BKbroiler

My TH350 trans was rebuilt and has a shift kit etc and works fine. The modulator has a tiny screw in it which can be seen after removing the vacuum hose. Turning that screw changes WOT shift points. Does that seem right? I thought the governor changes shift points. Any insight would be appreciated.

7T1vette

A modest amount of shift point adjustment is available via that original design modulator (amazing that you still have one that works!). But, the proper way to change shift points is by installing different springs on the shift valves in the valve body. You can 'bias' the overall shift schedule by changing the springs/weights in the governor as well; but that isn't the best way to get that job done if you want to adjust it very far.

BKbroiler

Thanks for the response. Actually, the modulator was replaced with an aftermarket unit. I have only changed the shift points a few hundred rpms using the modulator screw, but it does work. I was reading somewhere that only the governor weights/springs would change shiftpoints.

Vette5.5

An adjustable vacuum modulator can help a little, with a governor spring/weight kit being the next step. This is really just a band aid though, and gear's can hang up way to long in normal street driving, if not careful with kit part's selection. Really think guy's like "7T1Vette" dealing with at the core is best, but beyond my understanding.

ajrothm

BK, go read this buddy....

http://forums.corvetteforum.com/c3-t...-governor.html


Its worth the time once you get it right


BTW adjusting the modulator WILL affect the WOT shifts SOME....the more you screw it in, the higher/harder it will shift, but it will also affect the part throttle shifting. Same thing with the springs, they will REALLY screw up your part throttle shifting.

I would start with adjusting the weights only first, then see how close you get. I am still using the stock springs and have my kickdown detent hooked up and working. I know a T350 doesn't have the electric detent solenoid but your "kickdown" cable probably affects line pressure also so make sure its hooked up and working BEFORE you start adjusting weights.

BKbroiler

Thanks guys. What I was looking to hear was confirmation that adjusting the modulator will make a small change in WOT shift points. AJ, I read that thread and I feel your pain. My part throttle shifts are fine, before and after adjusting the modulator.
This all started since I changed intakes (Performer to RPM air gap). I ran the car Saturday and in good air my best time was 12.70 @ 104.5 versus best overall times (with Performer) of 12.53 @ 106. On Sat the car was short shifting 1-2 and 2-3 by about 300 rpm. That's with no adjustment to the trans, shifting itself. Why would a manifold change cause that? I equalled my best 60 ft time, 1.74 and my mph gain from 1/8 to 1/4 mile was the same as the 12.53 run. So I'm losing something in the middle and I think shifting too low might explain it. I'll be trying it again this Saturday.

ajrothm

Can you post the incrementals from both runs? particularly the 1/8 mph? Did you change the tire diameter(ie. taller)?


Not sure why it would start shifting 300rpms lower.....Vacuum at WOT is minimal so even a change in manifold vacuum(due to the intake swap) "shouldnt" have effected the shift patterns but....sometimes, you never know. I know a few months ago my tranny all of the sudden started shifting about 300-400 rpms LATER at the track....it kept getting worse and worse....eventually getting to the 6400 chip....Turns out it had a stuck spool valve in the governor. I replaced the governor valve with a new one from a tranny shop for $20 and put my weights/springs on it from my old governor and it was back to shifting at 6000-6100.


Short shifting will definitely hurt your MPH and ET though....

7T1vette

The manifold change will change the vacuum characteristics of the engine a bit. And that will change the level of vacuum at the modulator at specific engine rpm. If you can adjust it with the modulator can, do so. If not enough, you can increase the spring strength just a bit on the governor or add a bit of weight to both flyweights. Trying to make an incremental change via the governor is 'touchy'.
----------

Sorry, I erred on the strategy above. To make the shift point at higher rpm, you would need to take a little weight off of each flyweight (that way it would take more rpm to get them to swing out a specific amount). You may be able to do that with a little Dremel grinder. Remove material from and area of the weight with some "meat"...not at the outside perimeter.

ajrothm

He would need to take weight OFF of the inner weights to increase his shift points but you are correct in that the manifold swap "could" have messed with his shifts some....but I can't see 300 rpms....

