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| Performance https://www.smwebhead.com/phpBB3/viewtopic.php?f=2&t=8262 |
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| Author: | Flyboy [ Fri Aug 19, 2011 6:50 pm ] |
| Post subject: | Performance |
I've got a 1988 261 Liberator with twin 460 Cobra engines recently overhauled and I was told this was a 70 mph boat. I took my GPS with me last time out and at full throttle I'm doing a woppin 49mph. I tried to vary the trim on the outdrives but not much results. I have a 22 pitch prop. I was running 91 octane gas. Motors sound good but why doesn't it go fast? |
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| Author: | 230 Mike [ Fri Aug 19, 2011 7:02 pm ] |
| Post subject: | Re: Performance |
Where are you located? Where did you buy the boat? |
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| Author: | RotaryRacer [ Fri Aug 19, 2011 7:59 pm ] |
| Post subject: | Re: Performance |
What RPM were the motors running at at WOT? |
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| Author: | ric [ Fri Aug 19, 2011 9:06 pm ] |
| Post subject: | Re: Performance |
My 3.0 H180 with a whopping 130hp and a 19 pitch prop GPS's 39-40. With a 21 it does mid 40s. 22 pitch with that much HP is just not enough prop. It's like driving a car around in 3rd gear with a 5spd transmission |
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| Author: | RotaryRacer [ Sat Aug 20, 2011 7:04 am ] |
| Post subject: | Re: Performance |
ric, Comparing prop pitch as you have, doesn't make much sense. Outdrives have different gear ratios depending on which engine is connected to them. An outdrive with a 4 banger connected is very different than the ones that have 460 Fords connected. Prop pitch from one boat to the next is not something that you can really compare...especially if they have different engines and outdrives. |
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| Author: | LouC [ Sat Aug 20, 2011 9:49 am ] |
| Post subject: | Re: Performance |
The only way to know for sure is if the poster lists his wide open rpms, then we can say if it's prop pitch related. Outdrive gear ratio, and prop pitch are directly related, it's like the relationship between final drive ratio and tire circumference in a truck or SUV... Too high a prop pitch is like trying to tow with a 3.42 rear gear vs the 4.10 you should really have in a truck or SUV. It will not let the engine rev high enough to produce the power needed to move the load. In a boat that will really strain the engine and cause a lot of heat in the combustion chamber and burned or tuliped valved. In a truck it's bad too but at least the torque converter will unlock and the trans can down shift.... Hopefully both of those outdrives have the right gear ratios in them. My 88 OMC manual lists the overall gear ratio for the 460 Ford at 1.43:1, and the wide open throttle rpm is supposed to be 4400 to 4800 rpm. Keep in mind even if the drives have the correct ratio and the right prop, if there are other issues (fuel system problems especially) the engines may not hit max rpm.... |
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