| Re: [GMCnet] Gmclist Digest, Vol 42, Issue 114 [message #157025] |
Tue, 17 January 2012 07:04  |
Gary Casey
Messages: 448
Registered: September 2009
Karma:
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Senior Member |
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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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