Octane requirements for Yamaha's

[boscoe99]

Here is what Yamaha says about the octane rating for gasoline for use in their motors:


Here is what BRP says:


Are we in agreement that the documents are saying the same thing, recognizing that one states 86 and the other states 87?

Does the BRP wording make it a bit more clear that it is OK to use gasoline with a higher octane rating if that is what one wants to do?

[Zara Spook]

I think it's clear that it's optional to use a higher octane rating, but I was told that in the lower-temperature environment of an outboard engine, higher octane results in more unburned gas and carbon deposits. Is that true?

[boscoe99]

I think it's clear that it's optional to use a higher octane rating, but I was told that in the lower-temperature environment of an outboard engine, higher octane results in more unburned gas and carbon deposits. Is that true?

My thought is that it is possible, but not a given.

I have read all sorts of stuff about gasoline with a higher octane burning slower, faster, hotter, cooler, etc.. It is hard to know what the truth is for certain.

Innerweb rumor is one thing but some fuel suppliers say something else. One fuel supplier says that gasoline with a high octane rating can be formulated to burn faster than normal. Reason being is that in a race engine turning upwards of 9 - 10,000 RPM there is less time for the combustion event to occur so they formulate gasoline with a high octane to burn completely in the short time available.

I sent a suggestion to Mythbuster's about the Octane mystery several years ago to see if they could do a story on it. Alas, they never did it. Not exciting enough stuff I suppose.

[99yam40]

I wonder how you could test the different fuels to see how fast they burned.
That may be why they did not do it.

How could you tell when it was burned completely and time it?
Seems the compression ratio of the motor would have a bearing on it also.
how about elevation and O2 available, and temperature

[fairdeal]

The history of development of "anti-knock compounds" is interesting.
I imagine similar to Edison's incandescent lightbulb filament quest : just keep trying everything, until we find it...

Boscoe may appreciate this tidbit:

"World War I shifted Midgley’s early fuel research to aircraft fuel at
the Army Air Corp’s airfield in Dayton. Midgley found that some types of
fuels could be used in high compression engines while others would knock
violently.
On the list of antiknock fuels, pure ethyl alcohol was most effective,
followed by aromatic petroleum compounds (benzene, toluene, xylene), then
petroleum olefins, parrafins and ethers. Kettering and Midgley rejected
some choices as unsuitable for aircraft.

Benzene, for example, froze at 40 degrees F above zero,
while temperatures aloft could go as low as 76F below zero.

Ethyl alcohol was eliminated because of its lower BTU value,
which meant that an airplane might have to take about
one third again as much fuel to accomplish the same mission. "

[Zara Spook]

My thought is that it is possible, but not a given.

I have read all sorts of stuff about gasoline with a higher octane burning slower, faster, hotter, cooler, etc.. It is hard to know what the truth is for certain.

Innerweb rumor is one thing but some fuel suppliers say something else. One fuel supplier says that gasoline with a high octane rating can be formulated to burn faster than normal. Reason being is that in a race engine turning upwards of 9 - 10,000 RPM there is less time for the combustion event to occur so they formulate gasoline with a high octane to burn completely in the short time available.

I sent a suggestion to Mythbuster's about the Octane mystery several years ago to see if they could do a story on it. Alas, they never did it. Not exciting enough stuff I suppose.

I try to find a happy medium by getting 87 half the time and 91 half the time. I figure 89 is the safest bet.

[boscoe99]

I wonder how you could test the different fuels to see how fast they burned.
That may be why they did not do it.

How could you tell when it was burned completely and time it?
Seems the compression ratio of the motor would have a bearing on it also.
how about elevation and O2 available, and temperature

Not a direct measurement of the speed of the fuel burn but I did test three different Yams (F70, F150, F200) with 87 and then 91 octane. I saw the exact same exhaust gas temperatures (told me that one did not burn hotter or colder than the other), I saw the exact same air/fuel ratio's, and I saw the exact same power being produced in the form of the same top end wide open throttle engine RPM and boat speed.

It is my understanding that the advance timing is set so the motor produces its maximum HP. If the timing is further advanced or retarded the motor makes less HP. The timing is set so that the location of peak cylinder pressure occurs at a certain point shortly after top dead center. Usually about 20 degrees. My thinking was that if gasoline with higher octane burned slower than gasoline with less octane then the location of peak cylinder pressure would be later than it would with the lesser octane gasoline, and since it occurs later then the same HP would not be made. My tests did not show any change in engine performance with gasoline having the higher octane rating.

If it leaves more carbon deposits behind I have no way of determining if that is so or not. There are many variables that contribute to carbon buildup other than the octane rating.

[Zara Spook]

One thing to consider is how much compression the engine is generating. One would think an older engine with perhaps lower compression might be a poor match with higher-octane gas.

