Questions about trans

[ctucker]

I have a 1968 Camaro with a 327 and Powerglide trans. I was told that this tranny was made to downshift at high speeds and then upshift to get better speed. Does anyone know if this is true and if so where to find the info on it. My mechanic said that you got it up to 80 then downshift to low, get it up to 100 and then upshift. I am afraid to try this. HELP!

[CNorton]

Don't blame you for being a little dubious.  I've been around these cars for a long time and that particular part of the myth has escaped my attention.  Can't say conclusively that it is complete crap but I can guarantee you that I'd never consider it and I've beaten several Powerglides to death in race cars over the years.

Think it over before you try it.

[Nate Biery]

When I was in my teens I had an 67 Impala with a 327 and a powerglide that I could much the same thing, but not at that speed. If I wanted to pass someone who was doing 50 or so. I could drop it into low and stand on the gas. Pass the person and then up shift at 60 or so. it seemed to handle it fine. Never had an issue with the tranny shifting. Did it many times, but never tried it at the speeds you talking. I don't thik I would have tried it even when I was young and dumb.

[lakeholme]

Read the powerglide section of the 68 Chassis Service Manual.  And I quote, "A forced downshift feature provides a passing gear by returning the transmission to low range."  There is a chart there that tells what the shift points are.  And yes, there is a shift point range at full detent on the base 327 in the 60s and even into the 70s on the L30.  But you need to listen to the guys above.  Do that enough times even in the 40s range and you will eventually understand by what was meant by the phrase "my transmission is slipping" back in the day. 

[JohnZ]

I have a 1968 Camaro with a 327 and Powerglide trans. I was told that this tranny was made to downshift at high speeds and then upshift to get better speed. Does anyone know if this is true and if so where to find the info on it. My mechanic said that you got it up to 80 then downshift to low, get it up to 100 and then upshift. I am afraid to try this. HELP!

Trying that will leave you with a lot of aluminum and steel parts scattered on the ground behind the car. The kickdown linkage provides the downshift automatically from 2 to 1 when you floor the accelerator below about 70mph; above that, it won't downshift. If you manually downshift it to Low at 80 mph, it's NOT going to go any faster - it'll just leave you with a big repair bill. I don't know where this stuff comes from, but it's really stupid.

[tom]

In the day I had a 66 cutlass 2 speed auto. When stomped at speed even above 60 at times, it would kick down and go. On a wet highway, it even broke loose occasionaly from over 50 when I punched it. Loads of fun, nothing left of that one.

Tom

[JoeC]

upshift/downshift also depends on rear end gear ratio

In the 70s I bought a 1963 Impala 4 door 327 4bbl (i  think ) 250 hp with PG.
I bought it just for the engine but drove it for the winter and that thing would downshift at high speed and that 327 would screem until it up shifted again.

[rich69rs]

...as JohnZ stated, hard to believe where this all gets started.  Not only do you run risk of destroying the 'glide with a forced to low downshift above 70 mph - the rest of the drivetrain, including the engine, isn't going to like it either.  Take a look at the following link which has the 1971 Chevy "Theory and Diagnosis" auto tranny manual - in detail describes how a Powerglide and a TH350 work.

http://www.tocmp.com/manuals/trans/TTransChev1971/TTransChev1971.html

Quoting from page 14 in the Powerglide section: 

"Through-Detent" Forced Downshift. At all road speeds below approximately 70 to 62 m.p.h. the transmission can be automatically shifted to low range by depressing the accelerator linkage "through detent". This causes the production of maximum TV pressure and moves the detent valve to open TV pressure to the detent passage. The resulting full TV pressure over-comes governor pressure acting on the shift valve. This action shifts the transmission to low gear as previously described (fig. 22PG). The transmission will remain in low gear until either the accelerator is released, or the car speed reaches the maximum upshift point (68-74 mph). At road speeds above approximately 63 m.p.h., governor pressure acting on the shift valve is high enough to prevent TV pressure from moving the shift valve. Therefore, regardless of accelerator position, the transmission will remain in high gear at all road speeds above approximately 70 m.p.h."

.....and again from page 15:


"....a forced downshift can be made by moving the manual linkage into low range.

CAUTION: Do not force a downshift at speeds above normal "through detent upshifts" speed. See the Power glide shift point chart in the appropriate Chassis Service Manual for specific vehicle speeds."

I have a 'glide in my '69RS.  The manual kickdown linkage, which controls the tranny TV valve, is adjusted so that if I keep my foot in it, the car will upshift from low to drive between 60-65 mph.  If I am cruising below 50-55 mph and put my foot back in it, the tranny will downshift to low...however, I do not do this routinely.  Too much time and resources invested to have an original aluminum trans reduced to scrap.

[ctucker]

  Hey, Thanks you guys for your input on my question. I am really glad I asked before trying this. I was afraid that I would tear the tranny out if I did! I haven't had the car that long and would really hate to have to set it up after waiting 20 years to get it. Again thank you!!!