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Cylinder Head Comparison Test - Volume Control
We Test The Effects Of Four Different Cylinder Heads On Two Small-Block Engines.
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?
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.