| Re: [GMCnet] Transmission shift schedule [message #88453] |
Sun, 13 June 2010 19:32  |
Gary Casey
Messages: 448
Registered: September 2009
Karma: 0
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Senior Member |
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Note: The following is relatively verbose and there is a large attachment that I don't know how to post. You then have two choices - ignore this post (recommended :-), or ask me to send the attachment to you directly. Or tell me how to publish the attachment. Sorry for my lack of web skills.
Some time back someone asked about the characteristics of the transmission shift schedule. I finally got around to digging out one that I have. I couldn't make it work as a PDF, so I scanned it into JPEG format, making for a very large attachment. Sorry about that. The pattern is for a '68 Chevrolet, which is one of the more "aggressive" shift patterns at the time. Aggressive meaning that the shifts were at a relatively high rpm compared to those of Oldsmobile models. But there isn't that much difference, so the shift schedule still relates to ours.
The left side shows modulator pressure, controlled by manifold pressure and modified by governor pressure. The bottom is governor pressure, vehicle speed and line pressure. On the right is transmission output shaft speed. Let's see, where do I start? First, ignore the penciled lines - that is from a torque convertor clutch project I once worked on. The two-stage curved line is governor pressure as a function of output shaft speed(engine speed depends on which gear it is in - the ratios are 2.48 in first and 1.48 in second). There are 3 "N/V" lines that relate output shaft speed to vehicle speed for N/V's of 32.5, 37.5 and 42 (N/V is output shaft rpm divided by vehicle mph. I haven't figure out the N/V of the GMC). The diagonal family of lines is modulator pressure as a function of governor pressure at different manifold pressures (16, 18, 20 IN.HG., etc).
Now for the important part: The shift schedule is shown by the lines labeled 1-2 DR, 2-3 DR, etc. DET 1-2 is the shift point in the "detent" position, which means with the full-throttle switch closed. Note that there isn't much difference between the upshift and downshift points, meaning that you can always force a downshift at a fairly high speed, a good thing in my opinion. But note the part-throttle shift points: At 28 inches MAP the 2-3 upshift is at 2000 rpm at the output shaft (less than 3000 engine rpm), while the full throttle shift point is at 2800, a big difference. The 1-2 isn't quite so bad. Note that the 1-2 LO doesn't exist - the transmission will not upshift from first at any speed if in the LO position. Also note that the 2-1 happens at the same speed regardless of manifold pressure - there is no part throttle 2-1 downshift. Even the 3-2 downshift is very much inhibited at low manifold pressures.
Note that the modulator pressure is a function of manifold pressure, not manifold vacuum. For those of us that drive at high altitude, that is a problem since, short of going full throttle, the manifold pressure will be lower than at sea level at any given throttle opening. Barometric pressure drops at the rate of about 1 inch per thousand feet, so at my elevation of 7,000 feet full throttle occurs at about 21 inches compared to 28 inches at sea level. that means that there are no part throttle 3-2 shifts and unless I go to full throttle the upshifts are at hopelessly low rpms (at 21 inches the 2-3 upshift occurs at about 2000 engine rpm. Follow that on the chart? If so, you get an A in the course!). I suspect that many transmissions have had the absolute pressure modulator replaced with a vacuum modulator, as has mine. In that case, replace the IN.HG numbers with vacuum numbers by subtracting from 30, shown by the penciled numbers by the MAP
numbers. Then the high-altitude shift pattern becomes essentially more aggressive, a very good thing. How do you tell which your transmission has? Look at the lower right part of the transmission where the vacuum line connects. If the gadget is large and (usually) black it is an absolute pressure modulator. If it is small (usually) yellow and looks like a vacuum diaphragm is a vacuum modulator.
Conclusion: Even though the shift schedule shown is the most "high performance" version at that time, it is, in my opinion, very conservative and won't produce optimum performance. It will keep the engine noise down, a high priority at that time. What to do? There are people that build "performance" modifications that generally make the shift pattern more aggressive, but there isn't a lot you can do without making changes in valve diameters, etc. Most of the vertical and horizontal lines are by virtue of spring loads and they can easily be changed. The governor curves can be easily changed by adjusting the weight of the components and the springs values. Except that our governors are inside a dome, so a take-apart dome has to be made.
Sorry about the excessively long post. Hope it was useful to somebody out there.
Gary Casey
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| Re: [GMCnet] Transmission shift schedule [message #88461 is a reply to message #88453] |
Sun, 13 June 2010 20:24  |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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The GMCnet email list which you are using does not allow attachments. To get around this shortcoming the gmcmhphotos site was created. It was assumed that attachments would be pictures.
In your case all you can do is convert it to a jpeg and post it as a photo the the GMCMHPHOTOS site.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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, or ask me to send the attachment to you directly. Or tell me how to publish the attachment. Sorry for my lack of web skills.