HP losses from Bravo 1X drive
12-20-2006, 02:26 PM
#21
Re: HP losses from Bravo 1X drive
So why not measure the amount of HP required to turn a Bravo drive. Oh Yea you can't measure HP, So why not take a TORQUE wrench and measure how much TQ is required to turn over the drive. Synthetic lubes will lessen the TQ required and free up some HP. Who has a drive sitting around and can test this. Or you can test this in the boat in neutral and also in gear to see the added rotational requirements of the engine(the plugs need to be out and the throttle held wide open).
12-20-2006, 02:46 PM
#22
the Flightmaster
Platinum Member
Re: HP losses from Bravo 1X drive
The difference between the X and the XR drive are the straight cut instead of bevelled gear teeth, and the 1 1/4" instead of 1" propshaft (and a few other features).
The straight cut gears can handle greater load because the contact point between the gears is flatter and therefore a larger effective surface. The disadvantage is more friction and more noise. I think 30 hp is about right for the XR and 15 - 20 hp for the X. An IMCO SC saps 50 - 60 hp but can handle 700+, as a comparison.
As far as the HP ratings of the Mercury power units go, I can only tell you what is written and what I have experienced.
When Tyler Crockett tested the stock 496HO before fitting CMI headers to it, he recorded 425 hp at the crankshaft, added the headers and saw 485 hp. Other dyno tests have also recorded the 425 hp at the crank. Raylar, for example. If the hp rated by Crocket had been propshaft, that would make a 496 w. CMI´s more powerful than a 500EFI, which it isn´t.
Mercury racing officially promotes its 525 with exactly that amount of propshaft horsepower. The picture taken at the Miami Boat Show this year shows that!
My own 25 Outlaw with the 496HO did 67 mph GPS maxed out on a good day. X drive, labbed bravo 25" @ 4900 rpm.
It now does over 80 mph GPS fitted with a 2006 525 and an IMCO 3" lift extension box. XR drive, labbed bravo 28" @ 5300 rpm.
Let 4 mph be the result of the ext box/ higher x. That´s still 9 mph due to the motor.
The effective horsepower difference between 496HO and 525EFi is about 115 hp.
I can´t explain the speed difference otherwise.
The straight cut gears can handle greater load because the contact point between the gears is flatter and therefore a larger effective surface. The disadvantage is more friction and more noise. I think 30 hp is about right for the XR and 15 - 20 hp for the X. An IMCO SC saps 50 - 60 hp but can handle 700+, as a comparison.
As far as the HP ratings of the Mercury power units go, I can only tell you what is written and what I have experienced.
When Tyler Crockett tested the stock 496HO before fitting CMI headers to it, he recorded 425 hp at the crankshaft, added the headers and saw 485 hp. Other dyno tests have also recorded the 425 hp at the crank. Raylar, for example. If the hp rated by Crocket had been propshaft, that would make a 496 w. CMI´s more powerful than a 500EFI, which it isn´t.
Mercury racing officially promotes its 525 with exactly that amount of propshaft horsepower. The picture taken at the Miami Boat Show this year shows that!
My own 25 Outlaw with the 496HO did 67 mph GPS maxed out on a good day. X drive, labbed bravo 25" @ 4900 rpm.
It now does over 80 mph GPS fitted with a 2006 525 and an IMCO 3" lift extension box. XR drive, labbed bravo 28" @ 5300 rpm.
Let 4 mph be the result of the ext box/ higher x. That´s still 9 mph due to the motor.
The effective horsepower difference between 496HO and 525EFi is about 115 hp.
I can´t explain the speed difference otherwise.
12-20-2006, 03:24 PM
#23
Re: HP losses from Bravo 1X drive
I was just looking in an old stern drive manual I have and it specifies:
1989: 7.4L: 330 crankshaft HP
1989: 454 Mag: 365 crankshaft HP
1990: 7.4L: 300 propshaft HP (Bravo One)
1990: 454 Mag: 360 propshaft HP (Bravo One)
1991: 7.4L: 300 propshaft HP (Bravo One)
1991: 454 Mag: 350 propshaft HP (Bravo One)
So why a loss of 30 HP on the 7.4l and only 5 HP on the 454 Mag in 1990 through a Bravo One drive ?
And then a loss of 30 HP on the 7.4l and only 15 HP on the 454 Mag in 1991 ?
I think Mercury Marines HP ratings are a little inconsistent, which makes it difficult to calculate the loss at WOT in the drive.
