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Old 02-18-2012, 10:49 AM
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Octane Change

Looking for information on the effects of changing octane in an engine after a significant amount of use. I have an 85 Stars and Stripes, 351W, with roughly 1960 hours. We've done only tune ups and minor repairs (rebuild the starter, replace the fuel pump) since the boat was new. Bought it from my father last year. It's always run on 87 or 89 octane, despite the manufactures recommendation of minimum 93 octane. When I brought it to Wisconsin I have access to 91 octane or 87 with no ethanol. I feel that it likes the 91 better, as it doesn't have that slight hesitation at take off under heavy throttle.
My brother used to work at a marine dealer and was pcm certified. He tells me that you should never change octane after this long on an engine, it's asking for major problems. Car mechanics seem to have never heard of such a thing, but the marine mechanics I've spoken to seem to be split around 50/50 on this. But none can explain why this would cause a problem.
Does anyone have any experience with this? I would love to know if this is a real problem, and if so why. Also, what specific damage would it cause?
Thanks.
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Old 02-18-2012, 11:11 AM
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I have always operated on the "if it aint broke dont fix it" philosophy, so its worked this long this well on the lower octane, so why change would be my thoughts. That being said, I certainly dont think it will harm anything to put what the manufacturer suggests in it.
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Old 02-18-2012, 11:58 AM
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I don't believe any factory 351W ever needed 93. You can fun 87 in it all day long and not worry about it; it's like an 8.5 to 1 compression motor. I do not believe your brother is giving you correct info.
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Old 02-18-2012, 12:10 PM
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Premium gas 91-93 burns more in a more controlled manner - 87 octane can detonate in the compression stroke of a piston which can cause engine knock and piston/cylinder damage. I don't know how the 85 351W controls engine timing. Newer cars will detect ping/engine knock via a sensor and adjust timing to eliminate ping/knock.

If you made it to 1960 hours what is making you want change the fuel octane? Higher octane fuels are for higher compression engines... hence racing fuel aviation fuel....

+1 on Cant' repeat's advice.
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Old 02-18-2012, 12:28 PM
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I understand how octane works, higher octane, higher combustion point. Less perignition, greater volume in cylinder for explosion= more power. I'm not sure how changing it now could cause a problem. The original PCM service manual and MasterCraft owners manual call for minimum 93 octane, which is hard to come by. The reason for considering is the afore mentioned slight hesitation when getting into the throttle to pull my 285 lbs out of the water, particularly in April and November in Wisconsin. As I mentioned it seems that several mechanics believe this, though since they can't say why I believe it is likely to be an old wives tale.
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Old 02-18-2012, 12:39 PM
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Quote:
Originally Posted by madcityskier View Post
I understand how octane works, higher octane, higher combustion point. Less perignition, greater volume in cylinder for explosion= more power. I'm not sure how changing it now could cause a problem. The original PCM service manual and MasterCraft owners manual call for minimum 93 octane, which is hard to come by. The reason for considering is the afore mentioned slight hesitation when getting into the throttle to pull my 285 lbs out of the water, particularly in April and November in Wisconsin. As I mentioned it seems that several mechanics believe this, though since they can't say why I believe it is likely to be an old wives tale.
Is the hesitation always there or only in April November as you state...Cooler air might cause a lean fuel mixture....just enough to cause a slight hesitation under load....

I don't know the 351W's all that well but calling on my knowledge of other issues I had in other engines. You have the ignition control box I assume - which might be detecting ping on acceleration and retarding spark...hence why you don't get the hesitation when using premium fuel.

Could also be a carb issue or carb adjustment.
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Last edited by mikeg205; 02-18-2012 at 12:43 PM.
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Old 02-18-2012, 12:42 PM
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More or less a cold water issue. I'm sure it's past due for a carb re-build, but I'd rather push it off if I can just get away with switching to 91, which I believe I can. Still less than the recommendation, so why not. Just figured I'd run it past my boys here to see if anyone can raise a specific reason not to.
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Old 02-18-2012, 12:51 PM
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Quote:
Originally Posted by madcityskier View Post
More or less a cold water issue. I'm sure it's past due for a carb re-build, but I'd rather push it off if I can just get away with switching to 91, which I believe I can. Still less than the recommendation, so why not. Just figured I'd run it past my boys here to see if anyone can raise a specific reason not to.
I can't see running higher octane ever hurting an engine... I would also start adding seafoam to each tank - its something I do and have done for a very long time. I have engines that sit in Ontario Canada for 49 weeks a year...Always had hesitation in my outboards for the first week when refitting for fishing time...ever since seafoam - it's like the 49 weeks never happen.

I agree - you can avoid a carb rebuild - I could...when you do you can go twin supercharge? This setup would have no problem getting you out of the water...

http://www.youtube.com/watch?v=cgBc0o73CyM
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Old 02-18-2012, 01:01 PM
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I'd like to see in writing where 93 is recommended by either PCM or Indmar for an 85 351W. I just look at the specs and it's an 8.3 to 1 motor. In stock form there is zero reason to recommend or even consider running 93. It would be a complete waste of money.
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Old 02-18-2012, 01:02 PM
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Quote:
Originally Posted by madcityskier View Post
More or less a cold water issue. I'm sure it's past due for a carb re-build, but I'd rather push it off if I can just get away with switching to 91, which I believe I can. Still less than the recommendation, so why not. Just figured I'd run it past my boys here to see if anyone can raise a specific reason not to.
A $20 dollar rebuild kit verses an entire season of poor performance and overpriced gas that isn't needed? How is that even a choice?
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