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Porting Engine Heads - Head Porting: Is It Really Worth It?

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Based on the previous CA discussion, it's no surprise that-whether with small or big valves-port-matching provides little airflow benefit. As Hupp puts it, "A chain is only as strong as its weakest link, and a port will flow only as much as its smallest part." With the CA in the bowl still smaller than the port opening or exit, opening up the intake face and exhaust exit doesn't help. However, match-porting can be beneficial when the throat CA is ported-out larger than the intake entry or exhaust exit sizes.

Bowl Work
Hupp next ported the bowl and short-side-without port-matching-and once again compared both the large and small valve sizes after porting. "Every experienced heat porter knows how to make a head flow at low lifts or at high lifts," Hupp says. "One takes from the other." He chose to port this head for high lifts, "where most street race guys run, anyway."

Hupp believes that "for every valve size, there is one and only one perfectly designed port." The optimum port has "all the top-secret curves and angles, and it blends. It does all the fancy high-velocity, highflow fuel atomization and special angle of attack stuff. But ultimately it is directly related to the size of the valve. When you have got this perfect, the flow will jump in size from lift to lift, and when it doesn't, there is a restriction somewhere else."

Achieving an optimum runner shape mandates considerable metal removal from the bowl, the CA, and the short-side radius. For example, the stock 1.580-inch intake CA diameter was moved out to 1.710 with the 1.94-inch valve; the 2.05 valve required removing an additional 0.110-inch of metal (for a final 1.820-inch diameter) to maintain the "perfect" port.

After the bowl work, the 1.94 intake port flowed slightly better at low lifts but really came alive above 0.350-inch lift. It achieved its peak 225-cfm flow number at 0.650-inch lift. The big 2.05 valve's intake port was still down compared to the 1.94 valve below 0.350-inch lift, equal to the 1.94 at 0.350-inch lift, slightly better between 0.450 and 0.460, and about equal from there on up. The 2.05's average flow numbers-at low lift, at high lift, and overall-are marginally superior to those of the 1.94.

The exhaust side is a different story. There, the 1.50-valve runner demonstrated over a 30-percent max flow improvement compared to stock, while the 1.60 port was up over 45 percent! A peak 180.4 cfm blasted out of the 1.94 port at 0.700 lift, while the 1.60 port hit 200 cfm at 0.650 and 0.700.

Yet, the big intake and exhaust valve ports become saturated at 0.650-inch lift. On the intake side, that's because with the big valve there is now a new critical area-namely, the runner's pushrod bulge. That's also why the 2.05 valve shows only marginal gains over the 1.94, even with bowl work. This is an example where port-matching the intake face would be beneficial.

The exhaust side doesn't have a pushrod bulge, but for this test the port exit was not enlarged, either. Nevertheless, further improving the exhaust flow without corresponding improvements to the intake flow is not recommended because it would be detrimental to optimum intake/exhaust flow balance (see sidebar).

Fully Ported
Finally, Hupp completely ported both the big- and small-valve runners. "Full porting" included the good valve job, intake entry and exhaust exit port-matching, bowl and short-side porting, and complete runner shaping and polishing. On the intake side, this resulted in peak flow numbers of 249 and 263.6 cfm at 0.700-inch lift for the 1.94 and 2.05 ports, respectively. Of course, the really big gains occur over 0.350-inch lift, but both valves are up throughout the entire tested rpm range. Flow peak to flow peak, we're looking at a 19.7 percent increase compared to stock for the 1.94 port, and a whopping 26.7 percent increase for the 2.05 valve. The port saturation problem has been eliminated at least through 0.700-inch lift, the highest lift point tested.

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