Originally Posted by dave1969chev
Hello, I'm having some serious issues getting my buildup to run right. I have a 383 4 bolt .040 over with flat tops. Sportsman 2s with 64 cc chambers, strip dominator(also have a air gap on the shelf) . I'm in high altitude so I'm running a avs thunder 650cfm. Lunati voodoo hyd flat tappet 268in/276ex 110lsa. Scorpion 1.5 in/1.6ex rollers. 1 7/8" sidemount headers with 4" pipes. Hei dist with a 7 amp module. 11mm livewires. Bm holeshot 2400 stall converter. Built th350 with shift kit. Independent 3 series Posi with 3.55 gears. I put in premium unleaded but I can't get it to run right. It ran strong with the 69cc chamber Sr torkers, decided to move up to 200cc intake runners. I'm running 36 deg total timing. What am I doing wrong?
Ports are too big for the altitude air pressure. Solution is:
1) Go back to the SR Torquers with the 170 cc port to get the intake port velocity up.
2) Stiffer gears to get the engine wound up to where it can get the velocity in the 200 cc ports moving again.
3) Put some form of a supercharge on it.
4) This is important your exhaust system is too big. Reduce the size of the exhaust system from the header tubes back so there will be a harder pull from the exhaust during the overlap. 1-5/8ths header tubes into 3 inch collector of at least 20 inches. 2-1/2 inch head pipes to mufflers 2 to 2-1/4qtr tail pipes. Connect the head pipes just behind the collector with a 2 to 2-1/4 inch H pipe cross over. Do not use an X pipe. It might be necessary to use terminator boxs behind the collectors. You'll have to build the system then see how it runs then if necessary trim it to get the return waves when the engine needs them the most.
The problems of high altitude all come from the lower air density. This drops the velocities in all the porting systems intake and exhaust. It lowers the true compression pressure regardless of compression ratio, part of the solution here is less cam duration with more lift and more ignition both in terms of advance and volt/amps, a multi-strike can be helpful. The smaller carb you have was a step in the right direction as that keeps the venturi vacuum up which improves metering. The 11 mm wires are a great idea as the dielctric (insulating) value of materials goes down with altitute so a thicker jacket is needed to keep high voltage in the wires. A coil pack per cylinder is a massive help as it keeps the high voltage runs short and eliminates high voltage from the distributor cap where it leaks all over the place between terminals.