Lift is valve lift in inches;...
Lift is valve lift in inches; flow numbers are in cubic feet per minute. Intakes were tested with a port plate and clay blend; exhausts were tested bare and without a flow tube.
After Westech put both cylinder heads on a Superflow flow bench, there was no surprise on the exhaust side-both heads flowed nearly identically into the headers. The intakes are also close, but not as much as the exhaust. Up to a 0.400-inch lift, the Z304 head has a discernible advantage. It's fairly well-stalled from 0.500 inch and up. The RHS head has a softer stall and isn't quite stagnant up to 0.600-inch lift.
Just from this flow data, we'd normally guess the Z304 might make a bit more power than the RHS, but the huge 0.608-inch lift Comp Cams cam we were running likely gave the RHS heads a chance to compensate for whatever they might have given up in the mid-lift. Differences in combustion chamber shape, compression, and other cylinder head factors are also at play, but these numbers show the major airflow trends.
| | Z304 | RHS |
| Lift | Intake | Exhaust | Intake | Exhaust |
| 0.050 | 33 | 24 | 32 | 22 |
| 0.100 | 61 | 50 | 65 | 50 |
| 0.200 | 132 | 95 | 118 | 96 |
| 0.300 | 187 | 131 | 167 | 130 |
| 0.400 | 234 | 160 | 216 | 157 |
| 0.500 | 252 | 174 | 254 | 175 |
| 0.600 | 259 | 180 | 271 | 181 |
| 0.700 | 262 | 180 | 277 | --- |
Here are the numbers on the Racing Head Service heads we ran on our 347:
| Model | Pro Action |
| Application | Street/Race |
| RPM range | 3,500-8,000 |
| Valve sizes | 2.020x1.600-in |
| Valve material | Stainless steel |
| Intake port vol. | 200cc |
| Chamber vol. | 58cc |
For typical street applications, these heads are available in a 180cc intake port volume. They're easily the best choice for stock-cammed small-blocks. At the other end of the spectrum, the 225cc intake port version is a maximum performance head for either the largest stroker Windsors (more than 400ci) or trailered race cars.
Here are some ballpark prices for the parts and work involved in dressing a bare small-block Ford cylinder head, such as the Z304.
Add $900 for a pair of the Z304 castings, plus guide plates, and you're looking at $1,750 by the time the Z304s or other bare heads make it onto your engine. For bolt-on engines, it's faster and more convenient to buy cylinder heads as complete assemblies. For a specialty application, it might be better to start with a bare head.
| Valves | $300 |
| Valve job | $200 |
| Valve springs | $140 |
| Retainers | $55 |
| Seals | $30 |
| Keepers | $20 |
| Studs | $55 |
| Subtotal | $800 |
Valve-to-piston clearance is typically not an issue with bolt-on cylinder heads, street compression ratios, and multi-eyebrowed aftermarket pistons these days. But with the monster hydraulic 0.608-inch lift Comp Cam in our 347 test mule, the pistons and valves can get less than uncomfortably close. A relatively gentle reminder of this came when viewing the Z304 heads while prepping for this test. The valves and pistons had gotten together in a previous test.
Luckily, the interference was close and didn't have the valve-bending, piston-cracking crashing of full-on piston-valve interference. As seen here, the lack of carbon on the intake eyebrows in the piston indicates their proximity; the distress on the exhaust valves is easier to spot. Carbon has been swirled off the valves and pistons by the rotation induced by the coiled springs.
Claying the piston-to-valve clearance is the most foolproof method of verification. In our case, we could eyeball the relative position of the valves to the edge of the combustion chambers when comparing the RHS heads to the Z304s. The former valves were notably deeper in the combustion chamber than the latter, so we knew they wouldn't hit.