Octane rating - which fuel for bikes?

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oldnslo · #21 Unread post by **oldnslo** » Tue Jun 28, 2005 7:05 pm

If you want a long, dry, technical explanation, I can provide it, but the short version is that if your bike runs well on 87, don't bother wasting your money on anything higher. And skip the octane boosters, too.

#22 Unread post by **Sev** » Tue Jun 28, 2005 7:28 pm

oldnslo wrote:If you want a long, dry, technical explanation, I can provide it, but the short version is that if your bike runs well on 87, don't bother wasting your money on anything higher. And skip the octane boosters, too.

no no, we insist, long and dry please!

oldnslo · #23 Unread post by **oldnslo** » Tue Jun 28, 2005 7:37 pm

Since it's you, I know for sure you're pullin my chain.....you just want me to strain myself so you can laugh and make fun of me....

If you really want to read the thing, get into Motorcyclist archives, the issue is June, 1999, the name of the tech article is "Fuel Physics", written by Gordon Jennings.

#24 Unread post by **Sev** » Tue Jun 28, 2005 8:32 pm

oldnslo wrote:Since it's you, I know for sure you're pullin my chain.....you just want me to strain myself so you can laugh and make fun of me....

If you really want to read the thing, get into Motorcyclist archives, the issue is June, 1999, the name of the tech article is "Fuel Physics", written by Gordon Jennings.

Actually I'm serious. I don't have a subscription to motorcyclist, and obviously I wouldn't have been getting it that long ago. I've only been interested in riding since last year.

But I think that I know part of it. The higher the octane rating the easier it is for the fuel to combust under pressure. Now people here have been talking about precombustion or engine knocking when using a low octane fuel even if it is recomended. But that doesn't make sense to me. Because engines are designed to use the pressure of the cylinders in conjunction with the fuel mix from the carbs (or fuel injectors) and the spark to ignite the fuel and create combustion which in turn provides power and yadda yadda bike goes forward.
However, by putting a higher octane fuel into the engine you are reducing the pressure required to ignite the mixture. This means that it is possible for the fuel to ignite before the cylinder completes its stroke. So it's actually exploding INTO the cylinder while it's moving towards the top of it's stroke. So you're effectively increasing the wear and tear on the cylinder head. And the double ignition is what is known as knocking.

Anyways, this is me trying to reconstruct the ideas that I've had floating around in my head based on what I think I've heard before. If I'm close let me know, otherwise I'd honestly like a quick correction.

TechTMW · #25 Unread post by **TechTMW** » Tue Jun 28, 2005 10:27 pm

you have it backwards -

Higher octane numbers = less volatility.

Therefore, a lower octane fuel will cause preignition or knocking because it's easier to ignite.

Check out the link I posted earlier - if that's not long and dry enough for you, I don't know what is

BuzZz · #26 Unread post by **BuzZz** » Tue Jun 28, 2005 10:31 pm

If the fuel causes the bike to ping, it will damage it over time. Like Tech said, run what you can get away with in your particular bike. Even the same model of bike can run higher or lower octane fuel than others, every engine has it's own personality.

I can run straight regular in my FJ with no problems. Premium doesn't make it run any better or worse, so I run the cheaper fuel. Cause I'm cheap that way.....

High_Side · Wed Jun 29, 2005 2:03 am

oldnslo wrote:If you want a long, dry, technical explanation, I can provide it, but the short version is that if your bike runs well on 87, don't bother wasting your money on anything higher.

Thats what I thought until I discovered the increase in mileage......

poppygene · Wed Jun 29, 2005 2:16 am

High_Side wrote:
oldnslo wrote:If you want a long, dry, technical explanation, I can provide it, but the short version is that if your bike runs well on 87, don't bother wasting your money on anything higher.
Thats what I thought until I discovered the increase in mileage......

Interesting. My bike runs just fine on 87, mild compression and all (8.7 to 1). I'll have to see if I get a similar increase in fuel economy. The ol' girl is really thirsty - not exactly a cheap date.

oldnslo · #29 Unread post by **oldnslo** » Wed Jun 29, 2005 2:38 am

Maybe you should order up a set of period road-tests of your bike, Poppy. It will give you at least an idea of your bike is actually thirsty or if it just has a too-small fuel tank.
My old heap runs and lives on 87, gets mid-50's to low 60's, depending on riding aggression level.

oldnslo · #30 Unread post by **oldnslo** » Wed Jun 29, 2005 3:16 am

Historically, engines with very high compression ratios demanded high octane fuel. So, it seems logical that lower compression engines would be ok with lower octane fuel. The modern knock sensor would help modern higher compression engines to run on 87, I would think.
My CBR1100XX ran fine on 87. My unsophisticated butt couldn't tell one bit of difference between 87 and 92, either. My low compression old Seca runs fine on 87 also.