Yamaha 195 SVHO 91 vs 93 Octane

[Jeff L24]

Is anyone with a Yamaha 195 (any model) SVHO seeing any difference in top speed running 93 octane vs 91 octane?

 

[Crob83]

I've only used 91 octane 1 time, use 93 every other time. I could not tell a difference at all

 

[mrcleanr6]

You wont tell any difference. There is no more energy in 93 vs 91. Yamaha’s dont have a knock sensor to retard the timing which is what would cause lower power and the svho stock tune is set up for 91 octane so you should see no detonation with 91.

 

[jdbakerucf]

Yep no difference.

 

[FSH 210 Sport]

You wont tell any difference. There is no more energy in 93 vs 91. Yamaha’s dont have a knock sensor to retard the timing which is what would cause lower power and the svho stock tune is set up for 91 octane so you should see no detonation with 91.

Correct @mrcleanr6 ! If anything there will be a power loss and higher temps with the 93 vs. the 91 octane. 93 octane burns slower at a given cylinder pressure than 91 octane = less peak pressure in the cylinder equals less power. The burn time is longer creating more heat, possibly excessive heat causing damage to the top of the piston and valves etc.

 

[Crob83]

Correct @mrcleanr6 ! If anything there will be a power loss and higher temps with the 93 vs. the 91 octane. 93 octane burns slower at a given cylinder pressure than 91 octane = less peak pressure in the cylinder equals less power. The burn time is longer creating more heat, possibly excessive heat causing damage to the top of the piston and valves etc.

Really I didn't know that, I thought I was supposed to use 93. If 91 is better it is obviously cheaper

 

[Croker]

I only notice a difference between 91 and 98. 98 feels like it gives about an extra 4kts to top speed

 

[FSH 210 Sport]

Really I didn't know that, I thought I was supposed to use 93. If 91 is better it is obviously cheaper

A lot of people have this misconception about gasoline, its octane rating and horsepower output. Whatever octane level the OEM states in your manual is the best fuel to use in your engine that will produce the most power and run the best.

Its all about cylinder pressure and a given fuels‘ ability to resist detonation. Also, all engines and their rated horsepower is at sea level and standard atmospheric conditions where the pressure is highest.

For example the normally aspirated version of the 1.8 liter engine calls for 86 octane fuel, while the supercharged version of the 1.8 liter engine calls for 91 octane fuel. The reason is that the supercharged version has much higher cylinder pressure due to forced air induction. The higher the cylinder pressure the faster a given fuel / octane will burn, if the cylinder pressure gets high enough for said given fuel, it will not burn but rather it will detonate most commonly known as engine knock. This is what @mrcleanr6 is talking about in his post, and why the supercharged 1.8 liter requires 91 octane fuel.

Typically the whole burn process is completed within 30° of crank rotation after top dead center, think about the volume of the cylinder at this point between the top of the piston and the head, and the peak pressure created by the air / fuel burn. When the correct octane fuel is used this is how the burn process takes place. Now, if too high of an octane fuel is used, the burn process may not complete until 90° of crank rotation after top dead center, but because the volume of the cylinder between the top of the piston and the head is greater the peak cylinder pressure is lower creating less power.

As you stated, the 91 octane is also less money than the 93 octane, so it’s a double win, best possible power / top speed with least amount of cash.