Duster346
Chris Andrews
Why it happens, and how to prevent it from happening to YOU!
A few years back at a local dragstrip, I happened to be standing by the starting line when a racer in a 66 Dodge had his 727 explode right off of the starting line.
I went to his pit area to examine the carnage, there was a massive crater in the floorboard/transmission tunnel area. Luckily the racer only suffered from a sore ankle. We were discussing the possible causes, when he told me he broke the ring gear in the differential the week before. He mentioned that his "transmission guy" checked out the transmission afterwards and gave him the OK to put the car back together. To my surprise the "transmission guy" was standing in the racer's pit area. I told him that these transmissions are not supposed to explode if they are built with quality components and the correct techniques. He didn't take kindly to me offering up my opinion at the time, and he simply replied "If you race enough it will happen". That statement is flat-out WRONG.
Why it happens:
It all starts when the over-running clutch (aka the sprag) in the rear of the transmission case is damaged. The roller bearings will commonly "roll-over" the posts or fingers on the spring retainer. This action will cause the transmission's geartrain to over-rev the engine rpm multiplied by the first gear ratio, commonly 12,000 rpm or more. The stock OEM powered forged high gear/front clutch drum retainer will explode at approximately 9,000 rpm. The sprag can be damaged by one or more of the following:
1) Driveline failure such as:
-Broken ring and pinion
-Broken u-joint or u-joint straps
-Broken driveshaft
-Broken axle shaft
-If you have a driveline failure, remove the transmission and carefully inspect the case and tail shaft area for cracks. Disasemble the transmission and check the sprag in the back of the transmission case. If it is not damaged, it is recommended to replace the springs and rollers to be on the safe side.
2) Using a reverse manual or trans brake valve-body that does not apply the low/reverse band in first gear.
-Ask the manufacturer of your valve body if it has the low bandy apply feature. This is very important, and I blame the majority of transmission explosions on valve bodies that are not applying the low/reverse band in first. If your car "free wheels" in first (early turbo action valve bodies), it is not applying the low/reverse band.
3) Misuse/abuse on the street or strip in first gear.
-Remember you can only damage the sprag in 1st gear. Start your burnout in 2nd, and shift to 3rd. When on the street, repetitively getting in and out of the throttle while in 1st gear can damage the sprag.
In the photo above notice the two bent fingers on the spring retainer.
Photo courtesy of www.kbpi.com
Photo courtesy of www.kbpi.com
How to prevent this:
You can virtually eliminate the chances of having an explosion by using a valve-body that applies the low/reverse band in first gear. By having the low/reverse band applied in first, the band helps to "cushion" and protect the delicate sprag. Remember with low band apply, the engine will compression brake when the selector is moved from 2nd to 1st, so make sure your speed is substantially lower prior to shifting down to first.
It is also a good idea to replace the OEM powder forged high gear/front clutch retainer drum with a billet steel drum for the street, or a billet aluminum drum for "race only" vehicles. Although the aluminum drum is made from strong 7075 material it is not recommend for street use as it will wear quicker.
A bolt in sprag is a common upgrade for the racer's peace of mind. It will not, however prevent a transmission explosion. I would recommend the "Ultimate" sprag as an upgrade from the previous 4 bolt, 12 roller design. The "Ultimate" sprag is a 6 bolt, 16 roller design, with 4 extra springs and rollers to provide that much more sprag protection.
6 bolt, 16 roller design "Ultimate" sprag
Finally, all race cars should use a SFI approved transmission scatter shield to prevent debris from entering the vehicle should you have an explosion.
If you have any questions regarding this very important topic, please by all means feel free to reply here, PM me on the board, send me an e-mail or call me at 720-939-4896.
Thanks for reading,
Chris Andrews
A few years back at a local dragstrip, I happened to be standing by the starting line when a racer in a 66 Dodge had his 727 explode right off of the starting line.
I went to his pit area to examine the carnage, there was a massive crater in the floorboard/transmission tunnel area. Luckily the racer only suffered from a sore ankle. We were discussing the possible causes, when he told me he broke the ring gear in the differential the week before. He mentioned that his "transmission guy" checked out the transmission afterwards and gave him the OK to put the car back together. To my surprise the "transmission guy" was standing in the racer's pit area. I told him that these transmissions are not supposed to explode if they are built with quality components and the correct techniques. He didn't take kindly to me offering up my opinion at the time, and he simply replied "If you race enough it will happen". That statement is flat-out WRONG.
Why it happens:
It all starts when the over-running clutch (aka the sprag) in the rear of the transmission case is damaged. The roller bearings will commonly "roll-over" the posts or fingers on the spring retainer. This action will cause the transmission's geartrain to over-rev the engine rpm multiplied by the first gear ratio, commonly 12,000 rpm or more. The stock OEM powered forged high gear/front clutch drum retainer will explode at approximately 9,000 rpm. The sprag can be damaged by one or more of the following:
1) Driveline failure such as:
-Broken ring and pinion
-Broken u-joint or u-joint straps
-Broken driveshaft
-Broken axle shaft
-If you have a driveline failure, remove the transmission and carefully inspect the case and tail shaft area for cracks. Disasemble the transmission and check the sprag in the back of the transmission case. If it is not damaged, it is recommended to replace the springs and rollers to be on the safe side.
2) Using a reverse manual or trans brake valve-body that does not apply the low/reverse band in first gear.
-Ask the manufacturer of your valve body if it has the low bandy apply feature. This is very important, and I blame the majority of transmission explosions on valve bodies that are not applying the low/reverse band in first. If your car "free wheels" in first (early turbo action valve bodies), it is not applying the low/reverse band.
3) Misuse/abuse on the street or strip in first gear.
-Remember you can only damage the sprag in 1st gear. Start your burnout in 2nd, and shift to 3rd. When on the street, repetitively getting in and out of the throttle while in 1st gear can damage the sprag.
In the photo above notice the two bent fingers on the spring retainer.
Photo courtesy of www.kbpi.com
Photo courtesy of www.kbpi.com
How to prevent this:
You can virtually eliminate the chances of having an explosion by using a valve-body that applies the low/reverse band in first gear. By having the low/reverse band applied in first, the band helps to "cushion" and protect the delicate sprag. Remember with low band apply, the engine will compression brake when the selector is moved from 2nd to 1st, so make sure your speed is substantially lower prior to shifting down to first.
It is also a good idea to replace the OEM powder forged high gear/front clutch retainer drum with a billet steel drum for the street, or a billet aluminum drum for "race only" vehicles. Although the aluminum drum is made from strong 7075 material it is not recommend for street use as it will wear quicker.
A bolt in sprag is a common upgrade for the racer's peace of mind. It will not, however prevent a transmission explosion. I would recommend the "Ultimate" sprag as an upgrade from the previous 4 bolt, 12 roller design. The "Ultimate" sprag is a 6 bolt, 16 roller design, with 4 extra springs and rollers to provide that much more sprag protection.
6 bolt, 16 roller design "Ultimate" sprag
Finally, all race cars should use a SFI approved transmission scatter shield to prevent debris from entering the vehicle should you have an explosion.
If you have any questions regarding this very important topic, please by all means feel free to reply here, PM me on the board, send me an e-mail or call me at 720-939-4896.
Thanks for reading,
Chris Andrews
