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Torque Converter Swap to Increase Stall Speed?

 
Old 02-03-2014, 02:26 PM
  #31  
James Bailey
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top speed is limited by the aerodynamic and rolling drag vs available power. The S-4 5 speed with 2:20 final drive is pretty much maxed out at top speed in 4th gear at redline and the shift to 5th drops horsepower down to the point where it pulls very slowly from that point. Has been described as "hitting the wall" .

Porsche never used a speed limiter on the 928 they just eventually run out of power
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Old 02-03-2014, 02:57 PM
  #32  
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Originally Posted by James Bailey View Post
top speed is limited by the aerodynamic and rolling drag vs available power. The S-4 5 speed with 2:20 final drive is pretty much maxed out at top speed in 4th gear at redline and the shift to 5th drops horsepower down to the point where it pulls very slowly from that point. Has been described as "hitting the wall" .

Porsche never used a speed limiter on the 928 they just eventually run out of power
When I was a youngster and heavy into 356 Speedsters, I found an article about the "joys" of driving Speedster. It talked about driving in the rain, with a huge wall of water sloshing back and forth from the back seat to the pedals, crashing over your feet, when you hit the brakes.

Was never too difficult to figure out why the floorpans were rusted out, 10 years after the cars were brand new....

Also talked about "hitting the invisible wall of Saran Wrap" at 90MPH.

For some reason, that image of hitting that invisible wall of Saran wrap has always stuck with me....all these years.
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Old 02-03-2014, 05:31 PM
  #33  
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Just couldn't see clear to go faster?

My first car, a '65 SC cab, lived it's first several years with a 50hp VW engine nestled in the engine bay. Top speed was 70ish. When I finally gained the means to put a Porsche engine in it, it was a 912 engine with about 100 HP. (vs 90 or less in the earlier Speedsters) That would get the car to almost 100 MPH sometimes. Seemed HP and RPM limited, and the HP curve for the engine pretty much confirmed it. Faster was going to require more power and longer legs.

Fast-forward: My '89 S4 sails easily past 100, with acceleration rate falling sharply as it goes to 4th gear around 120 or so on its own. It still pulls nicely to top speed (about 171 indicated, 165 GPS), but certainly no need for whiplash protection for that last upshift. Looking at torque/horsepower curve and gearing, the top speed is not going to be noticeably different with 2.20 final drive, vs the 2.54 installed now. The zero-to-125 times should be slightly shorter with the 2.54, and the last pull in top gear from 125 is definitely shorter. If I had 2.20, that top gear pull would be starting about 9 MPH higher though. You guys can bash MK for his 'gearing doesn't make any difference" preaching, but the reality is that net zero to top speed times are very close to the same assuming the stock torque map.


Driving a 928 in traffic with the higher-stall converter can be a blessing or a curse depending on your mood at the moment. IMHO, the gearing and available torque in my car don't warrant a higher stall speed. If I want it to go more quickly, I can manually select a lower gear. First gear is over pretty quickly and so is very seldom used, and after that the RPM's under hard acceleration would never drop down low enough that the converter would bypass much at all. Right now I have the TV cable adjustment set loose enough to avoid downshifts in traffic unless I stab hard at the throttle. When I got the car, I followed group guidance and tried tightening the cable by a few turns, then slowly backed it off to original plus another 2 turns. Right now, it would seem soggy/doggy to anyone else that might drive it. But for moping along in L.A traffic it's great, taking advantage of the available torque at the lower RPM's, rather than spinning up for more available horsepower that I can't use, then shifting up quickly as the throttle is lifted.

When the car is offered for sale, I'll put the TV cable back to at least the original position, maybe one turn tighter. That makes for a much more serious launch experience, more in line with what the world expects from a V8 supercar. In the meanwhile and at the current altitude, two-loose is fine. Moving higher than a couple thousand feet, I'll go back tighter one turn for normal driving to compensate for the change in air density.
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Old 02-05-2014, 12:50 AM
  #34  
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I'm a bit of a gun bunny when coming off a stop so a higher stall fits more my driving style Since S3's start in first anyways, options are limited to bowden adjustments or adding power.

Lucky Porsche never uses speed limiters. I'd really hate to top out the same as my Nissan Altima (around 120), though top speed of the 928 is around much of where BMWs are electronically limited (155).
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Old 02-05-2014, 04:42 PM
  #35  
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Do your tires hold pretty well in a full-throttle first-gear start? Mine, um, slip a little. And the first-to-second shift at hard throttle comes fast and hard. If you want the car to launch faster, press harder on the skinny pedal. When the car isn't quick enough the first fifty feet at full throttle, and you have solid traction the whole time, then look at functionally offering more launch torque with a higher stall speed in the TC. That's the only time a higher stall TC will help, unless you drop below converter stall speed with full-throttle redline shifts. If you find yourself needing more low-end grunt any other time, select a lower gear so the engine makes more horsepower at the speed you need it.
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Old 02-05-2014, 04:48 PM
  #36  
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Anniversary (almost) link: https://rennlist.com/forums/928-foru...converter.html


Here is a link for one of the places I mentioned that says they can rebuild with a modest uptick in stall speed. I did have some email exchanges but didn't get as far as asking about how loose/tight the result would be and what the heat penalty would be. I took this place fairly seriously as their approach seemed fairly standardized, their claims for improvements seemed reasonable and modest, as were their prices IIRC. If so inclined, you can drill down to the722.3 application, which I think properly distinguishes between S/S4 applications, and then under a separate tab (racing? performance?) you can revue the different degrees of modification they say they can apply. Some info on the web indicated that this operation was doing the actual work/ rebuilds for a number of resellers.

http://www.gopnh.com/Transmissions.c...ssion=%20722.3

I have a core and an interest, but don't have the resources/hutzpah at present to stare down at TT R&R...especially given the WYAIT factor.
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Old 02-09-2014, 09:53 PM
  #37  
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Old 02-09-2014, 10:53 PM
  #38  
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Just need it in auto! I know what you mean about all the WYAITs when all that comes off. I've got half-axles, trans mounts, shocks on the list and that's just what I researched. Probably a whole load more when I ask everyone what else.

I definitely get some rear spin and a nice chirp at full-throttle from a stop and a commanding shift from 1st to 2nd. Stickier tires will help with the grip.
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