Muscle Mustangs & Fast Fords
Cylinder Head Comparison Test - Volume Control
We Test The Effects Of Four Different Cylinder Heads On Two Small-Block Engines.
Test motor 2 might be labeled a typical performance street 5.0L build-up. Like the 363, the 302 was run in carbureted trim and had a forged steel crank from RPM, 5.40-inch forged connecting rods from ProComp, and forged flat-top pistons from Probe Racing. The engine was machined by L&R Automotive and assembled by Demon Engines. At the intended engine speeds and power levels, the factory components would suffice, but the combination was built with the ability to withstand plenty of boost (at least to the limit of the factory 5.0L block) from a previous test.
The forged 5.0L short-block was augmented with a Comp XE274HR hydraulic roller cam. The XE274HR cam featured a 0.555/0.565 lift split, a 224/232 duration split and a 112-degree lobe separation angle. In keeping with our mild (but powerful) street application, we topped the 302 with an air-gap-style, dual-plane Qualifier intake manifold (from Pro Comp) and Holley 750hp carburetor. Also present was 13/4-inch Hooker Super Comp headers, a ProComp billet distributor, wires and valve covers, and a Meziere electric water pump. Having already run this motor, we filled the stock oil pan with Lucas 5W-30 synthetic oil and we were off.
Just as with the 363, we ran the cylinder head test on the 302 in descending order. Equipped with the Dart 225 Pro 1 heads, the 302 produced peak numbers of 399 hp at 6,200 rpm and 370 lb-ft of torque at 4,100 rpm. Torque production exceeded 350 lb-ft from 3,300 to 5,700 rpm.
For this testing, we loaded the motor all the way down to 2,700 rpm to ensure we could register any loss in low-rpm power with the larger heads. Dialing in the Holley HP carburetor was easy using the Percy's Adjust-a-Jet system. As with the 363, we tested the motor with total timing values ranging from 30 to 36 degrees. The 302 produced best power at 35 degrees of total timing, while the air/fuel curve was optimized at 13.0:1. Though seemingly oversized for the application (since the 225 Pro 1 heads will support over 650 hp), the 302 managed to produce 315 lb-ft of torque, even at 2,700 rpm.
Next up were the slightly smaller 210 Pro 1 CNC heads. Could the 400hp 302 take advantage of cylinder heads flowing over 300 cfm? Using the same procedure, we dialed in the timing curve with the 210 heads and were eventually rewarded with the very same 399 hp at 6,200 rpm and 370 lb-ft of torque at 4,100 rpm produced with the larger 225 heads. The 210 heads did manage to offer slightly more torque below 3,800 rpm (a difference of 5 lb-ft), but beyond that, the curves were identical. Torque production exceeded 350 lb-ft from 3,300 rpm to 5,800 rpm.
It must be pointed out that both the 225 and 210 heads were run with larger 17/8-inch step headers used to fit the wider 3-inch exhaust bolt pattern, while the smaller 195 and 170 cc heads were run with Hooker 13/4-inch Super Comp headers. The headers themselves are partly responsible for changes in the power curve lower in the rev range, but the difference between the 210 and 225 heads was strictly port-volume and flow dependant.
After running the 210 heads, they made way for the first of our pair of as-cast heads. The as-cast, 20-degree heads featured 195cc intake ports combined with 65cc exhaust ports. Though the 195 heads lacked CNC-porting, the peak flow numbers were impressive. Given the peak flow numbers of 288 cfm, even the 195 heads were capable of supporting much more power than our 302 could dish out. Equipped with the smaller 195 as-cast, Dart heads, the 302 produced 390 hp at 6,100 rpm and 373 lb-ft at 4,300 rpm.