“What I Learned Today,” With Jeff Smith: Avoid The Destruction 

This is what can happen to any three-speed automatic if shifted into Neutral at the end of a drag strip run. The direct drum over-speeds and explodes, causing damage that looks like a grenade went off inside your transmission.

Never shift into neutral at the end of a drag strip run to save the engine or shut off the engine to read the spark plugs. When a TH400 or 4L80E (or nearly any three-speed automatic) is shifted into neutral, the direct drum in the transmission spins backward at 2.5-times the driveshaft speed. If you click the transmission into Neutral at the end of a pass at 7,000 rpm, the direct drum will be spinning backward at 17,500 rpm. This could instantly result in a catastrophic explosion of the cast drum since it was never designed to spin that fast. This could also potentially lock the transmission, which will lock the rear tires and send the car into a crash. So racers need to be aware of the enormous forces at work in the transmission when shifted into Neutral. 

Several drag race transmission companies now modify three-speed automatics with what is called “clean neutral”. This is a modification where the Neutral shift position in the valvebody allows line pressure to be applied to the direct clutch while disengaging pressure in Neutral to the forward clutch, which unloads the transmission from the engine. Consult with your transmission builder to find out if your trans has been modified to include clean neutral. If not, or if you are unsure,  don’t shift into neutral at high speed. 

About the author

Vinny Costa

Fast cars, motorcycles, and loud music are what get Vinny’s blood pumping. Catch him behind the wheel of his ’68 Firebird. Chances are, Black Sabbath will be playing in the background.
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