Downforce Question

[ksluna]

So I am getting bored in quarantine and have been picking up some small projects here and there to do on my 911.

I am thinking about doing away with the bumperettes and getting IROC style front/rear bumpers.

Upon my research I've read (and seen) mixed information about the science behind the downforce for the front and rear of our 911s...

My 89 has a whale tail that I intend on keeping. I've read that if you have a rear tail, you need to have a front lip to match the downforce for better stability.

However, I've seen many cars with the IROC style bumper with no splitter and rear wing like this:

- Example 1

- Example 2

I much prefer this look over the IROC style bumpers with the built in splitter:

- Example 1 (TRE with built in splitter)

- Example 2 (Rennspeed's bumper)

So my very basic question is, will it be that big of a deal to go with the splitter-less IROC bumper despite having a whale tail? Will the loss in downforce be that apparent?

[Mothy]

IROC like the white car was as per development by Factory for 3.0RS and Penske for the IROC series (larger rear wing)

I'd be inclined to believe that the development work (in wind tunnel) by them was pretty good and there are some further developments done by others such as putting a wicker lip on the tail and a blade like splitter on the front to give better separation of airflow.

So it really depends on how fast you plan to go - for street use the difference will be relatively small. And keep in mind that most of these mods in the day were more about reducing lift by introducing air or turbulence into areas of negative pressure - few of the tails actually push down.

An interesting observation I had with my IROC style bodywork was that even when racing any water drops on the rear window did not move - just stayed in the same place = no airflow. So for ducktails and whaletails it was only the outside edge of the tail that did any work. For IROC that is probably why they are much wider and are pushed back so far past the rear bumper (to get them into the airflow to gain some down force as well as create turbulence.

Besides - I like the 3.0RS or IROC look better .....

[josephvman]

I think the majority of the downforce engineering in an air-cooled 911 involved putting the largest, heaviest battery they could possibly fit in front of the wheels!

[Oneredspyder]

I have an 86 Carrara with the whale tail, and if I glance in the rear view mirror I can see the rubber lip moving at any speed over 60mph. At 80-100 it is really moving. I kind of like it.

[RSBob]

Unless you are planning to take it to the track, go with your original plan. Shouldn’t be any effect at normal speeds. Post pics of your progress.

[Uwon]

Originally Posted by ksluna

Upon my research I've read (and seen) mixed information about the science behind the downforce for the front and rear of our 911s...

I much prefer this look over the IROC style bumpers with the built in splitter:

Unless you are going to track it at a high speed course, go with the one that suits your fancy and forget about downforce. You will never know the difference.
By way of example, I used to track my ‘86 which a 3.4L turbo at Mosport, where I could reach 265 kph on the back straight with no front end lift. Here is a pic of the front end:



No lip!
Like I said, choose the front valance/splitter that suits your fancy.
Stay safe.
Johan

[r lane]

I had read that early on in wind tunnel testing, an incident occurred where the decklid had been inadvertently left propped open when the fan was turned on. What they immediately noticed was that the yarn tufts taped to the rear quarters were lying flat and relaxed. The rear wind obstruction, the raised deck lid had stabilized the air running along the back sides of the car. So aside from what ever down force it was creating, rear stability seemed to be the real benefit

[dfhtrhjn]

My IROC clone doesn't have a front splitter and I don't get front end lift. I have a 2 degree rake though so my front end is slightly lower than the back.

[ksluna]

Thanks for the advice guys. I'll be going with the one w/o the splitter. Most likely going through with Rennspeed.