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The load on the drive must increase to hold an engine at 5000 RPM. How 'bout this example. You run a 17 pitch prop and turn the engine up to 5000 RPM. Now switch to a 30 pitch prop and spin the engine up to 5000 RPM. Obviously it takes much more torque to spin a 30 to 5000.The 30 pitch prop is trying it's darndest to keep the engine from achieving that RPM. The entire load is being absorbed by the drive. Does it make sense that the friction created by the gears would be much greater in the second scenario than the first? This increased friction is what would be costing the more HP. If you put the boat in the water without a prop and pushed the throttle forward to achieve 5000 RPM, how much HP is the drive using now? In the case of the dyno I program what RPM to hold the engine to. If I say 5000, it keeps adding load as the power increases to maintain that RPM. The drive would be absorbing every bit of that power.
In the case of the stock 496 we went from 431 HP at the crankshaft to 387 at the prop. That's 44 HP, or roughly 10%. I recently dyno'd a supercharged small block. It made 555 HP at the crankshaft. I took the engine straight off the dyno and put it in the boat. I redyno'd it at the propshaft. This was with a standard Bravo drive, not an X. It made 490 at the prop. There were some gibson mufflers on the boat that probably added a small amount of loss, which may be why we were showing over 11% loss in this example. Hope this helps. Bob
Originally Posted by checkmate454mag
(Post 2103685)
Bob,
Great information with interesting results. I am a bit confused at one thing. You said.. "One last Myth to bust. How much horsepower does a Bravo drive absorb? For those of you that are observant, you’re probably ahead of me. It’s certainly not 25 or 30 HP like many have come to believe. It is a percentage. In this case with a Bravo X drive, right at 10%. I’ve done testing on 600 HP engines and lost 60+ HP to the drive." I am certainly not doubting your claim to this, but how can a drive absorb more hp at a given rpm, with the only variable being a more powerful engine? Wouldnt a drive require the same hp at 5000 rpm regardless of what is turning it? I can see the additional horsepower turning the drive to a higher rpm, thus requiring more horsepower to turn it. I dont understand that a drive absorbs power on a percentage scale. Vinny |
Originally Posted by checkmate454mag
(Post 2103685)
I am certainly not doubting your claim to this, but how can a drive absorb more hp at a given rpm, with the only variable being a more powerful engine? Wouldnt a drive require the same hp at 5000 rpm regardless of what is turning it?
Vinny Michael |
What was the Torque loss thru the bravo drive? is that constant with rpm? Do you have any curves showing HP and TQ with stock and with Dana exhaust?
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All the dyno sheets are posted on my web site.
Originally Posted by cobra marty
(Post 2104112)
What was the Torque loss thru the bravo drive? is that constant with rpm? Do you have any curves showing HP and TQ with stock and with Dana exhaust?
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Okay, so now that we know the Bravo's scrub 10% off the ponies, what can we do to lessen that.....There in lies the question of inquiring minds...Synthetic oil? Lower weight oil? Dialing in the toe/heel adjustment on twins?
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Great job man...I can't wait for more test!!!!
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Originally Posted by bobl
(Post 2103706)
The load on the drive must increase to hold an engine at 5000 RPM. How 'bout this example. You run a 17 pitch prop and turn the engine up to 5000 RPM. Now switch to a 30 pitch prop and spin the engine up to 5000 RPM. Obviously it takes much more torque to spin a 30 to 5000.The 30 pitch prop is trying it's darndest to keep the engine from achieving that RPM. The entire load is being absorbed by the drive. Does it make sense that the friction created by the gears would be much greater in the second scenario than the first? This increased friction is what would be costing the more HP. If you put the boat in the water without a prop and pushed the throttle forward to achieve 5000 RPM, how much HP is the drive using now? In the case of the dyno I program what RPM to hold the engine to. If I say 5000, it keeps adding load as the power increases to maintain that RPM. The drive would be absorbing every bit of that power.
In the case of the stock 496 we went from 431 HP at the crankshaft to 387 at the prop. That's 44 HP, or roughly 10%. I recently dyno'd a supercharged small block. It made 555 HP at the crankshaft. I took the engine straight off the dyno and put it in the boat. I redyno'd it at the propshaft. This was with a standard Bravo drive, not an X. It made 490 at the prop. There were some gibson mufflers on the boat that probably added a small amount of loss, which may be why we were showing over 11% loss in this example. Hope this helps. Bob I know you talked about this when we did mine, what the bravo really eats. We dyno'd mine at 509hp at the prop (and i was a little disappointed) but going on the 383 buildup (555hp/490pshp) That would put mine over 560hp. I just wish i had the dyno sheets for the higher hp :D :D |
Thought I would bust another myth here in this thread since it was mentioned. GM has no current plans to drop the 496 8.1L engine for the Marine or other non light truck uses. That information comes straight from officals at GM Powertrain who build and supply this engine. Thought I would stop that rumor before it goes any further. I hope the CMI's get to you in time to finish a complete comparison.
Best Regards, Ray @ Raylar |
Awesome, I was hoping that was a rumor!
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I removed the turbulator rings when I polished the elbow. I heard they were more for condensation in colder climates. I live in Florida where there is supposedly less of a risk. It's about a year so far.
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