Well...in respect to the 320, you WANT to keep it under 4000. In my blurb above I was basic and brief, I could write pages and pages about this stuff... but the 320 is longer and taller and bigger. Mass in motion tends to remain in motion. The dynamics in that engine get outta hand fast, in the 248 and 263, which are more compact, it can be controled better. There is some supprisingly clever enginering in these engines and understanding whats really going on is key to the whole deal. Don't lay awake at night 'cause ya don't have a 4140 crank, work with whats there. Its good steel in there. All the old hot rodding tricks work, I use 'em because these engines respond to it. Of all the engines I've built the buick str8's are the most responsive. Virtually everything, you can feel in the seat of your pants. Forget "chevy think". So, all the stuff I relate here applies to all 3 engines, and its not "feel good" modifications. Gettin the power is not about doing 1 thing 100%, its about doing 100 things 1% and that 1%, 100%. These babys respond. Now, the reason your 320 flattens out at 3600 or so, is the top end. It just wont flow. The first thing you should do is get a buick duel carb set up. Every str8 should have 'em. The single carb runs the center rich and the ends lean, and the exhaust won't clear the cylinder. I have pulled more destroyed pistons out of str8's because of this than I can count.
A little more on pistons. I droped the 4th ring long ago and it works fine, but these engines badly need moderen pistons, ring placement and rings. The boys on the HAMB site are in the hunt but if the piston that fits their engine fits others...but 320's and 248's, no. Ive looked into custom pistons, the actual piston is not such a problem, the dome configuration is. Flat top pistons are best for flame travel but the chamber is WAY to big, and we have to live with it. Plus the little detail that if one of those pistons gets hurt, another must be made. This lead me to harley pistons. Everything is made for harleys. This piston gives everything ya want. Off the shelf. Plus 310 inches after its in there. But it only works on the 263. The drawback? Gotta make rods. The pin is higher and so the shorter 263 must be lenthened. I had access to a wire feed EDM, so I cut blanks out of 6061 aluminum and made my own. The harley piston has a hemi type dome on it, clears everything and gives 9:1 comp. Is it perfect? Probably not. But this baby makes power. I've also tried ceramic coating piston tops and it works when its fresh but as it gets covered up by carbon the effect diminishes. Plus the engine has messy combustion and a mirror polish everywhere seems to work the best. So, I did'nt do it in this engine, but I did teflon coat pistons and bearings, str8's have a lot of internal drag, it helps.
Ahhh...the head. What have I not done to the head? Morphidite thing. Bunch of plumbers cobbled this thing togather during break, I'm sure. That said, it can be made to work. Don't sweat that its siamesed, the inactive port acts as a plenum. Its all the square corners. The exhaust valve is ok. Intake needs to be bigger, I have a bolt in pt # for it. Gives 75% or so intake/exhaust ratio. I made my own valve guides, cut them on the top for PC teflon spring clamp seals, on the exhaust because the exhaust blows UP! the guide. Tried it in the intake, did'nt work so well, still got a lot of coke build up on back side. Flow loss! No good. So, I'm now trying umbrella type, just deflect it. Blunt conic on end of guide in port. 4 angle valve job, back cut valves, swirl polished. Highly polished exhaust. Drop the port floor about 100 thou, stoping about 1/2" carb side of the left/right corners at the end of port. About 7/8R on the corners, nicly rounded, and ramped up on the floor to giv a short side radius. Raise the roof about 100 thou all the way into the plenum, creating a nice rounded bowl shape at each end. Do not cut straight down to the seat, leave a rounded venturi. Thats the basics. Takes a few hours to do. However, this head will not work for spit with a stock intake/exhaust system. Factory duels, modified, at the minumn. I'm working on dual quads and stainless steel header, should really rock. I think this is enough for now. alleycat