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Torque Converter Swap to Increase Stall Speed?

 
Old 01-26-2014, 10:58 PM
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Avar928
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Default Torque Converter Swap to Increase Stall Speed?

I know there isn't too much interest in this but still some based on threads I've read regarding increasing stall speed to give the 928 better off the line response (yes, they are GT cars but can't hurt to get them to nice cruising speed faster from dead stop). Our cars can be made quick, they've got the power but it seems the transmission at least is what's holding it back.

Aside from having the TC sent off and machined to achieve a higher stall speed, has anyone swapped their US one for a Euro/ROW?

A28.04s for the S3s have a stall range from 1650-2050rpm while ROW/Euros A28.08 and a bunch of others have larger ranges. I think the top is 2200-2500 with the A28.08. Any experience with this modification in swapping or is machining the better route? Has the rattling normally associated with increased stall been fixed for those who've done it?

I was also thinking of getting a whole used ROW/Euro transmission with the gearing and stall I want then having it rebuilt and just swapping it into my US early '86...unless those Corvette transmissions become available for autos to bolt on.
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Old 01-26-2014, 11:12 PM
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I think Todd has a converter that is rated a bit higher. I am not familiar with people swapping the OEM for OEM to get a different stall. If they are otherwise basically the same, it may work and you may like it.

The domestic drag guys go up to 4000rpm and beyond!
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Old 01-26-2014, 11:15 PM
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Originally Posted by BC View Post
........
The domestic drag guys go up to 4000rpm and beyond!
Yes and good thing they only drive 1/4 mile at a time
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Old 01-26-2014, 11:22 PM
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I'm making a 2015 summer list of stuff to do when I have money and dropping the transmission to be rebuilt/swapped is on top of the list in addition to WYAIT rear suspension and half axle replacement (since I have to drop these anyways to get to the trans).

I read a 2200rpm stall is optimal to keep the car streetable. Not sure if I have this completely right but you don't want your stall speed to be too close or above your highway cruising speed because the trans will slip a lot and be more prone to failure? I'm guessing swapping for a EURO/ROW trans is better due to different gearing? So that means different cruising speed relative to stall speed to keep things healthy?

Note: I really have no idea what I'm talking about in that last paragraph.
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Old 01-26-2014, 11:33 PM
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Go for an S2 transmission, they have the best gearing for a 4 speed auto and aren't to hard to get from a parts car in the UK.

Your top speed will be lowered to 144 MPH from 155 and your cruising RPM will be a smidge higher.

The only other option for a good trans swap is a 89+ transmission, those have (correct me if I am wrong) 2.95 gears. They are larger than the pre 87 transmissions though.
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Old 01-26-2014, 11:54 PM
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IIRC, KillaV had a shop rebuild his torque convertor to a 2800 rpm stall. He seemed to like it. You might PM him and ask him the specifics
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Old 01-27-2014, 02:28 AM
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Originally Posted by soontobered84 View Post
IIRC, KillaV had a shop rebuild his torque convertor to a 2800 rpm stall. He seemed to like it. You might PM him and ask him the specifics
Cruising at 60-70 mph on the highway, you are below that rpm...wouldn't that be annoying ? contantly engaging..disengaging...to maintain hwy speed?
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Old 01-27-2014, 02:40 AM
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Dunno. I never cruised in his car at that slow a speed.
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Old 01-27-2014, 12:37 PM
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I had the turbine machined .250 and it increased the stall to about 2400. My converter seemed overly "tight" with a very low stall speed before I had it rebuilt. The stator was fine (which I suspected might be the issue).

But, then I added the turbo, and the extra 100 ft/lbs or so, made the converter stall to high for my tastes, so I swapped it back to a stock converter about a year and a half ago.

The modified one worked great with stock type power (my car dyno'd at 290 rwhp with x-pipe, rmb, and Roger's EIS system). It will feel weird to somebody that is used to the factory converter. It's not bad, its just different. My car launched much better from a stop, but it stalls a little more on the highway, and will cost you about 1 to 2 mpg......

The slippy converter is sitting in my garage collecting dust. I haven't tried to sell it, because right after I added the turbo in 2010, one of the torque converter bolts backed completely out, and would periodically get caught up between the converter and the aluminum case (happened about three times total before Sean led me down the right path). It put very slight dents in the exterior of the converter. I replaced all the converter bolts, and motored on for a year and a half with no issues though before I swapped it out....
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Old 01-27-2014, 12:43 PM
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Originally Posted by Tony View Post
Cruising at 60-70 mph on the highway, you are below that rpm...wouldn't that be annoying ? contantly engaging..disengaging...to maintain hwy speed?
I used to have an old 1968 GMC truck with a 383 stroker and I was running 373 gears, and a 3000 stall converter. The converter was 10" diameter too, which makes a huge difference in the characteristics of the converter. Plus, the truck was HEAVY.

