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2.55 vs 2.68

digginfool

digginfool

Well-known member
I'm in the unique position of being owed a gearbox from one of the manufacturers (doesn't matter who; at least for now). After I sent that gearbox in for repair, I bought another brand in 2.55 (same ratio). After several years, that gearbox failed and because at that particular time I was in dire need of my boat, I bought a second gearbox to use while what would become my backup was being rebuilt. ASSUMING I ever get my original gearbox back (a story I've been sitting on for going on 4 years), I'm contemplating having it sent back as a 2.68. I've always wondered how my boat would perform with the higher ratio. It's a 15' DB with a well worked over LSA (fully forged rotating assembly, LS9 cam, springs and 10% overdrive pulley for the supercharger; theoretically is capable of 675 hp but realistically around 600) turning a 4 blade 81.5" NGR. Currently, pitched at the 3 mark (which is around 19 degrees of pitch), I'm spinning it at 5,600 RPM. I know the NGR likes RPM but I've also been told it likes pitch as well. With the 2.68, the prop would spin only 100 RPM less but there would be more torque. Is it worth the experiment? I certainly don't need three gearboxes in 2.55 but this does provide an opportunity to check it all out.
 
digginfool said:
I'm in the unique position of being owed a gearbox from one of the manufacturers (doesn't matter who; at least for now). After I sent that gearbox in for repair, I bought another brand in 2.55 (same ratio). After several years, that gearbox failed and because at that particular time I was in dire need of my boat, I bought a second gearbox to use while what would become my backup was being rebuilt. ASSUMING I ever get my original gearbox back (a story I've been sitting on for going on 4 years), I'm contemplating having it sent back as a 2.68. I've always wondered how my boat would perform with the higher ratio. It's a 15' DB with a well worked over LSA (fully forged rotating assembly, LS9 cam, springs and 10% overdrive pulley for the supercharger; theoretically is capable of 675 hp but realistically around 600) turning a 4 blade 81.5" NGR. Currently, pitched at the 3 mark (which is around 19 degrees of pitch), I'm spinning it at 5,600 RPM. I know the NGR likes RPM but I've also been told it likes pitch as well. With the 2.68, the prop would spin only 100 RPM less but there would be more torque. Is it worth the experiment? I certainly don't need three gearboxes in 2.55 but this does provide an opportunity to check it all out.
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Get the 2.68 and turn the motor 5900 RPMs. The prop will be turning the same RPMs. Also you will have to add pitch because you will be making more HSP. I doubt your cruise will go up much since you are adding pitch. You actually might get better fuel mileage since you are taking load off of the motor.
 
Get the 2.68:1 set prop to where it stalls the engine at the flatline of the torque curve before it drops off.
 
Seems like that engine would do well with the 2.68 being a upgraded LS platform.

If 2,200 at the prop is a good fit with your blades then the 2.68 at 5,900 engine rpm looks like good math.
 
I wish I could get it up to 5,900 RPM but I'm running MEFI6 and don't have access to the part of the program that sets max RPM; it's hard set at 5,700 from Marine Power and the company that sold me the unlocked ECM and software to control it won't unlock the program completely. I offered to mail the ECM back for them to adjust it but they don't want the responsibility. I do have access to the fuel and timing maps as well as a host of other parameters but I'm pretty much stuck. So, will the lost 100 RPM be overcome by the increase in pitch?
 
A 2.68 turning a 4 blade NGR at 5600 pushes good. My opinion is more gear is better for bigger props and less gear for smaller props. Personally I would install a Holley HP system and get away from the MEFI6. You could reuse the timing and fuel tables from the MEFI6 as a starting point. Programming the Holley ECM isn't that hard given these tables.
 
digginfool said:
I certainly don't need three gearboxes in 2.55 but this does provide an opportunity to check it all out.
Click to expand...
Diggin,

I think you answered your question in your original post. Your probably gonna end up selling something, if it doesn't work out then sell the 2.68.
 
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