EXP119 + UL94?

Hi all,

Has anyone ordered/is it possible to order an EXP119 that will run on 94, perhaps through lower compression pistons? Any idea what kind of power loss would be the result?

Regards,
George

Good question.

Wikipedia shows the IO-390 with 8.9:1 pistons. Assume the EXP119 is the same. The O-360 is GTG with 8.5:1 pistons, but dont know if 8.9:1 makes a difference.

Interested in the answer.

It will probably run just fine -- subtract out another degree or two of advance if you want.

My guess work is as follows, using 100LL, Pistons with 8.5:1 -> are 25°, 8.7:1 -> 22°/20°, 8.9:1 -> 20° and so on. Drop a few points of R+M/2 "octane" rating and you get 8.9:1 @ 19 to 17° BTDC.

Now this is totally without any scientific study or experimentation, or other data analogues, so don't throw things too hard

The main difference between the 390-A and the 390-EXP119 (aka "C") is the size of the fuel servo throat and the induction sump.

FYI, the Lycoming IO-390-C Engine Installation & Operation Manual specifies a minimum 100 octane.

georgemohr said:

Hi all,

Has anyone ordered/is it possible to order an EXP119 that will run on 94, perhaps through lower compression pistons? Any idea what kind of power loss would be the result?

Regards,
George

Click to expand...

Stock EXP119 runs just fine on 91 with ignition advance set at 20 deg in relatively low temperatures. Not sure how much of safety margin remains though.

Stock timing on the 390 is 20 deg. Almost certainly safe to run 94 octane at 18 degrees of timing at high MAP as long as the CHTs are below 380F and IATs are below 120F.

Dropping CR will hurt power and economy. You want all the CR you can get up high as the effective compression pressure drops with altitude.

Variable ignition timing is your friend here. Limit max timing at high MAP, advance timing at lower MAP for best economy and power.