Re: Turbo 400 experts please step in....
 

I took it out for a test drive after running the screw in a little further.no change , at least nothing noticeable . I'll spend time with the governor as you suggest , however I don't want to change the way it acts under full throttle , now. I can see this is going to require a fair amount of trial and error.........in other words a PITA. But , I'll do it none the less.
Perhaps I should just consider a 4L80E and be done with it.

Re: Turbo 400 experts please step in....
 

OK , thanks........

Re: Turbo 400 experts please step in....
 

One thing I'm surprised no one has mentioned is that most OE vacuum modulator feed lines have a restriction ,usually a crimped area in the tube to help regulate the speed in which the modulator responds to throttle changes .
Also , if you are using the short , small 2" diameter modulator , There are many different spring calibrations for different applications identified by the color stripes ,usually black ,white ,red , red-red ,red-black are common identifiers .
Each one has a different spring rating ,the external adjustment only addresses the range of that particular spring.

Re: Turbo 400 experts please step in....
 

Seems that the factory had its work cut out for itself fine tuning this trans to fit all the various engine combinations it found itself behind. The basic three speed planetary gearset came behind 250 horse 327's up to 400 horse plus big blocks, plus Pontiac,Buick, Olds and Cadillac, all high torque motors with various characteristics. Vacuum modulators, modulator steel line sizes, and all kinds of valve body springs all went together to make this trans work for everybody. A certain amount of trial and error is to be expected in order to fine tune it to suit a particular user. Of course, most drag cars have a manual valve body so modulators and other spring applications turn into a moot point . The age old axiom is correct here when we find that the best dyno is the drag strip. Sounds like an assortment of modulators and springs and a trip to the track for a test and tune session would be your ultimate best bet. Just don't change anything on the engine's state of tune while you are working on the transmission. Good luck on your project,thanks for introducing a subject that resulted in quite a bit of intelligent discourse.

Re: Turbo 400 experts please step in....
 

Exactly , and I have the single red stripe , dual red stripe , black stripe and on and on and on , and I have no way of knowing what's doing what ......in other words , apparently I have to try everything and then MAYBE.......LOL

Re: Turbo 400 experts please step in....
 

Recall , this thread is only about part throttle early shift 1-2. But good points.......

Re: Turbo 400 experts please step in....
 

The basic differences between the modulators is a spring force , and the size of the diaphragm . It works by vacuum acting on the diaphragm area to back off the load on the spring . If no vacuum (WOT) then just the full load of the spring acts against the modulator valve which regulates a pressure that makes line pressure and also works on the shift valvetrains . This is max modulator pressure . When you have high vacuum (closed throttle) ,then the diaphragm works against the spring , reducing it to zero . This means zero modulator pressure which gets you minimum line pressure , usually 60 psi , and minimum shift points . So all these different modulators are just combinations of spring loads , and Diaphragm areas . The problem in this thread is sounding like the diaphragm area is too big so under medium engine load it backs off the modulator pressure and gives an early shift . It would be nice to have a chart of different modulator part numbers with spring load , diaphragm area , and ID stripes . Also note that the adjustment screw is usually adjusted at the factory to get a certain load at a certain vacuum , back when these were more common there was not a screw , but the adjustment was done by dimpling the top of the housing .

Re: Turbo 400 experts please step in....
 

OK guys , thanks again........
I'll conclude with this:
I have the "hard" line that has a "necked down" area at each end , but I'm having a hard time understanding how that would effect the 1-2 shift?
I'll replace the full length rubber hose it has now BUT , it seems to me those reduced areas joining at modulator or nipple on manifold top of runner , the only point of that feature was to locate the tube securely on the production line ( slipping inside the mating device ) and the short rubber hose at each end just kept it all together ?

Re: Turbo 400 experts please step in....
 

You are partly correct. The tube was necked down to locate it to the fittings. This allowed for just a straight short piece of rubber vacuum line to seal the connection. If you look closely into the opening of the tube, you will most likely see a restriction on the end by the modulator. I would open up the ends of the metal line with a drill of appropriate size.It’s possible you may have to change the 1-2 shift valve spring in the valve body to get the trans to shift where you want it to.

Re: Turbo 400 experts please step in....
 

Thanks.