Patro46

I have an LQ4/LS3 build in a 74 Corvette, and recently installed a ZL1 supercharger. I'm somewhere around 600 hp to the wheels (ish as I haven't put it on a dyno. She will blow the tires up at 70 mph) I'm running 3:73 cogs. The slushbox is a 4L60e. I've beefed it up with the usual goodies (sunshell, planetaries, servo, SD2). The transmission had about 30 miles on it when all hell broke loose. It appears I took out the sprag. I had sent the convertor in to FTC for a complete rebiild, and it is spec'd at 3600 rpm, single disk w/lockup.
After receiving ut back from FTC I learned that there was a difference in a boosted convertor vs a normally aspirated convertor?

So It seems that maybe I'm shocking the transmission?

This said, what say you? I'm in so deep on this build. I feel married to this transmission if for no other reason than I modded the tailshaft to provide both a VSS sensor as well as a conventional cable speedometer drive.

The car is runs street tires and I don't plan on hooking up hard. If I did it's a tossup if the rear end would give before the transmission.
Also, I.must admit when I hit it hard, I was in drive and not 3rd. I've heard this may have contributed to the sprag unloading. If I had it in 3rd when I hit it it takes the sprag out of the equation. Do you agree?
I'm pulling the transmission tomorrow and would love to hear suggestions involving maybe softening up the shifts or simply never hit it in overdrive.
What say you?
Thanks in advance!

performabuilt

iTrader: (25)

If you have installed the Sonnax HD 2-3 valve use manual shifts or leave car in D3 for racing the overun clutch will help to support the sprag and prevent this issue, If you do not have the sonnax HD valve get one and install it.

mrvedit

Without the Sonnax HD 2-3 valve, it is best to drag in OD/D4. The forward sprag will be equally loaded in 1st and 2nd gear in D3 and OD. When racing in D3 the overrun clutch will apply during the 2->3 shift, but in a stock trans this unfortunately also robs fluid pressure to the 3/4 clutch allowing it to slip a bit during the shift. Since more people have 3/4 clutch failure than sprag failure, it is best to drag race in OD with a stock-ish trans. A performance build will have more fluid flow the the 3/4 clutch and it therefore won't matter much between D3 and OD.
The Sonnax HD 2-3 valve keeps the overrun clutches also applied in 1st and 2nd gear and it is then best to drag race in D3. With this valve the load is split between the sprag and the overrun clutches, and no fluid is robbed during the 2->3 shift.

A failed sprag unfortunately often contaminates the entire trans, converter and cooler with metal fragments. This requires not only a trans rebuild, but the converter should be sent out for an open/clean/reweld operation. An aftermarket cooler is best replaced and the cooling lines flushed too. Without these (painful) steps the next transmission may fail prematurely too.

PBA

Make you replace the inner & outer race. Make sure that the inner race is perfect shape. It must be flat and very smooth. Use 600 - 1,000 - then 1,500 grit sandpaper or finer, to get "mirror finish". Outer race is just the opposite. Use 36 - 40 grit sandpaper, sand in direction of rotation until rough. Now install the late 29 element sprag that has the Nylon cage using brass retainers. This late Borg Warner sprag setup has 12.7% wider sprag elements. This late design sprag along with the modifications, has solved quite a few problems from sprag failures in the past.

bbond105

I followed these instructions from PBA and so far it has solved my sprag problems.

Patro46

I pulled the transmission last night, and today got it tore down. I must have called it as the input sprag literally "rolled over", meaning all those one way teeth rolled around the direction intended to lock instead of locking up.
Everything else looks great, and no metal shavings anywhere. The sprag I used is the newest Borg Warner 29 tooth unit, and from my understanding, it's the best thing available.
Thoughts?

mrvedit

Not to be "insulting", but are you sure the sprag "rolled over" and it wasn't installed upside down?
Perhaps PBA or one of the other experts and confirm whether a sprag can even "roll over".
In any case if there are no metal shavings, install a new one and get the Sonnax HD 2-3 shift valve to reduce load on the sprag.
Good luck.

Patro46

No insult taken. When we pulled the sprag, it was intact. You could also spin it either clockwise or counter clockwise. It did "roll over". You can now roll it either way.

mrvedit

Wow. Interesting.
Good that there are no metal shavings.
I have no idea if PBA suggested mod helps reduce the chance of a roll-over; he is very experienced and has customers using his 4L60E builds in 9 second cars.