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Cylinder Head Comparison Test - Volume Control
We Test The Effects Of Four Different Cylinder Heads On Two Small-Block Engines.
We used the same set of 1.6 ratio roller rockers from Comp Cams. In addition, all heads were run at the same air/fuel and timing, though we did perform timing sweeps to see if any of the head configurations responded to different timing levels. To maintain consistent compression during testing, all four of the test heads were ordered with the same 62cc combustion chambers. Basically every effort was made to isolate the port volume and attending flow as the only variables.
The cylinder head test was run on the 363 stroker in descending order, starting with the largest 225 CNC Pro 1 heads. The Pro 1 CNC heads offered impressive statistics, including 225cc intake ports, 87cc exhaust ports and a 2.08/1.60 valve combination. Peak flow numbers checked in at 325 cfm for intake and 235 cfm for exhaust.
Equipped with the 225 Pro 1 CNC heads, the 363 produced 568 hp at 6,700 rpm and 489 lb-ft of torque at 5,300 rpm. Torque production exceeded 450 lb-ft from 4,200 rpm to 6,600 rpm. For this testing, we loaded the motor all the way down to 2,800 rpm to ensure we could register any loss in low-speed power with the larger heads. The air/fuel ratio was adjusted using conventional jetting on the 950 hp. To find the ideal timing value, we tested the motor with total timing from 30 to 36 degrees. The 363 produced best power at 35 degrees, while the air/fuel curve was optimized at 13.0:1.
Next up were the slightly smaller 210cc Pro 1 CNC aluminum heads. Having performed a few of these in the past, the head swap went like clockwork. The Pro 1 CNC heads featured 210cc intake ports, but slightly smaller (76cc) exhaust port volume. Also reduced on the 210 heads was the intake valve sizing, down from 2.08 inches on the 225 heads to 2.05 inches. All the heads shared the same 1.60-inch exhaust valve diameter. As expected, peak flow numbers for the 210s were down slightly compared to the 225 heads, checking in at 305 cfm for intake and 208 for exhaust (both measured at 0.600 valve lift).
Using the same procedure, we ran timing sweeps and were eventually rewarded with 566 hp at 6,800 rpm and 484 lb-ft of torque at 5,200 rpm. From 4,200 rpm to 6,800 rpm, the larger 225 heads offered slightly more power (as much as 8-9 hp), but the curves were identical above and below those engine speeds. Like the larger 225 heads, the 363 pumped out over 450 lb-ft of torque from 4,200 rpm to 6,500 rpm. Either Dart Pro 1 CNC head would be an excellent choice for the 363 stroker, but the larger 225 did offer slightly more power with no loss in low-rpm torque.
After running the 210 heads, it was time for the first pair of as-cast heads. As with the other Pro 1 heads, the 195 Pro 1 heads got their name from the 190cc intake port volume. In addition to the intake port volume, the 195 head stepped down in exhaust port volume (from 76cc to 65cc) and intake valve diameter (from 2.05 inches to 2.02 inches). As expected, the peak flow numbers dropped to 288 cfm and 185 cfm for intake and exhaust respectively. Unfortunately for us, the 195 heads were shipped with a 1.437-inch-diameter valve spring package designed to work with a hydraulic-roller cam.
We quickly swapped over the larger 1.550-inch springs from the CNC Pro 1 heads to accommodate our Cam Research roller cam. Though the airflow data suggested these 195 heads would support nearly 600 hp, the peak power numbers dropped on our 363 test motor to 528 hp at 6,500 rpm and 482 lb-ft of torque at 5,200 rpm. Note that peak torque occurred at the same engine speed as the larger heads, but peak power dropped by 200-300 rpm (to say nothing of the loss of roughly 40 hp). The smaller 195 heads did manage to offer more power from 3,700 to 5,200 rpm, but we suspect the smaller 13/4-inch headers had something to do with the shift in the power curve.
The final set of heads was definitely out of place on this high-horsepower application, but they performed surprisingly well. The 170cc, as-cast heads combined the smallest intake port volume (at 170 cc) with the same (as the 195 heads) 65cc exhaust ports and a 1.94/1.60 valve combination. As expected, having the smallest intake port volume combined with the smallest intake valve size produced the lowest flow numbers of this impressive bunch. The 170cc heads offered 261 cfm on intake and 172 cfm on exhaust; demonstrating their ability to exceed 500 hp by producing peak numbers of 510 hp at 6.500 rpm and 472 lb-ft of torque at 5,100 rpm. Torque production exceeded 450 lb-ft from 4,100 rpm to 5,700 rpm.
Interestingly enough, the 170cc heads offered less power than the 195 heads, even down at 2,800 rpm. The two produced equal power from 3,200 rpm to 3,600 rpm, but the larger 195 heads pulled strongly away thereafter. On this 363 test motor, the larger heads demonstrated a clear advantage, though the difference between the 210 and 225 heads was 8-9 hp.