10.25:1 pistons- what octane rating would you suggest?

Yea, I know this is probably a can of worms.

I had 93 in the tank, so thats what I went with for the first 20-some miles. Better safe than sorry anyway I guess. I had to pull the exhaust to change the oil (after 20 miles, yes) and I noticed the once-immaculate exhaust ports were already all sooty. The bike was running very close if not right on, it seems, and the plugs weren't sooty at all.

Now, will any grade of fuel burn like that? Or should I be running, and should I be able to run, a lower grade of gas?

My car has 10:1 compression, stock, and that thing runs great all day on 87. Of course it has a knock sensor and an ECU that will retard timing. It is also liquid cooled.

The bike, of course, doesn't have those saftey nets.

Post edited by: Trav, at: 2006/06/23 00:53

rule of thumb you want to run as low octange as poss. that u can get by with with out sacraficing the performance of the bike. checking the plugs will always tell you if it is running rich or lean.

i only have a 1075 kit and stret cams and i run 93 for the street and the plugs look fine. but for the track days i mix 112oct and 93 oct 50-50.nothing like the smell of some race fuel.

What compression is that setup?

If everything else was the same and I had stock pistons, I should be able to run 87 no problem, as that's what it was designed to run on. Stock compression was around 9.5:1 I believe though.

I'd like to think I could run 89. I guess I could fill the tank with 89 (about a gal of 93 in it now) and see how it runs, then fill it with 89 the next time and see..

my commpression is the same as yours. i sure your ok to run 87 or 89. i have put 89 in my bike a few times due to the gasstation a block down the road having a broke pump(the 93oct). i isnt real going to hurt any thing to put a gallon or two of 87 or 89 in just to run it through to see how it dose.

what size is your piston kit? cams? bigger carbs? hot ignition and coils?

most of that stuff is in my signature, but:

-553 to 615cc kit
-carbs are 1mm bigger than stock, GPz550 carbs (but mainly easier to find so I use them)
-the cams are stock GPz550 cams, more lift and duration than stock KZ550
-dyna coils, wires, ngk caps, stock part # ngk plugs, factory IC ignitor box and pickup coils

Signs of detonation:

Tinny rattle under heavy load

Spark plugs working loose

Flecks of metal on plug electrodes

Don't want to make you paranoid but if you notice any of those things then it's time to run higher octane.

I do not understand why a higher octane fuel would produce additional soot if the mixture is correct.

Post edited by: trippivot, at: 2006/06/26 08:24

The higher the octane, the more controlled and cooler burning the fuel is.

Nobody told me this analogy so I don't know if everybody will like it, but from a very rough perspective, think of it as analogous to running a spark plug that is a lower heat range than you could possibly run safely. Unburnt deposits will start to build up. Obviously on your plugs as well, but like I said its not the same. However it helps me wrap my brain around it.

You want the hottest burning fuel your engine can take (just like in theory you want the most compression, and the hottest spark, etc.) however it all must be in proper balance, right? So sometimes other factors reach a point where another factor has to be lessened to creat some slack, IE alot of compression and the same ignition timing and everything else = cooler more controlled burning fuel (higher octane) is needed.

If the bike was designed to run perfetly fine on 87 octane (which these bikes were, when stock) then it is best to do that. Think about it as making the job easier for all the engine components. The engine isn't running nearly as hot as it would if it were modified for higher performance, and so the higher octane fuel isn't burning the way it would at higher temperatures. However the low octane is.

You want everything to burn at the 'right hotness' inside the engine, so the proper grade of fuel is important to that.

Well, take everything I said with a grain of salt, but that is the sum knowledge I have come to know.

I may be wrong.

What will dyna coils cause?

Also, aircraft engines are not high compression by any standard. Around 9:1. Avgas also contains tetra ethel lead and the plugs will foul.

Why not run the tank low then put in half a tank of 87 and see if it will work? If it causes a problem then finish filling it up with Premium and see if that works. If half Premium and half 87 works ok, then run it low and fill the tank half way with 90 to see if it works. Trial and error ought to get a result

I had a H1 500 triple that ran like crap on Premium but ran great on regular (a long time ago) that's when I discovered that more expensive was not always better

Post edited by: steell, at: 2006/06/23 12:24

lead is not the only difference in av-gas
it will not foul plugs. lead does lubricate non hardened poppet valves and bronze guides exposed to combustion temps. we now have superior materials in our heads

fuel and octane needs are defined more by the engine load (racing?), ignition timing, temperature,elevation,
more so than just the compression value.
under 12:1 a bike will go bar to bar or to work just fine on 85 - 87 O fuel. but go to the drag strip you will have a problem. (BMEP goes up octane need to go up)

8:1 bike will pre ignite on 105 O fuel at 100 deg F at full throttle,little above sea level. ask me how I found out at Bonneville lake.
I nuked a cb 750 all to hell and lived to not get a trophy.