Alternative Iron

Testing Pro Topline's Iron Replacement Heads

Steve Dulcich Jan 23, 2004 0 Comment(s)
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It seems kind of odd that a manufacturer with one of the broadest selections of aftermarket performance small-block Chevy heads is based in New Zealand. Regardless, this company has the savvy to produce some heavy-hitting hardware. Although best known for its massive aluminum race heads, Pro Topline also carries a range of iron castings in a mind-boggling array of sizes. Distributed in the USA through Shaver Racing, the Pro Topline small-block Chevy range includes everything from iron replacement heads to big-port 305cc iron monsters that would make a sandbagging street racer get big-eyed. Yes, that's a 305cc small-block head. Lest we give the impression that this company focuses on iron heads, the variations of small-block aluminum race heads is no less staggering. Pick a valve angle: 12 degrees, 14 degrees, or 23 degrees; high ports, offset valves, runners from 180 to 305 cc--essentially about anything you can dream up can be had from these Kiwis.

We were thinking about an affordable cast-iron stock-style head for a street 383. Pro Topline offers true stock-replacement small-block Chevy heads with 1.94-inch intake valves. We looked one step further up the application chart and zeroed in on the iron heads with 180cc inlet runners, 2.02-inch intake valves, and compact 64cc chambers, the most conservative pieces in Pro Topline's extensive line of iron performance and race heads. While GM Vortecs provide an impressive bang for the buck when considering output and price (based on the heads alone), we were >> already set up for the earlier generation of cylinder heads and wanted to avoid the additional costs associated with changing to the Vortec-style intake manifold and center-bolt valve covers. The Pro Toplines are a bolt-on swap for a traditional-style head. They offer the promise of much improved output, even when compared to some of the desirable OE cylinder heads, such as the 461, 462, 292, 040, 186, 041, or 492 castings--all of which are getting pretty thin on the vine these days.

The 180cc heads are sold under the Pro Lightning brand with either 64 or 72cc chambers, or with 50cc chambers by special order. They are also available with straight-plug or angle-plug positioning. A variety of valve and spring configurations can also be specified to accommodate the requirements of the engine combination. For our 383, which includes a moderate hydraulic cam, we opted for the straight-plug heads with 64cc chambers and the K1 assembly kit, which uses 1.460-inch single valvesprings and flat-wound inner dampers. This spring provides a 110-pound seat load, perfect for a hydraulic cam. Other spring packages can be specified to work with more aggressive solid-lifter or roller-cam applications. We like the wide product line and the flexibility these choices afford the engine builder. The 64cc heads put our compression ratio at a healthy 10.4:1.

The 383

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Our 383 is a very basic low-buck build. Starting with a 350 factory block, a Scat cast 3.75-inch-stroke crank and a 0.030-inch overbore creates the cubes. Filling the holes is a set of Probe Factory Performance Series 3831-F forged flat-top pistons. The rods are the factory stock 350 5.7-inch units, resized and upgraded with ARP bolts. The combination is balanced with a 400-weighted flexplate at the rear, and a neutral-balance damper up front. We went with a flat-tappet hydraulic camshaft to keep the combination simple, selecting a Competition Cams Xtreme Energy 274H. The XE274 has aggressive ramp rates, providing more area under the lift curve by virtue of quick lobe action. The specs show a healthy duration of 230 degrees intake and 236 degrees exhaust (at 0.050), while keeping the overall rated duration at a fairly modest 274/286 degrees. With factory 1.5:1 rockers, the lift works out to 0.487/0.490 inch on the intake and exhaust lobes, respectively. Our only extravagance was the addition of a Comp Cams three-piece aluminum timing cover. This cover allows quick camshaft changes or adjustment without having to disturb the oil-pan seal and makes the short-block look trick.

To finish the assembly, we selected an Edelbrock Victor E intake manifold; it's a large-runner single-plane design, which has a relatively low profile compared to other high-capacity manifolds of its kind. We topped the intake with a Mighty Demon 750-cfm carb. The Mighty Demon bridges the gap between the Speed Demon line of carbs and the Race Demon series. Like the Race Demon, the Mighty has no choke horn, instead featuring an airflow-enhancing, profiled air entry into >> the carb barrels. Also like the Race Demon, the Mighty has replaceable air bleeds, allowing a high degree of fine-tuning to the metering requirements of the engine. The intended application for this carb is more radical street/strip combinations, typically running camshafts in the range of 240-260 degrees at 0.050-inch lift. Though we were under that mark, we wanted a carb we could grow with if we upgrade our combination later. Completing the package, we ran an MSD distributor and wires, which we coupled to the dynamometer's ignition system.

On the Dyno

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While assembling the 383 proved to be very straightforward, the question remained--what kind of power would it make? We were hoping to find a solid 400 streetable horsepower from this combination, but only time would tell. The engine was loaded onto the SuperFlow 901 dyno at Westech Performance to find the answer. The engine was fired and the timing set at 36 degrees total. Since we had installed a new camshaft, the engine was put through a break-in cycle, running between 2,000 and 2,500 rpm to allow the camshaft and lifters to bed in. We weren't doing any comparison testing, so our objective was to simply find the best tune and run it for a number. Before long, we had the engine down to an idle and adjusted the air/fuel mixture. The 383 showed 11.5 inches of manifold vacuum at an 800-rpm idle--more than acceptable for a performance street engine. The Comp cam produced a mild lope, but with a little tuning of the mixture screws, the engine was remarkably smooth.

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We did a steady-state pull at 3,500 rpm to check for detonation and a proper air/fuel ratio. The numbers looked good for a sweep, so the dyno controls were set to run a short test from 3,000 to 5,100. We recorded 390 hp at the top of the pull (5,100 rpm) and it was still rising. Instrumentation showed a clear rich condition, so the jetting was pulled back from the 89/90 combination that was in the carb to 86 square. We repeated the short pull to 5,100 and output increased to 395 hp--again at the top of the pull. On the next pull, the upper rpm was raised to 5,900, and we recorded 402 hp at 5,400 rpm. We had over 400 hp on tap, and the 383 didn't have to rev to the moon to get there. The air/fuel ratio still looked a little rich, so the jetting was dropped down again, this time to 84s all around. Again the engine responded with better output, posting 410 hp at the same 5,400-rpm peak. The mixture curve looked close to ideal. A backup pull repeated the previous test's numbers, showing 411 hp--happily a few ponies more than we had hoped to extract from this basic iron-head combination.

Flowbench Results

Before installing them, we had the opportunity to test the Pro Topline 180cc heads on Westech's SuperFlow 600 bench. The heads were tested as they were delivered, with the stock, as-cast ports and no modifications.

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