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madcityskier
02-18-2012, 11:49 AM
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.

jhall0711
02-18-2012, 12:11 PM
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.

CantRepeat
02-18-2012, 12:58 PM
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.

mikeg205
02-18-2012, 01:10 PM
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.

madcityskier
02-18-2012, 01:28 PM
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.

mikeg205
02-18-2012, 01:39 PM
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.

madcityskier
02-18-2012, 01:42 PM
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.

mikeg205
02-18-2012, 01:51 PM
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... :D

http://www.youtube.com/watch?v=cgBc0o73CyM

CantRepeat
02-18-2012, 02:01 PM
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.

CantRepeat
02-18-2012, 02:02 PM
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?

mikeg205
02-18-2012, 02:06 PM
found this... and it calls for 93 using RON.

http://www.planetnautique.com/CorrectCraftManuals/PCMEngineOwnersManual.pdf

CantRepeat
02-18-2012, 02:14 PM
found this... and it calls for 93 using RON.

http://www.planetnautique.com/CorrectCraftManuals/PCMEngineOwnersManual.pdf

You've link to page with manuals that only go back to 2007. None of the information in those manuals will be relevant to your boat.

You'd be better off with the Indmar 1993 or older manual then using one of those.

http://www.indmar.com/service-support/manuals.aspx

CantRepeat
02-18-2012, 02:20 PM
Looks like my browser was messing up. Kept sending me to the main manual page.

And from the manual:

Fuel Requirements
Use any good grade automotive regular or premium gasoline with a minimum average octane rating of 88* (93 research) in your PCM engine.
An 86* average octane (90 research) gasoline may be used if the gasoline described above is not available; however, the ignition timing MUST BE retarded 41 to prevent harmful detonation.
*New U.S. Regulation requires posting average of research and motor octane.
PCM reserves the right to refuse warranty on parts which are damaged -when using improper gasolines.

It clearly says use any regular automotive fuel and can have an average of 86 octane OR premium 88. There is no doubt that 87 is just fine for that engine.

wrobins1
02-18-2012, 02:47 PM
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.

as another >250 lbs rider/skier all I can say is when I notice that slight hesitation I remember back to learning to wakeboard behind a 17' Monte Carlo with a 4 cyl...

madcityskier
02-18-2012, 10:35 PM
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.

Like these pages from the manuals?

Jeff d
02-18-2012, 10:53 PM
It says 93 Research Octane (RON) though. RON is more common worldwide but gas pumps in the US are labeled with (R+M)/S (AKA Anti Knock Index). 93 RON = 88 (R+M)/S. Look at the buttons on the pump next time you get gas. They will say (R+M)/S in the fine print under the "87", "89", etc. So, 93 RON = "Regular Unleaded" in most US regions.

CantRepeat
02-19-2012, 09:00 AM
It says 93 Research Octane (RON) though. RON is more common worldwide but gas pumps in the US are labeled with (R+M)/S (AKA Anti Knock Index). 93 RON = 88 (R+M)/S. Look at the buttons on the pump next time you get gas. They will say (R+M)/S in the fine print under the "87", "89", etc. So, 93 RON = "Regular Unleaded" in most US regions.

Yep, there is no reason to waste money on 93 when clearly it will run on 87. I would be more concerned about the valve seats and running unleaded fuels.

Here is a link to a nice RON/MON conversion.

http://www.btinternet.com/~madmole/Reference/RONMONPON.html

With this you can see that a 93 RON is close to 86 or 87 so using regular 87 is going to just fine. Once again, lead seems to a bigger concern unless hardened seats have been installed.

oldairboater
02-19-2012, 10:39 AM
Run the octane level that does not knock under load in the summer. Too low an octane level in a hot engine helps cause early ignition. The only problem with running lower octane levels is that most people can't hear engine knock or don't recognize it. If you ask people what ping, valve chatter, or engine knock sounds like the answers are not always right. The higher octane gas will not hurt your engine and changing to a higher octane will not hurt your engine. You will be spending more of your dollars but not harming your engine.

madcityskier
02-19-2012, 11:27 AM
All these posts and no response that says it will hurt. I'll take that as the answer.

mikeg205
02-19-2012, 12:12 PM
.20 more per gallon more for the best octane controlled burn is too cheap to even debate. Unless of course you're buying on the water. I buy about 180 gallons a season so that equates to $36 - $40 bucks a season...cheap cheap for minimizing any chance for detonation, my gas always sits, I don't run my boat every week, made a few new friends so we alternate. I am kind of an additive nut so I just added Lucas Octane Boost to the gas I have been saving all winter in my boats gas tank. Weather caused be to have more gas left than I usually like to.

I have a station locally that has some racing fuel, which I might use to top off the tank.

It's a great discussion for us boat owners who want keep our older boats running for darn near forever. Considering for me a slalom skier a new PS 197 sells for about a $49,900 + sales tax + higher insurance...

CantRepeat
02-19-2012, 12:27 PM
.20 more per gallon more for the best octane controlled burn is too cheap to even debate. Unless of course you're buying on the water. I buy about 180 gallons a season so that equates to $36 - $40 bucks a season...cheap cheap for minimizing any chance for detonation, my gas always sits, I don't run my boat every week, made a few new friends so we alternate. I am kind of an additive nut so I just added Lucas Octane Boost to the gas I have been saving all winter in my boats gas tank. Weather caused be to have more gas left than I usually like to.

I have a station locally that has some racing fuel, which I might use to top off the tank.

It's a great discussion for us boat owners who want keep our older boats running for darn near forever. Considering for me a slalom skier a new PS 197 sells for about a $49,900 + sales tax + higher insurance...

It's your ride and your cash. Do whatever makes you feel good at night. :D

mikeg205
02-19-2012, 12:29 PM
It's your ride and your cash. Do whatever makes you feel good at night. :D

In this economy... I wish I could sleep at night...but that's another thread....;)

1redTA
02-19-2012, 10:55 PM
higher octane gas is harder to burn, going to high can hurt performance but take that with a grain of salt though