So if our heads are done flowing at 0.600-inch valve lift, it doesn't make sense to try and stuff a cam in that has 0.750-inch lift. Sure, you can go a little over the top, say 0.050-inch, but don't try to run too much more valve lift than your heads can flow. Also, pay close attention to the low-lift flow figures, because that's where your engine spends twice as much time as it does at max lift. A head that flows good low-lift figures will respond well, especially with a good dualplane intake and a smaller carb. Then look at the exhaust flow figures and compare them to the intake flow figures. You'll want a head that can flow a good ratio of exhaust to intake; somewhere in the 65-80 percent range is a good figure to shoot for. Anything more won't hurt, but anything less will.
Ways To Improve Performance Without Porting
While new heads are cool, there are other ways to improve power using the heads you already have. A while ago, Joe and I tested a rather mundane small-block iron headthat'd spent more than one day on the flow bench and Serdi valve machine being modified and tweaked in steps to measure which mods worked and which didn't. We did things like a three-angle valve job, back-cut the valves, port and polish the bowls, grind and polish the ports, and finally add new high-flow valves. What we found was that just by performing a threeangle valve job along with adding a 75-degree throat cut to smooth the transition from the end of the port to the valve seat, the head picked up as much as 6 cfm at .500 lift.
When you swap rocker arms to a higher ratio, you'll typically increase airflow through the head due to the additional valve lift and duration the higher ratio creates. But make sure you're not increasing lift to the point of pushing it past the head's optimum performance level, i.e., opening the valve past .600-lift when the head only works to .500-lift.
Comp Cams supplied the chart on this page that shows how changing from the stock small-block Chevy ratio of 1.52:1 (Comp says it's actually around 1.46:1) to a true 1.60:1 will increase your total valve duration by almost 20 degrees. This geometric increase is true for any cam in any engine.
Factory vs. Aftermarket heads
Earlier I alluded to a big-block example of hand-porting's downfalls compared to CNC porting. But there's not always a CNC-ported option for every head. Hand-porting, when done properly by an expert, is still a great way to improve a stock cylinder head's performance. I mention stock heads here because I don't recommend hand-porting a set of aftermarket CNC'd heads unless you can't afford a new set. The cost of porting a set of older heads versus the cost of a new set of CNC heads might work out to be about the same amount. Sometimes it'll cost you more to port the heads than buy new ones, so check all your options before plopping your money down.
You'll often overhear gearheads talking valve angles, as in, "I'm gonna buy a new set of 18-degree Chevy heads." It seems that the aftermarket is changing valve angles faster than some of the related parts can keep up, and many are finding out too late that the new splayed-valve heads they just nabbed are going to cost a lot more to run than they thought.