It would be helpful if people could also post their empty CG and what engine they have.
I'm trying to figure out how much the CG shifts between using a 320 and a 360.
This question can be answered analytically. You need the following input data:
1. Predicted empty weight and CG of the aircraft with the O-320.
2. Engine weight and CG location for each engine model. The FAA Type Certificate Data Sheets (TCDS) for these engines. Note that the weight and CG varies with the exact engine model. The TCDS gives engine CG location as inches aft of the prop flange. You would then need to figure out where that is in Van's datum.
Once you have the above info, it is a simple math exercise to calculate the new CG. For example, using fictitious data (you need to replace these numbers with your data):
Let's say that based on studying the weights and CGs of the large number of RV-4s listed in the
database given in an earlier response, you guess that an aircraft with your equipment fit, with an O-320, would have an empty weight and CG of 1,000 lb at 70 inches. 1,000 lb times 70 inches is a moment of 70,000 in-lb.
Looking at the TCDS, you see that an O-320-D1A has an empty weight of 255 lb and the CG is 14.25 inches aft of the prop flange. Lets pretend that you determine that the engine CG is at 35.25 inches in Van's datum. We are going to fictitiously remove this engine, so we'll use a negative weight of -255 lb, at 35.25 inches, for a moment of -8,988.75.
The O-360-A1A has an empty weight of 258 lb (only three lb heavier than the O-320-D1A, if you can believe Lycoming), and the CG is 13.88 inches aft of the prop flange. Let's pretend that this is 34.88 inches in Van's datum. We are going to add this engine, so we'll use a positive weight. 258 lb times 34.88 inches has a moment of 8,999.04.
Add the weights: 1000 - 255 + 258 = 1,003 lb.
Add the moments: 70,000 - 8,988.75 + 8,999.04 = 70,010.29.
Divide the new moment by the new weight to get the new CG: 70,010.29 / 1,003 = 69.8". So, using these fictitious numbers, the CG has moved ahead 0.2 inches. You can repeat this calculation with the correct numbers, and get a more accurate answer.
