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Not just Harleys.. my Yamaha Raider also needs premium fuel, as do other bikes that typically have higher compression ratios
 

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Between the compression ratio and heat generated by the air cooled engine, the engineers figured out what octane was needed to prevent preignition.
 

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High compression engines require high octane fuel to avoid detonation, aka- premature ignition.
The higher the octane, the less prone it is to ignite by the heat generated by compression and slows the burn.
 

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Most Harleys don't "need" higher octane as most are 9.5 to one compression, even my XR1200 with 10 to one doesn't "need" it. As I understand (I may be wrong) up to 11 to one or a little more shouldn't need super unleaded, but I have used it in both my XR1200 and KTM 990SMT. The one bike I had that would need even more help as in octane booster added was a 2006 Husqvarna SM610 that would preignite (knock and ping) without it at 11 to one. I haven't minded paying more for the super as all three bikes got well over twice the miles per gallon as my Ranger pick-up.
 

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Head design.

Pinging has a mostly to do with head design, which includes the combustion chamber and heat dispersal. Valve angle and size and inlet and exhaust holes are all part of it. 11.5 to 1 and 87 octane can work.
87 and 97 octane make the same power. 97 helps stop the pinging, so does retarding the spark timing. But that can reduce giddy up.
Harley may have many reasons for requiring a higher octane.
Modern vehicles get about 35% of the available power from fuel. Modern ships with big diesels get about 55%. WOT is often 250 to 400 revs.

All my bikes run 87. The ethanol you guys have, makes less power than real fuel.

Unkle Crusty*
 

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The main reason I run premium in all my bikes and other small engines is because here it is the only grade of gasoline that can be had with no ethanol added. Since I have several bikes, they can often sit for long periods with no use. Same for the small engines I own. Ethanol sucks.
 

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Harleys and a Raider do not have higher compression ratio..........they have longer strokes

Gasoline engines want certain things about combustion, they want the highest pressure at 20* after top dead center, complete fuel burn at 70* after top dead center. Lower octane fuel is a faster burning fuel, higher octane is slower burning.

With a long stroke it takes more time for the piston to reach the points after top dead center therefore needs a slower burning fuel.

High compression engines are compacting the molecules closer together and thus have a faster combustion and need a slower burning fuel.

Read your owners manual, if it says something like "use only 86 octane or higher" then you will get better performance out of 87 than you will 93.......however if it says use 91 or higher then it won't run well on 87.

A short stroke engine NEEDS liquid cooling because it has way more booms causing heat, whereas a long stroke doesn't NEED liquid cooling because it has a lot less booms.

There is no big technological advancement that allows the use of lower octane or requires higher octane.

However if you are riding a bike with a shorter stroke and lower compression(like my Yamaha Stryker and Kawasaki 900's) then you are loosing power potential.
 

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A longer stroke allows more contact time between the piston and rings as it passes up and down in the cylinder, which normally makes a long stroke engine run cooler: Less piston travel.

At 70 mph, my Rocket is only turning 2,500 rpm's. I remember that my past Honda NT700 was turning 4,100 rpm's at 60 mph and some bikes like the 250 and 300 smaller sportbikes are much, much higher than that, creating more wear and heat.

Do the math on these bikes at 60 mph, in top gear (1 mile per minute) at whatever rpm and you'll be surprised how many times those pistons go up and down in just a mile. more less in a hundred, thousand, hundred thousand or more!

Lots of things dictate whether a certain engine actually need premium fuel. The ECU's of most fuel injected bikes, cars and trucks normally have anti-knock sensors in the circuitry that retards timing if a pre-Ignition is sensed, even on some very high compression bikes.

Static timing with set fuel-air ratio's must be fine tuned to avoid damage, heat and or erosion of the piston crown.

The dyno winning Big Twin Harley engines (EVO's) that I built in Southern CA, all ran on 87 0ctane fuel---go figure:)

Most of us just feed our Babies (our rides) the best because it makes US feel better:)

Premium is a waste of money unless there is pre-ignition.

Sam:)
 

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The main reason I run premium in all my bikes and other small engines is because here it is the only grade of gasoline that can be had with no ethanol added. Since I have several bikes, they can often sit for long periods with no use. Same for the small engines I own. Ethanol sucks.
That may be true where you live, but here in Jersey ALL grades of gasoline have ethanol!
After having the carburetor on a new leaf blower engine ruined by ethanol, and a nice chainsaw screwed up by that crap, I now go to the local airport and buy a couple of cans of aviation gas to put in anything that's going to sit for any length of time. Avgas is pure LEADED gasoline, just like what you used to get years ago. Expensive but worth it.
I was just outside cleaning up leaves. The leaf blower that I put away with a full tank of avgas last fall started up on the first pull and ran like a champ!

P.S. Soon it will be time to park my bikes for the winter too. Once I know that they won't be used again 'til spring, they will get a tank of avgas too.
WARNING! My bikes are oldies, 1970s vintage. Don't put avgas in any newer machine that has a catalytic converter. The lead in avgas will destroy the converter!
 
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