Torque convertor considerations

[imjoesnuffy]

Have some questions regarding torque convertors. Am I on the right track thinking the following:
1. Torque convertor too low of a stall for the turbo = lag?
2. Torque convertor too high of a stall for the turbo = faster spool up but softer launch during normal driving unless you floor it/ launch off the brake?
3. On average what are the costs of restalling a TC? Say from 3000 to 3400? Would I be better off just buying one with a higher stall if so why?

Thanks in advance for any help.

 

[bison]

Have some questions regarding torque convertors. Am I on the right track thinking the following:
1. Torque convertor too low of a stall for the turbo = lag?
2. Torque convertor too high of a stall for the turbo = faster spool up but softer launch during normal driving unless you floor it/ launch off the brake?
3. On average what are the costs of restalling a TC? Say from 3000 to 3400? Would I be better off just buying one with a higher stall if so why?

Thanks in advance for any help.

1. If the converter is too low if a stall for your application it could cause lag, but other things could be causing it also.
2. Too high a stall for the turbo will cause a driveability issue and excess heat build up where you would have been fine with a lower stall.
3.$200-400

There are a lot of things that should also be considered when ordering/adjusting a converter besides the stall. Two of the most important ones are the efficiency of the converter as you are nearing the end of the quarter mile and its ability to keep the engine in its sweet spot on ratio changes. A really good properly selected and adjusted converter will slip less than 10% as you cross the line. Some are reporting 6% on really high tech high $ converters these days. This may not sound like a lot but 4% is a lot when the car is going 140mph+ in the quarter. As far as the ratio change rpm goes the converter will begin to slip (usually a lot) as you shift into the next gear. Not to worry as the car is being sling shotted by the converter as the engine is hovering right where it wants to be to provide its maximum acceleration ability. You may actually not gain more than 200 rpm but the car will go a few hundred feet before the converter starts to couple. This is fine as torque is being multiplied. Then the converter will begin to couple and the engine rpm will climb as you approach the line. As you increase the gear number (deeper gears) the converter will couple (reach its peak efficiency) sooner in the run. This could be accomplished by using higher numeric gears or shorter tires. Usually you will lose more rpm on shifts under this situation also. Doing the opposite will cause the later coupling and or even less efficiency. This is why a lot of guys running older converters with their new engines making 200 ft-lbs more torque than they were 4 years ago have an efficiency problem up top. Adding a taller tire just aggravates this as the converter will slip more for a longer distance in the quarter causing a lower than desired trap speed or you simply run out of gear at 1:1 wasting e.t. You should never be in OD when drag racing. If you are then you have converter/gear issues. The ultimate is to have a converter that will stall easily under torque, be efficient cruising, and reach a really good efficiency in the top of 3rd gear.

 

[imjoesnuffy]

Thanks Bison. I don't think it could have been explained any better.

John

 

[John Wilde]

Bison,

Excellent post!

 

[Jerryl]

Bison,
Excellent post!

Should be a sticky! :biggrin:

 

[Bens87tr]

Ok, I'll bite... Now how does one go about figuring out the converter to match? Is there a general rule of thumb about efficiency related to brand, size, or other parameters? Seems every builder has a different answer or
preference.