Propshaft VS Crank HP ratings..
 

Is there a typical amount of drop-off when rating an engine's HP?

20 - 30HP drop from crank to propshaft?

Does it vary from engine to engine or is it a fairly consistent drop?

It is usually 30 hp with a bravo set up and then it drops and changes depending on the drive etc...:D :D

A better rule of thumb is to figure about 6% loss between crank and prop with non-transmission drives like the Bravo. (The higher the load, the more friction you have to deal with.) Losses through transmissions will be higher.

Example: Hp500 x .94 = 470 propshaft hp (as rated by Mercury)

The higher the load...
 

C_Spray - Does that mean that if I put 600 HP through a bravo the loss will be more than 6%? Is it also true that losses through gears are related to RPM as well, maybe RPM squared or cubed?

Where's Rambunctious....what do you say?

tomcat, I would say that the 6% covers the additional loss.

6% of 500= 30
6% of 600= 36

I've heard that the transmission drives, absorb more hp the heavier they are built. I think I read where the Merc drive absorbs 20hp and the Borgwarner absorbs 30hp. Some of the guys that work on these Trans may know a little more exact.

We have a dyno that actually measures the power required to turn a transmission (or crash box for that matter). See Trans Tester for details. On the average, the torque required to turn a stock Velvet drive (current model, the early ones are worse) is about 18 ft lbs any place between 4000 and 6000 rpm, it does go up a bit after that. One of our modified units takes about 14 ft lbs and our dry sump is about 10, and goes up to 12 or so at 7000 RPM. A regular crash box is about 5 ft lbs. When in forward there are no gears involved, so as far as the engine is concerned the transmission is just a straight connection to the drive (with the above drag). The loss as a result of the transmission is fixed and not related to load or any kind of percentage, but of course HP = RPM x TORQUE/5252 so the loss does go up with RPM.

One of these days I will make the adapter to hang a drive on it and at least get an idea of the no-load power required just to turn it.

Marc, That's exactly what we need!! Make the adapter and then test the TQ required to turn the drives.

XZ, XR, IMCO Extreme, IMCO SC Extreme, B-Max.
1.5 and 1.36 ratios.
3000-6000rpm.
Mercury lube and then amsoil synthetic lube.

OK, are you done yet. We need this data. Hey Tomcat, are you interested?

Marty - I'm interested too. If an HP500 loses 30 HP @ 5200 then it loses 30 ft-lbs in the drive, but I wonder how this changes with RPM.

This info could help explain the speed loss some are experiencing with the XR and B-MAX drives. The effect of ratios, rpm's, and synthetic lube would be very enlightning.

the math is pretty clear--every right angle eats up a curtain percentage--thats why the motorcycles (that race/go fast) use chains--less right angles--the V-MAX kit from driveshaft to chain makes a big difference as far as rear wheel hp.