That thing was terrible to drive on the highway. It would stall and slip constantly.

The modified converter in my 928 was not like this at all. It was very subtle. It did slip more, but it wasn't like riding the clutch til 2400 RPM then instant lock up. It's hard to explain. It just loosened things up a little to let the engine get more into it's torque curve but wasn't radical by any means....
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Old 01-27-2014, 10:05 PM
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Default Torque is way more important than HP

In the wayback machine, a user "Pappy" left a pretty good trail about his modified torque converter- not great results in the end but diagnosed as poor workmanship on the initial modifications, IIRC. There a couple other threads but a general paucity of real information/testing.

As for calibration, IIRC the rule of thumb is to shoot for TC stall speed close to the engine TQ peak, and refine from there.

A Euro TC should be good for a slight improvement....I inquired about scoring a used one during the recent seasonal sales, but no luck. As for the S2trans...well, might as well just score one of Gregg's S3 RoW, if they aren't gone! The cost/benefit balance might be tough to swallow though.

I have picked up a core, on the off chance that I ever find a shop I am confident can rebuild to match my intentions and that I'll have a way to undertake the R&R work...pipe dream right now.

Worth noting that, unless the MB/Porsche TC is some kind of new mystery apparatus, within certain boundaries a higher stall speed does not necessarily require a looser TC. Also stall speed and lock-up are different parameters.
http://www.ls1gto.com/forums/showpos...64&postcount=1

I've corresponded with a shop in NJ that advertises some MB and Porsche mods, asking about both valve body and TC mods. They say they can do the TC but I haven't gotten solid info on their approach to the mods. There is also a rebuilder that claims they can rebuild to higher stall, but again I haven't had the temerity to give it a shot.

Last edited by SMTCapeCod; 01-27-2014 at 10:30 PM.
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Old 01-27-2014, 10:20 PM
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I was directly involved with both Pappy's and Rob Rossitto's converter modifications. I was the one taking the damn things in and out.

Turned out it wasn't a science, but a guessing game....with zero ability to duplicate a second time.

As I recall, Pappy found one that he could live with and left it in. Rob eventually gave up and we installed a Euro convertor into his transmission.

I still have one or two of those euro transmissions with the shorter ring and pinions, limited slips, and higher stall convertors....for the '85/'86 vintage cars. Beautiful to behold....every piece is brand new. I think I even have one with the uber short ring and pinion.
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Old 01-27-2014, 11:20 PM
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So I take it with machining it's still a gamble but if done right and at the proper RPM will be what I'm looking for.

Very very tempted Greg. I could pull the trigger but I know it'd be very unwise financially right now. I actually was just looking at that thread not too long ago. You could basically eat off of it, so clean and fresh. I don't know much about transmissions but how do those special EURO trans affect the overall performance, top speed, etc...? Shorter gearing so the engine should stay in the power band more and you get better response, acceleration and top speed?
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Old 01-27-2014, 11:35 PM
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Notsomuch.

Higher (numeric) gear ratio in the differential delivers better torque multiplication- stronger standing starts and stronger torque but will limit top end at least in one respect, the (numerically) higher ratio means that the engine is operating at a higher rpm for a given speed/velocity. So all other things notwithstanding, top speed of a car with a 2.73 diff would be lower than a car with 2.20...again assuming we are just talking about redline as the limiting factor. However, the 2.73 should provide a better 1/8 mi, 0-60, and 1/4 mi time.

Given dc/md habitat, unless making a lot of roadtrips, I'd definitely favor a diff in the 2.7-3.55 range over 2.2. So much so that a new trans is the best COA? Well, that's a personal decision...

But if we aren't careful...this conversation will lead straight into Dinsdale's thread on retrofitting the vette six speeds....that have the potential to offer a steeper low gear and taller top gear...
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Old 01-27-2014, 11:55 PM
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Yea, I can always read that thread about Corvettes trans. I get what you're saying, I'm a stickler for trying to get it all so I'd probably need to compare all that stuff before deciding which route to go. I wouldn't mind sacrificing several miles off the top end to get much better acceleration on the lower end as well since our area doesn't really allow you to open them up.
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