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Propshaft VS Crank HP ratings..

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Old 03-23-2002, 09:46 AM
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Default 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?
 
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Old 03-23-2002, 10:02 AM
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It is usually 30 hp with a bravo set up and then it drops and changes depending on the drive etc...

Last edited by thunderdan; 03-23-2002 at 10:16 AM.
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Old 03-23-2002, 10:09 AM
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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)
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Old 03-23-2002, 12:01 PM
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Default 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?
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Old 03-23-2002, 03:35 PM
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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.
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Old 03-23-2002, 04:49 PM
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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.
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Old 03-23-2002, 05:12 PM
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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?
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Old 03-23-2002, 06:21 PM
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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.
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Old 03-23-2002, 06:58 PM
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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.
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Old 03-23-2002, 09:24 PM
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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.
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