642 HP Budget Iron Head Build - Iron-Headed Rat

You can make a lot of horsepower with good iron heads, as we prove on this big-block.

Richard Holdener Jun 11, 2014 0 Comment(s)
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Old-timers must look around at the proliferation of affordable and power-producing aluminum cylinder heads and think to themselves, “These young guys have it easy.” Back in the day, aluminum big-block Chevy heads sat on the shelf right next to the supply of hen’s teeth. Yeah, they were that rare—and even more expensive.

It seems like every big-block buildup in the last 20 years has been blessed with aluminum heads. Despite this, we should remember that with the exception of very few, Rat motors came from the factory with iron heads, and those same iron heads won thousands and thousands of races. On the plus side, for the budget conscious, there are still great iron cylinder head castings available from the aftermarket—all the latest technology in port design applied to fancy aftermarket aluminum heads has also been applied to their cast-iron counterparts. A cost analysis of iron versus aluminum made us wonder: Can you build affordable big-block power using iron head castings? To find out, we made a casting call.

The first order of business was to determine the parameters, or more specifically exactly what we meant by big-block power. Way back when, it was impressive to have a factory powerplant produce 1 hp per cubic inch. Chevy offered both small- and big-block versions that eclipsed this magical specific output, but time and technology have marched on. For this build, we decided on a nice round figure of 600 hp.

Iron Head Big Block 2/14

Do Chevy enthusiasts still build big-blocks with iron heads? Heck yes, and we’re going to prove they can make big power on a budget.

To help us reach this goal with our iron heads, we decided to further enhance the displacement. The three-fold benefit of displacement is that it adds little if any additional engine weight (the enemy of performance), more displacement makes it easier to reach any given power goal, and said combination can run in a milder state of tune (meaning increased driveability compared to a smaller engine at the same power output).

With our power goal in mind, we set out to get the necessary components. The first order of business was to get our hands on an affordable set of big-block castings. That casting call went out to Summit Racing, which supplied PN Sum 152125. These as-cast iron heads featured 308cc intake runners, 121cc combustion chambers, and a 2.25/1.88 valve package. According to our flow bench, these iron heads flowed 340 cfm, or enough to support more than 680 hp (easily exceeding our goal of 600 hp). At $789.50 at summitracing.com, they’re quite the bargain. You could easily triple that at a machine shop refurbishing and upgrading a set of stock heads. Of course, what the heads flow and what the engine makes are two different things, but starting with enough flow to support the intended power level is mandatory for maximum performance.

To get the displacement we wanted, we secured a 4.25-inch stroker crank and matching 4340 forged steel rods and teamed them with a set of 18cc dome, Probe forged pistons. Sealing the pistons was a set of Total Seal rings. The result was a forged, four-bolt short-block displacing 496 ci. Even at just 1 hp per cubic inch, we were creeping up on 500 hp, but we had more tricks up our sleeve to inch it closer to 600 hp.

Big power and big displacement required adequate cam timing. No RV cam was going to get us the power we needed, so we gave Crane Cams a call to secure a beastie bumpstick. Knowing our goals, Crane supplied a solid-roller cam that included 0.714-inch lift (intake and exhaust), a 254/262-degree duration split, and a 112-degree LSA. Crane also came through with the necessary roller lifters, hardened pushrods (8.6/9.5), and Gold roller rockers. Completing the valvetrain was a double roller timing chain, cam button, and ARP retaining hardware.

The job of feeding the beast was handled by an Edelbrock 454-R (Victor Jr.) intake and 950 Ultra HP Holley carb. MSD supplied the billet distributor and ignition amplifier while spent gases exited through 2.25-inch dyno headers. Additional touches before testing included a Milodon oil pan and windage tray filled with 30W (break-in) oil.

Once assembled, the 496 was installed on the dyno in preparation for a pair of break-in cycles to properly seat the rings and bearings. Though it was equipped with a roller cam, we filled the motor with high-zinc break-in oil and primed the motor with a drill to ensure every surface had plenty of oil before start-up. The iron-headed big-block fired up on the first try, and settled in for a pair of computer-controlled, 15-minute break-in cycles. Once we were satisfied that the rings were seated, we began the time-honored mix of jetting and timing to unearth the maximum power. The initial load in at 3,600 rpm netted 550 lb-ft of torque, but this eventually climbed to a maximum of 591 lb-ft at 5,100 rpm.

What can we say? We overshot our goal of 600 hp, which came and went at just 5,300 rpm, and by more than just a few ponies. The iron-headed 496 exceeded our expectations by reaching a peak of 642 hp at 6,200 rpm. The stroker also excelled in the hunt for grunt, as torque production exceeded 575 lb-ft from 4,200 rpm to 5,800 rpm.

Does this test mean cast-iron BBC heads will replace their aluminum counterparts? Not likely, but if you are looking to build a stout street/strip BBC on a budget, these iron castings from Summit Racing should definitely be considered.

Mark Iv Block Rods 3/14

1. The bottom end consisted of a four-bolt Mark IV block stuffed with a forged crank and rods.

Milodon Windage Tray 4/14

2. To improve oil control, we installed a Milodon windage tray and pickup along with a Melling HV oil pump.

Summit Iron Heads Timing 5/14

3. With the Summit iron heads capable of supporting nearly 700 hp, we needed sufficient cam timing to properly motivate the valves. The Crane solid-roller cam offered 0.714 lift, a 254/262-degree duration split, and a 112-degree lobe-separation angle.

Adjustable Double Roller Timing Chain 6/14

4. Running a solid roller cam in the early Mark IV block required use of a cam button to eliminate cam walk. Note also the adjustable, double roller timing chain supplied by Crane.

Cast Iron Port Heads 7/14

5. With an endless supply of alloy heads available for the Rat, we often forget their cast-iron counterparts. These cast-iron, rectangular-port perfor-mance heads from Summit Racing fea-tured 308cc intake ports, a 2.25/1.88-inch stainless valve package, and a multi-angle valve job.

Spring Package Roller Cam 8/14

6. The heads were supplied with a spring package designed for a flat-tappet cam of less than 0.660-inch lift, so we upgrade the spring package for our roller cam.

Crane Cams Solid Lifters 9/14

7. Crane Cams came through with solid roller lifters and hardened pushrods for our buildup.

Crane Gold Roller Rockers 10/14

8. Accurate valvetrain geometry was ensured with the installation of 1.73:1 Crane Gold roller rockers.

High Zinc Break In Oil 11/14

9. Before running, we filled the Milodon pan with 6 quarts of 30W, high-zinc, break- in oil.

Holley 950 Ultra Hp Manifold 12/14

10. Feeding the iron-headed stroker was a Holley 950 Ultra HP and single-plane intake manifold. The induction was designed to optimize power production in the upper half of the available rpm range.

Dyno Iron Head Engine 13/14

11. Run with dyno headers and pump gas, the iron-headed 496 produced 642 hp at 6,300 rpm and 591 lb-ft of torque at 5,100 rpm.

Iron Head Dyno 14/14

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