1991 RS/SS

I know that this can vary somewhat, but I was hoping that I could get some estimates on the percentage of power that is lost in my drivetrain.

I have a stock 4L60E / stock torque converter (1995), both with about 75,000 miles on them, stock steel flexplate, stock steel driveshaft, and a GM 7.5" rear end - moser forged axles, auburn pro limited slip diff, 3.73 gears.

Any ideas are appreciated. Thanks.

DJP87Z28

With a 700R4 the drivetrain loss is between 20/24%.

kdrolt

Stock B platform fullsize GM cars (Caprice, Impala SS) that had the LT1 typically dynoed 210 to 220 peak hp @ 4700 engine rpm, as measured at the rear wheels. GM rated the Bcar LT1 at 260 fwhp @ 5000 rpm. Keeping the math simple (which sidesteps the 4700 vs 5000 rpm delta), the drivetrain throughput is in the range of 210/260 or 80.7%, to 220/260 or 84.6%. That means the drivetrain loss, as a percentage of input power can vary from 19.3% to 15.4%.

Most people just take the average, 215/260 or roughly 83% throughput, and therefore around 17% loss from flywheel to rearwheel. This also assumes that all GM Bcar LT1s had 260 fwhp... which is obviously false, but we'll assume that the 260 fwhp rating is a median value (roughly an average) and move on from there.

The 4L60E trans used on the GM Bcars is just the electronically shifted version of the 4L60, which was formerly named the 700R4. Manual trans typically have less loss doe to the lack of fluid slip in the torque converter (because there isn't one). If you dig through the web you can probably find examples of dyno tests on stock LT1 Fcars having 6spd MT and get drivetrain loss estimated from those tests.

Actual drivetrain loss, in the most picky engineering sense is the combination several losses, some that are constant, some that are rpm dependent, some that are load dependent, and some that combine these.

The dominant loss in the drivetrain happens to be the load-dependent loss associated with the forces between gear teeth (in the trans and in the rear end), as evidenced by the increased heating of the trans and differential with increase in the power delivered to them. If the loss were fixed, as has been previously and wrongfully claimed on TGO before, then the trans + rear end would not increase in temp no matter how much power was being delivered through them.... nor would it matter to the trans temp if you were towing, or towing uphill. Clearly that's nonsense -- the drivetrain load increases if you attempt to drive at constant speed uphill, and that's why you need extra cooling capacity (a trans fluid cooler) when towing heavy loads.

So that's why (to first order) the "percentage loss method" is a better approximate method for reconciling the output at the flywheel vs the output delivered to the road by the driving wheels. The method is better for use with street driven vehicles than it would be for racing cars that have very nonlinear behavior tires. HTH.