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Old 08-04-2019, 08:43 PM
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Default Wrong drive ratio?

Trying to help out the owner of a 29' Scarab that has a 598 motor, rebuilt and dynoed this spring. I think it came in around 650hp @ 5200 (Not my boat, so not positive on numbers)
Boat has an IMCO SCX drive, upper and -2 lower, with a 1.34 ratio. 24" Bravo One prop. In the water, this boat will not turn over 4000-4200. New distrib, ignition box, everything. Carb opens fully. Fuel pressure good. This carb and a dyno carb where both used on dyno and no issue running 5200. Motor sounds and runs good. It just hits a wall at 4200. To see if the motor would even spin higher, I took it out and trimmed it higher than normal and was able to get the rpms to increase. So we know it can spin higher.
This lead us back to the ratio being the wrong one, as a few I have talked with have agreed with when trouble shooting. Have been trying to try a B1 lower with a 1.5 ratio, but this requires the $180 adapter to work with the SCX upper. Cheap test, but this has its concerns as the Bravo lower is 2" longer and the boat on trailer sits low and may end up with damaged skeg. So thats been shelved. Owner is about to just screw it and just have the IMCO lower sent out and regeared to 1.5
I'd like some input before he goes that route as it will be an costly way to find out if this is/isnt the problem.
So, those with experience on ratios and whatnot, would you also agree this 1.34 ratio could be the issue? My train of thought is that its possible, but then I keep coming back to the small 24" prop, thinking that would offset the lower ratio. Boat was purchased 2 yrs ago and have never ran over 4200, thus the motor was looked into and some changes (new cam and heads) were made this spring, but to no luck.

Thoughts?
Thanks
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Last edited by US1 Fountain; 08-04-2019 at 08:46 PM.
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Old 08-04-2019, 09:07 PM
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His engine builder left a LOT of HP on the table with his 598. I was getting 650 HP with a 10 to 1 CR, hyd roller 509. MY 598 with 8.6 CR and a solid roller is putting out 800+ HP.
Now to the subject. I am running a 1.34 ratio Bravo X drive upper with a -3 IMCO lower. I am spinning a 28 pitch labbed Bravo prop to 6100 RPM.
I know different boat hull but I would say he has the wrong cam in the engine. I think his engine builder built him a stump puller that just falls on it's face.
How far are you away from GR MI. My drive is still not on the boat. I got a very late start on some upgrades.
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Old 08-04-2019, 10:27 PM
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I don't know the specs of the cam or heads. Originally it had a real mild cam and this one was recommended by builder. I want to say its on par with the old Crane xxx751 hyd roller cams. Owner did actual rebuild. Pretty sure its 9.5:1 motor at most, with main objective to be 100% trouble free turn key motor. My initial thought was previous owner built it low CR with thoughts of blowing it later on, thus the SCX drive and the 1.34 ratio thinking it would end up big power, but never transpired. I don't know though. Beyond my knowledge. It has the IMCO transom assembly, so a bravo will not bolt up to it. Thanks
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Old 08-05-2019, 05:36 AM
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Yeah I forgot they use a bigger helmet.
i have a spare 1.5 ratio -2 drive.
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Old 08-05-2019, 05:30 PM
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From what I've seen first hand, the SCX lower is a lot like dragging a 5 gallon bucket through the water. Spend the money on the adapter and put a SC lower with 1.50 gears on it.

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Old 08-05-2019, 05:35 PM
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Does the boat wander all over the water while cruising ?
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Old 08-05-2019, 08:45 PM
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Which rpm card is on the MSD?
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Old 08-05-2019, 09:23 PM
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Yes, we have also been told about the drag of this lower unit. Boat does not wander at all. Been using a MSD with a 5200 chip.

But back to the main question, is it likely the 1.34 ratio be the main culprit keeping this motor from going over 4200rpm, even with a 24" Bravo One prop?
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Old 08-06-2019, 03:02 PM
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Try a lesser pitch prop, if it picks up speed and RPM then that would be your answer.
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Old 08-06-2019, 03:23 PM
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Originally Posted by US1 Fountain View Post
Yes, we have also been told about the drag of this lower unit. Boat does not wander at all. Been using a MSD with a 5200 chip.

But back to the main question, is it likely the 1.34 ratio be the main culprit keeping this motor from going over 4200rpm, even with a 24" Bravo One prop?
I think so
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