Your plan to use dual carbs is probably where you're going to encounter the biggest disappointment. An inline six with a conventional firing order of 1-5-3-6-2-4 and only three intake ports is simply not compatible with two carburetors- - - -it's much better with either one, or three. Since the firing order only allows the volume of one cylinder at a time to flow through each port, it makes much more sense to go with three small carbs if you intend to get a small, but measurable power gain over the conventional single carb. By choosing a carb that can handle one cylinder's volume on each intake stroke, you'll keep the venturi velocity up through a much wider range of RPM without encountering the dreaded flat spots, backfires, and other gremlins the dual carb system will create. The only hair-pulling frustration a triple setup creates is trying to keep all three throttle plates synchronized. Jaguar, Austin Healey, Nissan, Toyota, and a few exotics such as Aston Martin have all opted for triple side draft carbs on inline sixes, and the 235 Corvette also ran three Carter side drafts. If you've got a few thousand bucks burning a hole in your pocket, scavenge up a Corvette setup, if you can find one that's not worn out or cobbled up to the point it's completely useless.
The only reason to raise the compression ratio above around 9:1 is to compensate for a long duration camshaft profile. The compression pressure (not ratio) stays approximately equal to stock compression, as a lot of the compression stroke id used up by the intake valve duration, resulting in a shorter distance for the piston to actually squeeze the mixture before the spark plug fires. A short duration cam and high compression promotes detonation. That requires the ignition timing to be retarded, which negates the advantage of matching compression, cam profile, and timing advance to the desired RPM where you want the power gain to happen. One of my customers about 40 years ago insisted on running 12:1 compression pistons and a stock cam in a 1968 Morgan +4 (Triumph TR-4 engine). He detonated the tops out of the pistons in a very short time, even with the ignition timing set AFTER TDC!
I'm working on building a 3-carb setup for a destroked 230 engine that's still on the drawing board- - - - -around 200 cubic inches, NASCAR connecting rods, forged pistons, some extensive port work, a roller cam, and three Keihin side draft carbs designed for an 80 cubic inch Harley Davidson EVO engine. If everything comes together as planned, the engine should pull strong from just off idle to over 7K RPM, with no noticeable flat spots. I'll probably use a head for a 194 engine, with smaller combustion chambers, ports, and valves to begin with, to avoid losing low and midrange performance to ports that could ventilate a Kentucky coal mine! It will be one of several engine options in an Excalibur fiberglass kit car, running anything from a Mercruiser 4 cylinder, to various inline and V type 6 cylinders, and a 250 cubic inch small block V8. All those engines share the same bell housing bolt pattern. I'm not against high performance- - - -I just hate to see a newcomer to stovebolts waste time and money on projects that have disappointed people for decades.
Jerry
