MY second run this weekend will be with 10%. Running YS60RE short stroke back in the day, I used 5 - 10% nitro. I didn't start using 20/20 until the AR's came out which are long stroke. I think 20/20 would be to much for this motor on a initial run. The pipe/header length are at their max base on last weekends run. I'll play it safe and keep it there. The neat thing about this new motor, the word "Classic" on the head. BTW, the 4" header length I spoke about, came from Richard's trail testing. Richard indicated he was getting 14100-14200 on his runs. My runs seemed only what my ear was telling me and I could be off somewhat. To really get an accurate reading, I'll use a tachometer this weekend. One thing for sure, this motor is every bit of the old school performance and then some. I've also got a Classic in an Aurora - that's next after the Summit