does octane matter?

[RJS]

Re: does octane matter?

Octane matters if you have a higher compression engine (high performance), most engines are not high compression; therefore, only require regular gas. Doesn't matter how big the engine is, it's the compression. In a majority of the outboards sold today, you are wasting your money if you use any thing above 87 octane.

 

[sort]

Re: does octane matter?

My 1995 Yamaha 250 V6 requires 89 octane minimum per the owners manual. Would there be any benifit to running 92-93 octane as this is one step up from the minimums. With gas prices at over $2.00usd/gal it makes one look at just what is required to make the engine perform to it's best levels without wasting money. Engine runs great on 89octane but maybe it will run better on 92...

 

[walleyehed]

Re: does octane matter?

Fireman, don't waste money on 92-93, it'll run with less power.<br />I see several posts up, guys talking about avgas.....well, #1, 130 hasn't been made for several years, and what is being called "107" avgas is just 100 Low lead (100LL).<br />Octane rating methods differ on aviation fuel vs. auto fuel. 130 (Green in color and not manufactured anymore) is actually "107" by automotive standards, and the 100LL (Blue in color) is about "95" by auto standards and very, very "dry"-more highly refined than auto fuel and much less lubricity, causing alot of Valve issues over the years in four stroke aircraft engines. The average aircraft engine has an 8:1 comp ratio, but these are air-cooled engines, and the purpose of 100LL is to prevent vapor-lock<br />Nothin' better than having someone come in wanting to buy 2.75/gal fuel that isn't much-if any different than 93 as far as performance properties.<br />In all, I know of only 2 outbords (carbed) that require 91 or 93, and that's the JohnRude GT/XP engines...

 

[gss036]

Re: does octane matter?

That sicky yellow stuff in your tank is probably sulphur. BP gets the majority of its crude from the Alaska north slope which has a very high sulphur content. Several years ago they had to retrofit the local BP refinery here to handle it.

 

[walleyehed]

Re: does octane matter?

gss036, do you have any thoughts on Air BP...I understand their av and auto are coming from 2 different locations, as in: someone is manufacturing the av end of it. Any chance you would have insight??? I'm also interested in the sulphur in auto/marine...is this present in all????

 

[EFIS]

Re: does octane matter?

Originally posted by FlyBoyMark:<br /> Howbout' we use JET A kerosene 'wit no oil additive in two cycle diesel, kerosene does the lubing??

There is an aircraft engine available now that does just this see this link<br /><br /> http://www.centurion-engines.com/index.htm <br /><br />5 GPH and 140HP<br />As soon as merc make a commercially available one i will be buying one. There is already one Merc engine produced in OZ that runs on JetA1 (kero)or(Diesel)

 

[jleus]

Re: does octane matter?

So what is that cranking compresion threshold (in everyones experience) that is the point of concern? Not a ratio but a cranking compresion. Not trying to get the feathers flying (kinda like when you ask about oil and ratio) but I have a DT25c that I have been modifying over time so that I am not beat in a horse power restricted area. I go to some pains to get 93 octane that I know is fairly fresh, it would be easier and probably fresher fuel to use 87 at a pump that has a pretty good volume of business. I am averaging 130 psi on all three. Where is the threshold, the point of concern. Thanks.

 

[Dhadley]

Re: does octane matter?

The static number varies from motor to motor. It all depends on cylinder head chamber design and the port areas, especially the exhaust ports. Sometimes you gain power by opening, widening and/or raising the exhaust port. Its possible that this will actually lower the static (cranking) comperssion number but you may still have more compression in the cylinder head chamber from head/chamber changes.<br /><br />And obviously timing plays a very important role here too.

 

[jim dozier]

Re: does octane matter?

To expand on what Dhadley was alluding to (I think). You can't determine dynamic (firing) compressions from static (non-firing) compression tests. Most engines, 2-stroke and 4-stroke, rely to some degree (higher perfomance rely more) on the effects of inertia to charge the cylinder with mixture. That is the inertia of the air mixture at high speed will cram more mixture into the cylinder space than it will at low speed. This is why there is valve (and port) overlap (intake and exhaust open at the same time). This is what makes port and valve timing so critical on high performance engines. Then if you add the effects of port or valve design, combustion chamber design, and ring sealing into the picture, static pressures have little to do with dynamic pressures.<br /><br />In jleus' case you need to be alert for signs of pre-ignition and detonation. If you melt a piston you'll know your compression ratio was too high for the octane you were running. When a manufacturer sets the compression ratio for their engine they are looking at performance, reliability, engine life, and reasonable availability of adequate octane. They test for these in the field. You can always raise compression ratio for more performance but you'll have to do sacrificial testing to know how far you can go.

 

[JasonJ]

Re: does octane matter?

jleus, 87 is fine for your compression ratio. If you were up around 150 or more, you would need more octane than 87. This of course also depends of spark advance. If you have advanced your spark, then the higher octane would be needed, but if you are at a relatively stock spark advance, then 87 would be fine. I had a small pickup truck with a 4 banger in it. I had installed a rebuilt head that had been overmilled, and I had nearly 200 psi. I could not run less than 93, and I had to almost retard the ignition just so it would run. It did have more power, but in the end I replaced the head and was back down to the more normal 140-ish psi. The engine soon failed because all that high compression had overstressed the bearings for the crank and pistons, and they were knocking bad within a month of the proper head being installed.

 

[jim dozier]

Re: does octane matter?

