Like our 306, the 331 featured a production 5.0L block equipped with forged internals from Probe Racing (flat-top). We stepped up th cam timing for the larger 331 with another grind from Crane Cams. The hydraulic-roller cam offered a 0.542/0.563 lift split, a 224/232-degree duration split, and 112-degree LSA. Run once again with an Edelbrock Performer RPM Air Gap and 750 carb, the combination produced 352 hp at 5,300 rpm and 400 lb-ft of torque at 3,800 rpm with the stock E7TE 5.0L heads.
The head swap for the 331 came courtesy of Edelbrock in the form of its aluminum E-CNC 185 heads. Equipped with a 2.02/1.57 valve package, the Edelbrock heads offered an additional 135 cfm over the stock heads. The head swap on the 331 increased the power output from 352 hp and 400 lb-ft to 449 hp (at 6,100 rpm) and 444 lb-ft of torque (at 4,500 rpm). This represented a jump in peak power of 97 hp and 44 lb-ft over the stock heads, but (as with the 306) gains were even more significant higher in the rev range.
Extra head flow is only as good as the supporting cast of parts.
14. The final test involved a 393 stroker that featured a full forged rotating assembly fr
15. Our head choice for the 393 stroker was a set of CNC-ported 205s from Airflow Research
16. Recognizing the airflow and rpm needs of the stroker, we installed a single-plane Edel
The final test to illustrate the gains offered by swapping heads was performed on a 392 stroker. Scat once again came through with a rotating assembly that included a 4340 forged crank, rods, and JE flat-top pistons. After boring and stroking, the 351W was successfully punched out to 393 cubic inches.
We stepped up to a solid-roller cam on this stroker application. Crane supplied a healthy solid-roller profile that offered a 0.579/0.598-lift split, a 246/254-degree duration split, and a tighter 110-degree LSA. Crane also supplied the necessary roller lifters, timing chain, and Gold roller rockers.
Given the high-lift roller cam, the stock heads required a few modifications. The stock spring seats were cut to accept larger- diameter springs to accommodate the required spring rates. The stock heads were also upgraded with rocker studs and guideplates. Knowing the cam timing was designed for power production higher in the rev range, we equipped the 393 with an Edelbrock Victor Jr. intake. The combination required a larger 950 carb, while we retained the 13⁄4-inch Hooker headers.
It should be obvious that this combination was never designed for stock E7TE heads; power suffered greatly because of this. Equipped with the stock heads, the stroker produced 387 hp at just 5,000 rpm and 461 lb-ft of torque at 3,500 rpm. After swapping 205 heads from AFR, the peak numbers skyrocketed to 555 hp at 6,600 rpm and 515 lb-ft of torque at 4,900 rpm.
Credit the full CNC porting, impressive chamber design and big 2.08/1.60 valve package if you will (and you should-they are excellent heads), but the real reason for such insane power gains was the combination. Basically, the engine had everything it needed (intake, cam, compression, carburetion, and displacement) and was lacking only head flow. As soon as you provided that in the form of the AFR 205s, the motor responded with a huge power gain.
The question now? Is a head swap worth the 45 hp offered on the 306, the 97 hp offered on the 331, or the 168 hp offered on the 392? The answer is all of the above or as we said before, the power offered by a head swap depends on the combination.