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hcalmar

B&S Vanguard Premium or Regular?

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hcalmar
Just purchased the Snapper YT 400 with the Vanguard 21hp. The manual says 85 octane or greater. Any advantage using premium? I know some professionals who only use hightest in their machines.

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rsnik
After years of jaw dropping repairs bills on boats, motorcycles, tractors etc. I took a 475 hour marine engine mechanic program (outboards, inboards, diesel) at a local community college. Two evenings a week and Saturdays. Cost a total of maybe $2500 in tuition. I have probably got that back already doing my own repairs. I was shocked to hear repeatedly that if an engine, be it an outboard or a 4 stroke tractor motor is designed to run on regular gas it will not run better on premium and use of premium is discouraged, if it is a regular gas outboard, as premium could shorten service life. Instructors say there may be some exceptions, but generally an engine (usually lower compression) designed for regular will run worse on premium.

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SteveP
Here the way I understand it.... Premium burns at a higher tempetures compared to regular gas. High performance (higher compression) run at higher temps. Lower compersion engines do not fully burn premium gas, resulting in lower efficenticy, higher deposits and most importantly wasted money. IMHO

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HubbardRA
The lower the octane, the faster the fuel burns. Gasoline also speeds up the burn rate as it becomes compressed to higher pressures. A low compression engine, 8.5:1 or less usually runs fine on 87 octane. From about 8.5:1 up to around 9.5:1 will usually run good on 89 octane. 9.5:1 up to 10.5:1 will do good on 93 octane. Usually above that you will need Cam2 or Aviation Fuel with singificantly higher octanes. If you run a higher octane than is needed, you will actually lose performance in the engine. The higher octanes burn much slower in the low compression engines, and a portion of the fuel will be spit out the exhaust unburned. Yes, Premium (high octane) fuel has more energy than regular, but it takes higher cylinder pressures to make it burn fast enough to use that extra energy. On the other hand, low octane fuel will have ping or audible detonation in a high compression engine because it burns so fast that the peak pressure is produced in the cylinder before the piston gets to TDC and the pressure starts trying to push the piston back down before it gets to the top. This is why slowing down the timing helps when a car is pinging on regular, same as putting higher octane in helps by having a slower burn rate.

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andrewk
Depending on the engine, compression ratios aren't that low in some of this stuff. Take your average port style 2 cycle- You won't audibly hear it ping with the low octane stuff, but you sure can read the plug after a few hours. There is little duration, and hence low cylinder pressures, in small engines, but the compression ratio is still higher than most think, not that this really pertains to the topic. As far as performance claims or harm claims, the engine will consume what the mechanical limits make it consume. The idea behind octane is not to increase performance, but to give an engine a fuel that won't detonate before it is supposed to. If you run 91 in a 270000 series briggs, where 87 will work, you aren't doing anything to the engine except wasting money, because that engine won't detonate with 87. I don't have a ton of marine experience, but I have a hard time believing that long term use of a higher than recommended octane will lead to shortened service life. The gasoline provides the same btu's, regardless of octane. Octane is nothing more than the fuel's ability to resist detonation. Nothing more. There is no more, or no less energy in it. There is alot of misconception, and misinformation on the topic too. Andrew

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lndscpr
my neighbor is telling me to go with a higher octane gas with a fuel stabilizer for all my small engines. He reasons that the new regular gas(whatever the heck they are putting in it now) retains more water and breaks down faster. He says most small tractors and mowers wont run well on regular gas due to the water content. I dont know if this is true. I go through 10 gallons of fuel a week in my mowers so i dont worry about the fuel breaking down.

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andrewk
Well, depending on where you are, and what fuel stations are available, he may be right. In the shop I work in, I have found 5-7 percent alcohol in 87, non ethanol blend fuel as of late. I have found 14-17 percent in E10, and none in the premium, but I haven't tested much premium, just what is in my motorcycle. Seems like I read, or heard, that after Katrina, the gov't lifted fuel regulations, and has yet to reinstate them. Some fuel companies are bumping the octane of their fuels with alcohol, meaning that the base fuel is worse than normal. I have seen fuel in the shop break down in as little as 20 days- Not good. It's sort of like a big black hole for fuel- whatever they can put in the take with x octane, is good enough. You don't notice it in your car, because cars are not nearly as sensitive as small engines. Some stations around me have worse fuel than others. I know it all comes off the same pipe, or at least the base does, but there is a difference somewhere. Most small engines will run great on "low octane" so long as it is real 87, and not something that was 78 and has been bumped with alcohol. No matter what you use, I would use a stabilizer (NOT STABIL, use Seafoam, or briggs makes one) if you won't use the fuel in 3-4 weeks. Going back to cause and effect of high octane fuel, if in fact premium does not completely burn (and I have yet to see such a thing), leaving unburnt fuel in the cylinder, could that not be fixed by adjusting the carb to use a lesser amount of fuel? Enough rambling for now- Andy

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dhermesc
Problem with buying high octane fuel - half the time or more) you don't get what you are paying for. I used to live in a small town in NE Kansas and had a 67 Mustang that had been modified "a bit" - to run correctly it needed at least 93 Octane. It seemed that half the time the thing would barely run with lots of knock and pings and other times it would run like a striped **** ape. I'd screw the timing get it wrong then get it set back then suddenly it would run fine. I finally realized that if I filled up at one station in town I got 93 if I filled up at the other two stations their "premium pumps" were filling the car with regular. Once I realized what the issue was it ran fine. When traveling it was almost funny to fill up at a gas station then a mile down the road have the engine start pinging from their mislabled fuel. You would know immediately who was stealing from their customers.

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rsnik
As far as how the premium shortens service life in a low compression outboard like older Johnson's and Evinrudes, well, I got an A on the test, but as far as remembering it now...I'll give it a try. As I recall it is that carbon is positively charged and I think it is the rings that are negatively charged so a carbon build up occurs (more rapidly with the premium) on the top ring. With a 2 stroke firing on every stroke there is a constant flame jet above the piston. The flame jet super heats the carbon bonding to the compression ring. The compression ring fails. The flame jet shoots past the destroyed compression ring and destroys the second compression ring and the oil ring and now the flame jet is shooting entirely down the sides of the piston skirt. The heat is intense, the piston and bore begin swapping metal and catastrophic engine damage happens shortly thereafter. Up until, I think 1971, they had tetra-ethyl lead as a gasoline additive which was negatively charged; carbon would bond to the TEL and be carried away with the exhaust instead of bonding to engine internals and causing premature failure in 2 stroke motors. As well as avoiding premium in low compression outboards, marine mechanics recommend additives such as Yamaha's Ring Free that do what TEL used to do in terms of bonding carbon to an additive that goes out the exhaust instead of bonding to the rings.

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