I see I did it again. The 48s had moderate porting but stock valve sizes. Sorry for not catching that earlier. A set of '67 326 2 bbl heads flowed 180 cfm (intake) on my flow bench, same as the OHC six heads. My fully ported OHC intake ports flow 230 cfm at .450" lift with a 1.94" valve. Maybe that will help the comparison by providing some baselines, Tom.

I flow at 28" of water corrected to STP, which is now the industry standard but wasn't always. The biggest difference is usually because of a lower test pressure on the flow bench. Some were tested as low as 10". Some newer flow benchs still test at low numbers but convert to the 28" standard. I question how accurate those numbers really are but I don't have access to one to check.

Again, numbers can be most reliably checked on the same flow bench even with corrected numbers. I know my flow bench tracks quite well with the SF-600 numbers on the intake but is conservative on the exhaust, up to 17% lower at certain points.

To correct old readings to 28" from say 20", divide 28 by 20 (or whatever the old test pressure was), take the square root and multiply the cfm reading by that number.

Vikki, in a perfect world where the valve opens to the full lift number instantly and the air fills the cylinder just as fast, we wouldn't need better ports. Unfortunately the valve reaches peak lift for only a small part of the lift cycle and the air has inertia. That reduces the amount of air that actually gets through the engine using good stock heads to about 80% of what is possible at the torque peak and declines from there as the rpm builds. More flow means more go because in this imperfect world that gives the engine a better shot of air and allows more airflow at higher rpm, which is where hp is made. The right engine combination and proper intake and exhaust tuning can take the volumetric efficiency (how full the cylinder actually was) over 100%, effectively supercharging the engine. And 8000 cylinder fills per minute equals 16,000 rpm! Would that it were true!

Mid-range flow is very important! That's another thing to examine when checking flow numbers, not just the big numbers at .600" or .700" lift. I have software that shows the average cfm over the lift range as well as the individual numbers so I can see what is happening. I'll take a port that flows great up to .400" lift and then flatlines over one that only excels at high lift.