Richard Holdener
December 22, 2010

One of the big three, cylinder heads are a critical ingredient in any performance build. Working with the cam profile and intake manifold, the cylinder heads help determine not only the peak power numbers, but the overall shape of the horsepower curve. They are so important that installation of the right set of aftermarket 5.0L heads on a given combination can be worth 90-100 hp or more.

Are the stock heads really that bad? No. In Ford's defense, the factory 5.0L (E7TE) heads were never designed to support 400 hp (or more), but it's amazing what the proper heads can do to wake up a 5.0L or stroker combination. In the past, we've tested nearly 30 different sets of cylinder heads on different test motors based primarily on their port volume and market competition. Basically, the bigger heads were run on bigger and more powerful engine combinations. This testing left some important questions. How would the smaller heads fare on the bigger motors, and vice versa? Are the larger heads really out of place on a milder combination, and just how much does a small (but efficient) port hurt a wilder combination?

Answering such questions meant dusting off the old airflow bench and engine dyno. In order to test the effect of port volume, we decided to run a series of tests on four different sets of cylinder heads. Supplied by Dart, the four Pro 1 theads offered intake port volumes ranging from 170 cc to 225 cc. Then we'd run the four different heads on two different test motors to illustrate what happens if you run cylinder heads that are either too big or too small on a given application.

We suspected that the larger CNC 225 heads would offer more power than the smaller as-cast 170cc heads on our wilder 363 stroker, but we were curious to see how well the only slightly smaller 210cc heads would fare. The same can be said for the as-cast 190cc heads, as they flowed enough to support the intended power output of the 363 stroker, but would they offer the same power as the larger CNC versions? How would the milder 302 combination respond to the four different head configurations? Would the additional airflow offered by the CNC heads add power, or would we see a loss in low-rpm torque from the change in port volume?

Wild 363
Though the test could certainly be run on a larger 408 or 427 stroker combination (the Dart Pro 1 CNC heads were plenty capable of supporting these displacements and/or power levels), the 302-based 363ci stroker was plenty powerful. It was essentially a bored and stroked 302, but not one that featured the self-destructing production block.

Our 363 SHP came from directly from Dart and featured a Dart four-bolt block combined with a forged crank, rods, and pistons. The SHP short-block combined a 4.125-inch bore with a 3.40-inch stroke to achieve the 363ci displacement. The big bore improved breathing potential of the combination, allowing our stroker to take full advantage of the free-breathing CNC-ported Dart Pro 1 heads. The 11.0:1 short-block was stuffed with a solid-roller cam from Cam Research Corp, featuring 0.692 lift (with the 1.6 rockers) combined with a 256/260-degree duration split (at 0.050) with a tight 105-degree lobe separation angle.

The Cam Research Corp cam was combined with Comp Cams solid-roller lifters, a double-roller timing chain, and chrome-moly pushrods. All the heads were secured using Fel Pro MLS head gaskets and 1/2-inch ARP head studs. The testing was run with a Parker Funnel Web intake and 950hp Holley carburetor. Thanks to the Carb Shop for the loan of the dyno headers to fit the wider Dart exhaust bolt pattern used on the Pro 1 210 and 225 CNC heads. Oddly enough, the 170cc heads featured both exhaust bolt patterns.

Prior to running the different Dart Pro 1 heads, we took the liberty of checking the pushrod length and made certain to have adequate spring pressure for both combinations. As we planned on running a solid-roller cam in the wilder 363, we configured our test heads with a spring package suitable for the roller application. The 195 pounds of seat pressure might be a tad on the high side for a hydraulic roller application, but this much pressure eliminated any trace of valve float. As it turned out, all heads required the same pushrods on the hydraulic roller 302. The solid-roller cam required slightly longer pushrods.

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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.

Mild 302
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.

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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).

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Horsepower Numbers

Dart Head Test 363
RPM170195210225
3,000214217221220
3,300251252254253
3,500276276279279
3,800315319317316
4,000340346340340
4,300375381369372
4,500393403384391
4,800421433423428
5,000446456454458
5,300474485487494
5,500483496503508
5,800493505524527
6,000502514533539
6,300509525549557
6,500510528559565
6,800506525566567
7,000497518561562

Dart Head Test 302
RPM195210225
2,800159171168
3,000186189187
3,300217222221
3,500237241239
3,800267266266
4,000283281281
4,300305301301
4,500318311311
4,800336325328
5,000346341340
5,300366361361
5,500374372371
5,800383387385
6,000389395395
6,200386399399