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Cylinder Head Comparison Test - Volume Control
We Test The Effects Of Four Different Cylinder Heads On Two Small-Block Engines.
Not surprisingly, the smaller heads offered less peak power (390 hp versus 399 hp), but slightly more torque and at a slightly higher engine speed (4,300 rpm versus 4,100 rpm). Oddly enough, the smaller heads offered less power than the larger heads down low (up to 3,700 rpm), then the 195 heads offered more torque through the mid-range through 5,600 rpm. The smaller head then fell below the larger CNC-ported heads up to 6,300 rpm. We suspect the headers played a part in the respective power curves, but without a set of 17/8-inch headers to fit the larger heads, it is difficult to speculate how much.
The final set of 170cc as-cast heads were actually never run on the 302, as catastrophic failure occurred before we could finish the test. While swapping the heads, we developed an intake leak. The author is to blame for not ensuring the intake gaskets were sealed for the final installation. Water leaked into the adjacent intake port, which unfortunately had an open valve prior to start up. Having water fill the cylinder resulted in a hyrdo-lock situation that actually snapped a connecting rod after start up!
Though the motor ran, an inspection revealed that the piston in No. 1 cylinder had left the party. Thus, we are left to speculate even further about the 170cc head results on the 302, but we know that it would produce less power than the 190cc heads despite the fact that they offered more than enough flow to support our 400hp small-block. It would have been interesting to see if it improved torque production down low compared to the 195cc heads, but judging by the results of the 363 test, it seems unlikely.
Even without running the 170cc heads on the 302, the results are certainly helpful. They illustrate that the relationship between port volume, airflow, and power is an important one. Looking at the results of the testing on the 363 stroker, we see each step up in cylinder-head flow and port-volume increased the power output. Stepping from the 170cc heads to the 190cc heads improved output by 18 hp and 10 lb-ft of torque, while the jump to the 210cc heads offered another 38 hp and 2 lb-ft of torque. As expected from cylinder heads in this power range, the gains were most prevalent at the top of the rev range, but the 190cc heads improved the power from as low as 3,600 rpm over the smaller 170cc heads.
On the 363, basically more flow equaled more power, despite the increase in port volume. Stepping up to the largest 225cc heads improved the peak power numbers by just 2 hp and 5 lb-ft of torque over the 210 heads. The reason is that the 210 heads offered more than enough flow to support the power needs of the 363, but the increased flow offered by the larger 225 heads did improve the power output by 8-9 hp and lost no power to the smaller 210 heads anywhere in the curve. On the right application, bigger really is better.
Looking at the 302, we see a similar pattern, as the 195cc heads produced 9 hp less than the larger 210cc heads. It is important to look at the entire curve, as the 195cc heads offered more power from 3,800 rpm to 5,600 rpm, but lost out to the larger heads above and below that rpm spread. Some of the improved mid-range might be attributed to the 13/4-inch headers being a better match for the mild 302 application than the larger 17/8-inch headers run on the larger CNC-ported heads, but the 195-cc head is an excellent choice for this mild application.
This was reinforced when we installed the largest 225cc heads from Dart. Though the 225 heads certainly flowed more than the 210 heads, they offered no power gains. Minor differences of 2-3 hp below 3,700 rpm were registered, but oddly enough it was the larger 225 heads that offered the small power gains lower in the rev range. It should be pointed out that the 225 head lost no power down low compared to the 210 (or 195) heads, though the results would be different at part throttle. The larger ports would certainly be a tad more sluggish on the milder application at part throttle. Had we run the smaller 170 cc heads, we suspect that they would have offered less power than the 195 heads, but the difference would not have been as dramatic as the test on the wilder 363 (probably 10-12 hp at most with a small but consistent loss registered through the second half of the power curve-from 4,000 rpm on up).