275-40-17
1.004 4th gear ratio
3.37 Fd
7000 rev/min

7000 rev/min / 60 sec/min / 3.37 / 1.004 x 6.72 ft (OD) x 0.97 (avg slip) ~ 225 ft/sec ~ 153 mi/hr

 

I can see 150+ in 4th happening. If all situations are just right, you have a strong clutch, and your tires grow some...I belive it is possible.

 

My comment was in response to the on-going discussion below, not directed at your previous post.

As several of us have calculated and acknowledged – at 7k rpm and w/ 275/40/17 tires, it will do that. But w/o initially knowing the rpm and tire size from the original post, I thought it sounded questionable too.


Your specific speedo may not be off by 10%. But some of the VDO speedometers on these cars can vary at least 10 and upto 15mph at 150mph. Tachs also have error in them. So it should be known there can be a noticeable difference in the indicated numbers that people are posting for mph @ rpm. I have had enough of these cars over the past 17 yrs to see dramatic variation at high speeds. Its proportional error, so an 8% error at 50mph is 4, but 12mph at 150. I dont think I can tell the difference between 46 and 50, or between 138 and 150 without an external reference, like another car.

Regardless of error, a speedo is the measure of the transmission output shaft speed not ground speed. It doesnt account for tire growth either, and they are calibrated for stock tire size – you put on a different sized tire and they can indicate really far off from actual.

My gearing/speed calculations are correct. Ratios are ratios. As far as debating other factors - I will offer this: we are not dealing with top fuel drag tires here, low profile tire growth is not significant compared R/P changes, 500 rpm higher revs, 1" differences in tire diameter. If you really care, you would also need to worry about tire pressures, tread wear, and tire temps too (and the effect of ambient and road surface temps), compression due to the weight of the car (sidewall construction of a particular brand), etc, etc.


In any case, when a poster is trying to determine if they have a Turbo or S2 transmission in their car, is it practical for him to check the r/p ratio by running the car upto 7k in 4th on the way home from work tonight? A chart of engine speeds at 60 mph for both the Turbo and S2 trans is probably more helpful. But it can be harder to interpret because the charted numbers will only be ballpark compared to indicated numbers, due to speedo error & tach error; then dealing with indicated speed vs actual speed requires additional considerations (like tire sizes and your tire growth concern), and the ability to accurately measure actual ground speed.

For instance, assuming stock tire size, a Turbo will run a calculated 2290 rpm at 60mph in 5th. And an S2 will run 2460 rpm (assuming the .778 5th). Now if the S2 has 3% speedo error and the tach is accurate, it will be indicating 2340 rpm. 2340 compared to 2290 or 2460? Pretty hard to tell which trans is in the car. If you can and hold the S2 to an actual 60 mph (by GPS or other means), but the car has 275/40/17s instead of stock tires, then it will be at 2370 rpm. Again, not a real decisive indicator.

Probably best to look at the lower gears, something like 50 mph in 2nd (4730rpm Turbo, 5430rpm S2), or 60mph in 3rd (3860rpm Turbo, 4430rpm S2). That way, various factors and errors will not cause as much confusion with determining the trans ratio.

Or pull the trans and read the case code off the top of the bell, or open it up and count the teeth on the R/P.....

 

the claim was made of 7k+ and 145 in 4th. otta be able to do it again and post a pic of the 951 speedo/tach.

i leaned a long time ago, don't make a claim if you can't prove it and a lot of folks here questioned the initial claim until "over 7k and growing tires" was mentioned.

granted, the tires grow some with heat but you couldn't see it with the naked eye..

87951
95sl320 dd

 

although a picture would put this to bed, we do know that a 7k limiter is readily available with a chip. not like this is a 3.8 ford that redlines at 5200rpm.

mathematically it works perfectly.

i have 225/60/15 tires on the back of my car and with my turbo box it will pass 130 at 6000rpm in 4th. takes about a week and a half to get there though, but mathematically at 6500rpm with that size tire i can get to 143. (this is why my location says what it does)

 

I am not going to go out and run it to 145 on public roads to prove anything to you.

Having ran it out in 3rd numerous times I have no reason to doubt it will run over 145 in fourth considering it matches what all the gearing charts come up with in third.

What was said in my first post was that it was dyno proven.

Here are the sheets


Here is an old dyno sheet one has speed the other rpm. Sorry for the OT

 

Asking someone to go out and run their car that high while either a) having someone take the picture or b) attempting to take the picture themselves is a fairly stupid request just to try and win a pissing match you in fact started.

 

read my post again, nothing said about on the open road, TAKE THE PIC WHILE ON THE DYNO OF TACH/SPEEDO.

87951
95sl320 dd

 

Of course, going to the dyno is as easy as going to the store Once again, to attempt to prove your point you're telling him to go out of his way. GENIUS!

 

"Having ran it out in 3rd numerous times", sounds like he spends a lot of time on the dyno or someplace in warp drive.

a bold claim was made by the op and i questioned it.

many folks piped in with computations, i liked the one guesstimating 192 in 5th.

the op lives in colorado/denver at 5280msl, where manifold pressure is 5" lower than sea level.
have you ever driven thru denver in your 951, how'd it perform, as well as sea level because it's turbocharged?

87951
95sl320 dd

 

FYI, even my 147HP NA can wind up the top of 3rd gear in good time. pretty much every time i merge onto a freeway i am at least 6000rpm in 3rd by the end of the onramp.
a 275/40/17 tire has a total height of 25.8". this gives it a circumference of just over 81". there are 63360 inches in a mile, so each full revolution of that tire covers .0013 miles.

at engine speed of 7000rpm, in 4th gear (1.034:1), the output shaft is spinning 6770 times a minute (7000/1.034). the axle spins 1 time for every 3.375 turns of the output shaft...2005 times per minute (6770/3.375).

since we measure in miles per hour, that number is multiplied by 60.

that makes 120,300 revolutions of the axle shaft per hour maintained at that engine speed and tire size. since each revolution covers .0013 miles, those numbers multiplied equates to 156 MPH.

keep in mind that my calculator uses much more precise numbers in its calculations and comes to 154 top speed in that gear, that rpm and that tire. i was rounding off to the nearest whole number or decimal place.



the same math can be used for 5th gear. 7000rpm with a .829 gear makes 8444rpm at the output shaft. 8444/3.375 = 2502rpm at the tire, or 150,120rpH. 150120*.0013 = 195 (rounding error)

that dyno chart was done at that altitude, over 400 to the wheels despite altitude disadvantage. that thing would scream back down on earth. if given enough time and space im pretty certain he could feel a 7k rev bounce in 5th.

 

I personally feel a stock 951 box at roughly 7k rpm (+/- 100rpm) should only hit 180-185.

Either way, I would be more then happy to get you a picture of 149-150mph in 4th gear at 7k rpm...the again, my car is down to the short block at the moment. Maybe when the S2 is done, I'll attempt it. Just hate how long it takes in an S2...

 

i'm waiting. show me 7500 in 4th on tach/speedo.

87951
95sl320 dd

 

Much easier way to do this. Traqmate data using GPS on 17 inch wheels, not the "always reads high" speedo in the 944. The GPS data in the graph lags the real time RPM data slightly so the top point on the speed curve is 140.19MPH @6717 RPMs. So at 7K RPM, you add on 6 MPH for a speed of 146 MPH at 7,000 RPM.

 

thanks, great graphs, 7k/140, i mentioned that way back in the thread, double the rpm for speed in 4th with std gearing/xmsn.

so i guess that would make about 7250 for 145, my original claim of over 7k rpm.

how many of you folks spend time above 7k in your 951?

87951
95sl320 dd