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| omc 75hp max rpm https://hpbc.ca/viewtopic.php?f=19&t=2350 |
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| Author: | johnny rotten [ July 21st, 2014, 7:16 am ] |
| Post subject: | omc 75hp max rpm |
What should the maximum rpm be on a 1977 johnson 75hp with stock 125psi compression and timing at 19 degrees ? We are using the Lucas semi 2-stroke oil @ 40:1 with 91 octane and hitting 7000rpm at full speed our SSV Baker. Thanks - John |
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| Author: | daveee [ July 21st, 2014, 8:55 am ] |
| Post subject: | Re: omc 75hp max rpm |
John, I'd say you are on the money for accelleration, 2-300 hi for max HP (Speed). The 1976-77 blocks had the best exhaust port windows in our experience. Dave |
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| Author: | johnny rotten [ July 21st, 2014, 4:25 pm ] |
| Post subject: | Re: omc 75hp max rpm |
Thanks Dave. We are running a 12 3/8 x 22 cleaver with a nitro lower unit with 1:86 gear set. At 7000 rpm nand full trim we are at 58mph. Do you think the engine might be too high on the transom and not gripping enough water? |
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| Author: | LittleCharger [ July 21st, 2014, 6:26 pm ] |
| Post subject: | Re: omc 75hp max rpm |
johnny rotten wrote: Thanks Dave. We are running a 12 3/8 x 22 cleaver with a nitro lower unit with 1:86 gear set. At 7000 rpm nand full trim we are at 58mph. Do you think the engine might be too high on the transom and not gripping enough water? I'd confirm that you don't have 2.42 gears in that gear case. That's a pretty high slip number if you have the 1.86 gears. |
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| Author: | LittleCharger [ July 21st, 2014, 6:26 pm ] |
| Post subject: | Re: omc 75hp max rpm |
johnny rotten wrote: Thanks Dave. We are running a 12 3/8 x 22 cleaver with a nitro lower unit with 1:86 gear set. At 7000 rpm nand full trim we are at 58mph. Do you think the engine might be too high on the transom and not gripping enough water? I'd confirm that you don't have 2.42 gears in that gear case. That's a pretty high slip number if you have the 1.86 gears. |
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| Author: | johnny rotten [ July 21st, 2014, 8:35 pm ] |
| Post subject: | Re: omc 75hp max rpm |
Thanks Little Charger I checked it the way Dave told me about, which was to rotate the crank and note how much it turns for one complete of the prop shaft. That was 1 and 7/8 turns of the flywheel. I should note that it is not at 7000 trimmed in. It is about 5800 trimmed in and 7000 trimmed out. We have the through transom single ram trim. |
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| Author: | LittleCharger [ July 21st, 2014, 9:48 pm ] |
| Post subject: | Re: omc 75hp max rpm |
That's some serious slip then. I'd expect you to be in the mid 60's Where is the bullet (at neutral trim) relative to the pad? What other props have you tried and what kinda numbers did you get out of those? |
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| Author: | johnny rotten [ July 21st, 2014, 11:57 pm ] |
| Post subject: | Re: omc 75hp max rpm |
I'll have to measure to be certain (the boat is at the lake) but from memory, the top of the bullet is about 1 inch above the pad at neutral trim. We tried a 12x23 and ran 60.2mph but the motor spun at 7800rpm at full trim(not for long once I saw the rpm) The higher pitch should lower the top rpm, so I'm wondering if its too high on the transom. |
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| Author: | Hevi Kevi [ July 22nd, 2014, 1:34 am ] |
| Post subject: | Re: omc 75hp max rpm |
Too high. Get the center of the bullet below. Start at 2" or so below and work your way up testing. Hard to believe you have water to cool the engine at that height. |
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| Author: | daveee [ July 22nd, 2014, 10:06 am ] |
| Post subject: | Re: omc 75hp max rpm |
Agreed, too hi. 26% slip confirms it. We run our entire bullet under the pad. |
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