Todd T

I was reading a thread started by Mongo about installing a different torque converter to change the stall speed for increased performance:
https://rennlist.com/forums/928-foru...h-us-cars.html

I was going to post in that thread, but figured it would be better to start my own as not to distract from that original discussion.

Stall speed is only part of the equation, you also need to take into account torque converter flare. Torque converter flare is when the torque converter slips or uncouples between gear changes. The action of flare keeps the engine RPM's higher and closer to the peak HP of the engine after / during a gear change. Because naturally, everybody knows the closer you are to peak horsepower the faster the car is accelerating. This is all about going fast!

Higher stall speed typically goes hand and hand with more flare or slip, however it's not really that simple. Some assume or think it's the same thing while it's not always a 1:1 relationship.

When I first started playing with different torque converters in my 941rwhp/751rwtq twin turbo 928, I installed a stock 11" from an early car (S4 torque converters are 12", typically the smaller the converter the higher the stall speed & increased flare). I could feel the stall speed went up a little bit higher and because I'm making so much torque I could feel it flare a bit more too, keeping the RPM's higher between gear changes but not enough.

So then I modified an 11" torque converter by cutting and angling the pump veins (about the last 1/2") so that it wasn't such a direct hit on the turbine. I also increased the clearance between the pump & turbine, clipped back the entry / exit of the stator by changing the shape of the little "wings", and on the pump moved the angle of the veins by cutting, reshaping & silver soldering them into the new location.
All in this took about 100 hours, I had to create my own tooling to make these modifications.
This increased the stall speed a bit, not as much as I was hoping, maybe another 800rpm and also helped with the flare.

The next step was using a 10" torque converter from a 190E Diesel. This is NOT a bolt in application.
The bolt pattern to the flex plate and the stub nose is the same, however, the transmission side is different. The pump nose is much smaller so I had to cut that out and weld one in from a 928 converter. The turbine spline also had to be exchanged from a 928 unit, and the stator itself has to be exchanged form an 11" (which also had to be modified to fit, it's too big in stock form).
Once everything was modified to fit together, I could not source the appropriate rivets to hold the turbine wheel to the 928 drive hub, so I had to design and make my own. This took some trial and error testing 3-4 different types of metal until I found the right formula to be as strong and malleable as the stock rivets.

Initially I setup this converter very loose via modifications to the pump veins. The results were an alright stall speed (around 4k rpm), but the flare was to much. To remedy this, I started with another 10" torque converter, made the modifications necessary to fit the 928 and changed nothing else. This worked out good but I discovered the clearances between the pump and turbine housing were too tight causing them to rub. This meant I had to repair the turbine veins, the outer edges had to be straightened and re-silver soldered. This additional silver soldering added more strength so it worked out good in the end.

Now the stall speed is around 3,500rpm and the flare keeps the engine above 5,900 - 6,000rpm at wide open throttle between shifts to my 8,000rpm redline.

I have an 89 transmission with 2.54 R&P and 255/50-16 tires. At 70mph or so I'm spinning about 2,800rpm. The converter is slipping a little here, but I've never had issues with transmission fluid temps in daily driving or down the drag strip.

To summarize, this is why swapping around torque converters is not an exact science. You won't really know exactly what you'll end up with until you try it. The person that taught me the torque converter science once said, he usually isn't happy with a custom torque converter project until he's tweaked or changed it about five times, and ironically that's about how many variations I went through.


After all of this was dialed in, the next challenge was the drive shaft after I cracked a stock one (which was rifled drilled from both ends, not all the way about 8" in)

So I made a new driveshaft using drawn over mandrel tubing, shown in this thread:
https://rennlist.com/forums/928-foru...-the-back.html

For those new to this project, this setup ran the 1/4 mile in 9.5 @ 152.7mph.

The last year or so I've concentrated on putting this car on a major diet. So far I've knocked off 400+lbs from the configuration which ran down the strip in 9.5 seconds. Even at the heavier weight, the data logs supported this car is capable of running down the strip much faster but I have yet to apply full power in 1st gear and I'm still taking it easy on the launch which has a significant effect on ET & trap speed down the 1/4 mile.