I can run my modulator screw almost all the way in and all the way out and only have MAYBE 100 rpm swing.....but the part throttle shifting will get all jacked up AND the tranny will shift much softer the further out the screw goes....

BK, try running the screw in 1/2 turn each time, it should raise the shift points and make them firmer. I doubt you will get 300 rpms without a weight swap. You may just need to change one of the weights though to one slightly lighter then one of your stock weights.

You can also remove your stock inner weights and grind the edges of them to remove weight but do this systematically as you can't add it back on if you go too far..... I would buy the kit from B&M so you can always go back to stock. But if you are trying to get it ready for a race this weekend, start grinding.....just a little bit at a time, then test it....A little goes a LONG way in this...

BKbroiler

I see you guys have experience with the governor adjustments, but I'm going to hold off on that for now. Yesterday I was under the car 3 times turning the screw in the modulator (clockwise) and trying the car on the road each time. I got the shift points back up to where they were before the manifold change and the car feels better. Also, tires are the same as last year. The only other difference might be a degree or two of timing. I don't like to make too many changes at once but I did set the total advance back to 34*.
Here are the incrementals:
11/6/10 ......................... 3/19/11
60 1.75 .......................... 1.75
330 5.083 .......................... 5.145
1/8 7.934 ............................ 8.046
mph 85.66 ............................ 84.31
1000 10.421 ........................... 10.565
1/4 12.535............................ 12.706
mph 106.61........................... 105.02

larrywalk

Using the vacuum modulator to change WOT shift points is not the best as it mostly affects part throttle shift points.

Shimming the detent regulator spring will help with WOT shifts since detent oil is activated when the throttle cable pulls the detent lever internally at WOT.

To further adjust shift points, the governor can make a large difference. The 1-2 shift is most affected by the small internal springs - the lighter the internal spring, the higher the rpm if the 1-2 shift.

The 2-3 shift is most affected by the weight of the inner flyweights. The lighter the inner flyweights, the higher the rpm at the 2-3 shift. As a caution: I'd recommend shimming the detent regulator first, followed by lightening the inner governor weights to get the 2-3 shift rpm; and finally by trying different springs to get the 1-2 shift at the rpm you want.

Finally, all these changes interact somewhat with other shift points, so it's not a pure process - just have patience and Good Luck!

ajrothm

BK, if adjusting the modulator got your shift points back to where they were or better yet, where they need to be now with the new intake, then leave it as is.

Looking at the slips, you are .06 off in the 330', .11 and 1.34 mph off in the 1/8....so that likely means you "could" be losing the ET in the shift points IF your car is shifting to 3rd before the 1/8 but.....my guess is something else is slightly off....either the DA was different OR something different in the tune up..... Timing changes, jetting off....something. As long as you have the shift points back to normal, now you can start to track tune it and see if you can pick it back up..... I am really suprised by what it did.....I would have thought it might 60' worse with the RPM air gap but pick up MPH in the 1/8 and 1/4 with the 20hp you should have gained.... You may have to move the timing around a bit and see what it wants....Also you may have to jet it up since you have more air flow and possibly better DA.....but I kind of doubt it. Either way that intake swap "should" have picked that thing up .1/1 mph if not more....

Make sure you are jotting down the DA on the time slips every time you race....you could easily go up or down 1-1.5 mph with a big swing in DA...

BKbroiler

Larry - I will give it another shot at the track on Sat. If the shift points are still off, I will have to try the governor.

AJ - DA numbers were fairly close those 2 days so that doesn't account for the difference. I am into 3rd before the 1/8 mile mark, so shift points are in the part of the run where I lost most of the time/mph. I'm off 1.34 mph in the 1/8 and "only" 1.59 in the 1/4. So between the 60 ft point and the 1/8 is where the shift points are and where its laying down. After I get this corrected I'm sure I will have to experiment with tuning.