from about 200 to 50 into pitlane and I saw the roof flapping .. I thought it was a driver roof hatch that open for a split second , but he then showed me the hidden flaps in the roof (and front hood iirc) and the clear spoiler than ran all along one side of the car ..
I hadn’t noticed it from 20 feet away , but the clear “wall” only traps air when the car is sideways , and directs it into the first flap to lift it , and from there there is a tunnel sending it to the second flap .
Basically they pop up like wind brakes to keep you planted if you spin .
Pretty ingenious .
After talking to the nascar guy I found the tab and looked again at the singer .
The roof and rear window have been subtlety yet drastically been revised .....

freesaints · 11-01-2019, 04:08 AM

ian, the level of engineering and ingenuity at the professional race level never fails to amaze me. As a big fan of F1 (heading to COTA this weekend), I've long appreciated the subtle tweaks and constant improvement of both mechanical and aero advantage. I had no idea about those Nascar channels and purpose.

If the diffuser is there to help with the low pressure underneath to help the aero aids on top, it would seem that the Singer diffuser might be more about aesthetics (like most modern cars with air scoops which are non-functional) than true race proven tech. I don't imagine the ducktail provides the most downforce and if the diffuser is a counterbalance to that... Especially true if as Jack mentions, only 1% of Singer owners will ever track the car.

Not that the aesthetics of the diffuser is a bad thing considering I think it looks bad a**.

stownsen914 · 07:44 AM

I put a fairly large diffuser on my 914 racecar. I had a bit more freedom to do so than you will on a 911 since the engine is farther forward. I did have to move some things around to get it to fit - mostly the exhaust and shift linkage. The car handles really well, but without data acquisition it's hard to quantify. On our cars without serious wind tunnel or CFD time to optimize, we're not going to get thousands of lbs of downforce like the big boys get. Hundreds could be a more realistic goal.

There are a few things to keep in mind with underbody aero:
- It won't do much to have a diffuser if you haven't flat bottomed the entire belly of the car. A diffuser is intended to control fast flowing air under the car and gently guide its passage out the back of the car and return it to "atmospheric." The underside of older Porsches has all kinds of undulations and protrusions that slow down air flow under the car. Putting a diffuser behind that will greatly reduce its effectiveness.
- One of the great things about underbody aero is that it can actually improve your aero drag while providing some downforce. If you flat bottom your car, you're reducing drag under the car which can offset the drag that downforce from a diffuser would add.
- This part isn't so easy on a 911, but having a low wing can help "pump" the air out of a diffuser and have some benefit to downforce. This may sound unintuitive since usually a high wing is better, but the gains in improving the downforce of the diffuser can offset the loss of extra downforce that you'd normally have with a higher mounted wing. Back in the old days of IMSA GTP, they used to run two wings to get the best of both worlds!

For my 914 diffuser, I mostly copied existing designs as best I could. I did put a low wing and flat bottomed the entire belly of the car with a sheet of aluminum. I need to do some data acquisition to see what it's really doing. Would love to do some CFD to optimize it. I'm sure I've left a lot on the table.

Scott

ian c2 · 06:03 PM

Quote:

Originally Posted by freesaints

ian, the level of engineering and ingenuity at the professional race level never fails to amaze me. As a big fan of F1 (heading to COTA this weekend), I've long appreciated the subtle tweaks and constant improvement of both mechanical and aero advantage. I had no idea about those Nascar channels and purpose.

If the diffuser is there to help with the low pressure underneath to help the aero aids on top, it would seem that the Singer diffuser might be more about aesthetics (like most modern cars with air scoops which are non-functional) than true race proven tech. I don't imagine the ducktail provides the most downforce and if the diffuser is a counterbalance to that... Especially true if as Jack mentions, only 1% of Singer owners will ever track the car.

Not that the aesthetics of the diffuser is a bad thing considering I think it looks bad a**.

I’m assuming you have the full series of photos ?
If not look on bbc news site , top gear section for an article .

Edit :
No longer on my phone so have the ability to edit and add a few photos

The diffuser might not be “true race proven tech” , but the Williams team knows a little about what works and what doesn’t .

They obviously use wind tunnels and computers , but A quick and simple way is to add cotton tufts and get somebody to photo you at speed .

Pics added to reply below ..

Note the skirts changed between the red and the white car .

But ... Look again at the roof .
It’s channeled and has a upper spoiler on the rear .
This guides the air on top of the car down the rear window so it actually hits the ducktail .

ian c2 · 05:52 PM

https://www.topgear.com/car-news/big-reads/it-911-reimagined-singer-and-williams

faapgar · 11-01-2019, 09:52 AM

I have a full ground effects 914-6 built on a chrome-moly tube frame chassis.Chassis was built in the 90,s in Texas with a stock floopan to make it appear like a 914.Ground effects were set up by an engineer.It made 1100 lbs.of downforce at 100 mph.It was brand new with 1.25 inches of ground clearance.Full side skirts and slick tunnels under the drivetrain.I put a tie wrap on the shocks and went testing until the tie wrap moved 1 inch. I changed springs 6 times until I got it right.1100 lb.rears and 950 lb. for the front.It hit bottom at around 100 until I got the springs right.It was a trip to drive but became tenuous if you bumped a curb and lost some air.VERY quick to 100 then slow to accelerate because of the drag.Scott Townsend knows this car.I will get a picture of the car and rear diffusor.The big rear wing was designed for the air to pass underneath it.Motor was 2.8 on 3.0 turbo case.Ti everything.45mm slidevalves,Motec.373 hp at 9300 rpm but it needed that Rawknees Turbo Powaaa.Fred

ian c2 · 11-01-2019, 07:06 PM

I’d like to see pics of that Fred !!

