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Octane rating, rate of burn and ignition timing

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I work in the motorcycle industry and there has always been a lot of debate around using 98 octane PULP in bikes that have been designed to use 95 octane PULP from the factory. Very few bikes run knock detection and none that I know use a flex fuel sensor. Is there any point in running 98 in a bike that is meant to use 95, other than adding greater headroom against knock, if you don't alter the ignition timing or compression?

Also, does the rate of burn/flame speed vary enough between 95 and 98 octane fuels to warrant a rethink on ignition timing?



If the ECU can't take advantage of the higher octane rating then there is unlikely to be any performance advantage from changing from 95 to 98 except for the added safety margin.

I couldn't tell you what the difference in burn rate between 95 and 98 octane fuel is, however you can still use a load bearing dyno to tell you when the ignition timing is optimised. If the engine isn't knock limited I'd be surprised if the engine would need/want the timing changed if you moved to 98. For example I've tuned a number of race cars that had been running on 100 octane av gas that needed to be retuned for local 98 octane unleaded to meet race regulations. The leaded av gas has a slower burn speed and hence needs more timing than 98. When retuning for 98, less timing is needed to reach MBT and I actually found in many cases we made marginally more power on 98 than av gas.

98 burn rate is marginally slower than the one of 95. The lower the octane number, the higher the energy content of regular fuel. Having said that, a lower octane fuel "could" result in more power if detonation would not get in the way, given the nature of high performance tuning this is a hypothetical statement and not that I'd advocate using lower octane.

On a sidenote, not directly related to your question, I see 15.5 crankshaft degrees of rotation on E85 (on a particular engine and load / rpm) from the point where the spark is being fired until the cylinder pressure starts climbing above the curve of compression if no combustion would have taken place. In the screenshot the pressure starts rising at 4.2 degrees BTDC. The firing event took place at 19.7 degrees BTDC. The energy release / burn rate can be seen in the second screenshot, pink line.

At first glance this is a remarkable delay but it illustrates how relatively slow a controlled burn propagates throughout the chamber. It can be compared to a camp fire where we use matches to light up the first couple of sticks, it will take quite some time until the campfire is fully lit up.

Attached Files

Actually octane has no relation to burnspeed.

Burnspeed are a result of how the fuel is put together... There are high octane fuels that burn "slowly" and other that burns fast.

You would generaly want one that burns fast.

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