by Ferrous_Head » Mon Nov 02, 2020 9:24 pm
I only use the ratios as an proximate guide. I always check the cranking pressure and then if someone asks me I'll work it out as a ratio but a lot of people just go by what the piston is SUPPOSED to provide.
The stock Sportster 900 is listed as having 9 to 1 compression. That would be appropriately 135 PSI but the numbers can vary due to so many things. Ambient temperature, altitude above sea level, leakage, cranking speed.
The actual numbers are that important to me as the limiting factor for so many engines will be the fuel your using. All my race engines for the past 10 years have run on Methanol so I can run very much higher pressures.
I'm not sure how good the gas you guys are getting is but ours is not great. It doesn't matter in the newer engines that have exceleent (comparative) combustion chambers designs.
But our poor old hemi's in cast iron need a little help.
The 900 I ran on gas I ran on Avgas with a touch of benzene. Not pump gas. Even then I holed a piston when I had the timing out a couple degrees.
"I know only too well the evil that I propose, but my inclinations get the better of me."