Calculating Transmission Losses

[trevorlawrence]

Chassis dynamometers are the devices that measure the power and torque of an engine, without removing it from the vehicle's chassis.

A chassis dynamometer has one or two steel drums, called rollers, and the vehicle's wheel is placed between them.

But, after the engine's installation in the vehicle, it connects to several other items and devices, and it has to drive them, which absorbs some of its power. When the engine is tested with an engine dynamometer, which measures the power straight from the engine, it will show readings that are higher than the ones collected from a chassis dynamometer. In some cases, the power 'at the wheel' was 2 thirds from the power measured at the engine. Also, the fact that the rollers are made of steel is important, because the tires may slip, an the results will be quite altered in this situation.

Many of the parts from the transmission system absorb a small amount of the the engine's power. This is called transmission loss, and the modern dynamometers are built in such way that they can calculate this transmission loss aproximately.

How do they do this? - With the help of special computer software.

This dynamometer software can calculate and predict transmission losses, and it can also simulate real life driving conditions, or even racing conditions. Or at least that's what it's developers say, because according to some mechanics and engine tuners it's not accurate at all, and its results are not true.

So, the most accurate and sure method of calculating transmission losses is to measure the engine's power with an engine dynamometer first, and then with a chassis dynamometer. The difference between these represents the real transmission loss.

Also be sure that the tires are properly inflated, because tires that are not inflated can absorb a lot of power and the dynamometer's readings will be inaccurate.

 

[fishy68]

I haven't had any dyno experience but i do know a lot about torqueflites and your right they do absorb some engine power, just like any other auto trans. As far as figuring torque converter loss I'd imagine they do that by how much rpm drop there is in high gear between the engine and rear tires. I have watched a bunch of dyno videos and always see the drive tires strapped down super tight when dyno'ing so I'd think there shouldn't be any tire slip on the rollers. But again I haven't been there in person to witness this. Just my thoughts. It's possible there is a set of transmission losses programmed into the dyno computer using average losses figured on trans. dyno's for each transmission and the dyno opp selects which trans. it is. I remember yrs. ago a magazine dyno testing every popular auto trans. there was and posting the hp loss through each. If I remember right the 727 TF lost about 56 hp and the 904 lost about 42 hp but it's been yrs. so I may be off a couple hp.

I'd assume your statement to figure loss by dyno'ing the engine first then in the chassis is mostly correct but remember when doing this the engine is in different surroundings so the results can be somewhat skewed.

 

[POS Dakota]

Try the loss through an overdrive supposedly in the 20-23 percent range.