1989: 7.4L: 330 crankshaft HP
1989: 454 Mag: 365 crankshaft HP
1990: 7.4L: 300 propshaft HP (Bravo One)
1990: 454 Mag: 360 propshaft HP (Bravo One)
1991: 7.4L: 300 propshaft HP (Bravo One)
1991: 454 Mag: 350 propshaft HP (Bravo One)
So why a loss of 30 HP on the 7.4l and only 5 HP on the 454 Mag in 1990 through a Bravo One drive ?
And then a loss of 30 HP on the 7.4l and only 15 HP on the 454 Mag in 1991 ?
I think Mercury Marines HP ratings are a little inconsistent, which makes it difficult to calculate the loss at WOT in the drive.
12-22-2006, 10:18 PM
#25
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Re: HP losses from Bravo 1X drive
Originally Posted by cobra marty
Someone please put a torque wrench on a drive and see how much to make it turn.
12-22-2006, 10:47 PM
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Re: HP losses from Bravo 1X drive
I have a boat I'm just completing. It has a pair of Whipple supercharged small blocks. I dynod both engines after completing them. I am going to put the prop shaft dyno on it next week and measure them again using a Bravo X drive. I should get a pretty good idea of what power is consumed. The engines made 570 HP at the flywheel. I'll be using the same dyno to prop shaft test them so It should be very accurate. I'll post the results.
12-23-2006, 10:55 AM
#27
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Re: HP losses from Bravo 1X drive
Originally Posted by cobra marty
Someone please put a torque wrench on a drive and see how much to make it turn.
The bearings are not acting hydro-dynamically at that point. If you look at the friction curve of any journal bearing system, it approximates a logarithmic function plus a constant: Very high at near zero speed, then approaching a lower constant as speed increases.
12-23-2006, 12:00 PM
#28
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Re: HP losses from Bravo 1X drive
Originally Posted by bobl
I have a boat I'm just completing. It has a pair of Whipple supercharged small blocks. I dynod both engines after completing them. I am going to put the prop shaft dyno on it next week and measure them again using a Bravo X drive. I should get a pretty good idea of what power is consumed. The engines made 570 HP at the flywheel. I'll be using the same dyno to prop shaft test them so It should be very accurate. I'll post the results.
That will be very interesting! The way the dyno loads the outdrive gears (hopefully similiar to the way a prop would load them) we could also expect to see the logarithmic reduction of propshaft HP as you increase the RPM. Do you use the Land & Sea dyno?
12-23-2006, 01:34 PM
#29
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Re: HP losses from Bravo 1X drive
Originally Posted by cobra marty
Someone please put a torque wrench on a drive and see how much to make it turn.
12-23-2006, 01:39 PM
#30
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Re: HP losses from Bravo 1X drive
Originally Posted by tomcat
What you say sounds logical for just the "rolling" resistance of the gear box, and probably to wet sump "pumping" losses as well, but I don't believe it applies to losses at the gear teeth.
If you are spinning the input shaft at 5200 RPM with a 50 HP electric motor, then you are spinning the prop in air or spinning a very small prop, and there is next to no load on the propshaft. Under these conditions it will not take 30 HP to spin the drive since much of the loss comes from the gear teeth having to carry a load.
If you are spinning the engine at 5200 RPM with 1000 HP, then the prop is putting a huge load in the gears and the loss will be higher. And with 1000 HP you are definitely exceeding the design of the gear, so more deformation of the teeth under load, more contact area, more friction and higher percentage losses. A loss of 6+% or 60+ HP seems reasonable in this case.
The higher losses on the XR straight cut gears supports this argument. They are stronger due to more tooth contact area, but more contact means more area for friction to happen.
I admit to having reached the limit of my meager knowledge on this subject, and would appreciate a better explanation from someone who really knows this stuff.
If you are spinning the input shaft at 5200 RPM with a 50 HP electric motor, then you are spinning the prop in air or spinning a very small prop, and there is next to no load on the propshaft. Under these conditions it will not take 30 HP to spin the drive since much of the loss comes from the gear teeth having to carry a load.
If you are spinning the engine at 5200 RPM with 1000 HP, then the prop is putting a huge load in the gears and the loss will be higher. And with 1000 HP you are definitely exceeding the design of the gear, so more deformation of the teeth under load, more contact area, more friction and higher percentage losses. A loss of 6+% or 60+ HP seems reasonable in this case.
The higher losses on the XR straight cut gears supports this argument. They are stronger due to more tooth contact area, but more contact means more area for friction to happen.
I admit to having reached the limit of my meager knowledge on this subject, and would appreciate a better explanation from someone who really knows this stuff.
Last edited by articfriends; 12-23-2006 at 01:46 PM.
