trebor_quitman

Hello all,

With all the tranny posts I've seen today, I became curious about somethings. Much of the follwing I assume will be very expensive, say in the realm of handing over your first born and your man-hood immediatley there after as well. Anyway, are there any methods of getting diferent gear ratios for our cars? Aftermarket gears, adjustable gear boxes (pricy, I'm sure), mixxing finals and other tooth counts from different trannys, etc... Can I get ahold of one of these 968 turbo gear boxes. Those things supposedly enable speeds exceeding 190.

Tabor

If you get a 924 gearbox of the correct year, at 7,000RPM you will be traveling in excess of 200MPH.

Other than that, Quaife makes an aftermarket gear box, but it isn't adjustable, and it isn't made specifically for our cars.

We don't have a bunch of afermarket gears for our cars like the 915 guys. I think there is 1 company in Excellence that advertises aftermarket gearing for the 951.

944Fest (aka Dan P)

FWIW, I read an article back in C&D when the Turbo S's first came out, where a customer had ordered shorter gear ratios in his Silver Rose S. The car was the C&D test car, and the shift to third came right at about 56 MPH, so the 0-60 time was no quicker than a normal S, but the car was noticably faster with less of a top end, I'm sure. So, somewhere, out there is a silver rose 944 T S with shorter gears. (If it is still alive) I wonder if the present owner knows it. And, if it was wrecked, if the junkyard/next owner knows it. Anyway, I don't know if there is an option code for such a factory mod, but it does exist! (C&D believed it was a wise mod, because you rarely use the top end in the USA, yet a few tenths from 0-60 are always nice!)
I think it ws C&D, maybe was R&T I'd have to check the archives to be sure.

Tom

I believe a modification was to use the 944 ring and pinion with the turbo transmission. The catch was that that r & P is not strong enough so you need to have a ready supply of replacements.

Danno

Roberto, what are you trying to achieve? Taller or lower gearing? Our cars have gearboxes optimized for their output, such that power-peak is geared to coincide with the theoretical top-speed (as computed from max-power).

Unless you have dramatically different power-outputs than stock, changing your gearing will most likely make you slower in top-speed. A lot of the Turbo guys have put NA trannies in their car for the lower-gearing and they don't last.

There's a gearing-chart on my website at 951 RacerX

Michael Stephenson

Sounds like the S2 gearing. I have heard of several Turbo guys slap an S2 tranny in their track cars to help them keep on boost coming out of turns.

I know that the Turbo gearing will allow you to get to about 100 in third while the S2 is good for only about 80 in third.

trebor_quitman

Great chart Danno/RacerX,

For the most part I was just curious, I think it would be neat to find taller gearing, but I obviously won't need it on US roads. I realize the HP has been matched to gearing for optimum performance, but when I have 20k to spare I will buy a Powerhaus turbo kit so I could push those taller gears to 190 or 200mph, while I'm at it I'll need a few mill so I can build my own personnal track to reach those speeds on. Dreeeeeam, dream dream dreeam...

86944turbo

Roberto,

I was looking for taller gearing in my 968 6 spd. The only place I found was PowerHaus II. The cost per gear was approx. $850. I was contemplating on increasing my top speed, so I was considering ordering new 4th, 5th and sixth gears. I decided against it for the time being. I do not have an opinion on this company or it's products. My car pulls readily to the RPM cut-off in sixth. Porsches come from the factory with fairly ideal gearing for the power output of a stock motor. As someone else posted, taller gearing can lower your top speed. Both the Viper and the 'Vette attain their highest speed in 5th, not 6th. The factory for these cars use 6th for meeting CAFE goals. The 944 turbo needs around 400 or so H/P to reach 190, approx. 470 to reach 200. Less if the car is lowered and mirrors tucked in, etc. But, it will also require taller gearing.

adie

I'm still working on this dilema myself, I get 102mph in 3rd but it's taking some 10.4 sec to get there so I really need to make the gears taller, still looking around to find someone who can provide reliabe advice to the strength of S2 gearboxes.

Robby

The C&D 951 S test car that Dan P mentioned had the Euro final drive- a 3.89 vs a 3.38. Powerhaus II has them, and many others, but, generally speaking, people usually go w/shorter gearing (vs taller), as it makes the car accelarate faster (like the C&D car- 13.9 @ 1/4 vs the normal 14.2). Ever notice how Porsche claimed a 13.5 @ the 1/4? That's b/c they were testing Euro cars- still, I don't think you'll EVER find a stock Turbo S that will run that fast, even w/the shorter gearing- the 0-60 times didn't benefit b/c of extra shift, but the 1/4 times did, as the shift into 4th will happen w/either gearing before the 1/4- w/US gearing, you shift just before you cross, costing several 1/10's- w/Euro gearing, you shift earlier, giving you more time in 4th, AND, more overall leverage. As for top speed- C&D said 5th redlined about 140mph- my math shows about 145mph, but whatever...