[zenoahphobic]

Boscoe it seems from your tests that higher octane initially burns slowly but as pressures increase from the burning that suddenly octane rating matters not as all fuels will burn rapidly in that high pressure?

[TownsendsFJR1300]

The cheapest, non-ethonol fuel at the marina, 89 octane @ $3.82 gallon, ouch!

[99yam40]

we just had a gas station down the road from me start selling non ethanol .
I have not stopped to see what octane it is yet as it is 2 times as much in cost per gallon.

I do not let my tanks sit long with fuel in them and have not ran into a problem yet so maybe I will some day

[99yam40]

Not a direct measurement of the speed of the fuel burn but I did test three different Yams (F70, F150, F200) with 87 and then 91 octane. I saw the exact same exhaust gas temperatures (told me that one did not burn hotter or colder than the other), I saw the exact same air/fuel ratio's, and I saw the exact same power being produced in the form of the same top end wide open throttle engine RPM and boat speed.

It is my understanding that the advance timing is set so the motor produces its maximum HP. If the timing is further advanced or retarded the motor makes less HP. The timing is set so that the location of peak cylinder pressure occurs at a certain point shortly after top dead center. Usually about 20 degrees. My thinking was that if gasoline with higher octane burned slower than gasoline with less octane then the location of peak cylinder pressure would be later than it would with the lesser octane gasoline, and since it occurs later then the same HP would not be made. My tests did not show any change in engine performance with gasoline having the higher octane rating.

If it leaves more carbon deposits behind I have no way of determining if that is so or not. There are many variables that contribute to carbon buildup other than the octane rating.

Did any of these motors have knock sensors or have a way for ECU to change the timing if needed?

A dyno may have been a better way to measure HP produced.

How and where did you measure exhaust gas temp?
you would think that if you measured right where it come out of the cylinder it would show if fuel was still being burned.
maybe even the lower octane fuel was still being burned there also

[boscoe99]

Did any of these motors have knock sensors or have a way for ECU to change the timing if needed? F70 and F200 have a knock sensor. F150 does not.

A dyno may have been a better way to measure HP produced. True. Wish I had a dyno but don't.

How and where did you measure exhaust gas temp? Exhaust gas temperature probe screwed into the port on the motor designed for same.

you would think that if you measured right where it come out of the cylinder it would show if fuel was still being burned. A number of piston motors with short exhaust stacks I have seen will usually throw flames out of the exhaust pipe.

maybe even the lower octane fuel was still being burned there also

Regarding the knock sensor, I once wanted to tie a boat to a tree on the edge of the lake. Then run the motor at full power to see if I could get some detonation and see what happened. First with a motor with a knock sensor and second with a motor that has one. Just to see what there is to be seen. Life got in the way of me doing the test.

Now what I did do was to remove the oil line to the number 2 cylinder on a big block HPDI. I was trying to fail the cylinder via oil starvation to see what the piston and cylinder looked like. Ran the motor in a test tank at about 4000 RPM. Ran the damn thing for 25 minutes or so and then gave up. It was too loud and too many exhaust fumes. Nothing happened. Was strange to me that it ran that long just fine without any oil.

[TownsendsFJR1300]

Well the fuel itself is some lubrication by itself... But that is impressive..

[rodbolt17]

back in the day of the 220special if you ran an octane above about 93 you could expect up to a 20%power loss.
Yamaha even had a tech bulliten out about NOT using 93 or higher octane.
now some of the F motors require a MIN of 89 octane.

you simply CANNOT compare a two stroke to a 4 stroke as far as octane vs power.

typically a two stroke has a much shorter stroke.
typically a two stroke fires every time the piston comes up.
typically a two stroke has intake and ex ports that must be closed off BEFORE secondary compression can start.
means a very short window to compress,fire and ex.
remember the ex is still occurring as the next AF charge is entering.
its called scavenging.

typically two strokes tend to require a lower octane number to keep the flame fast enough to burn the maximum amount of fuel for that short power stroke window.

kinda like smokeless powder VS black powder.
typically we don't use compressed loads of smokeless in most rifle loads.can be done with certain powders.
smokeless has additives and coatings to control the burn rate.
means the whole time that bullet is moving down the barrel the powder behind it is still burning adding pressure as the bullet accelerates downrange.

Black powder MUST be compressed,MUST BE.
black powder is very LOW octane as it burns ALL at ONCE.
BANG.

just like various gun powders.
the engineer of that rifle/cartridge is just like the engineer of that motor.
you make the pressure curve at the maximum pressure and temperature for the optimum burn rate in the space/time allotted by the physical mechanical paremeters.

to much pressure or temperature and it goes BANG instead of PUSH.