Fuel mixture

Does anyone know what roll the fuel octane has on the fuel mixture of an engine, Does higher octane make your engine run leaner or richer or does it not affect it. Also what octane are you running? With gas prices rising is hogher octane cost effective? I have to go but I will check back in on this subject latter. Thanks

 

This weekend I went away on me 750. On way there I had Shell V-power in and got there on a tank and had another 10 odd miles before hitting reserve. On way back I filled up with cheaper stuff and hit reserve about 50 miles from home, actually a shorter route as I took a de-tour on the way there. Lucky I got home as the bike does less than 50 to the gallon, and theres less than a gallon on reserve, managed to get home some how, probably my carefull use of only 1 gear, even setting off in 5th (sorry engine, clutch, driveshaft and gearbox).

Yes it a difference, a big one.

 

Octane rating does not affect fuel/air ratio.

Higher octane fuel is less prone to preignition (ping, knock, detonation) and burns slower. Higher octane fuel only helps if you have preignition trouble.

Your 650 will be perfectly happy with 87 fuel.

 

octane is not a power adder, an "oxygenator" or an "enrichener".

higher octane fuel is NEEDED mainly when running higher compression. there are other circumstances that will require a higher octane, such as super lean carb settings, extremely advanced ignitions....but for the most part, if your engine is "stock" (exhaust and pod filters dont make a big enough difference) and it isnt pinging, dont go up on octane.

your power output will actually suffer from running too high of an octane (slower burning) and although it might not be noticeable on a bike, i have seen a significant drop in power at a dyno day when a guy ran his mildly built mustang on 87, and then again on c-12. he lost about 15hp at the peak. his 8.5:1 compression had no need for 108 octane fuel.

a common myth about higher octane fuel is that its "racing" fuel. the race gas i run in the quad is vp's u4.2. its leaded, heavily oxygenated (5 jet sizes up to compensate) and is 102 octane. if i ran 110 rated pump gas i would see about a 10% loss in power, and it would probably ping. race fuel is not run for the octane rating (well...sometimes it is) its run more for its extra fun GO BABY GO additives, and its extremely clean burning properties.

vp's site has some good info on fuel theory, and clears up some common misconceptions. i would say, if your bike isnt pinging, stay with the 87.

 

I found that my bike would turn lower RPM's with 92 +.

65 MPH = Approx. 4700 RPM with 92+

65 MPH = Approx. 5200 RPM with 87

75 MPH = Approx. 6000 RPM with 87

75 MPH = Approx. 5300 RPM with 92+

So I would have to assume that I have greater HP with higher-octane fuel.

The lower RPM's would then equate to better mileage, If I was not stuck on the (California Parking Lot) FWY.

 

how could your revs at 100km/h change? Did the fuel change your final drive ratio or something?

The latest range of fuels like V POwer can change your mixture setting as they are a denser fuel

 

Shamus had a good question.

I also don't understand how your rpms could change at different mphs just by using different fuels.

But hey...if it works for ya...all the better.

I've heard stories of Maxims getting 50 mpg, but even running a constant 60 mph, I've never gotten better than 44-45 with any of my Maxims, brand new or older ones (but no chain driven 550s).

I surely would like to get 50. I get 39-40 on the hwy between 70-75 mph (with a windshield and rear seat luggage).

Rusty

 

I figure I get 40 at 70mph.

I got 50 once. Long trip on 2 lane road at 55mph with minimal stop signs.

 

Amy manual I've seen on the xj650/xj750 says to use 91 octane.

If you ride very easily you might get away with 87 octane but I wouldn't use it unless in a bind and couldn't get 91 octane. If you like to rev these bikes then I'd run what they were built for, 91 octane, they'll hang together a lot better.

You can get into all kinds of trouble with pre-igntion etc using a lower octane fuel.

 

Octane is only a benifit in HIGH compression vehs. Our bikes are NOT HIGH compreesion vehs.
Octane value is related to the ignition of the fuel mixture. Higher octane prevents preignition


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