With the occasional torqueflite explosions we hear about I've got a few questions. How much rpm will a stock front clutch drum withstand before you have to head for the bomb shelter? I know no one can give an exact RPM figure but has anyone spun one up on purpose and a machine to see what they'll take?
I should be heading to the strip with my timebomb in June sometime. I've been shortshifting it at 7000 rpm and may try some different shift points but I'd rather not blow a tranny. AND YES, I'm running a Lofgren trans blanket just in case. AND, I've got the low band apply in 1st. Dave

over at moparchat there is alot about it. this is the little bit i know about it. the drum spins at 2 1/2 times the engine rpm's which they are capable of handling untill the sprag fails and the rpm gets out of hand. yes there are tests on it. anything that unloads the sprag fast will hurt it. like a broken driveshaft or ujoints, axles, ring and pinion etc. a burnout in first gear where the sprag freewheels then stops when you hit the dry pavemant. short shifting at 7000rpm's is impressive to me.

Here you go.
Drum destructive test

I think bob's post should read :
When the sprag fails the rpm gets out of hand, the drum will spin at 2.2 times the engine rpm's which some are capable of handling, but some explode.

Thanks Leigh! 14,000, so 7000rpm is more than enough to send one to the promised land if the overrunning clutch fails. I'll cinch the blanket down extra tight before heading out to the strip. Dave

To help understand what is needed or not I've tried below to explain the problem of sprag roller clutch failure on a Torqueflite.
Let's see if I can go through this sprag clutch/drum explosion Torqueflite scenario and explain why the drum "may" explode with a sprag clutch failure and what can be done to help prevent this type of failure. This is going to be long and pretty basic for many, but you really should understand how something operates before you can take steps to prevent problems.

First off, a short tutorial on planetary gearsets. Planetary gearsets consist of a central sun gear that is in constant mesh with two or more planetary gears that "orbit" the sun gear and are in turn in constant mesh with and enlosed by an outer ring gear. Various gear ratios and direction changes can be seen by holding one member, driving another and taking the output off the remaining member. No need to go into specifics but the main idea with a planetary gearset is that in order to work one member is held while another is driven. The only exception is if two members are driven then the entire gearset turns as a unit (1:1 ratio).

In a Torqueflite there are two plantary sets mounted together that share a common sun gear. This arrangement is known as a Simpson geartrain and is also common in Ford C4's and C6's and many other automatics. Two planetary sets are required in order to get the required number and ratios needed for use in an automatic. Clutches in automatic transmissions are used to drive OR hold a planetary element while bands and sprag clutches are strictly holding devices. The torqueflite Front Clutch(sometimes refered to as the High/Reverse clutch) is the drive device for the common sun gear. The Rear clutch is the driving member for the front planetary ring gear. The intermediate or kickdown band is the holding device for the common sun gear while the rear band and the sprag clutch share the holding duties for the rear planetary carrier. The output members are the front planetary carrier and the rear planetary ring gear(splined to the output shaft).

Now I could go through the various power flows while in different gears, but for this discussion it is more important to know what clutches, bands and sprag clutches are in play and what happens when a sprag fails. Sprag clutches are basically one way clutches that allow movement in one direction only. The "Sprag" in a torqueflite is not a true sprag but a roller clutch. It works by forcing rollers up on hardened ramps that in turn cinch down on an inner race stopping it's rotating motion in one direction. If the rotating motion on this roller clutch reverses direction, the holding force is immediately released. This is one of the main reasons for it's use in an automatic, the instant release feature when the load is removed makes it simpler to adjust shift firmness as there is no overlap to contend with.

Torqueflite's use a roller clutch that is mounted in the rear of the case. The outer race of this clutch which contains the ramps is splined to the case. Clamped between the case and the outer race is a metal retainer that has small "legs" that are bent upwards betweeen the ramps and the inner race of the clutch. These "legs" are the anchor for the wave springs that serve to push the clutch rollers up the ramps when the inner race tries to turn clockwise and thus clamp the inner race from turning. If there is a counter clockwise motion on the inner race, the rollers push back down the ramp and collapse the wave springs and release the inner race.

