I just sold off 2 - YS .63's that I had for about 5 years. Nothing wrong with them just went electric for the smaller stuff. The .70 may ( but I doubt it ) have the same power as a .63 and the .81 probably does but I can tell you NOTHING will match the .63 for speed of transition from idle to full. I had these engines for 5 years and the speed at which they transitioned still scared me. I thought they would blow up - but they never did