Originally posted by Curls
Now that someone with tech knowledge is here can you explain what a low stall converter is/does please?
Not surprisingly a low stall converter does the exact reverse of a high stall one.
In simple terms when the engine accelerates, and RPM is below the stall speed, the fins on the engine side of the torque converter are moving faster than the fins on the transmission side and this is what allows a torque converter to slip in much the same way as a clutch does in a manual transmisison. The downside of this slippage is that it generates heat - the natural enemy of the transmission.
The "stall" speed is reached when the fins on both sides of the torque converter are spinning at the same speed (or close enough to) and you then have positive drive with no slippage.
Whilst not a representation of the true stall speed you can do a simple test by applying the brakes hard in gear and accelerating until the car either stalls (thus the name), the wheels start to spin or there is no extra increase in tacho RPM. The RPM this happens at is a guideline indication of the torque converter stall speed.
Typically, manufacturer stock units will stall between 1200-1800 rpm as this provides good driveability characteristics and minimises the heat generation at cruising speeds.
For drag racing application the stall speed is often altered to 2500-3000 rpm. This allows a quicker launch in much the same way as building up revs and dropping the clutch does in a manual transmission as the engine is producing more torque at the higher RPM point.
Recent years have seen a trend from manufacturers with high torque engines (like the Boss pair) towards low stall converters that stall between 800 and 1200 rpm. These are easier on driveline components as they do not allow the torque to build up and place extra strain on either the transmisison or rear axle and they have the added bonus of being very driveable on the street.
Like all ideas this concept was pinched - in this case from the diesel world where low stall converters have been commonplace for some years due to the torque characteristics of diesel engines - typically high torque at very low RPM. The reason though is different! In diesels the aim was to maximise the torque multiplication of the converter and to ensure that it didn't stall above the maximum torque point.
Hope this helps.