Todd T

Here are the photos of the modified Mercedes 190E diesel torque converter. You can see where I had to silver solder the veins:

andy-gts

wow where do you get the time......pretty cool....though

GregBBRD

Interesting.

An 11" convertor would barely move Andy's car forward when full throttle was applied. A 12"convertor was better, but still way too much slip to be of any use.

Must have more torque right at the stall speed?

SwayBar

Todd's work is ultra-impressive as usual, he never ceases to amaze...!

Just thinking out loud, I wonder how practical it would be to swap-in an 8-speed C7 Z06 auto tranny?

According to this thread, post #4:

https://www.corvetteforum.com/forums...mum-hp-tq.html

..they are rated at 885 ft lbs of torque.

But perhaps an 8-speed automatic is not needed?

Mark Anderson

The first time I met Todd was at road America maybe 15-20 years ago. He was telling me about his “home made “motorcycle. It looked a bit Mad Max but the more he talked I quickly realized his genius

Landseer

Hacker, can you repost the links to the turbo car build pictures for us?

(Just because they are really cool pictures and happen to also drive-home the 'genius' aspect Mark mentions.)

ptuomov

Is the car still designed to use nitrous shot at low rpms on the drag strip?

I think that an optimally set up automatic transmission will in the end have a higher potential on a drag strip than a manual transmission. That's because the automatic transmission allows the engine to operate closer to the peak power rpm more of the time. This is in my opinion not the case with an unmodified stock S4 automatic transmission, which I believe would be beaten by an unmodified stock S4 manual transmission even off the line. However, an optimally modified automatic transmission allows for the engine rpm and wheel rpm to decouple in a way that applies more power at the wheels than a manual transmission.

Analogously, I think that the stock S4 automatic transmission requires more torque at the low rpms than the stock S4 manual transmission for similar driver experience. However, if the transmission is modified for drag strip, then the opposite is true: The modified automatic can deal with a peakier engine than manual transmission.

Because of the fact that the manual transmission doesn't slip, I want my turbo car to hit the 1st gear traction limit at as low an rpm as possible and then maintain that level of torque at the wheels. This is one of the reasons why I like the stock S4 intake manifold for a turbo car with a manual transmission, or unmodified S4 automatic transmission. The stock S4 manifold moves great deal of air on its own in the flappy closed mode at and under 3000rpm, which helps to spool the turbo at lower rpms.

123quattro

Nice work Todd. That's pretty amazing stuff.

Yes, and auto will be faster at the strip. This is the case with the new Vette. It's easier to launch harder and not shock the tires with clutch engagement.

Imo000

Excellent write up Todd.


This is only the case with the more modern automatics, the ones made when and after the LT1 engine cam out. Now if you are running really high horsepower in older cars at the drag strip, automatics are a lot more consistent so a lot of people prefer them especially in bracket racing.

Chris Lockhart

Todd you truly are a genius, 928 or not. Your turbo 928 is the Holy Grail car IMHO.

IcemanG17

hacker-pschorr

Todd's car has over 750 ft/lbs of torque. How much does Andy's car have?

WestInc

Found this tread today. Very interesting and informative redaing. Super impressed with the craftmanship that went into this. Another level all together, is the car still with you Todd today?

hacker-pschorr

Todd doesn't pay much attention to Rennlist anymore. The vitriol criticism he's witnessed towards other people's projects and snide comments made about his projects offline caused him to lose interest in the 928 forums. He's local to me (just met with him today) and he'll never stop developing this car.
Over this past winter he installed a Motec M150 along with a new intake system to feed the turbo's and initial testing it's making more power everywhere and that's with the boost turned down.
His ultimate goal is a dropping his car to 2,700lbs or less while maintaining a 58% rear weight bias for the best possible handling. He's easily over 1,000rwhp right now and that figure will be increasing as the Motec tuning continues and the boost is turned up.

If you are new to this project, check out these threads:

https://rennlist.com/forums/928-foru...ht-design.html
https://rennlist.com/forums/928-foru...-the-back.html
https://rennlist.com/forums/928-foru...hart-lake.html
https://rennlist.com/forums/928-foru...nd-shocks.html
https://rennlist.com/forums/928-foru...-a-veyron.html

Todd said something to me today that only Todd could say: "Making over 1,000hp with the 928 engine is easy, getting it to handle with that power is the challenge"