JasonJ, how did you know wha jleus' compression ratio was? He said he had modified the engine, so I'm assuming he has modified the head, but by how much? So how do you know that 87 octane is enough? For stock engine yes, but what about modified? Also, he's getting ~130 psi with his gage not yours and they all read a little different. Also as Dhadley alluded you can't compare 130 psi in two diffent engines, there are too many different parameters involved. He may be OK with 87 octane, but it depends on a lot of unknowns. I just think its a waste of time trying to judge octane requirements off of the results of compression test in the driveway.

 

[JasonJ]

Re: does octane matter?

I only said that 87 would be fine considering the information that has been given. We do not know what mods have been done, or what advance he is running. Even with higher octane fuel, you still more than likely do not deviate much from stock as far as advance goes, 2 strokes have a bad habit of blowing up that way. He needs to read his plugs and check his pistons/cylinders to know if detonation is occuring, you don't always hear it like in a 4 stroke.

 

[Hawaiian]

Re: does octane matter?

I was told that if the gas in your tank sits for a while, it looses it's octane. That is why I put 89 octane. If it sits for a month or so, it should be around 87 by the time I use it.

 

[Mark42]

Re: does octane matter?

Check your owners manual. Although mine lists 87 octane as the minimum requirement, Evinrude explicitly states that higher octane gasoline is strongly recommended for extended service life.<br /><br />When in doubt, go with the manufactures suggestion. I run 89 regularly, and 92 when the price is right. Of course, I am buying at the local gas station and not the higher priced marina gas. And with only 12 gallons to fill, a few cents a gallon doesn't make much impact on my wallet.

 

[Mark42]

Re: does octane matter?

Does the oil in the 2 cycle engines affect octane rating? <br /><br />I would think it should raise octane because it is slower burning than gas. Maybe not enough mixed in to matter, but I would think it does have some affect.<br /><br />Anyone know for sure?

 

[LubeDude]

Re: does octane matter?

Originally posted by Mark42:<br /> Does the oil in the 2 cycle engines affect octane rating? <br /><br />

Yes it does as you describe, but it is VERY minimal and the engine manufactures have taken that unto concideration with there reccomendations.

 

[rodbolt]

Re: does octane matter?

I am not a petrol anganeer. I jus fix dem broked ones. but I can tell you after thousands of dyno runs and playing if your motor can burn it, the lower octane reating will normally allow more RPM and power at that RPM. we can control how much air and fuel enter, we can control how much we squeeze the mix, we can control when we light the mix off. the only thing we cant directly control is when the fire goes out. so we have about an inch of piston motion to convert the air/fuel mix into a burned expanded gas. then the fire needs to go out. yamaha and most other motors with knock sensors tend to keep advancing engine timing looking for a feed back from the knock sensor. sometimes if 93 octane was used,especially with some of the excel motors, not only was the burn to slow but the timing ended up to far advanced and engine output suffered. we verified this on a dyno after yamaha sent out a tech bulliten about high octane fuel power loss. I also watch the timing curves on some of the EFI motors. some will go as high as 38* BTDC trying to find the best spot for the sensor data inputs to make the system work at top efficiency. so mostly my reccomendation is use what the manufacture reccomends. todays fuel is nasty of that there is no doubt. the old car trick for bad/low octane fuel was to add a qt or 2 of motor oil to the tank. while this wont affect the actuall octane it will raise the flash point and assist with detonation. this was in the days of 10- 11 to one stock factory compression ratios. was a fun time but ate a lot of fuel. nothing like a good running 425 olds with a 10.25 or a 10.5 compression ratio <br />some of the older posters may remember the rocket emplem and thge ultra high compression stickers on the air cleaners.was normal to see 175 to 225 psi on the compression gauge. I think the 225 OMC looper runs about 80psi. most of the stern drive engines have gone to 7.5 to 8.5. a few of the EFI will run 9.

 

[muskyone]

Re: does octane matter?

ya but what about the ethanol there puting in 87 and 89 octain now days i put 87 with 10% ethanol last year and the boat ran like crap till i ran the stuff out

 

[rodbolt]

Re: does octane matter?

ethanol really does not increase octane but rather raises the flashpoint. the octane rating of ethanol is about 100 about the same as methanol. high rpm motors dont like it without further mods. the burn mix is about 7-1 for alcohol and 14 to 1 with gas. takes twice as much alcholol to do the same job. nice thing is alcohol will tolerate 13-1 compression ratios. its been my finding that most v4 and v6 two strokes really dont like the ethanol blends. nor do the rubber components in them. octane is very misunderstood about what it is. the reason for the additives in the av gas to raise it to 112 and beyond is altitude. to make power at altitude ya gotta compress air. there aint as much so you raise the mechanical compression. to make the motor live at low altitudes ya gotta run a richer fuel mix. a richer mix will help decrease detonation.when was the last time ya saw a mixture control knob on a boat or a car? technology has improved some and made running these things a bit easier for the average guy. but as fuels change its hard to redisign a motor that is already some years old.<br /> by the less gasoline per gallon its almost correct. a slower burn normally means a less powerful burn and a lower total BTU output for the same volume. but the flames out the exhaust stack are cool . also the reed vapor pressure specs of av gas are all wrong for sealevel operation. there are many books out on the subject of fuels and why and how. some make good reading and some will glaze your eyes like a doughnut.

 

[devildoc]

Re: does octane matter?

The advantage to AV gas is there is no ethonal. More expensive but less corosive to older rubber parts and carbs. No need to stablize AV gas after the season. I use it in ALL my two stroke motors, lawn, weed eater, outboard 67 merc 1000.

Doc