Fuel ratio and jetting question.

[Laker]

For fun lets say were running a 24:1 gas oil and switch to a 32:1 or 40:1
Is their a guideline on how much richer (more gas less oil) the jetting (or ECU) will be?

[BarryStrawn]

Carbs and fuel injection both meter fuel by volume so a simple calculation will account for the different mix percentages.

24:1 = 4.167% oil and 95.833% gas
32:1 = 3.125% oil and 96.875% gas
40:1 = 2.5% oil and 97.5% gas

So if you moved from 24:1 to 40:1 you would be 97.5/95.833 or 1.7% richer.
24:1 to 32:1 would be 96.875/95.833 or 1.1% richer.

The common mistake is double oil or moving from 50:1 to 25:1 which is 2.1% leaner. OK if you were rich but not so good if you are close to stoich.

There is also an opinion that the gas/oil viscosity changes enough to effect the flow. I would have to see a flow test to believe that one.

[Laker]

Im thinking the flow rates would be affected because the specific gravity of oil Vs Fuel will change with the ratio…?
I know my fuel here SG is .687… our 110 race fuel 7.23ish... (Cant remember exacty) ......know Idea what oil is. My hygrometer has not come in yet.
Great info Barry!
THANKS

[BarryStrawn]

Specific gravity of the fuel doesn't make much difference to metering it through an orifice. The carb or injection will do a pretty good job of delivering a reliable volume.

Specific gravity is important because the combustion chemistry is by weight of the fuel and air, not the volume. But the fuel is metered by volume. So tuning for different fuel and air densities can be fun. The next part of cipherin' would be to account for the changed fuel weights. Then factor in air density. Maybe humidity if you are a masochist. Good record keeping is the only way to have much chance of getting close on the first jet change.

[Laker]

Must finish my Physics text books I see...
Thanks Barry!