Aerodynamics!!!

This is way into outlaw territory so don't flame me too much, please!

When dreaming of the 300+ HP I could possibly get out of my liquid cooled engine if I spent the dough, I began to worry about collateral damage. First were the tires. Will I need to get 18" rims with low profile, wide speed rated tires? Yes! Will I need to modify the body to accept those big rubbers? Yes! Will I be able to keep the car from flying? Perhaps, if I drive really slow (not)or add aerodynamic features.

I came up with an air dam which replaces the front bumper and over riders for track runs. I suspect a fiberglass or carbon fiber unit will be really cool looking but that I could fab one of sheet metal very quickly here at the shop and test it much easier. For the rear end, I came up with a deployable rear deck ducktail (manual hydraulics) which would look almost stock when lowered except for the flip up at the trailing edge.

Can you imagine it? Has it been done?

Tomm

Tomm, Im beginning to worry about you. Mor power is almost always a good thing, but everything else will need to be upgraded to keep all four on the ground. It can be done, at what cost, and will it function or be undriveable, will it be "safe". I guess thats an oxymoron. Is this IM for the street? or are you going to track it? Im sure you'll have fun with Henry discussing this. By the way, my frame is done and hopefully Henry will get it mounted up. Spring is here!

I sure hope you're a man of means, Tomm.
It adds up. Ask me how I know.

Howard and Cory, I have much bigger aspirations than cash of course. These are just wicked ass ideas. I got a quote for a dynoed 300HP engine for approx $11k today and it got me scheming. I also have a friend who tracked his Audi S4 on Friday as part of a driving school. You both know how it goes...

Anyway, the 901 transmission can handle anything thru 300HP and the engine can put out that much given enough time and money so why not think about it.

Tomm

Tomm,
Be very careful of 1st gear in a 901 with that much power. 1st gear is unsupported and will shrapnel on hard launches with less power than you are thinking. 2nd gear up, go ahead and hammer, maybe just drive it like a 4 speed except for uphill stoplights. The 915 is substantially stronger (and bigger).

The older 901's also had an issue with something called a "simplified differential." Apparently everybody who is anybody corrects this when the transmission is overhauled, but if you don't update the transaxle and a failure occurs, the ring and pinion will shear your transmission into large pieces. If you think this might be an issue for yours, I'll track down year/model # for you.

May I also suggest a very healthy brake and suspension update prior to the big power? By the way, LOVE the idea of 300 hp in a speedy!

Also for aerodynamics here's a clue one of the RUF guys came up with. An underbody spoiler that is about at the midpoint of the car, just ahead of the engine. RUF proved it to be very effective at correcting high speed stability. I've got a picture somewhere if you need it.

angela

I have seen pictures in magazines of front bumpers lowered and even canted downward, like a front air splitter. The claim was for an aero advantage, but it may have been just talk and eye candy. The bumpers were also carefully trimmed to contour closely to the front body, with minimal air gap.

If you can "instrument" the front and rear suspension, and measure how much the body lowers (or rises!) at a given speed, then you can throw in the spring rate and actually calculate the amount of down (or up!) force your body is generating. The test drive needs a really smooth road with no bumps, easy acceleration and easy braking to remove their respective effect. I like your thinking!

Tomm,
Why don't you just start from scratch and figure out a way to attach a Speedster body (ala Cory)? It seems like you're trying to build something "over" something else, design the car, build it (or have it built) and then just bolt a Speedster body to it - done and done. Why would you go through all the mods that'll be needed to support a 300+ hp engine on anything other than something specifically designed for that purpose? I may be wrong but I don't think any of the manufactuers ever considered that kind of power when they did their designs.

Angela, et. al.,

If I could find a 915 at a reasonable price for my build, I would go that direction. Are there not kits to build a 901 1st gear less prone to failure? Cool ideas on the RUF aerodynamics, I also liked the instrumentation idea. I can say, almost with certainty that the chassis I am building my options into is one of the strongest out there, that, combined with the empirical data from 3.2l Porsche engines being used in the same frame (modded for mount points and the different tranny) with success lead me to believe that I do not need to build from scratch.

Tomm

Tomm,
I'm sure you know better than me and no doubt it'll be an incredible car. Keep us posted.

Mickey,

How could I know better than you? You have your car, I don't!

Tomm

Patience Tomm, patience. It'll be worth the wait, that you can be sure of.

Tomm,
For the money to build a 901 to strengthen first and the intermediary for the HP you propose, you could buy a rebuilt 915 and already be ahead of the game (except that the 915 is a little bigger and heavier.)

