Different sources give different numbers for best burn speed, all seem to be in the mid 11:1 - low 12:1 range for gasoline --- far too many variables to give an absolute number I guess.
Ignition advance ( indirectly an indicator of burn speed ) also depends on combustion chamber design and squish etc. as we all know. Interesting chart on the change in required ignition advance in one of the NACA reports also.
Check out Figure 5 In NACA report E5E18 for a good example of both the change with afr and engine rpm. On the Aircraft engines they were working with they got minimum required advance at a fuel air ratio of .088 or approx 11.36:1.
In the book How to Tune and Modify Engine Managment Systems he plots a curve on page 127 that shows best power at fuel air ratio of about 0.083 (12.05:1) and fastest burn speed at 0.09 (11.1:1)
John you raise a very important point that people need to keep in mind --- every time they change fuel air mix or in our case WI rate or water/methanol mix you are in effect changing engine ignition timing by changing when peak cylinder pressure occurs in the cylinder.
At a given engine rpm and manifold pressure and AFR, there is only 1 ignition advance that will optimize cylinder peak pressure with the mechanical best crank angle for max engine effeciency (somewhere near 14 deg ATDC)
It is much better to be a bit late on ignition timing in a high power engine than it is to be a bit early. If your on the ragged edge and you reduce your WI spray rate or add alcohol to the mix, you are for all practical purposes advancing ignition timing.
The window of acceptable ignition advance for best power/effecincy is only a few degrees wide so you don't have a lot of room to play around with if your pushing the engine hard.
In figure 6 in the NACA report E5E18 on the next page following figure 5 you can see that at max power output (863 lb/hr air flow) the curves get a sharper peak and the max power nose of the curve is only about 8 deg wide, where at a lower boost pressure (437 lb/hr air flow) the same zone was almost 14 deg wide.
Larry
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