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Old April 6th, 2004, 21:09   #12
GoFaster
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Join Date: Jun 1999
Location: Brampton, Ontario, Canada
Default The absurdity of fixed percentage drivetrain loss

Before this thread gets over-complexified, I'm gonna make one very simple statement:

When installed in a vehicle, engine horsepower does not matter to the end user. Wheel horsepower matters because that's what is propelling the vehicle.

It simply doesn't matter to the end user how much driveline loss there is. It's irrelevant. In the world of tuning motorcycles, which is the one I'm more familiar with, nobody pays attention to engine horsepower. Everything in the aftermarket is gauged by wheel horsepower as determined by a suitable chassis dynamometer.

If you make mods to the engine that result in 5 more wheel horsepower, that's great. If you make mods to the drivetrain that result in 5 more wheel horsepower (good luck, but the way!! very difficult), that's equally great. IT DOESN'T MATTER.

Sometimes it's nice to know how much power is being lost at different places, in order to pursue areas of improvement, and this is where it's kinda nice to dyno the engine out of the vehicle and then dyno the whole vehicle. But invariably, the engine installation at the engine dyno differs from the installation in the vehicle, and that muddles the situation. Result, you know something but not everything, and this approach isn't really practical for anyone short of professional race teams with lots of $ and time on hand.

Dave is correct that driveline loss is not really a "percentage". There is a component that's proportional to the torque transmitted (friction between the gears and in the bearings - but the bearings are all rolling-element and gear-sets are typically 98% - 99% efficient except maybe for hypoid rear-ends that are a bit less). If there are two gear-sets to go through and no 90-degree gearsets, which is typical of transverse front-drive gearboxes, then the mechanical efficiency of the gears not counting other types of losses can be in the 97% range.

There is a torque component of the losses that depends on the rotation speed (typically, churning and pumping losses of the lubricant - these losses increase with rotation speed but aren't affected by how much torque is being transmitted), and there is a torque component of the losses that is almost constant regardless of rotation speed or torque transmitted (typically, sliding friction in seals). And automatic transmissions have complex patterns of losses (typically much greater than manuals) due to the torque converter and the pump that operates all the hydraulics.

There are things that can be done about some of these losses (different lubricant viscosity, for example) but much of it is set by the design - nothing can be done about it other than different lubricant viscosity or oil level, and that's limited by what is specified by the manufacturer. Bottom line: Transmission losses are there, they vary and are not a set percentage, and little can be done about them, so the most suitable course of action is to simply not pay attention to it and only pay attention to wheel horsepower.

The bit about wheel and rim weight and other inertial loads deserves comment: how much difference this makes depends on what type of dynamometer is used. If a brake dyno is used, in which speed can be held constant while the torque is absorbed by a (typically) water brake, then Dave is correct, nothing you do about the inertia will make any difference at all. But if the most common aftermarket-type dyno is used (the Dynojet inertial dyno, or similar) then inertia of those parts CAN and WILL make a significant difference.

Finally, the most complex element of all: Like it or not, there are variations from one dyno installation to the next. Some of it is variance due to the dyno itself, but Dynojet claims that it's small. Cycle Canada did some testing a while back which found that perhaps the variation is greater than Dynojet admits to. For sure, there are variations due to the installation, and there are variations due to the way the testing is done, and there are variations due to temperature and barometric pressure.

If you are doing serious tuning, then comparison tests should always be done on the same dynamometer by the same operator using the same test procedure, and preferably under ambient conditions that are as close as possible. Comparisons between different dyno installations aren't really valid.
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