One last thing to know is what devices are in play when in various gears. First off a stock transmission.


Shift lever in Drive, Low gear - Rear Clutch and Roller Clutch
Shift lever in Drive, 2nd gear - Rear Clutch and Intermediate band
Shift lever in Drive, 3rd gear - Rear Clutch and Front Clutch
Shift lever in reverse - Front Clutch and Rear Band applied

Shift lever in Low (1) - Rear Clutch and Rear band+roller clutch

With the shift lever in 2, the car will start in low just like it would in the drive position, shift to 2nd and stay in that gear.

For a manual valve body, the difference is all the automatic features have been removed so putting the lever in low, the car starts and remains in low until the shift lever is moved. In second, the car starts in second and stays in second while in third or drive, the unit starts and stays in third. The difference in the Low Band Apply (LBA) valve bodies is that they apply the rear band in low while the non-LBA valve bodies do NOT apply the rear band in low. This is an important difference as you will see shortly. The reason for the non-LBA manual valve bodies is to speed up the one-two shift since there is no overlap to contend with using the roller clutch as a holding device.

Now we have all the basics in place we can continue with what happens that causes a roller clutch failure and ultimately a Front Clutch drum explosion. Anytime there is an excessive shock applied to the roller clutch there is a possibility of the shock load being transmitted to the "legs" that hold the springs in the roller clutch. This shock could be something breaking in the drivetrain, the tires suddenly gripping coming out of the water box etc. The key is the sudden shock load is only a problem when the vehicle is in low gear and utilizing the roller clutch only as the holding device. If you break an axle or shock the trans coming out of the burnout box in second gear, there is little to no chance of damaging the roller clutch. This is why the old-timers will say to always do a burnout with a Torqueflite in second gear and never roll out of the burnout box while letting off the throttle allowing the tires to grab suddenly. Same goes for the LBA valve bodies with low gear burnouts or driveline breakage. Since the rear band is also applied there is less of a chance of damaging the roller clutch because the band is assisting and absorbing some of that shock.

The above explains what not to do, but what happens if you disregard this advice and shock the driveline? Again, the problem is the small tin legs in the roller clutch. The shock load can bend these legs back which reduces the spring pressure on the rollers. Now the rollers are not being pushed up the ramp or can simply flip out of place which in turn allows the inner race to rotate in the direction it would normally be locked. The problem with this is that now the wheels become the holding device and the weels are connected to the output shaft which is spined to the front planet carrier as well as the rear ring gear. If you follow the power flow with the Rear clutch applied and the front planet held and calculate the ratios involved, you will see that this combination results in the Front Clutch drum rotating in reverse at a ratio of over 2 times the input speed. The stock clutch drums in a Torqueflite are made of powdered iron, basically iron filings that have been heated and squeezed into shape. It's a cheap way to make complex shapes but it is not as strong as a billet piece. At some RPM point these powdered iron pieces fly apart with devastating power.

So what do you need to do to minimize the chance of witnessing first hand one of these explosions? If you read and undertood the above, you can see that a simple bolt in sprag/roller clutch will do little to nothing to prevent a roller clutch failure of this type. Rarely is the problem of the outerrace turning in the case a factor in a drum explosion. Not to say that a bolt in roller clutch is not a good thing, just that it will not help with this type of failure. The stock and most aftermarket roller clutches for the 727 utilize 12 rollers and springs. The super sprags from A&A and Coan have 16 elements. These units may offer some relief from this type of sprag failure simply because with the added elements there is less room for the legs to bend back far enough to allow the rollers to slip or fall out of place. The 16 element clutches are probably a good idea not only for this reason but also their added holding power. Next would be to consider a billet aluminum, steel or steel jacketed aluminum front clutch drum that will handle the extreme RPM that a roller clutch failure may transmit to the front drum. A trans blanket or shield is always a good idea and a requirement in some classes. Last of all, heed the advice of the old-timers!! They've run these 727's for years and still have their toes. Knowing how to do a burnout properly and checking the roller clutch after any driveline failure regardless of what parts you have in the transmission only makes good sense.

