Hello Capt,n
I am running the 51 at 16:1. I ran the ST 40 for about 2 hrs at 20:1 and upon thorough inspection found no discernable stress nor unusual wear. That one is almost 20 yrs old though so I don't know for sure what the newer 51 will tolerate. 20:1 is as low as I think I will try and only after some hours of breakin. It has maybe 1 1/2 hrs now so I am thinking maybe when it's 5 or 6 hrs old I will start taking some oil out. It doesn't leave as big a mess on the plane as glow but it does still put some residue out. (Kind of a honey golden brown stuff) I don't have an explaination as to why but I am blending 50/50 Pennzoil with Zenoah synthetic. Some of my untralight buddies have shown me tests with the pennzoil being superior to the synthetics in every respect including deposits. I have seen the pics of the engines after torn down and the pennz had less wear and less deposits. Flyboy Dave has some very interesting points about synthetics from his many many years of motorcycle technician experience. My experience pretty much mirrors his. Ken Lambert and Keith at BME put needle bearings in the 90's so you can get the oil ratio even lower for less mess and put walbros on them for better fuel control. Since borna did both I am quite certain he is going to be real happy with his conversion. My power did not suffer as much as I thought it would. The 51 pulls my Super Sportster around on gasoline better than the 40 did on 15% glow and burns about 5 oz in 30 mins of general sport type flying. In another thread I mention it turns an 11X6 prop 13,400 on gasoline and was turning that same prop 14,100 on 15% glow. If you decide to convert I think you will be happy. I have a 90 as well and it is on a 60 size Super Sportster. I am thinking of put bearings in it with a walbro and running it on gas. Good luck with yours.