Quote:
Originally Posted by Andyman
I thought of another modification that can be added to the "Drivetrain mods" section. Use a tight or low slippage torque converter. This could also be called a low stall speed torque converter. Stall speed is the engine RPM when the automatic transmission is in gear with the brakes on to keep the car stopped and throttle opened completely. This shouldn't be done for more than a few seconds since it creates a lot of heat in the transmission. It's also important that there is nothing to hit in front of your car in case your brakes fail during the test.
If the torque converter slips less, it wastes less energy making heat in the torque converter. This should help improve mileage in slow local driving. On the highway, most torque converters engage a clutch to eliminate slippage. This happens at 35 - 50 MPH on most cars. On my car, it's close to 50 MPH. I get a lot of slippage when I climb hills at lower speeds. I estimate that I'm losing about 1/3 of my engine power in the torque converter when I climb a typical hill at about 32 MPH. My engine runs at 2000 RPM in this condition, but if the torque converter is locked, my car can go 48 MPH at 2000 RPM. So only 2/3 of the engine speed is passed to the transmission when I climb the hill.
Cars with larger engines and engines that run slower (such as diesels) are likely to have tighter torque converters. This would require some research to figure out which torque converters are available for your car and which ones are tighter. Keep in mind that a tight torque converter is likely to reduce your acceleration from a stop because the engine may not be able to get to the speed of maximum torque immediately.
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Interesting to hear it put this way. Torque converters with a higher stall speed are a common performance modification for muscle cars with automatic trannys. In my Formula for example, the stock stall speed is 1800 rpm, many people (and perhaps me eventually) replace it with an aftermarket unit with a stall speed around 3000 rpm, even higher for cars used primarily for drag racing. The improvement in acceleration is very large, although of course there is a mpg hit at lower speeds when the converter is unlocked.
On my Escort I can program an X Gauge that shows the ratio of output shaft speed to input shaft speed, so I can see torque converter efficiency expressed as a percentage. Sometimes when climbing steep hills it gets down around 60%, which is somewhat in line with what you are describing (which car btw?). This is also when I see the highest trans fluid temps, between 200-215*.