M40 adjustments

[TimG]

I’m afraid I know very little about M40 transmissions and my ’70 Z28 has the original one installed in it. I’ve heard that there is a way to adjust the firmness of the shifting. This transmission doesn’t have a shift kit in it. Is there a way to make adjustments to the transmission aside from installing a shift kit?

[Hotrodpaul]

I believe the TH400 uses a vacuum modulator to vary line pressure based on engine vacuum. There are adjustable modulators available that allow the driver to vary the line pressure with the turn of a wrench. The modulator is mounted on the passenger side of the tranny and has a steel line going to the intake manifold vacuum fitting.

Paul

[WILMASBOYL78]

The factory CW code TH400 trans was the unit that was installed in the L78 cars and the Z28. Hi-shift point with the large can style modulator...the adjustment screw is sealed on these units with a glue/wax type product. I have never tried to loosen the factory fitting...the aftermarket small gold colored units have an internal adjustment screw. Just remove the hose and turn it in to restrict vacuum and create a later and firmer shift. There are shift kits available for the TH400, but the factory hi-perf unit is pretty good "as is".

wilma

[RPOLS3]

You can also buy aftermarket kits with different weights - similar to distributor advance weights - that will alter shift points.

[John Brown]

[ QUOTE ]
You can also buy aftermarket kits with different weights - similar to distributor advance weights - that will alter shift points.

[/ QUOTE ]

I take it that you're talking about the weights in the governor assy.

[RPOLS3]

Correct - I have a "CD" coded low perf M40 in my 72. By adding a shift improver kit (not a full blown shift kit) and tweaking these weights I was able to get it to shift where and how I wanted it to. It is a little bit of a trial and error process but is not overly difficult. It has performed flawlessly behind a GM crate 454/425 for 8 years now.

[TimG]

Thank you for the information. It is the original CW transmission and it shifts very well now. If it is adjustable, I was going to play with it to see if I could make it bang a bit harder. http://www.yenko.net/ubbthreads/images/graemlins/3gears.gif

[Mr70]

Tim
Which posi rearend ratio are you running...3:73...4:10?

[TimG]

The original 4.10, it's kind of a beast.

[Chevy454]

LOL...all these guys wanting firmer shifts from there 400s, and I'm going the opposite direction... http://www.yenko.net/ubbthreads/images/graemlins/hmmm.gif

http://www.yenko.net/ubbthreads/images/graemlins/naughty.gif

[WILMASBOYL78]

I don't htink the governor will have a big impact on normal driving upshifts....adjusting the modulator or modifying internal pressure with a shift kit will work better. The governor has more of an impact on WOT shift points and down shift for passing gear.

Tim, I would try using an adjustable modualtor(aftermarket) and not mess with the factory unit. The more you restrict the vacuum....the firmer the shift and it will hold a little longer.

CW tranny and 4.10 gears are a trip...we have that setup in our 70 L78 car and it really works http://www.yenko.net/ubbthreads/images/graemlins/grin.gif...and, with the stock modulator. http://www.yenko.net/ubbthreads/images/graemlins/beers.gif http://www.yenko.net/ubbthreads/images/graemlins/3gears.gif

[TimG]

It sure does, if I hit it just right, I can leave long black marks in second gear.

[RPOLS3]

Wilma you are correct.

The governor revisions affected the shift points (up or down). I was able to dial in WOT shift points that were substantially higher than stock since I was starting with a pretty lo-perf trans. The shift improver kit is what really firmed up the shift itself as I chose not to adjust the modulator at all. Both things were pretty low buck modifications I could do myself that I felt really worked well for my application. My goal was to duplicate as close as I could the performance of an LS6 M40.