honestly, its not that modified...
the stock engine has 10.5:1 pistons and required high octane fuel...when we put the motor together, i was 16 or so, and i couldnt afford good, high compression pistons, so we bought the least expensive 30 over 340 pistons that we could find. those pistons were 8.5:1 compression. the "good" 340 cylinder heads (j heads, x heads, etc) typically hold large intake valves (2.02 intake 1.60 exhaust)..my heads are late model 360 heads, ported and polished, but they only have 1.88 intake valves, which limits the engine quite a bit. so those two items actually hurt the performance...
to counter that, the engine is 30 over, so it actually displaces about 346 cubic inches. i run hooker super competition headers as opposed to the restrictive exhaust manifolds. for a long time i was running the stock cast iron intake manifold, although it was ported to match the heads, and i ran the stock thermoquad carb. with that setup, i was in the low 14s. I am now running an edelbrock air gap intake, which is a flow-through style intake manifold, and a proform "race carb". the carb is 800cfm, where as the stock thermoquad was (i believe..its been a while) 625 cfm? I am also running a much bigger than stock camshaft, and roller rockers. but in comparison to what most 340 racers are using, my build is very, very mild...
we are considering going to a better piston, perhaps 9 or 9.5 to 1 this winter...and someday id like to have a good set of heads with the bigger valves, but the car runs well for now, and if i would just stop buying cars, i could put more into the mirada, but amanda and i would really like a cruiser. eventually we will have the mirada here, and here, it is street legal.
i would like it to be into the lower 12s, nearing the 11s before it makes the move here...plus, my dad, who is THE reason that it runs so well with his decades of experience and of course his labor on the car, is starting to run it since its stored at the shop, and hes having alot of fun with it...so i dont want to take it off of him.