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Off Road Torque Converters

Two commonly asked questions about torque converters and off road driving are:

  1. What kind of torque converter should I get for off road driving and why? And,
  2. What is the significance of the "stall" speed.
For off road driving, you want low stall. Exactly what number depends on what kind of engine torque you've got. If you've got a torquey engine, you can get a tight converter. If you're rock crawling, you probably have a torque monster engine already. If you're not rock crawling, it doesn't' matter a whole lot other than what you prefer. Something with a stall speed of around 1600-1800 would probably be the best bet. OEM converters are usually low stall, and probably good for most applications. You can determine your convertors stall speed by pressing hard on the brakes and with the tranny in drive, push on the gas and note the highest rpm achieved.

High speed stall convertors are used by drag racers since they allow the engine to climb higher up into its power band before fully coupling with the drive train. It is very much like revving up your motor and dumping the clutch.

Disadvantages of a high speed stall converter for off road driving are:

Greatly increased tranny heat. The slipping at low rpms will create gobs of heat.
Worse gas mileage since driving below the stall speed is much like driving around in low gear.
More difficult to moderate speed.
A lower stall is easier to moderate since it starts to engage without having to get your rpms up very high. If you have to get the rpms to up over 2000 before you start moving that's a bit of pedal you're applying. Once you start to move at this rpm, you may run into objects, break axles or other bad things could happen.

For street and highway driving, you want your converter's stall speed to be lower than your cruise RPM. Significantly more heat is generated below the stall speed than above it, so if you have a 2500 stall converter, and you cruise at 2000 RPM, you'll generate too much heat, which breaks down the tranny fluid, and that ruins trannies.

A high stall converter will slip more at highway speeds than a low stall converter, but probably not much.

When your engine accelerates, and your RPM is below the stall speed, the fins on the engine side of the torque converter are moving significantly faster than the fins on the tranny side. This allows the converter to "slip," much like a clutch, but it also creates turbulence in the fluid, which is what generates the heat. The rotating fins on the engine side cause the fluid to rotate, which in turn puts force on the tranny side fins causing the tranny to turn. Imagine spinning your hand in a bucket of water. At first there is turbulence, but eventually the water is moving close to the same speed as your hand. If there is something at the bottom of the bucket, it begins to spin as well.

A car with a high stall converter is really not much fun, except for racing. At low RPM, your engine feels kindof mushy. The engine tends to wind up to the stall speed before anything happens, and then it stays at that RPM until your car catches up to the engine speed. If any shifts occur during that time, you barely even feel them (of course this can be good if you've installed a shift kit).

I would consider 2000 RPM and below reasonable for the street, 3000 and above unreasonable, a the rest depends on the vehicle.





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