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I am going to use a Rotax 912 ULS engine in my single place Onex experimental. This engine weighs about 170 lbs and it will be replacing the VW conversion engine normally in the Onex which weighs 190lbs. So I am doing the math below to figure out how many inches I need to move the new engine forward to compensate for its lighter weight. Using the weight and balance diagram below, the number of inches that the cg of the engine is forward of the wing is 48.6”. So the moment is 190 x 48.6 = 9,234.
If the new engine is 170 lbs, to get the same moment I would need to have the arm be:

9234 = “X” x 170
“X” = 9234/170
“X”= 54.32
So I would need to move the engine forward about 6” (54.32” – 48.6”=5.72”)

Is it really that simple or am I missing something?

Looks like you are making a mistake in using the moment arm to the wing mass as the the CG of the the plane. The way you are calculating the differences here only make sense if you are equating the moment of the different engines from the CG.
The Center of Gravity of the Onex should be between 20% and 32% of the chord of the wing or 10" to 16" back from the leading edge of the wing (50" chord). You would want to calculate your shift based on those two points. Then you could adjust from there based on your expected pilot and baggage weights. I think this will work out to be somewhere between 4 and 5 inches.

I don't think the Aerovee weighs 190 Lbs. My data shows it to weight in at about 170 to pounds serviced and mounted. Did some work on mounting a Jabiru 2200 on a Onex and it only weighs about 130 Lbs and had to extend the engine mount by about 7" forward.

Don't forget to include the weight of the radiator and coolant and its location. Neither are required for the AeroVee and may affect the moment arm.

petep wrote:I don't think the Aerovee weighs 190 Lbs. My data shows it to weight in at about 170 to pounds serviced and mounted. Did some work on mounting a Jabiru 2200 on a Onex and it only weighs about 130 Lbs and had to extend the engine mount by about 7" forward.

When you did the weight and balance how did the numbers come out?

For what it is worth I moved my CAMit 2200 7 inches forward on my Onex. The CG moved a little forward from where it was with the AeroVee but still well in the envelope and that was a good thing for my situation. I am 230 lbs and have had a tendency to touch down on my tail wheel first. IMHO consider your pilot weight and don't go further back with CG than you are with the AeroVee. I have a Jabiru 2200 and AeroVee in my shop right now and will weigh them post results. The AeroVee will be weighed as flown in my Onex and the Jab has been run on a plane but never flown.

Just weighed the AeroVee that I had on my Onex and it came in at 177 lbs on a digital scale hanging on a chain fall. This weight is as flown without prop, Sonex Spinner, 4 EGT and 4 CHT probes. It includes cast iron jugs, the 2 in 1 exhaust, Rotec MK II TBI with throttle cable, top mounted oil cooler, and complete baffling. Sonex's advertised listed weight of 160 lbs dry must be very close and accurate. I believe the Rotec MK II is a little heavier than the Aeroinjector. Hope this helps.

That is very helpful, thanks Loren!