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nortySun Jun-19-05 09:19 PM

  
"octane rating"


  

          

bike 850 comando, with boyer black box

I have been using 93 octane, what do you folks use, anyone experimented with 87 or 89?

In theory under 10 to 1 87 shouldnt be a problem (on cars at least) but my bike seemed to kick on 87

  

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GreenyMon Jun-20-05 04:02 AM

  
#1. "RE: octane rating"
In response to Reply # 0


  

          

There is bound to be lots of info on the net regarding relationships with octane rating.
If you don't understand the implications, I would not suggest it to be a good idea to 'experiment' with lower octane.
Basically if you want to run lower octane, you would be advised to alter the ignition timing to suit. The result if you don't do this could be as bad as a hole in the piston (no it won't happen instantly).
Using around 96 to 98 octane you would be looking at ignition timing set to around 31 degrees Before-Top-Dead-Centre when fully advanced.
If you use 89 octane with this ignition timing, the ignition will be lighting off the mixture too early, and it will reach peak energy levels pretty much before the piston has reached the top of the compression stroke, placing enormous loads on piston, and all crankshaft & conrod bearings. This is why you are experiencing 'kick back' as you put it. The engine will 'rattle' or 'pink' under load (which is very bad!!!) and will not produce the equivalent housepower that can be obtained by retarding ignition the correct amount.
What is the correct amount? - in broad terms it is retarded just enough so it no longer 'pinks' under load. Hope this helps.

  

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GreenyMon Jun-20-05 04:37 AM

  
#2. "RE: octane rating"
In response to Reply # 0


  

          

In essence, lower octane fuel will burn more rapidly than a higher one, so if you use 87 octane when your bike has been tuned for 96 or 98, you will be firing off the mixture too early. Peak energy levels will be reached possibly before the piston has reached the top of its compression stroke. This is the reason you are experiencing 'kick back' as you put it. The engine will tend to rattle (or 'pink'), moreso under load and this is bad!!!
All that energy is trying to drive the engine backwards, but the flywheel inertia carries it over TDC. Enormous loads are experienced by the piston-conrod-flywheel bearings, and the longer term consequeces can be as bad as holed piston/s - no, it won't happen instantly.
I would suggest you stick with 96/98 octane, but if you still want to check out the lower octanes, then the timing must be retarded to suit.
Using 96/98 octane you'd be around 31/33 degrees full advance.
On 88 you need to reduce that back somewhat. How much?? - well in broad terms just enough so it no longer 'pinks' under load.
Hope this helps.

  

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iggyMon Jun-20-05 07:34 AM

  
#3. "RE: octane rating"
In response to Reply # 0


  

          

The boyer variations are easy to see on this graph.


Setting the advanced timing to 31º guarantees lots of variation at lower RPM (1500-2500) where the pinging normally occurs.
A stock compression 850 should be able to run on 87-89 octane.
One engine (boyer) might run fine and yet another pings like crazy.
It's probably one of those boyers with a flatter curve.

iggy
850 with rita running regular.

  

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nortyMon Jun-20-05 10:15 AM

  
#4. "RE: octane rating"
In response to Reply # 3


  

          

Thanks guys.

Greeny where do you get 98 octane gas?

The highest octane "pump" gas around here is 93

  

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