Final Porting Flow Numbers 390 Head by c322348 posted May 10, 2016 at 1:06 PM The head porting is finally done! You can see the numbers in the illustration above. The biggest concern was that at .300 lift the intake and exhaust flows were nearly identical. The exhaust flow should be between 75% and 85% of the intake flow according to the "experts." The porting was able to get it to 87.5%. The blue line is the actual intake flow, the purple line is the actual exhaust flow. The red and green lines may be confusing, but they show the upper and lower limits of the 75% - 85% range; that's where you want to be. As long as the blue line is in-between the red and green, flow rate is optimal. To recap, the engine is a 331 block with an offset ground 390 crank using SBC blower pistons (to get a piston pin height in a useful range). The block was decked .060 to achieve a -.010 deck clearance which will yield a .030 quench height with a .040 thick gasket. Displacement will end up at 399 cubic inches with a 3.910 stroke, 4.030 bore and 7.100 BBC rods (bushed to SBC piston pin diameter with the big-end narrowed to fit the crankshaft journal). All this will result in a 10.7:1 compression ratio. The application is a 2600 pound sports car. The power will go through a 3-speed '37 LaSalle top-shift trans and back to a 1950 Ford rear. It would have been a lot easier to use a 390 block, but I wanted to use a 1953 331 block like the car was originally built with. We've already got a Steve Long custom cam and we're picking out some better than stock valve springs. Final assembly should begin soon! I'd like to run the engine on a dyno, but I might need to borrow a better intake manifold. The fabricated manifold I plan to eventually use may be a bit restrictive for this engine.