Would the indicated BHP from a chasis dyno be different if there were different rear gears in the car?

Oh boy! It's a yes and no sort-a-thing.

The purveyors of chassis dynos like to run the vehicle in a lower gear than 1:1. This keeps the roller and resolver speed down, thus lowering the investment cost of the machine. But of course torque, which is really what a dyno measures is affected by gear ratio, tire size, tire to roller slippage, tire hysterisis, bearing losses, gear losses, oil pumping and a bunch of other functions.

All this Mickey Mouse motion is compensated for with a bunch of equations buried in the machine. But then it's torque thru equations that’s read as horsepower, so reality is what ever you’re willing to accept as such.

Mr. Watt invented horsepower as a sales gimmick for his steam engines, it's still best treated as such, unless you possess a God's eye view of the universe, in which case you know the absolute answer. The rest of us are just pass'n thru.

Bogie

[[Mr. Watt invented horsepower as a sales gimmick for his steam engines, it's still best treated as such, unless you possess a God's eye view of the universe, in which case you know the absolute answer. The rest of us are just pass'n thru.]]

Bogie, may I use this statement of yours? Of course giving you full credit, but it really explains something I try to convince people of and almost always to no avail. Everyone thinks horsepower is king, no matter how it is measured or how it is corrected.

In reality, power is torque over time. RPM is a variable, while the gear ratio affects torque, it also affects tire RPM. So this is compensated for by the equations buried in the computer and then related back to engine power at given RPMs.

If you could do back to back runs with the engine in the car with the car attached to a chassis dyno and then pull the engine and run it on an engine dyno, you would have an opportunity to see if the equations within the chassis dyno conversion factors were reasonable.

Of course all or any run on any type of a dyno is subject to the conditions I previously listed and a few more. I start to have consternation about any answer when the adjustment factors start getting to be a large percentage of the outcome, which has become rather standard practice over the past decade.

A general rule of thumb the chassis dyno guys use is that crankshaft horsepower is 20% greater than wheel horsepower. But in some Ford internal engineering documents going back to the 1950s, they state that the difference is more like 50% (47% published) based on the back to back type testing I mentioned a couple paragraphs above. But one has to consider the differences in manufacturing tolerances, tire construction, types of transmission and differential lubrication, and the big one is probably that engines before 1972 were rated in blueprinted condition, without accesories and coolant pump, and without the intake and exhaust systems as they would be installed in the vehicle. That has changed over the last 50 years with engines being rated as from ordinary production with accessories and coolant pump, and as installed induction and exhaust systems, so todays losses appear to be less, but are actually the result of techncial improvements and changes to how measuments are made. So this tells you that you need to consider whether the installation of your engine in the chassis is more or less similar to how the factory installs the engine. If you're running exhaust manifolds, single exhaust, low rise intake with exhaust heat like a factory installation then the power difference between chassis and engine is probably percentage greater than 20. If your engine is blueprinted, running a performance intake and exhaust and few if any accessories, then the correction percent difference is probably more like 20 or a bit less. 

Bogie