We opted for the 901 because we are mid-engined. Though there is a 915 mid engined conversion (WEVO), it was not something we were aware of at the time and is fairly expensive. So at this stage, I would HIGHLY suggest a 915 with the more robust clutch packages that are readily available for that transmission for your rear engined application.

A 915 is more affordable than you think. And whomever told you that a 901 will readily survive 300 hp was full of beanz. Remember thant a 901 in a porsche 911 was running behind a 110 to 190 horsepower engine. Unless it is 300 hp with absolutely squat for torque of course as it is torque that destroys the transmission not HP. On the street with decent width tires, most people find the 901 limited to around 250 hp. If you drive it as a 4 speed, a little more is OK. A 915 will take 300 hp and so will the readily available clutches. People far more knowledgeable than myself tell us repeatedly that we are really pushing the envelope with our 250ish horsepower and no significant transmission mods. Not trying to dampen your enthusiasm, merely trying to encourage you into a direction that will give you the most FUN for the bucks you spend.
angela

Angela,

I just can't seem to find a 915 for less than 5 big ones not counting the core trade in ($2500 bucks), ring gear and starter mods necessary. Am I looking in the right places?

Tomm

OH, and Angela, thanks!

Are you kidding? Ask how much for the non-gold plated ones. We have an 82 that we'll sell you as a core. It was removed driving but definately is worn and needs overhaul. All 5 forward and reverse gears engage. I'm in Atlanta right now, let me get back to town and see how much Steve wants for it. Should be cheap as we will sell it as a core only. It is an open differential, case is sound. Don't remember if the omega spring is still there or if we used it for the replacement transmission.

We've seen used 915's sell from $500 to $1800 depending on condition (open diff's). An LSD will add about $1500 to the transmission. LSD is not common and is a big time add when people sell these.

Getting to $5K on a 915 (plus core...) is awfully steep. What ring and starter mods are needed for your engine? Maybe that's what is soaking up the $$$.
angela

OK, Angela, I now am exposing my ignorance... What means open diff...?

I am interested in this core you have. Here is what one transmission rebuilder had to say:
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Tomm,

Pushing a 901 gearbox beyond 200hp is asking for trouble.

If you decide to try this make sure you cool the transmission fluid. I would also suggest that you use a good gear oil like Redline Heavy Duty.

An early 915 gearbox with a 7:31 ring and pinion will handle about 275hp without too much trouble. Above that you should add cooling etc.

A later 915 gearbox with an 8:31 ring and pinion will handle around 300hp without cooling.

You can pick up a used stock 915 gearbox for $1000 to $1200 US.

A rebuilt unit will cost closer to $2500 for stock ratios.

If you change the ratios you should count on $800US per gear set + labour, gaskets etc.

Changing 1st gear runs between $1200 and $1700US.

If your paying 5 G's for a 915, I'll pull mine out and sell it to you. Stock $1200 to $1800, like the lady said. I have seen them at half of those prices( of course I wasnt in the market for one when I did)

300hp in a fiberglass car (without a full cage) is interesting.

bt,

Interesting as in freaking insane or as in I would do that?

Tomm

Here is aero:

can I tell you how that would be a better picture?

Tomm as in insane. I've been in two major car accidents and walked away on both. I dont think number three is in the cards.

Tomm,
Open differential is the standard one most cars have. It allows all of the power to go to one wheel - the wheel with the least amount of traction.

The most common variation you will see on a 915 is called a "Quaiffe" or generically an LSD (limited slip differential). You will hear other terminology and arguments over the exact meanings, but powered shared between the rear wheels is the general idea.

Long and short - at 300 HP specify your 915 with some version of a limited slip. The driveability especially on wet pavement is dramatically better. I drove the spyder once in the rain with an open differential. Once. Never again.

angela

Tomm - whatever you do, DO NOT ask for a 'Queef' - oh, wait a minute, she said 'Quaiffe' - my bad!

Angela,

When you get home please figure out if you want to sell the transmission and contact me via PM. Jim, why not Quaife?

Tomm

"Interesting as in freaking insane or as in I would do that?"

No way. Absolutely not. I like living on the edge, but I'd draw the line on that one.

I'd do it. In a heartbeat. I'd want to pick my suspension, brakes and tires before the 300 hp, but yeah, count me in. I'd do it.

'Course, it might be better to NOT listen to me... I once shaved my head on a dare and another time rode a mountain bike naked (black berry vines suck...)

angela

The look on Vince's face as he reads the above post....Priceless!

If you are doing all this, you might as well go mid engine and displace the package tray. This will balance the car a lot and allow you to slam it. Make that front spoiler pliable and cheap to replace so you can drive it and forget the rear aerodynamics. It's the front air dam and side skirts that keeps it down on the road.