Lastly, the LBA valve bodies can also help prevent drum explosions because as mentioned they reduce the shock to the sprag. They may not totally eliminate the possibility of a roller clutch failure but even if the roller clutch did fail, the band would hold enough to reduce the overall freewheeling of the rear planet carrier that would in turn reduce the RPM of the front clutch drum.

Hope this helps clairify some of the misconceptions that the TF is a timebomb just waiting to explode.

chanes are that if you roll the sprag, your gonna blow it on the line not goin down the track.

You're correct. Since it's a low gear problem, it's going to show right at the initial hit.

OK ...so what is the rpm number that the billet aluminum drum comes apart ? Somewhere I think I heard 24,000.

Sooooo .... unless I spin my Mopar-moder up-to 10,000 rpm ...... I am "safe" ?

This does not say that I do not want to run any safety equipment like a shield .... but I want to be "safe" from any damage to the trans case, valve body, alum trans pan and a hi-$ converter.

TCS has this data on thier website, but in thier testing, here's what they found:

Stock Iron drum failed at 14,000 RPM
Aluminum drum with steel jacket 22,640 RPM
All Steel Drum 25,000 RPM
All Aluminum Drum 36,550 RPM

That was some of the best info ever written on the 727 I have ever read. Thank you.

Quote:

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and never roll out of the burnout box while letting off the throttle allowing the tires to grab suddenly.

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This is only when in low gear right?

I have had a stock drum up to 8400 rpm .at least untill I stripped out a planetary spline.
That being said I now have a aluminum drum and I sleep a lot better.

One of the times I had a roller clutch, or sprag, failure was after installing a 33X10.5 tire on the car. I was able to 60 ft. in the teens with 31" tires but wanted the larger tire. The very first trip to the HOUSE OF HOOK, Peidmont Dragway, I ripped the sprag out of the case blowing peices of sprag through the case and into the sheetmetal. The valve body was a TCI made by Tranzact but, was mot a Low Band Apply in 1st gear. Thank God I was smart enough to have and Alluminum Drum. I have seen stock drums in transmissions explode with blankets and still dame chassis and floor pans.

I prefer the alluminum drum not just because of the higher RPM explosion resistance. But, if it were to explode it will be like alluminum rods breaking compared to steel. You may have to replace the tranny but the car will be fine.

Correct.

So ...unless you spin your motor over 14,500 rpm and then have a sprag failure - there is no way to typically "explode" a 727 ?

I have seen my share of the "stocker" drums coming apart. The last one was at Milan Dragway in a Duster. Even with a blanket ... it blew-apart a fiberglass dash and shattered the windshield !

I'm sure there are many other ways to blow up a 727, but roller clutch failure and overspeed of the front drum is what was explained above. The usual scenario is a driveline failure of some sort that damages the roller clutch. The drum explosion does not usually occur at this time but instead the next time the car is launched. The roller may hold even when damaged but then suddently fail under load. Since this load is most likely a low gear launch, your foot is on the floor when the roller fails sending your engine screaming onto the rev limiter. Remember that there is a ratio involved so if the engine hits 7000 the drum is rotating the opposite direction at 2.2 times that speed or 15,400 RPM. This is well above the 14K limit seen by the tests TCS had done. That's why it is wise to inspect your Torqueflite after a drivline failure.

the engines rev so fast and faster with more power if you get out of the throttle later than .5 secandsi bet its done depending on the launch RPM my point is if there was a interupt that would kill the ignition if the engine reved past a set RPM speed rate. i think i have herd of 1 but not sure.
ps i guess that would be traction controle!