Mopar Muscle Association UK

In an earlier thread I mentioned reading about problems with the 727 gearbox and doing 1st gear burnouts and trying to explain it to Adam but could not find the info.
Well I came across it again ( its in Car Craft) and thought it might be useful for all you tyre shreders with 727's so I blagged it and here it is.
Blag.

Probably the most problematic feature of the 727 is the overrunning clutch in the back of the case, sometimes incorrectly referred to as the sprag. Simply put, the overrunning clutch, a steel housing splined into the aluminium case, acts to stop the gear train and lock up the rear planetary in low and Reverse gears. Failure is often caused by driver error while performing a burnout in First gear. The problem can arise after the water burnout is completed and the car rolls onto dry pavement. The resulting high-shock loads can be transmitted directly to the overrunning clutch, over stressing it and damaging the case. In order to eliminate this weakness, the design of the transmission would have to be completely re engineered, but durability can be improved by pinning or bolting the overrunning clutch to the case, which provides better support than stock soft aluminium splines. Both A-1 and Turbo Action perform this modification to their 727 TorqueFlites. The best way to avoid overrunning clutch stress is to always perform burnouts in Second or high gear.

Naughty take note how many boxes is that now?

Thought most people knew that or am I being unfair because I'm in the trade as it were? but have not managed to get a JCB to do a burn out (yet)

Ok all you members who knew this put your hands up
JCB with a 727 that must be an irish mopar then John

Yep! Been told as soon as I fitted a linelock! If any trans bits break, eg uj, halfshaft etc the over running clutch goes ballistic and the front drum overspeeds and explodes taking out the bellhousing, floor and probably your feet and ankles.....

Ok Bob , gotta admit i did'nt know , what I don,t understand is if you stick it in 2nd , as soon as your on the throttle the box goes into 1st.

Knightcharger wrote: JCB with a 727 that must be an irish mopar then John

I think it applies to manual valve bodies Adam since the kickdown will make it jump to first on full throttle....I could be wrong though?

POP in reverse manual valve body cars. Had been coached by Blue so have always done it; so does Juile.

TCS Products did this destructive test to compare their drums to the stock vesion. Interesting results.

With stick or column I know its stupid but if you stick it in 1and burn out uyou yu will leave the box in 1st and BANG.
IF you burn out in 2nd or 3rd it will change up and you will not leave the box in 1st

Dave wrote:TCS Products did this destructive test to compare their drums to the stock vesion. Interesting results.

Although those speeds might seem very high, what you have to remember is that if the overunning clutch fails in 1st gear then the drum will be spinning at engine RPM x 1st gear ratio. So if it lets go at 6000RPM x 2.45 gear ratio, the drum is spinning at 14,700RPM......

It happenned to a mate of mine back in the '80's, ended up with half the contents of the gearbox inside the car, he luckily was not hurt but it tore out most of the transmission tunnel, looked like a fragmentation bomb had gone off under it, a real mess.

Some scary failure pictures here for you gorehounds,

http://albums.photo.epson.com/j/AlbumIn ... 2568880&f=

Wow,

OK so basically the racers would not burn out in 1st and have shields and or blankets plus prop loops and uprated gearboxes but this is more likely to happen to an unaware road car outside the pub or on that RWYB outing.
Be lucky guy's and gals.

It is torque and lots of it that kills overunning clutches. You are unlikley to damage the sprag with the average road car but it still something to be aware of especially with manual valve bodies, avoid 1st gear burnouts. Manual valve bodies with low band apply are available to get around the problem, applying the band takes some of the strain off the sprag. A normal fully automatic torqueflite does not apply the band in drive but it does when manually selecting 1st gear incidentally.

Blue, is full throttle launch whilst in First gear a bad idea, then?

No that's not a problem. lets not get too carried away here, the Torqueflite is one of, if not the strongest automatic transmission ever made, it will take a lot of abuse. I would say that if you are making under 500hp there is little chance of seriously damaging it if it is in good condition. All that is being said is that the overunning clutch or sprag is it's Achilles heel if you hit it with enough torque. When the drum blew up in my mates Roadrunner, he had been starting his burnouts in Ist gear because we didn't know better back then. It was a 3700lb car with around 600hp and he Bananarama! it with a 175hp shot of nitrous off the line when it blew.