Gas Grades

[pwake7]

My marina has the high grade gas and i was fueling the other day and surprisingly i was bitching about the great price we have rigth now. The guy working there told me they might drop down to reduce the price to a medium grade gas.

What will this do to the fuel and will it have any effect on the boat and performance? any gas gurus out there?

 

[uncljohn]

I have read that high-test gas is not good for 2-stroke engines. It burns ring seals up. You shouldn't use anything higher than mid-grade.

However some 4-strokes want high-test.

This is just what I've heard. Of course, your local marina is buying gas for outboards, inboards, megayachts, etc, so he likely doesn't care if his gas burns your seals up a little faster.

 

[Grog]

Unless your motor needs the higher octane, you don't need it. I think the F250's are rated with 89 octane (probably Verado's too), the others run on 87.

 

[CJBROWN]

I have read that high-test gas is not good for 2-stroke engines. It burns ring seals up. You shouldn't use anything higher than mid-grade.

However some 4-strokes want high-test.

This is just what I've heard. Of course, your local marina is buying gas for outboards, inboards, megayachts, etc, so he likely doesn't care if his gas burns your seals up a little faster.

Ring Seal? What are ring seals???

2-stroke outboards will run fine on 87 octane, they are not highly tuned motors, nor high-compression, so there's not an issue with detonation.

They will also run fine on premium, or 91 octane, they'll even run fine on 95 or 100 octane, although they may run a little too rich as the anti-knock compounds really cool off combustion. When we tuned 2-stroke race bikes we would actually lean out the main jets for race gas or they would run too rich. You had to make sure you didn't try to run pump gas then, or they would get too hot from running lean. Those motors ran premixed fuel too, and if you put too much oil in the fuel they would run lean. 2-stroke combustion is cooled by fuel, not oil. If if the ratio of fuel is less, they'll run hotter. And they'll build carbon like crazy.

Carbon is an enemy of ring seats, or grooves, maybe that's what you thought you heard. That's what the ring-free is for, keeps carbon build up down so the rings don't stick in their groove. If the rings stick they'll wear unevenly, and they can get so thin on one side they'll break and catch on the ports. A fully synthetic two-stroke oil will also go a long way in eliminating carbon as it does not burn. It will actually clean out carbon from ring grooves.

Octane is largely misunderstood by the general public. Most people think 'hi-test' gas means they'll get something more out of their engine. All it means is there is more anti-knock compound added which cools the combustion process which in turn stops detonation or pinging in higher compression engines.

I'm not aware of any four-stroke outboards that require high-octane gasoline. I did notice Mercury suggests 91 octane for their higher HP Verados. That would suggest a little higher compression in that model to warrant it, but it also says it will run fine with 87. The computer compensates with spark advance control to eliminate detonation from the lower octane fuel. And because the spark is not advanced as much as with higher octane fuel, you lose a little power. That's why it says 'for maximum performance'. Doesn't mean it will hurt or help the motor by using either fuel.

Cars are the same way, the computer controls the fuel mixture, and spark advance, and with the MAF and O2 sensors adjusts accordingly. The reason some require premium grade fuel is due to their high compression ratio. If the manufacturer doesn't require 91 octane, then you're wasting your money on the extra cost for the additional anti-knock compounds. They do nothing for your lower compression motor.

 

[uncljohn]

[

Octane is largely misunderstood by the general public. Most people think 'hi-test' gas means they'll get something more out of their engine. All it means is there is more anti-knock compound added which cools the combustion process which in turn stops detonation or pinging in higher compression engines.

actually it doesn't say anything about "more anti-knock compound". But like you said, octane is misunderstood by the general public.

http://en.wikipedia.org/wiki/Octane_rating

The octane rating is a measure of the autoignition resistance of gasoline and other fuels used in spark-ignition internal combustion engines. It is a measure of anti-detonation of a gasoline or fuel.

Octane number is the number which gives the percentage, by volume, of iso-octane in a mixture of iso-octane and normal heptane, that would have the same anti-knocking capacity as the fuel which is under consideration. For example, gasoline with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90.

Definition of octane rating
The octane rating of a spark ignition engine fuel is the knock resistance (anti-knock rating) compared to a mixture of iso-octane (2,2,4-trimethylpentane, an isomer of octane) and n-heptane. By definition, iso-octane is assigned an octane rating of 100 and heptane is assigned an octane rating of zero. An 87-octane gasoline, for example, possesses the same anti-knock rating of a mixture of 87% (by volume) iso-octane and 13% (by volume) n-heptane. This does not mean, however, that the gasoline actually contains these hydrocarbons in these proportions. It simply means that it has the same autoignition resistance as the described mixture.

A high tendency to autoignite, or low octane rating, is undesirable in a spark ignition engine but desirable in a diesel engine. The standard for the combustion quality of diesel fuel is the cetane number. A diesel fuel with a high cetane number has a high tendency to autoignite, as is preferred.

It should be noted that octane rating does not relate to the energy content of the fuel (see heating value), nor the speed at which the flame initiated by the spark plug propagates across the cylinder. It is only a measure of the fuel's resistance to autoignition. It is for this reason that one highly branched form, or isomer, of octane (2,2,4-trimethylpentane) has (by definition) an octane rating of 100, whereas n-octane (see octane), which has a linear arrangement of the 8 carbon atoms, has an octane rating of -10, even though the two fuels have exactly the same chemical formula and virtually identical heating values and flame speeds.

 

[JUST-IN-TIME]

the new e-10 actually washes the oil away more because of the alcohol content in it

run 87 if you can