Severely OT Ford 390 octane requirement

[PlasticBoob]

Guys, it's almost 3am and I've been browsing around the archives and thinking about a few things. I heard that if your compression ratio is 9.5 or higher you need to run higher octane gas. Our '64 TBird is going to have a freshly rebuilt 390 with a stock 10.1 compression ratio, and hardened valve seats. Does this mean we need to use 91 octane gas? :confused: How does this work? P.S. That 10.1 cr puts my mucho macho 8.35:1 401 to shame!

[andy d]

yup, it will prolly ping on regular.

[Stuka]

You may be able to adjust the timing to get it to not ping on 87, but you will loose power. 89 would be easier to tune for.

[mandoneer]

the lower octane on gasoline means that it will act like diesel fuel and actually spontainously combust at a higher compression (pre spark ) so on a 10:1 motor like the 390 you should use a high octtain , on my 'bird I use the 91 octane and in the months that they add ethonal to the fuel here I even add some octane booster (the pump sais 90 or 91 but the motor pings a but and hesatates if I dont