2.68 VS 2.55 testing

[Waterthunder]

We did a simple quick test mostly because a customer was being missled by someone. We took one of our 430CUI EFI motors and ran it with a 82'' 3 blade JR with both a 2.55 then a 2.68 ratio OX BOX. The humidity was 45% ambeaint air temp was 74.5 and the air density or corrected altitude was 1,154.46. Both pulls were within an hour of each other. The 2.55 engine RPM was 6087 with a prop RPM of 2,387 and the 2.68 engine RPM was 6461 with a prop RPM of 2410. Remember this is with both ratios run the exact same prop and pitch, the prop was pulled off the ratio changed and prop was put back on. So the exact same engine turned the exact same prop more RPM's with the 2.68 ratio. This is the torque multiplication theory I have been preaching for 20 years.

 

[12ftgso480]

Would you mind elaborating more about how the customer was being mislead compared to your testing ? Would like to know more

 

[SWAMPHUNTER45]

Dave,

I must be a believer, because I would have expected that result. Now that said as long as the engine was designed to run in that RPM range (capable of power at rpm). ie cam, heads etc.

The hypothesis is proven to be factual.

I think there is another consideration in this formula for an owner to consider and that is “ride quality” and general function. Essentially 300 engine rpm higher through out your ride may be unnecessary. If not racing, I might opt for a 2.55 if the boat was not a barge.

In my personal experience with big inch, long stroke vintage power we started with a 2.55 OX and then swapped out to a 2:37 ratio drive. The result was a much more enjoyable cruise having lowered the engine rpm by a bit more than 200 rpm. I noticed no loss of power and pitch was near identical. Full disclosure this vintage engine is designed to run at much lower rpm than your modified LS platform.

Thank you for sharing your data and setting the bar high !

“THE PROOF IS IN THE PROP”

 

[Waterthunder]

Would you mind elaborating more about how the customer was being mislead compared to your testing ? Would like to know more

Customer was being told it would make less thrust and the prop would turn slower. Neither was true and we had to prove it, in order that our motor to get the gear ratio that will perform the best.

 

[Waterthunder]

The 2.55 ratio with some props produces a tad more thrust at cruise RPM's which if you have a bigger boat could be advantageuse. The rule of thrumb we like is 76 long and below 2.38 and 77 to 79 sometimes 80 a 2.55 and 80 and up a 2.68. As always nothing is always the best in every case such as if you run a power adder like a blower or nitrous then things change.