Richard Holdener
June 1, 2013

One of the most common questions we get asked here at MM&FF is: "How much horsepower is (insert part) worth?" The part in question can range from a throttle body, to stroker assemblies, to any type of power adders. For this article, the parts in question are cylinder heads. And while that narrows the field somewhat, those looking for a concrete, black-and-white answer as to which heads are best or how much power they make might be disappointed to hear our answer—it depends.

Have we lost our backbone? Are we afraid to hurt the feelings of the advertisers? Or are we just too stupid to accurately answer the question? The debate rages on about the validity of those three statements (who are we to suppress a good conspiracy?), but the reality is that the power gains offered by almost any modification depend on the combination it is tested on, the person testing it, and the manner in which the test is conducted.

Of course, some engine configurations are more receptive than others to modded cylinder heads, but big ol' ports with big ol' cfm numbers won't necessarily show big gains unless you have the cam, compression (or power adder), intake, and exhaust to make the combination run efficiently.

If we add a set of aftermarket cylinder heads to an otherwise-stock 5.0L Mustang engine, just how much power will they add? The question is an excellent one because we have recently tested a stock combination and have seen as little as 10 extra horsepower. Does this mean the heads in question are only 10hp better than the stock heads? Not likely, since the heads in question offered flow gains exceeding 60 cfm over the stock heads, or enough to support an additional 120 hp (or more). The reason the heads didn't improve output of the otherwise-stock 5.0L was that the stock cam and intake manifold were restricting the potential of the heads in question.

On the other hand, we've also seen head swaps (like the final test in this story) that offer over 150 hp gains on the right application. Does this mean every head will add 150 hp? These two extreme examples help illustrate that extra head flow is only as good as the supporting cast of parts. Of course, you also need the right gearing and suspension setup to maximize your newly found power, but that's another story.

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306 Short-block

The first test involved a mild 5.0L 302 bored out to a 306. Equipped with the stock heads, the 306 produced 1 hp per cubic inch, posting peak numbers of 306 hp at 5,300 rpm and 342 lb-ft of torque at 4,000 rpm. Not bad for a set of stock iron heads that only flow 165 cfm.

The head swap involved a set of Windsor Jr. heads from World Products. Designed as an emission-legal replacement head for the 5.0L, the cast-iron Windsor Jr. heads featured 180cc intake ports, 64cc exhaust ports, and a 1.94/1.60 valve package. Flow testing indicated the World Product heads offered an additional 45 cfm over the stock heads.

After swapping the heads, peak power jumped to 351 hp and 354 lb-ft of torque. This represented a gain of 45 hp and 12 lb-ft measured peak to peak, but the gains increased higher in the rev range. Based on the results of this test, you might be tempted to think these Windsor Jr. heads are only good for 45 hp.

That's not an insignificant amount, but the flow data tells us that the mild 306 combination was actually holding back the heads and not the other way around. Tested on something with wilder cam timing (especially something approaching or exceeding 0.600 lift), the combination would make full use of the available airflow.

13. Run first with the stock iron heads, the 331 produced 352 hp at 5,300 rpm and 400 lb-ft of torque at 3,800 rpm. Installation of the Edelbrock E-CNC 185 heads pushed power to 449 hp at 6,100 rpm and 444 hp at 4,500 rpm. Since the Edelbrock heads will support 600 hp, there is plenty of power left.

331 Stroker

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.

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392 Stroker

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.

17. Run on the dyno first with stock heads, the 393 produced 387 hp at 5,000 rpm and 461 lb-ft at 3,500 rpm. The stock heads were really limiting this big stroker motor. After installation of the AFR 205s, peak power jumped to 555 hp at 6,600 rpm and 515 lb-ft at 4,900 rpm.

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.