Want to go over my 72 Stage 1 TH400 transmission. 90,000 miles on her. Want to overhaul tranny and put B&M Shift kit in it. Could someone give me some pointers on what to have my trans. mechanic do to the tranny. it is original B B . But do not want to change the tranny from down shifting at 25 mph from 2nd to 1st.
The BB valve body calibrations are what makes the BB downshift from 3-1 at speeds under 35 MPH, so I wouldn't worry about doing anything to change that. This Spring, I am having a fixed pitch THM400 built for my car. I believe the switch pitch is costing me some power. I still have to talk with my builder about this, but at bare minimum, I will go with the best sprag I can find (34 element?), and HD racing clutches and steels.
Would you use a B&M shift kit. Larry if I have a tranny with high miles should I replace clutches? who sells them. Who do you like?
I think it's kind of like a bearing deal and the elements are like rollers in the bearing. I think stock is 16 element, I think you can get up to 40 element. From Yellow Bullet...." The most common failure in a Turbo 400 is the 2nd gear sprag. This is a mechanical part has to stop the direct drum turning in the opposite direction of transmission rotation the instant the 2nd gear is engaged. Imagine a 9lb drum turning at engine RPM and has to be stopped instantly. What typically happens is the sprag will wear and break or flip over causing a loss of 2nd gear. This will happen from a stock 400 to Hi HP units. The industry for years has worked to improve the problem GM built into the Turbo 400. The industry solution was to use a larger sprag out of a front wheel drive GM transmission referred to as (Super Sprag). This sprag is larger and has more surface area and it does extend the HP limitation of the 2ng gear sprag. The problem is the Super Sprag is still a mechanical part, it still wears and fails only at a higher HP level. No one seems to know what HP level this new sprag is reliable. When you shock a mechanical part it will fail period. That's what happens every time you pull the shifter on your 400 into 2nd gear".
I'm still learning myself. The builder I am using makes his own shift kit. Yes, you replace clutches. If you rebuild a transmission, you do it right by replacing all the parts that commonly wear. http://www.transmissionpartsusa.com/TH400_4L80E_transmission_intermediate_sprag_p/460-000046881.htm 34 element sprag. http://www.transmissionpartsusa.com/TH400_4L80E_transmission_intermediate_sprag_p/460-000046881.htm
TH400 shift kit I see a lot of old TH400s that have great hard parts. So I put in a "soft" kit ($30) that replaces all the rubber & metal seals. Or replace the clutches, but keeps the original steel plates, etc. And the Torrington thrust bearings and any that aren't brass. I would have to go out in the extreme cold to get part #s today. Maybe open up that hole in the valve plate to speed downshifts. Bruce Roe
B&M shift kits suck, I would not use or recommend them. TransGo makes a much nicer kit. Summit racing sells them.
I see that when I do the 70 BB trans that is out in the shop now, we need to go thru that together.. We talk the death out of the sexy motors, and ignore the thing that really makes the car go.. Steve.. Yes, Trans-go shift kit, weakest link in the 400 is actually the direct clutches, the Transgo 400 1-2 kit has a special separator plate that increases holding load on the direct clutch considerably. It is the best way to accomplish this, although there are other ways, and the world is full of trans tech's who fancy themselves engineers. Having done all these modifications every way possible over the years, I have found that for overall performance, installing this kit is superior to "engineering it" yourself. The kit allows you fine tune the clutch applications and timing, via careful hole size selection and spring calibrations. That, along with careful attention to clutch clearances during a major overhaul, is what makes the trans shift properly. Note I did not say "hard".. It is the superior kit for the 400, used by most transmission professionals. I use nothing else. Costs a little more, and that's the main reason why many guys say "Hell, I can do that for almost nothing"... but you cannot. You can certainly take a seal out and gain the increased holding power, but you sacrifice the fine control of the clutch apply. The kit is about $65 and can be bought thru jegs and summit. There is also a 45 minute video that shows how to install it. http://transgo.com/400_1&2Video.html Trans go was founded by a man named Gil Younger in the late 50's. Anyone that has ever read the directions that come with the kits understands he had a sense of humor. Here is a page on the man, who just died here recently, and the company that he founded. He was a pioneer in the field, and actually trademarked the term "shift kit". http://www.transgo.com/GilYounger.html _______________ Specifically, your '72 BB trans does not have the dog-bone element type sprag on the direct clutch drum..(often reffered to as the "intermediate sprag") This is the 16 element sprag that is routinely upgraded to 34 elements for performance use. You have a roller clutch on yours, GM went to that in '71. For serious performance use, the roller clutch's aggressive loading angles on the sprag race can cause it to fail.. most performance builders switch out the later roller clutch drum for an early sprag clutch one, and retro-fit the HD Sprag clutch. With radials and street usage, the roller clutch is fine.. just get a fresh on in there. If you have a serious build, and put sticky tires on it and go to the strip occasionally, then you should have the HD roller sprag in it. Leave the 36 Element 4L-80E "Super Sprag" to the boost fed racers.. the standard 34 element unit is fine for any street driven vehicle. JW
The biggest cars with the biggest engines used a direct clutch with 6 clutch discs instead of 5. Like 472, 500, probably others. Changing to 6 clutches is just a matter of using the special piston to allow room. I see it on EB++y 161200499273. Bruce Roe
Steve, Ask Jim Weise what manufacturer he uses. I'm going to try and get something that isn't made in China. Hope I can. Are you rebuilding your own transmission?
