Dan Cobb

I was having a discussion today with a guy who claims to know quite a bit about 911s. Let's assume the following scenario:
What is the top speed of a stock 3.2L, with 915 trans?
Would tire/rim size have any significant impact (positive or negative) to this value? How much impact per 1" of overall tire diameter?
How fast should it be able to go without serious modification?
I'm guessing 140-150, maybe more but not likely.

He also suggests that with the stock 915 gears, you would have to turn 10,000RPM to reach 160 MPH.
This didn't seem right to me, but he was insistant.
I'd like to be able to intelligently debate his claim (either way), but I am not certain what information would be necessary to calculate the correct value.

Any insight or mathmatical calculations/equations would be beneficial.
+++

Steve Weiner-Rennsport Systems

Dan:

FYI,..An '84 US-spec Carrera with 225/50-16 tires will go 158 MPH at 6500 RPM assuming that the engine is healthy.

With 245/45-16 tires, it theoretically goes 163 MPH, although thats not likely with a stock motor.

I have a special piece of software for doing gearboxes.

Dan Cobb

Thanks Steve, that's EXACTLY the information I was trying to locate.
+++

Corrales911

FWIW: My 85 USA Carrera Dealership glossy has a published top speed of 146 mph.

Cookie Porsche

Dan,
I would suggest that your 'friend' take up another field of expertise!

His 160mph/10k rpm would equate to 16mph/1000rpm! Not even a 1300cc Shoebox type city car revs that high at cruising speed. I suggest talking to him about the weather instead.

I haven't checked my manuals but as a rule of thumb all SC/Carrera 915 cars on standard ratio / tires (US may be different) run in the region of 22-25mph per 1000 rpm. I use 25 because it keeps me out of trouble on public roads (2 of my cars have NO speedometer!).

So - Steve is right (he always is...). His 158/6500 is 24.3mph per 1000rpm.

BTW your biggest enemy at that speed is the relative headwind, your cars aero dynamics - not your engine power or the weight of your vehicle.

You could tell your 'friend' that at 10k rpm you would cruise at some +/- 230mph - NOW that would be an interesting topic...

Tom

r911

"relative headwind, your cars aero dynamics - not your engine power"

- Well, close. The usual term is drag (total drag, not the Cd, which is dimensionless value used as a figure of merit for different shapes). The engine power is what will ovecome the drag. Drag is a force and the engine power will exert a force opposite to that via the rear wheels. More power means you can ovecome greater drag, and thus reach a higher top speed. All this is if the gears "let" you.

I would just ask the friend what is the basis for his assertions. maybe he can show you an equation... Ask him where the number comes from. There are equations for this, but I dunno where you'd find them. Race Car Vehicle Dynamics would be a good bet. You will still need various numbers to feed into the equations or into a software program based on the equations. Maybe just find the charts from an owners manual or the info from a test report in R&T and show him that.

d993

That makes no sense, because 245/45-16 tires have a slightly smaller overall diameter than the 225/50-16 tires, so going faster at the same rpm with a smaller diameter tire is impossible.
You'd actually be going slower.

d993

Redline on a 911 Carrera 3.2 (unchipped) is 6300 rpm.
I had a '89 3.2 Carrera, and I had it up to 6300 rpm in 5th gear.
I had a set of Fuchs 16x7 front with 205/55-16, and 16x9 rear with 245/45-16 AVS ES100's, and the speedo indicated 158 mph @ 6300 rpm. I don't know how accurate it was, but it seems about right if you do the math. My rear tires were about 0.8% smaller diameter than stock, thus showing 158 mph, when it was actually going about 156 mph.

911-3.2

The owners manual for our '86 states top speed as 146mph. I wouldn't be surprised if that is conservative considering it's the factory published number. Are the g50's geared differently than the 915 cars?

zsolt

I have an 85 model Euro Carrera, and before I brought it over the pond I drove it 260km/hr at redline on the autobahn. My speedo could be off, so this probably is not an accurate number, but it should be pretty close.

Zsolt

imcarthur

If you want to play with speed vs rpm . . .



Go to the website & download the excel file & put in the necessary figures . . . gear ratio, tire diameter etc. The figures on the left are for a stock 87 with G50 & 15' tires (except the redline, I noticed . . .). Ignore the middle & right hand side, my figures are bogus because I couldn't find them anywhere.

Ian

stv951

They say a picture is worth 1000 words. What about 3 pictures of my thoughts ! Sorry about the poor/shakey quality, but wife was scared. My car has a 6800 rpm rev limit. Bought it like that. I run 205s and 245s on 7x16 & 9x16 Fuchs. I have seen an indicated 260klms ++ a bit. That 360 spyder was still faster... Damn... Someday... For those asking it was the Autobahn - but ofcourse...

zsolt

Yup, I got a couple of those. Looks about the same.
Zsolt

stv951

ZSOLT give us a clue

afinepoint

Dan,

In the real world the best I have gotten my 84 up to is 125mph or so on the track. I'm sure I coud go faster but that would take alot more road. My engine is in very good shape but the 3.2Ls of my vintage are horsepower limited as far as rocket acceleration is concerned.

If I ever get out west I'll let you know what it can do.

Reg