I haven't really experimented with using smaller orificing and 2 checkballs so you could be the guinea pig
One thing I've learned about doing VB mods...
Sometimes trying to outthink the GM engineers will get you in trouble.
In many cases the checkball is a downshift control. It may have little function on a normal upshift, but it may seat in a hole to force the fluid through a smaller orifice on a forced downshift. It acts as an accumulator in this fashion, sorta a one way valve to allow hard upshifts, but control the downshift apply.
What many guys don't realize is that many auto transmission failures happen on a downshift. Manually forced or detent, accel and decel.
We can pretty well figure out how to eliminate most of the VB's function (look at a Griner brake for a TH400) and make it a simple, fully manual fluid transfer device for upshifting only.
However it takes a smarter guy than me, and lots of trial and error to figure out how to control auotmatic and manual upshifts, AND automatic and manual downshifts without breaking parts.
Some of it is beyond the VB's control anyway. A forced 3-2 downshift under hard decel can be brutal to the intermediate sprag on a GM 3 speed. Same effect as the 4-3 downshift is for the 700/4L60 forward sprag.
It is relatively easy to find literature stating what checkball has what function, as far as "leave out checkball #x and the 1-2 shift will be harder", however nobody is saying what the side effects are of that modification. If that was all that checkball controlled, the OEM's would have just saved the 12 cents, left the checkball out, and had the plate made with a smaller hole.
Without actually sitting down and studying the hydraulics IN DEPTH it's a shot in the dark. Even after studying you can get it mixed up and be "thinking backwards" and realize your mistake when you've wasted another $40 sprag, outer race, $50 drum, as well as another filter, pan gasket, possibly pump gasket, 12-15 qts of fluid, and an afternoon of work.
My first TH400 I did for myself was in my '69 Chevelle SS. I was working as a tech at a GM dealership. Fairly newlywed, 3 small kids, in other words on a very limited budget.
I overhauled it. I had a very pitted original '69 core that had been in the weather, and another previously hot-rodded core.
Between the two I ended up with an early style drum with stock 16 element sprag and a deep pan. I was doing good. :biggrin:
I rebuilt it using the HP Books TH400 rebuild manual by Ron Sessions. I did all the blocking the 2-3 accum mods I had been doing for awhile, and used a set of B&M kit instructions to know which holes to drill and which two checkballs to leave in....
It worked good while the car was in process of restoration. Shifted very firm at all throttle openings. Great for a young guy with a musclecar...
I finally got the car together, had made a couple of partial runs with it, tuned it up good, and left to drive it to the local 1/8 mile track one Friday after work.
Enroute to the track is a nice section of road for testing. So not wanting to show up at the track and embarass myself, I made a test pass.
The 1-2 shift felt like double hit, and then it revved to the moon.
I shifted to third (column shift) and let off, all was OK.
I slowed to a stop and ran through the gears manually nice and easy, all was OK, so I proceeded to jump in it again, and what do you know, 2nd gear revved to the moon.
Classic symptoms of a rolled sprag but I didn't know it at the time. I knew I had wounded the TH400, but I was less than 2 miles from the track and went anyway.
I ran it probably 6-8 passes with low and 3rd. I would rev the piss out of the 427 in low, let off, shift to third (remember column shifter
) and get back into it, just in time to cross the 1/8th. It sounded like I had a 3 speed manual with a worn out column shift.
It still ran high 8's in the 1/8th at 80 mph.
I took it back to the dealership on Monday and asked the trans guy what he though, he told me the sprag had failed, told me what to get, and so off I went to get a 34 element sprag. Got the trans apart and also needed a drum. Ordered the drum and waited another day...
Didn't like the 3rd gear clutches, so ordered a set of those and waited another day. They had gotten heated from the big ratio change.
Finally got all the parts and a sparre trans a friend gave me that had the early style reaction carrier with a HD sprag in it (actual sprag).
I put the trans together all fixed up and used a TransGo -2 kit at the advice of a really sharp trans guy online. He explained where I had gone wrong, what had actually caused my sprag failure, and how to prevent it.
I backed the car out of the bay, put it in low and it wouldn't move, nor in any other forward gear. It was like a transbrake. I had experienced a similar situation before when I cooked the forward clutches together in another TH400 ( I learned to rebuild these things because I'm exceptionally good at breaking them :biggrin: )
Once again, I walked across the parking lot to ask the trans builder WTF I did wrong. He told me I installed the sprag backwards. I was SURE I had installed the sprag right, it shows the proper direction in the manual right?
Next evening, 5 pm sharp, I have a bunch of co-workers help me push the car into my lift bay. 5:05 it was going up. 5:45 it was out on the bench. I completely disassembled it and looked for my mistake. I took me probably 15-20 minutes to find it and I did so because I looked at how the regular roller clutch reaction carrier spun on the center support. I reversed my mis-installed low sprag, slid the thing back together, put the VB back on, pan, etc, Put it on the jack, put it in the car, let it down, re-installed my distributor cap, filled it with fluid, double checked the fluid level, and pulled out of the parking lot at 7:10 pm... With the transgo kit it would chirp going into 2nd and 3rd at anything above 2-3 throttle. I was using a $99 Summit 2200 converter at the time.
I've never done the 2 checkball method since. I've also since learned a better way to get the dual feed done and spend the money elsewhere instead of on the TransGo kit. I got away from the TG kit when I started building them for local racers and 500 HP cars would kill the directs in a season or less. I finally narrowed it down to a too slow 3rd gear feed, no matter how big the orificing was.
Many other builders won't tell the average guy what works, they want to sell you a transmission, and there is nothing wrong with that. However when i was a struggling novice with not much $$, someone was kind enough to help me along so I have passed most of what I have learned along to many others.
I still have to keep some secrets close, mostly due to requests by those who have taught me but the VB calibration that works, it's out there for anybody who wants it.
Lots of good stuff in this topic by several very good builders.