Data Recorder Shows 300 rpm 'droop' After Car Launches???

ss3011 · 06-14-2012, 07:21 PM

Torque converter slip is a function of engine torque and turbine shaft rpm. The turbine shaft (input shaft) is at zero rpm on the starting line and a torque converter can be analyzed using a formula k=n/sqrt(T) k is a constant that defines a particular converter n= engine rpm and T = engine torque So when the turbine shaft is zero k= (the stall speed) divided by the square root of the engine torque. This is why if you use the same converter, for example a 280 k converter, behind a big block that makes 500 ft lbs it will stall at about 6260. The same converter behind a small block that makes 450 ft lbs will stall at 5940. This is a balancing act because the torque curve changes with rpm, but that K number stays the same. It is a function of that particular converter. So if you launch the car and the engine is making 500 ft lbs, and something effects the engine and it drops some torque, you get a sag in rpm, on your data logger.
One other comment, is that the K is the same as long as the converter is hydraulically stable inside. If the converter cavitates then the K is not predictable. Cavitation can happen if the torque going into the converter is higher than the converters ultimate capacity. This is more likely to happen when the turbine shaft is at zero rpm. Capacity increases quickly as the turbine shaft picks up rpm after launch. So engine rpm could go higher that the predicted stall speed based on the K factor. This could be another reason you see a sag in rpm.

06-14-2012, 07:21 PM	#9
ss3011 Senior Member Join Date: Feb 2002 Location: Southeast Michigan Posts: 909 Likes: 70 Liked 239 Times in 114 Posts	Re: Data Recorder Shows 300 rpm 'droop' After Car Launches?? Torque converter slip is a function of engine torque and turbine shaft rpm. The turbine shaft (input shaft) is at zero rpm on the starting line and a torque converter can be analyzed using a formula k=n/sqrt(T) k is a constant that defines a particular converter n= engine rpm and T = engine torque So when the turbine shaft is zero k= (the stall speed) divided by the square root of the engine torque. This is why if you use the same converter, for example a 280 k converter, behind a big block that makes 500 ft lbs it will stall at about 6260. The same converter behind a small block that makes 450 ft lbs will stall at 5940. This is a balancing act because the torque curve changes with rpm, but that K number stays the same. It is a function of that particular converter. So if you launch the car and the engine is making 500 ft lbs, and something effects the engine and it drops some torque, you get a sag in rpm, on your data logger. One other comment, is that the K is the same as long as the converter is hydraulically stable inside. If the converter cavitates then the K is not predictable. Cavitation can happen if the torque going into the converter is higher than the converters ultimate capacity. This is more likely to happen when the turbine shaft is at zero rpm. Capacity increases quickly as the turbine shaft picks up rpm after launch. So engine rpm could go higher that the predicted stall speed based on the K factor. This could be another reason you see a sag in rpm.