As for 968 gearing- I would not advise it under any circumstances- the 1st gear is the same, and all the other gears are much shorter- 968 redlines 6th at ~155mph, whereas a 951 redlines 5th at 167 using stock rolling diameters- when you see pics of people running 175mph @ 6000rpm, then they either have different gearing, RD's, or their speedos are off... Anyway, a 968 6-speed box would be cool, but only w/custom gears (IMO)- 45 (1st), 70 (2nd), 100 (3rd), 135 (4th), 170 (5th), and ~210 (6th)- that's about the way I'd like it for a street car w/some track use- with a good 400+bhp of course A 951 should need about 400bhp @ the wheels to hit 200 (based on formulas I've seen)- someone mentioned about 470 (crank I assume-?) which could very well be about right. Hell, neither a 993TT nor a 996TT can get much over 190mph, and that's w/just over 400 (crank) w/slightly less frontal area AND Cx- although they have more rolling resistance (bigger tires), and (I asume) slightly more drivetrain loss (AWD).

BTW, Danno's Racer X site has the most complete listing of Porsche gearings I've ever seen- you might want to check it out. You can plug in the formulas for speeds in gears using the RD's, rpms, and overall ratios in individual gears, and figure it all out- there are several formulas...
One that I use is: RPM X RD X 2.96 divided by the overall ratio X 1000 = speed in that gear (at given rpm)... For instance, a 951 S has a 3.38 FD, a .83 5th, and a RD of 24.68 (regular 951 is 24.86), so, the overall ratio of 5th is 2.8054, SO... 6400 x 24.68 x 2.96 -> 467,537.92/2805.4 = 166.66, OR, 167mph @ redline in 5th...

Danno

That was a doctored photo...

Easy way to figure out speed in gears is to use the MPH/1000rpm column on my gearing chart. A 951 has 26.442mph/1000rpm in top gear. At redline it has 6.4x1000RPMs, so 6.4x26.442 = 169mph. I did all the other calculations for you assuming an average of all three tire-sizes available.

The only way I see that's easy to break through the top-speed limit of our cars is to re-program the rev-limit.

Robby

Sure, Danno, you could remove the rev-limiter, but if the power drops off above the redline, then you still might not get much more speed out of it. It also won't change the fact that each 1000rpms in 5th is worth a hair over 26mph. You've obviously done your research on this stuff, and I really like your website- great chart too!

I use the formula I showed b/c it's very simple, and, is VERY accurate too, but it's not perfect. You'd have to take static ride height into account, tire pressures, etc. I hope that's right- static ride height?- the RD when the weight of the car is on it-> the tire's height will be slightly different off the car, than it would on, when supporting it's weight...

As for the shot of that guy running 175mph w/~300rpm left, I'm not sure it was doctored (I didn't think it was) although you may be right(?). I just always assumed his speedo was off. I've heard of several people that have claimed over 170 with room to spare on the tach. Many people are now under the impression that these speedos are off at higher speeds almost as bad as BMW's, and maybe some are- mine is pretty much dead-on, as far as I can see. I've had it just over 150mph on two different occasions at just over 6000rpm.

BTW, European Car had a few 951 articles several years back ('96 - '98)- mostly Powerhaus ones. One was in the summer of '96, I believe. - they took 4 Porsches to the desert- 1 brand new 993TT, 2 heavily modded earlier 911 T's (one was a cab) and, David Raine's 951 (supposedly ~330WHP). They were trying to hit the magic 200mph using speedos only- none made it. The 951 hit ~175 and had several hundred rpm left- there were other articles that mentioned similar results, but many people understand that they're gear limited to under 170, unless they change their gearing, and/or RD's.

Oh, one last thing- I have an article in either C&D or R&T where they tested a Viper, an NSX, and a 993TT-S. All were tested 2 ways w/both the speedo and a radar. All of them were off by a little, but were consistent, except the 993. It was the fastest, but it's indicated speeds were both over 200mph. The radar showed that it was off by ~12 one way and by ~20 the other! It wasn't even consistent- the NSX was the closest (w/in ~5mph), and the Viper was almost the same as the NSX, but that 993 was far off- even though it's REAL top speed (average) was ~190mph!

