Can BHP be measured at the crank?

I'm having a discussion about this on another forum as I thought there was no way to measure HP at the crank on a motorcycle. However, someone has said:

I don't have a picture but I've seen videos where they have a special case housing with a shaft coming straight out of it and a flange on the end of it so it can be bolted straight onto the shaft of the dyno.

I though there was a thread on this but I can seem to find it searching! So...

Can HP be reliably measured at the crank? (not output shaft, but the actual crank)

Discuss.

As far as im aware it can be and im pretty sure thats where most manufactures messure the bhp output, but also the engine is out of the car / bike

http://forums.sv650.org/showthread.p...ght=33bhp+dyno

i believe it was talked about in this thread

[fastdruid]

Yes it can and it's not done by inertial dyno's but by a 'brake' dyno.

Obviously as it needs the engine out and mounted to a frame it's not easy or quick which is where the inertial dyno comes in.

Druid

So do the use special crankcases with a hole in the side of them?

Im not sure... you'd have to have the side case off so the oil could escape so id guess yes.

What is it, someone bragging about crank BHP? Who cares anyway, it only matters if you can actually compare it to other machines and who else would have crank BHP?

Originally Posted by ogden

Really, though, the power output at the crank is about as relevant as dry weight. It's great to quote in marketing literature but what really matters is how much you can put down on the road. As a rule of thumb, you'll lose between 10% and 20% between the crank and the tarmac - mostly in the tyre, but the gearbox and final drive do their bit. My thou's making 145 at the wheel which means the thick end of 170 at the crank but, frankly, who cares about the latter? It's only the lower number that matters.

Well said...

[ogden]

Yes, but you need a dyno set up to take an engine that's out of the frame.

Technically, I think the only way to measure *brake* horse power is in that manner.

Really, though, the power output at the crank is about as relevant as dry weight. It's great to quote in marketing literature but what really matters is how much you can put down on the road. As a rule of thumb, you'll lose between 10% and 20% between the crank and the tarmac - mostly in the tyre, but the gearbox and final drive do their bit. My thou's making 145 at the wheel which means the thick end of 170 at the crank but, frankly, who cares about the latter? It's only the lower number that matters.

Well technically crank BHP *does* matter lol...as it is proportional to the BHP you then get at the wheel....the "most correct" way of putting it would be that it matters but isn't easily measurable so the wheel BHP will do

It would actually be a better way of measuring if you wanted to tune the engine (which is usually what's being done) as although you will get loss through the drivetrain, in an ideal world you want to eliminate that so you can see the full effect of changes to the engine (i.e. see them directly) rather than have to factor in/out the (guestimated) drivetrain loss.

[Mr Speirs]

Im sure I read somewhere that BHP is a calculation rather than a measurement. Could be wrong though.

race teams and manufacturers run engines on test beds, this is how they get there crank measures. we loose power through the drive chain, this is why we have rear wheel power figures at it is more suitable to the real world.

[Red Herring]

There is a subtle difference between cars and bikes here. A lot of car race teams run engines on test beds where they measure output at the crank, and for them this is actually easier. When you consider the classic race engine the gearbox is detachable leaving a clutch housing where all they need to do is connect the dyno directly into the centre spline. If they tried to leave the gearbox on they would need to either lock off one driveshaft (enormous strain and power loss through the diff) or somehow connect the dyno to both sides. On bikes the gearbox is an integral part of the engine so it would be impossible to bypass without substantial modification to the original engine. It may be that Honda etc measure it at the crank when they are designing development engines but I've never heard of an engine tuner doing it on bikes. What they can do is mount the engine on a test bed and connect the dyno directly to the gearbox output shaft, thus avoiding the variables created by the chain and tyre.