Ernest Mena at Westech was the lead technician on our RHS cylinder head inspection. On our 347 dyno mule, the heads made 460 hp out of the box while breathing through a typical 750-cfm Mighty Demon carburetor and dual-plane Performer RPM II Air Gap intake manifold. Exhaust was our usual set of Hooker Super Competition long-tubes with 18-inch collectors and no mufflers.
Horse Sense: New small-block cylinder heads just keep coming. We've lost count, but there are, or have been, around 40 aftermarket cylinder heads since the 5.0 H.O. engine made the scene.
A long time ago at racetracks far, far away, T-shirts used to say "Old Chevys never die, they just go faster." Pictured below was a wheelstanding '55 coupe with a straight axle and flames billowing out its open exhaust.
Later, the people at this magazine said the 5.0 Mustang was "the '55 Chevy of the '90s." It was a minor heresy at first, but it increasingly became gospel. That brings us to the next millennium, and the 5.0 parts keep coming. Thank goodness.
One of the latest parts is a series of cylinder heads from longtime head specialist, Racing Head Service. The company has decades of history in performance heads. Now a part of Comp Cams, RHS recently introduced a line of small-block Ford cylinder heads. These Pro Action castings are sourced from Pro Topline, a New Zealand company that RHS recently purchased. The key to that deal was retaining the personnel and connections with the Pro Topline foundry.
At the same time, RHS went through the Pro Topline castings, which gave them detailed improvements throughout to make them comparable with other current small-block Ford heads. RHS says the resulting Pro Action heads feature tight-tolerance phosphor bronze valve guides, thick cylinder decks (around 0.800-inch) for good milling possibilities, improved water jackets, and hard valve seats.
RHS heads are derived from Pro Topline castings, but they're improved for greater power and durability.
The new Ford heads are available in cast iron or aluminum, bare or assembled, with either a 58 or 64cc combustion chamber and a choice of 180, 200, or 225cc intake ports. If you can't find what you want on this menu, you have something different in mind.
Thanks to the low labor and foundry costs offshore, the RHS heads are competitively priced. Assembled with familiar Comp Cams valves, springs, retainers, rocker studs, and guide plates, jobber on the heads we tested is a friendly $1,280 a pair. In this case, the jobber price is a fairly accurate real-world street price. Your mileage may vary, as they say.
We were curious how these heads performed, so we picked the one we considered most popular with 5.0&SF readers-the 200cc aluminum head with 58cc chambers. We gave it a whirl on our Probe Street Fighter 347 dyno mule short-block using Westech's 902 SuperFlow dyno. As a yardstick, we also ran our regular set of Z304 cylinder heads from Ford Racing with 59cc chambers. To keep you from skipping to the end of the article, the RHS heads peaked at 460 hp when coupled to an immense Comp Cams hydraulic-roller cam, Edelbrock Performer RPM II Air Gap intake manifold, and a Mighty Demon 750-cfm carburetor.
Available in multiple configurations-including iron or aluminum-and runner or chamber sizes, the RHS heads span the needs of almost all 289- to 400-plus-inch small-block Ford engines.
What's eerie is the Z304 heads made 461 hp. Actually, the two heads were separated by an inconsequential, inside the margin of error of 0.4 hp. To save the dog-eating-homework explanations for the sidebars, after reviewing the situation at length, we concluded that mid-400 hp is what the RHS and Z304 both make. There wasn't any indication of restriction or inefficiencies in any of the data, and a close inspection shows how the RHS heads have a small but distinct advantage across most of the powerband. In other words, we don't believe that any of the other items on the engine constituted a choke.
If there was a choke on the top end, it would be the dual-plane intake manifold. But we ran the Performer RPM Air Gap against single planes on this same short-block and it came close to single-plane power. We're certain our test documents the relative potential of these cylinder heads, especially in a representative bolt-on capacity.
As you inspect the dyno sheets closely, a small but distinct difference emerges. The RHS heads peak at 6,300 rpm while the Z304s do their best at 6,700 rpm. That's the top rpm listed in our chart; we ran the heads higher to determine the peak was at 6,700 rpm, then didn't run any higher to avoid the valve float that steps in shortly thereafter. Furthermore, the RHS heads are ahead of the Z304s all the way up the tach, but don't peak as high.
We opted for 200cc intake runners on our test heads. This is too large for a stock-displacement, stock-cam 5.0 engine, but just right for our big-cam 347. Go with the 180cc intake runner when working with a stock-cam street 5.0. We noticed this RHS 200cc intake port opening is smaller than on the Z304 heads, and the RHS heads ran a bit better than the FRPP benchmark heads until ultrahigh rpm.
To back up a second and look at the forest instead of the trees, this is good power. Our 347 is really a 342 because it has a standard, not a 0.030-inch overbore, so at 461 hp it's making 1.34 hp/ci. That's the upper limit for bolt-on parts in our 20 years of dynoing small-block Fords. It's also 4 hp or so behind the same engine with a towering Funnel Web single-plane intake. These heads are doing OK. Put on a street-car exhaust, a water pump, and an alternator, and you'd still have 440 hp or so to play with.
With both cylinder heads making nearly identical power, the differences between them boil down to cost and convenience. The Z304 cylinder heads come bare and must be assembled locally, while the RHS heads can be sourced as complete units. In fact, other than disassembling one cylinder to check the valve springs, our heads were run straight out of the box.
Getting the Z304s running, as with any other bare head, required ordering valves, springs, retainers, and keepers separately, then putting them all together. This usually means a trip to a machine shop for the average enthusiast to check the valve spring installed heights, pressures, and so on. That's when you'll discover the Z304 uses valves 0.200-inch taller than other small Ford heads.
We're not saying the Z304 is a lousy cylinder head. We've been running our set for more than a year and they're hard-charging, durable parts. It's just that they're sold bare and have to be dressed before being used. That time, effort, and treasure must be accounted for. We had nearly $1,800 invested by the time they were ready to go.
The same applies to the other heads that produce the same amount of power as the RHS assemblies. By the time you get a better-than-average, ready-to-go assembled head, especially if it's CNC-ported, the tariff is going to be far north of $1,300.
In short, the RHS heads offer the convenience of an assembled head coupled with the good value associated with relatively low-cost and high-power production.
Click here for the Mustang cylinder head dyno chart