331 cam

[bobcowan]

Because you don't have a common plenum to draw from, air is really only moving through the tract when the intake valve is open. So the air speeds up and slows down all the time. You're almost building 8 single cylinder engines - as far as the intake goes. That's why overlap is so important. If the intake and exhaust valves are open at the same time for too long, you'll get reversion of the intake charge. That famous "Fuel Cloud" you've heard about.

My throttle bodies are right at 2.019". Just about the same size as the intake valve. As you can see from the torque curve, that works pretty well. If I had the time and funds, I would like to try a 2.05 intake valve, and maybe a 200 or 205cc intake port. From some of the reading I have done, a slightly larger valve will work, but too large will not.

Intake charge velocity is not independent of displacement. Imagine if you were trying to inhale a lung full of air through a garden hose vs. a soda straw. For a big engine you need bigger ports and valves. For a smaller engine, you need smaller ports and valves. I don't have the experience to tell you what will work best for a 289 or 302. But if you want to maintain flow velocity though out the tract, don't put in valves that are significantly larger than your carbs.

Carbs of any kind are very sensitive to flow velocity. If the carbs and ports are too big, you'll get a poor vacuum signal and poor metering off idle and into the mid range. Also, if the velocity is too low, the fuel will fall out of suspension before it hits the chamber. That's why big engines with big carbs built for high rpm's often idle at 1200-1500 rpm's. They are not capable of running at 800rpm's.

For my engine, I suspect my throttle bodies are limiting my power production. If I had a 55 or 58mm TB, then I could use a 2.08 valve and 225cc intake port. I suspect that would increase the air flow in the mid and upper range = more power. If I had carbs, intake tracts that big would cost me a lot of bottom end power and drivability. But that's not as big of a problem with EFI.