| Re: [GMCnet] Gmclist Digest, Vol 42, Issue 114 [message #157025] |
Tue, 17 January 2012 07:04  |
Gary Casey
Messages: 448
Registered: September 2009
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Thanks, Bob, for the link. Very interesting. Like you say, there are some errors. A couple that might be of interest to us: The losses in the transmission can't be expressed as a single number - he said the THM-400 loses 45 hp. That might be true under some condition, but it has little or nothing to do with us. I know that there were some "heavy duty" transmissions for trucks built with straight-cut gears - would that be the THM-475? I don't know.. Yes, there were some built with different numbers of plates in the clutches, but almost all were the same. The differences in the myriad of different models were almost always very slight changes in the valve body and accumulator springs so that the shift feel could be dialed in for each engine. I'll bet that most (used for the same engine displacement) could be interchanged without anyone being able to tell the difference. The comment about the switch-pitch transmission was just plain
wrong. The converter will usually have a stall speed of about 2,000 rpm in low pitch and 2700 in high, but it WILL multiply torque in both positions. In low pitch (or do they call that "high" pitch?) it will have a high multipication at stall, but it will drop rapidly, as it "locks up" sooner. In low pitch it will have less multipcation at stall, but will carry it out further. So the normal control was to go to low pitch at idle and full throttle, the former to reduce creep at idle and the latter to produce some multipication at higher speeds. Usually, transmissions with switch-pitch converters would have less downshift capability, relying on the converter for the "passing" gear.
Gary Casey
From Bob:
This site has a lot of info on GM transmissions. Although there are a fair number of errors, it's still worth reading:
http://www.442.com/oldsfaq/oftrn.htm#TRANAutoTransmissionComparison
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| Re: [GMCnet] Transmission notes [message #157029 is a reply to message #157025] |
Tue, 17 January 2012 07:18  |
Gary Casey
Messages: 448
Registered: September 2009
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________________________________
From: Gary Casey <casey.gary@yahoo.com>
To: "gmclist@temp.gmcnet.org" <gmclist@temp.gmcnet.org>
Sent: Tuesday, January 17, 2012 7:04 AM
Subject: Re: Gmclist Digest, Vol 42, Issue 114
Thanks, Bob, for the link. Very interesting. Like you say, there are some errors. A couple that might be of interest to us: The losses in the transmission can't be expressed as a single number - he said the THM-400 loses 45 hp. That might be true under some condition, but it has little or nothing to do with us. I know that there were some "heavy duty" transmissions for trucks built with straight-cut gears - would that be the THM-475? I don't know.. Yes, there were some built with different numbers of plates in the clutches, but almost all were the same. The differences in the myriad of different models were almost always very slight changes in the valve body and accumulator springs so that the shift feel could be dialed in for each engine. I'll bet that most (used for the same engine displacement) could be interchanged without anyone being able to tell the difference. The comment about the switch-pitch transmission was just plain
wrong. The converter will usually have a stall speed of about 2,000 rpm in low pitch and 2700 in high, but it WILL multiply torque in both positions. In low pitch (or do they call that "high" pitch?) it will have a high multipication at stall, but it will drop rapidly, as it "locks up" sooner. In low pitch it will have less multipcation at stall, but will carry it out further. So the normal control was to go to low pitch at idle and full throttle, the former to reduce creep at idle and the latter to produce some multipication at higher speeds. Usually, transmissions with switch-pitch converters would have less downshift capability, relying on the converter for the "passing" gear.
And the triple-turbine dynaflow didn't shift at all, and the turbines didn't, as I recall, deliver power through any gears. They were set up at different pitches and were connected through free-wheel clutches. There was what was essentially a Powerglide gearset in the transmission, but that was downshifted only in manual low range - and used for reverse. There was a Dynaflow "4100" used in the Buick special that had no gears at all - reverse was obtained by locking the turbine and connecting the stator to the output shaft. How it ever got into production for the mercifully few years is a mystery. And there were two different gearsets used in the Powerglide the "light" one had a 1.82 low, while the "heavy" one was 1.76. The heavy one was very strong and is often used to this day in drag cars.
There were some interesting times at "The General."
Gary Casey
From Bob:
This site has a lot of info on GM
transmissions. Although there are a fair number of errors, it's still worth reading:
http://www.442.com/oldsfaq/oftrn.htm#TRANAutoTransmissionComparison
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| Re: [GMCnet] Lock-up converters [message #157036 is a reply to message #157030] |
Tue, 17 January 2012 08:58 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
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| Gary Casey wrote on Tue, 17 January 2012 06:21 |
A question on torque converters - has anyone looked into the possibility of adapting a late model converter with a lock-up clutch to the THM-425? Converters are all similar enough in overall configuration that I'd bet it is almost possible to do that. Anyone looked into that? It would perhaps be one of the best things that could be done to modernize the driveline.
Gary Casey
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I think Manny has done that. Hopefully he will chime in.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] Lock-up converters [message #157115 is a reply to message #157036] |
Tue, 17 January 2012 18:59 |
mannystrans
Messages: 209
Registered: June 2006
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Gary,
Early last year I called my converter shop and ordered my normal 20
converters. He wanted me to bring him the cores first. I only had
three.
This presented a problem because I ship so many trannies and don't get
the cores back because the shipping charges are higher than what the
core is worth.
We discussed the possibility of using a different converter like the
12-1/2" lockup that had almost the same stall. It has a different
spline.
I had a new stator shaft made and using the s/p passage it seemed to
work on the test stand. At that time I also inquired about 13"
converters and someone had a stash of 200 cores. I had them built me
converters for our application with my specs and out of that stash I
got almost 150 rebuilt converters. It was enough for last year and the
next two. Since then and until I exhaust these converters, I will
store the take-out converters for future rebuild.
Eventually i will have to go with the 12-1/2" converter but for now
it's on hold.
--
Manny Trovao
mannystrans@gmail.com
Manny's Trans / Power Drive
San Jose, California
408-937-1583
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