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I understand the role of higher octane fuel on a knock limited setup but on a non-knock limited setup what is the role. Does putting race fuel in (for example) on a non-knock limited engine allow you to run a leaner / less safe AFR which you may have chosen for comfort zone/reliability?
Hello i would be using it for its ability to cool the valves if not needed for knock limiting
it generally burns slower helping that side out
You cool down the temperature in the combustion chamber to avoid knock. If the engine is not knocking at all and can perform at MBT everywhere in entire RPM range then there is no need to use extra fuel for cooling the temperature in combustion chamber. All you can get out of it is just fuel economy... The only way to get more power is to find the air/fuel ratio that would allow to use less ignition timing but still create peak cylinder pressure close to the piston position at 5-10 degrees ATDC, but most likely it will be very minor gain.
Unless there is a significantly different stoic AFR surely there is minimal cooling benefit just in octane rating?
I belive higher octane fuel has a cooling affect because it lasts longer before igniting during which time it provides some cooling.
The reason for higher octane fuel is so you can support mechanical changes that lead to increased knock tendency. So the fuel allows more power to be made with changes in compression ratio, combustion temperatures, etc.
So basically if you’ve already optimized your afr and timing then there is no use for higher octane fuel on the track or anywhere? This is what I thought but wanted to verify.
While unlikely, you would may want to test, you will only find this out with careful testing on the dyno.
If you managed to dial in ignition timing up to Maximum Break Torque points everywhere across entire RPM range there is no much else you can get from higher octane rating fuel. What you can use fir getting slightly more power is the other fuel property - burn rate. It will have some impact on average torque value thus power.
Some are getting the general idea.
If the engine is in the same state of tune, and wasn't 'knock limited', a higher octane fuel won't give more 'power', but may even reduce it slightly because the potentially slightly slower burn would mean a later pressure peak and possibly less efficient use of the expanding gasses.
There may be a small advantage in advancing the ignition timing to optimise the pressure peak, but that is something to experiment with.
For fuelling, if the engine wasn't knock limited, and EGTs were acceptable, you should already be running at the lambda for best torque/power and adding or reducing fuel will only cause a loss of torque/power. However, if timing is altered, you may find a very small gain, one way or the other, if you trim fuelling with the timing.
I may have missed you mentioning it, but you may be able to run a bit more boost with the better fuel. If you're happy with what you have, there is an indication that you can run a little more mechanical compression on the next rebuild, which 'should' give a little better bottom end response.
Not all the high octane fuels have slow burn rate. When i was trying to dig some info on burn rate of VP q16 and VP import high octane racing fuels i couldn't not find the exact numbers but i found a topic where it was said that they were made specifically to match the burn rate of alcohol fuel brands. I cannot confirm it but if that is true the higher octane rating fuel not necessarily burns slower.
Well you guys have certainly answered my question. Many thanks for that!