Danno

Well, not remove it completely, but most of the aftermarket chips will raise your redline from 6400rpm to 6750rpm (giving you 9mph more top-speed headroom). This would be good for 176-178mph.

That speed would only require about 320hp at the crank. Of which, there are numerous examples of 951s easily producing this power level, even at redline (more like 400hp+ at power-peak around 6200rpm). Well, I like to confirm the speeds with calculating actual speeds from the tachometer as well. This is of course dependent upon the tire's rolling radius as well (I like to use the manufacturer's revs/mile rating as it takes out cummulative errors with such tiny numbers in the radius).

Anyway, I believe the electronic speedo in our cars are really accurate. I've tested mine by doing strafing runs at a police radar booth on a local highway. At 100mph, my speedo indicated 1mph over. At 150mph, my speed showed 153mph. Going through the gearing-ratio calculations showed that the tach was right on as well. I re-checked this after getting new tires and the speedo was now slow by roughly the same amount as well.

To get spot-on calibrations and to compensate for different height tires or tire-wear, you can get one of these devices:
Electronic Ratio Adaptor Yeah, I was wondering about that too. Perhaps it must use a mechanical speedo with gears and a spinning cable or something? Or perhaps picking up interference on the speedo-sensor line? Who knows...hmmm... Why thank you. It's always great to find that people are getting good use out of my work!

Robby

Danno,

Yes I've looked over it several times, playing around w/various ratios, etc. As I said, I'd really like to have MUCH more power (wouldn't we all) w/taller gearing, or, a 6-speed, so it wouldn't have to be too much taller, but still offer an extremely relaxed top gear, and speeds over 200mph (assuming enough power were available). I had looked for the 968 gearing for awhile, as I didn't have any tests on it, but when I found out what it's gearing was, I was pretty disgusted w/it

Anyway, I'm curious about the Torsen articles. When driving a 951 w/out an LSD, it seems to be much easier to get the rear end out and play around- it feels balanced that way. But, in my Turbo S, I don't feel that way at all. I realize my suspension is shot, but I doubt that's all of it(?). When I floor it in a turn, it feels like it's picking up the front wheels, and the car starts to understeer. If floored hard enough, or at the right time (before the tires are loaded), it oversteers, but it doesn't feel controlled. Anyway, my point: it always seemed to me that LSD's would make a car understeer(?). But, when reading the article's on your site, it said the 300ZX w/out the Torsen understeered more. So, I was wondering, do you know if that car had any sort of LSD or not? I didn't see it anywhere, so I was thinking it was a test b/t a Torsen and a non-LSD, but, if so, the difference in times didn't seem as great as I would have expected. The story about the parking lot donuts was impressive, but it still seems weird to me. What do you think, and/or, do you have anymore info on the articles, or subject in general?

Danno

There are roughly three different types of differentials: open, friction-plate LSD and Torsen LSD.

You have to distinguish the Torsen LSD (standard only on '93+ 968 6-speed) from the friction-plate clutch-type LSD available for our cars. Mine had the optional clutch-type LSD and it was starting to wear out. Such that it was alternating between limited-slip mode and open-diff mode with only moderate throttle openings. This coincided precisely with maximum cornering-speed at turn-2 at Willow Springs. The result was that I had a wallowing rear-end that was swaying back and forth between oversteer and understeer, really disconcerting and annoying.

So the issue I have with the friction-plate LSD is that the torque-split isn't linear and progressive. In a straight line, it would have a 50/50 split, great. As you build cornering loads with throttle, the torque-split gradually gets to 60/40 and is still diverting more torque to the outside wheels with more traction, fine. But with more power applied with wider differences in grip between the outside & inner rear tires, it overcomes the spring-tension operating the clutches and then all of a sudden ZINNGGG!!! you're spinning the inside tire like an open-diff.

This behavior simply isn't acceptable. Sure there are other options on other cars like the 928 and 911 with 80% electronic lockup, but at some point, they too will turn into open-diff mode.

The Torsen on the other hand, will continuously and smooth split torque from a 50/50 straight-line launch up to a 0/100 split if you happen to get one wheel off into pavement (low-grip tire never spins). This transition occurs smoothly without any sudden breakaway behavior (like the friction-clutch action you're describing).

As for times, the tighter the course, the bigger the advantage of the Torsen unit. At the Streets of Willow Springs a couple of weeks ago, I counted at least 15-seconds per lap where I was spinning my tires (about 18% of the time). With a diff. that gets me more drive traction, I think I can easily shave off 5-seconds out of a 1:21 lap.