Stock 727 planetary with transbrake?

I just put a transbrake in the 727 I have in my Duster. The car weighs about 3300 lbs. at the starting line. It runs mid-10's. It's a 500-inch big-block making about 600 hp. 4.10 gears, 4000 converter flash.

I haven't taken the car out yet, and I read some info about stock aluminum planetaries being questionable with a brake. I wouldn't want to take the car to the track and strip out the planetary splines. The rest of the trans has been well upgraded with a billet steel front drum and 16-element sprag. Is the risk I'm taking worth the cost and trouble of putting in a steel planetary at this point?

If it were me, I'd do it now. "Peace of mind", "done before it takes out other stuff with it", "ready for more motor" etc etc.

If it strips it won’t take anything out. Probably not a bad idea to go steel carrier. We’ve stated to twist the splines out of them. Always caught during inspections. For sure you can put a lot of power thru the stock carrier on a foot brake leaving hard. 1.2x 60ft 3350-3550.
Doug

I had aluminium run through the 'box, cooler and lines after stripping one. It's just a PITA to clean out and my OCD had me freshen the 'box at the same.

I'v change from a 3 planet to a 4 planet with alumina carrier and had no problem (1.27-130 60') with 4.56's 3350lbs.....I'm also looking to upgrade to a steel planet /277 1.st gear.

6 pin comes in the Cummins trans that fits the 727. They cost under $50. Years back I bought the A&A 4 pinion for $500 after wiping out the aluminum!

Hemi-itis.....is those 6-pin planets in the cummings steel or aluminum?

I stripped one out shifting into 2nd and it was like missing a shift with a manual car. Engine instantly over revved. It may not hurt the tranny, but it could hurt the motor.

The Cummins planet is steel. But your output shaft has to have the correct spline angle to work. The amount of aluminum that came of the the ones we've hurt was gradual. Never saw much shrapnel. Over rev? A race car without a top end rpm limiter?
Doug

When starting out in 1st, brake or not, both planetaries are sharing the load. but. When shifted to 2nd, the front planetary absorbs the load, so that's the common failure mode.

Thaxs DVW

Very interesting. Puts it into perspective. I've got probably 80 passes on the car, shifted into 2nd gear on every one of them, lol. I'll be sure to inspect carefully next time I take the trans apart, but I think I'll leave it for now

Do you know how I would check for the correct spline type for the Cummins planetary? I know there are early and late shafts, mine is the later (bigger) type.

I am no expert by any means,,,,,,but I think the 66 to 71 spline is the same I wiped 2 aluminum sets foot braking off the line with 800 + HP @ 3900#

The output shaft spline included angles are as follows:

'62-'75= 90°

'76-'93= 75°

'94-on= 60°

so 94 on output shaft to use the 6 pinion planetaries ?

It happened on the street, actually getting onto I-10 after a car show. The car has a limiter, but it still redlined with no load which never feels good.

It's called inertia. If the engine is under load at say 6500 and the load instantly goes away, the inertia of the crank and everything turning with it can accelerate it well past its redline, if only briefly.

A rev limiter will have zero control of that.

Kevin

Zero control? So you don’t use one?
Doug

I don't currently have anything that requires one.

My point is if you lose the overrunning clutch anywhere towards the top of the RPM range, inertia can accelerate the engine past its redline regardless of any rev limiter.

Kevin

Do you have any data? I do use one. I've never seen the playback show RPM go over the setting by any noticeable margin during a drivetrain failure.
Doug

It would depend on what RPM range you run I guess. Probably a non issue with a 6800 rpm combo but a concern with a 9500 combo.

My engine guy does a lot of hydroplane engines that run in the mid 7000 range and the Racepak shows an instant spike to upwards of 10,000 every time the prop comes out of the water. Apples and oranges difference in applications I admit.

Kevin

It’s not inertia that does this. Inertia keeps things either at a standstill or a constant speed.
By a stretch it could be called kinetic energy

I’m betting you will find that it is caused by “fire in the holes”.
Fuel fed to the engine to maintain or accelerate under a given load (launching the car or in a burnout) then all of s sudden the load disappears.

Physics class was a LOOONNNGGG time ago so probably using the wrong name for the property. Stored kinetic or potential energy? Same principle that if you are going down the freeway at 75 mph and lose a wheel, causes the wheel to accelerate past you and disappear.

Kevin

Me thinks the increase drag on the car brake drum or disc rotor rubbing or dragging on the road, causes the car to slow down and let the wheel and tire go at the same speed as it came off of the vehicle

I once lost a tire on a vehicle in tow. In the rear-view I saw the tire go off into the next lane and trail far behind me. Then another car ran straight over it with the tire upright the whole time. It was like the car went over a jump. It was a terrible thing to watch, but the tire never went ahead of the vehicle, I can tell you that. I remember the incident vividly. Nobody was hurt, fortunately, but the car that drove over the tire was totaled