Been meaning to call you back , sorry bout dat .

Rawknees'Turbo · 11-01-2019, 09:31 PM

Quote:

Originally Posted by faapgar

. . .Motor was 2.8 on 3.0 turbo case.Ti everything.45mm slidevalves,Motec.373 hp at 9300 rpm but it needed that Rawknees Turbo Powaaa.Fred

Boost that bisch, FAAPtabulousFred, booooooost that bisch!!!

faapgar · 11-02-2019, 03:09 AM

I built in 2000 a twin-plug,twin turbo 3.5 for it then priced the center fan it would need to cool it.The fan was more money than the core motor and its build cost.Never put it in and moved to the beach on the west coast of Costa Rica.I thought 700 hp. at 1700 lbs.would work just fine with a Hewland gearbox.I will find some pics.Still have the car but the body work is off and it is buried in the shop.Fred

faapgar · 11-02-2019, 03:35 AM

This when I first brought it home less motor and trans.Windshield is extended forward like a 917.

stownsen914 · 08:08 AM

Fred - I found the old ad for your car. It still has all the pics. Click the small pic on this page, and it brings up the page with the other pics.

Porsche 914 GT2 Race Car Spec Sheet

911pcars · 11-02-2019, 08:53 AM

Quote:

Originally Posted by stownsen914

I put a fairly large diffuser on my 914 racecar. I had a bit more freedom to do so than you will on a 911 since the engine is farther forward. I did have to move some things around to get it to fit - mostly the exhaust and shift linkage. The car handles really well, but without data acquisition it's hard to quantify. On our cars without serious wind tunnel or CFD time to optimize, we're not going to get thousands of lbs of downforce like the big boys get. Hundreds could be a more realistic goal.

There are a few things to keep in mind with underbody aero:
- It won't do much to have a diffuser if you haven't flat bottomed the entire belly of the car. A diffuser is intended to control fast flowing air under the car and gently guide its passage out the back of the car and return it to "atmospheric." The underside of older Porsches has all kinds of undulations and protrusions that slow down air flow under the car. Putting a diffuser behind that will greatly reduce its effectiveness.
- One of the great things about underbody aero is that it can actually improve your aero drag while providing some downforce. If you flat bottom your car, you're reducing drag under the car which can offset the drag that downforce from a diffuser would add.
- This part isn't so easy on a 911, but having a low wing can help "pump" the air out of a diffuser and have some benefit to downforce. This may sound unintuitive since usually a high wing is better, but the gains in improving the downforce of the diffuser can offset the loss of extra downforce that you'd normally have with a higher mounted wing. Back in the old days of IMSA GTP, they used to run two wings to get the best of both worlds!

For my 914 diffuser, I mostly copied existing designs as best I could. I did put a low wing and flat bottomed the entire belly of the car with a sheet of aluminum. I need to do some data acquisition to see what it's really doing. Would love to do some CFD to optimize it. I'm sure I've left a lot on the table.

Scott

Quote:

Originally Posted by faapgar

I have a full ground effects 914-6 built on a chrome-moly tube frame chassis.Chassis was built in the 90,s in Texas with a stock floopan to make it appear like a 914.Ground effects were set up by an engineer.It made 1100 lbs.of downforce at 100 mph.It was brand new with 1.25 inches of ground clearance.Full side skirts and slick tunnels under the drivetrain.I put a tie wrap on the shocks and went testing until the tie wrap moved 1 inch. I changed springs 6 times until I got it right.1100 lb.rears and 950 lb. for the front.It hit bottom at around 100 until I got the springs right.It was a trip to drive but became tenuous if you bumped a curb and lost some air.VERY quick to 100 then slow to accelerate because of the drag.Scott Townsend knows this car.I will get a picture of the car and rear diffusor.The big rear wing was designed for the air to pass underneath it.Motor was 2.8 on 3.0 turbo case.Ti everything.45mm slidevalves,Motec.373 hp at 9300 rpm but it needed that Rawknees Turbo Powaaa.Fred

Aptly illustrates that extensive testing, data acquisition and valid replication is part of the equation, especially when dealing with "black art" topics like vehicle aerodynamics. Without such testing, many vehicles may appear to be "state of the street art" despite being only best guesses as to whether those efforts bear any reality of real-world effectiveness.

I can only shake my head and wince at the "aero" bits tacked onto sporty cars by the marketing departments of car manufacturers. These include rear wings, strakes, rear end "diffusers", spoilers, winglets, ornamental scoops and NACA ducts (many non-functional), etc. A substitute for good car design perhaps.

Sherwood