Cylinder Head Flow

We found some interesting tidbits when we dug into testing the flow of our new RHS cylinder heads. Only a few issues back (see "The Ironman," July 2007), we used a set of Summit iron cylinder heads, and were more than impressed with the numbers recorded when the machine shop sent them out for testing. What surprised us was that testing the RHS cylinder heads at Westech on its FlowCom-equipped SuperFlow bench showed nearly the same flow. However, we've preached the importance of not taking numbers from two different benches when making comparisons, and were curious enough to re-test the iron heads on the same bench to get a valid comparison. It seems the Westech flowbench is definitely less generous than the bench used to originally test the Summit heads. Testing back-to-back on the same bench gave us a valid set of numbers for the comparison.

RHS 320CC ALUMINUM HEADS
VALVE INTAKE INTAKE EXHAUST
LIFT cfm(poor) cfm(good) cfm
.050 37 38.6 29.3
.100 72.2 73.6 60.5
.200 131 129 114
.300 191 186 156
.400 248 238 193
.500 296 286 224
.600 320 330 252
.700 328 356 276

Checking P/V Clearance

One check that should not be neglected is the piston-to-valve clearance. Coming up short here can be disastrous, though our assembly was well within Outlaw's safe specifications. The proper procedure is to place a few dabs of clay on the piston valve reliefs, then coat them lightly with oil. Install the cylinder head temporarily on the block, install the valvetrain on that cylinder (using light "checker" springs), then rotate the engine over by hand. The valves will leave witness marks in the clay, which can then be carefully bisected, and the clearance measured. The p/v clearance measured .178 inch with the Thumpr cam, which is more than enough. You should look for a bare minimum of .100 inch, with .120 being preferable as a minimum.

SUMMIT/DART IRON HEADS
VALVE INTAKE INTAKE EXHAUST
LIFT cfm(poor) cfm(good) cfm
.050 35.5 32.4 31.5
.100 72 69 66
.200 144 143 103
.300 196 197 152
.400 247 249 188
.500 286 292 217
.600 307 320 240
.700 307 326 256

The Howitzer Vs. The Ironman

What's The Diff?Sharp eyes will notice there's a lot of commonality between The Howitzer and The Ironman, a 468-inch big-block we put together in the July issue. Both engines are one and the same. Sort of. When we put the iron-headed 468 on the dyno and made 579 hp, we decided to go back for a little more power and a little less weight before dropping it into the Chevelle. The primary differences: Swapping the iron Summit 308cc heads for the RHS 320cc aluminum heads, boosting the displacement from 468 inches (basically a stock .060-over 454) to 496 inches using a longer 4.25-inch stroke, and-surprise-stepping back from the brink in the cam department, forsaking a radical-for-the-street .647-inch lift solid roller cam for a street-friendly .570-lift hydraulic roller (COMP's new Big Mutha Thumpr). Another smaller change was a slight increase in carb size from an annular booster 825 Mighty Demon to a downleg booster 850.

The Ironman 468 made 579 hp at 6,200 rpm and 545 lb-ft of twist at 4,200 rpm, and had a build-it-yourself price of $6,633.09. We were initially concerned that the stroked 496 might blow the budget with a more expensive set of aluminum heads, but the calculator only registers another $890.28 over the Ironman's price, and produced 47 more hp and another 90 lb-ft of torque (coming in 400 rpm earlier). We like that trade-especially with the weight savings-and we're glad we made it. Andy Mitchell of Outlaw Racing says he can build The Howitzer for you just like this for about $1,500 over the do-it-yourself price we're listing, and interested parties should call him at 909-931-4612.