Quote Originally Posted by sfort View Post
What do they do and when do you need one? Just got my truck back on the road with more HP(425) turo 400 from a 78 suburban. My wife is grinning ear to ear I am a blessed man!
Many factors determine the need for a looser converter, among them rear tire size, rear gear ratio, vehicle weight, induction system, camshaft timing and so forth.

Let's say that you have a stock vehicle with the stock converter. It is matched by the engineers at the factory to be just loose enough to allow the motor to idle at a stoplight without pulling too hard against the brakes in gear and just tight enough to prevent allowing over-revving the motor on acceleration and wasting fuel. The stock camshaft is designed to make power from idle to 4000 rpm's or maybe just a tad higher.

Now, you swap the cam for one with longer timing. This new cam makes power in the motor from 2500 to 6000 rpm's, so there will be a dead zone from idle to 2500. With a stock converter that hooks up at maybe 1200 rpm's, the motor must pull through this dead zone, making little power and resulting in a slower car than you had with the stock cam in place. Also, the new cam wants to idle the motor at 1000 rpm's instead of 750 like the stock cam, so the motor is tugging at the brakes at every stoplight because the converter is up near its stall rpm.

At this point, a looser converter, one that stalls at around 2500 rpm's will be needed to deal with the longer cam. This new converter will allow the motor to idle higher without tugging on the brakes at stoplights and will also allow the motor to rev through the dead zone before the converter begins to stall at the higher rpm's. So, from a standstill, when you wood the loud pedal, the motor rpm's will flash up to about 2500, allowing the motor to be in the power-making zone and applying some real grunt to the drive rubber.

Converters are available in a wide range of stall speeds. A stall of around 3000 is generally considered by many to be perfectly acceptable behind a street motor, but much more than that is considered unfriendly on the street. Of course, a looser converter will make more heat, so that issue must be addressed with a cooler mounted in front of the radiator.