Join Date: Jun 2005
Location: Deep in the Heart...
'05 Auto Redline Red #20727.... Born on 05/05/05...
Torque Converters 101 by Greg Ducato
This is good information for those that need to know, Thanks to Greg Ducato with Phoenix Transmission for sharing his wisdom:
Originally Posted by Greg Ducato
Due to the very technical nature of the converter and it's variants, I will try and keep this simple and easy to understand, however, I realize this is a deep subject and will be happy to answer any other questions about the torque converter in this thread, so evryone can try and get a better understanding.
The stock torque converter in the SSR is rated at about 1600 rpm stall speed. Stall speed is the the point where the converter is hydraulically peaked in it's ability to transfer engine power to the drivetrain. Most converters are about 85 percent efficient at peak stall speed so there is always some inherent slippage in the converter. The 4L60E-4L65E uses a "lock-up" torque converter, this type of converter has a clutch inside which locks both halves of the the converter to eliminate any internal slippage.The computer controls the amount of lock-up and when the lock-up occurs according to speed and throttle demand as well as temperature.
A torque converter, by design, multiplies engine torque to the driveline and by altering components in the torque converter, a builder can raise or lower the stall speed of any given converter and also change it's torque multiplication ratio. By using a smaller than stock diameter converter as a base unit, a builder can achieve a higher stall speed without a great loss of efficiency and excessive slippage. It is the slippage in a converter that generates heat, and the higher the stall speed, usually the higher the heat generated, another important reason to keep the lock-up function which eliminates this heat.
It is a common misconception that a higher stall converter will not allow a normal take-off and that it must rev much higher to get the vehicle rolling. This would only be true of some converters that have been far over modified for their diameter and made very inefficient. The stock converter on the SSR is 300MM, the safest stall speed modifications on this would take it to about 2200-2400 rpm. To go to higher stall speeds and still be efficient, converters for the SSR can be built in 280MM, 258MM and 245MM housings with stall speeds from 2600 all the way up to 4500rpm.These converters will "flash" to the higher stall speed under harder acceleration, but will usually be pretty docile in light throttle operation if properly built and matched to the vehicle and it's modifications.
The LS based engines in the SSR's are a much higher revving engine than the old small block designs of the past and torque peaks are much further up the rpm scale than older design engines. Adding an aftermarket cam profile and free flowing cylinder heads will increase power and torque but usually at the expense of bottom end performance. Anyone with a stock SSR knows that these are heavy vehicles and are not particularly strong out of the hole, usually struggling to even break traction from a standing wide open throttle start. Part of this is because the stock torque converter has such a low stall speed that the engine struggles to reach a powerband that will propel this heavy vehicle faster.
By installing a higher stall torque converter, the engine can rev much quicker to it's horsepower and torque peaks giving much faster acceleration under wide open throttle. This is true of both off the line and also when dropping into passing gear on the freeway.
There are some around town MPG penalties with a higher stall converter, but if chosen and designed correctly these penalties are quite minor, and highway mpg will not suffer at all due to the lock-up design.The biggest MPG drop will be in the first week after the converter is installed since you won't be able to keep your foot out of the throttle, it makes that much of a difference. In fact, swapping to a higher stall converter gives as much or more performance gain as a gear swap, and will not alter the highway cruising rpm like a steeper gear will do. A higher stall converter works great on an otherwise stock SSR to give greater acceleration without extensive engine modifications and will work with a stock tune and lower octane fuel.