IROC aerodynamics

[911teo]

I am rebuilding my car and it will look very close to this one...




I am having a 3.6 engine induction modified and will have some long individual throttle bodies (51mm + horns + aircleaners) in place of the Varioram.

I was hoping I could use an IROC tail but i think the ITBs might be just too tall for the engine lid to close.

So as an alternative I was thinking of getting a 3.8 RSR tail.
The problem is that the RSR tail will actually create some downforce (or net negative lift.. let's not get into that argument )

I have read Paul Frerhe 911 book but could not find any data on the IROC front bumper. My gut feeling is that it does little to reduce lift.
So the combination of the Iroc front bumper and the 3.8 RSR tail might not be the best (lift generated on the front axle and downforce on the rear).

Has anybody had exprience with the IROC tail? Is there more room under there than a normal Carrera tail for me to fit the ITBs?
I know the original IROC was running carbs... but I don't think they were as tall...

If I have to scrap the IROC tail, can I decrease the lift in the front using splitters?

[Bill Verburg]

Look here

You would want to increase the efficiency of the front by only allowing air that flows thru the cooler matrix to go thru the grill, get the front as low as possible and use a splitter, hook large diameter smooth bore tube to the brake cooling intakes

[911teo]

The brake coolers will be hooked...

As per the front bumper... are u suggesting something like this?



This has the extension and the splitter... But I have never seen it on an IROC...
It might look ugly...

[Craig 930 RS]

This works well for brakes

[Bill Verburg]

Just some food for thought, each of these for differnt reasons are improvements on the base IROC front end

[Bill Verburg]

Craig's is good as well, except for the brakes ducts, unless they are hooked up and sealed they are a real roblem in so far as front end aero goes.

[911teo]

I would certainly do this as a minimum



The Martini RSR would be the best... but too radical for a street car...

The last one (the dark Blue one #55) looks like a normal IROC bumper.

[Bill Verburg]

The last one (the dark Blue one #55) looks like a normal IROC bumper.

If you look at it carefully , it's not

[Jack Olsen]

The IROC is a pretty effective tail. But I've run both it and the 3.8RSR tail on my car with the same front end. The difference between the two is not so great that it's going to effect the car's handling or lap times in any surprising way. In my experience, the two tails run within a second of each other.

If you run an extended 60-inch wing up above the RSR tail, then you'll feel a difference -- but even in that case, the front/rear impact is something you can tune away with swaybar adjustments. The big wing will improve your laptimes, but it won't make it seem like you're suddenly driving a different car.

I've also run the extended front lip splitter like on the green car, and an assortment of other front ends. There are subtle differences, but it's not like the car is going to get hard to handle all of a sudden if you make any of the changes you're discussing.

To put the rear wing's effect in perspective, the difference between the IROC and the 3.8RSR wing is probably only one half to one third as significant (in terms of handling and lap times) as the effect of putting someone in the passenger seat while you're driving.

[911teo]

Jack that's conforting. I hoped you'd chime in as I have seen pics of your car with the 3.8RSR tail.

My choice for the tail will be dictated by whether or not the engine will fit under the IROC one. If it does I'll keep it "original", if it doesn't then the 3.8RSR it is.

I was worried about having to much understeering due to the aerodynamic effects of a mis-match front/tail combo.

I realise the IROC solution is miles away from being the most aero efficient. But lap times are not the top priority (it being fun) for now.

[TRE Cup]

matteo
the original RSR ran with either high butterfly or slide valve injection. Either injection was pretty tall and i am certain you could run just about any itb's out there w/o having to change the tail.

[Jeff Alton]

The one I have has a splitter molded into it, I can post a pic when I get home....

Cheers

[jluetjen]

Originally posted by 911teo

I know the original IROC was running carbs... but I don't think they were as tall...

If I have to scrap the IROC tail, can I decrease the lift in the front using splitters?

First off, the original IROC cars used MFI, not carbs.

Secondly, when you have a car with a bluff front end (like an 911 with an IROC front spoiler), it tends to create a high-pressure area in front of the car. Putting a splitter under that high pressure area will generally create downforce in the area of the splitter. The bigger the splitter, the bigger the downforce. So it's a very tunable feature just by moving the splitter in and out.

[jmz]

Why the ITBs?
Are you chaning the internals too (wild cams)? otherwise Do you really need to change the intake?

Didn't the IROC have high butterfly injection?

[Jeff Alton]

Here is the bumper/spoiler that I have.....



Cheers

[911teo]

I was under the impression that IROC cars had a carb setup... Must be mistaken.






Sorry for the quality of the scan... the BBS would not allow me a bigger file...


Dave thanks for the heads up.. I should try the IROC tail then...

Jeff if you had pics it would be great!

[911teo]

Originally posted by jmz
Why the ITBs?
Are you chaning the internals too (wild cams)? otherwise Do you really need to change the intake?

Didn't the IROC have high butterfly injection?

Yeah... going 3.9L with Motec M600, Jenvey ITBs, DC-22 cams (halfway between Supercup and 3.8RSR), solid lifters, Ti retainers, ported heads, RS Valves and carrillo rods...

The above is a pic of whatseems to be an original IROC 3.0

[911teo]

Jeff I have seen that for a narrow body, havent seen one for 9' flares...

But I'd love to keep it IROC look if I can!

[911teo]

Originally posted by jluetjen


Secondly, when you have a car with a bluff front end (like an 911 with an IROC front spoiler), it tends to create a high-pressure area in front of the car. Putting a splitter under that high pressure area will generally create downforce in the area of the splitter. The bigger the splitter, the bigger the downforce. So it's a very tunable feature just by moving the splitter in and out.

That's great stuff! I think this is how I am going to do it! Thank you!

[Jeff Alton]

There is a pic above, I can go take another if you would like??


By the way the pic you scanned is a MFI motor in that car......

Cheers