[ I assume that a dyno corrects for , temp, humidity, baro ]
.
Does your typical Dynojet - Mustangdyno, make a correction
for driveline loss? [if no, ignore rest of post]
If so, is that correction incorporated in the dyno's printout?

---------------
people talk about Rear wheel HP.
people talk about 18 percent correction, to get flywheel power
power is measured at the rear wheels, so 'RWP', should
be obvious [except correct for temp, H, baro]
---------------
something does not add up, please enlighten me.

 

No; too many variables.

They report RW HP, corrected for easily calculated percentage errors caused by measurable variations in air density. They DO NOT report engine HP. There is no way to "calculate" engine HP based on a RW dyno reading.

18% might be a satisfactory rule of thumb for "drivetrain loss"; but that's not the same as "difference between engine HP as measured on engine dyno vs RWHP". Everything from exhaust system, to air intake tract, to the accessories on the front of the motor, and so forth, will affect that difference.

I was talking to a guy that owns a chassis dyno, who has a good friend who owns an engine one. A bunch of their friends get their engines built, and dyno'ed on the one guys engine stand; then install them in their cars, and get them chassis dyno'ed. He says that the lowest difference between the 2 he had ever seen was 22%; that was in a full race car (late model pavement stock car). He said that street cars usually run 28-30% different for a stick, and as much as 35% difference for an auto. He sees as much as 2% variation in the difference in the same car, just from adjusting the drivetrain angle correctly..... which should give you some idea of why you can't take the one and calculate the other. Too many variables.

 

Speaking of drivetrain loss, something I've always wondered but never gotten a good reply to, is if the tranny/rear/accessories are a constant, why is loss as a percentage?

i/e if it takes a 50HP to turn the tranny/rear, etc on a 250HP engine, why wouldn't it still be 50HP on a 500HP engine?

 

Some losses are a constant, proportional only to RPM; pinion bearing preload or fluid viscosity, for example. Others are a percentage, since they are in direct proportion to the forces applied to the parts; gear tooth friction, for example. So the total picture is a mixture of the 2 kinds of losses.

Yet another reason why there's no "conversion factor" possible between engine and RW HP.

 

I think dynojets can allow manual trans cars to do a coastdown and come up with a "frictional loss" number. That can then be factored into that specific result.