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If you quickly scanned just the title and photos of this story it may seem like Mike Petralia and the gang at Hardcore Horsepower in Tennessee did an iron versus aluminum block shootout with this pair of 427-cube Ford Windsor engines. But they didn’t. It just happened that two different Ford customers – from opposite sides of the planet – both wanted small block Ford 427 ci Windsor engines built for their rides right around the same time.
One customer from Australia wanted an all-aluminum, or “alloy”, as they refer to it Down Under, engine for his 1973 Falcon XB Coupe. The XB can be thought of as the “Mad Max/Road Warrior” car, but without the blower. As they are very fond of road-racing in Australia, and he would be running a manual transmission in the car, Petralia knew he needed to build the customer an engine that was light and reliable.
Left - The biggest difference between these two engines was total weight. The aluminum version scaled in at 350 pounds, while the iron version was 483 pounds. Middle - Both engines used identical Scat 4340-forged steel crank and connecting rods, and SRP forged pistons and moly rings. Right - Both engines ran the same roller lifters from PBM and the same Comp Cams roller rockers and pushrods.
“One concern was that if the engine made too much low-end torque it would just spin the tires and he’d be going around the track sideways. Ironically, they really like doing that down in OZ. I think they call them ‘powerslides,'” says Petralia. “Basically, going around the entire course sideways, laying down rubber the whole time, if I’m not mistaken.”
Here in the USA, that’s called ‘drifting’.
The SRP pistons in both engines were CC'd at .500-inch down-bore and measured exactly 115cc, which translates to around -5cc piston valve relief volume.
For this particular engine, Petralia wasn’t too worried about making tons of low-end grunt. Instead, he wanted an engine that could turn high rpm and keep on building power upstairs – and maintain good street manners too.
On the flip side, he had a customer from New Mexico that was building a heavy off-road 4×4 with 35-inch Baja tires and an automatic transmission. This customer had much different requirements – he wanted to run for hundreds of miles at W.O.T. and still be able to surmount any giant obstacle when needed.
The only major difference between the two engines, besides blocks, oil pans, and cost/weight; were the different Comp Cams roller sticks Hardcore choose for each specific application. We won’t reveal all the specs, but the cam for the iron engine was around .620-inch lift and the cam for the alloy engine was around .670-inch lift.
So the challenge for Petralia and Hardcore Horsepower became to build two 427 Windsor engines, each tailored to the best outcome for their specific owners. This was not meant to be an iron versus aluminum comparison, it just turned out that way in the end. Obviously, there would be a huge difference in the cost and weight between the two engines. More than 130 pounds of overall weight [350 pounds for the alloy engine vs. 483 pounds for the iron piece], and $4,500 on the final build sheet, to be exact.
Left - Dart's SHP iron block comes standard with billet steel main caps in all five spots. The center three caps are a splayed 4bolt design for higher strength. The front and rear caps are designed to clear a standard 351W oil pan - Hardcore used a pan from Canton Racing with built-in trap doors and a windage screen to help oil control when off-roading. Left Middle - Dart's aluminum block has billet steel 4-bolt main caps at all 5 locations, requiring a different oil pan; this one was custom-built by Stef's. Right Middle - Different head gaskets were needed for each engine. Fel-Pro 1134 gaskets were used on the iron block, but not on the aluminum block due to the cylinder sleeves overlapping the valve relief pockets in the gasket. Right - Round bore Cometic head gaskets were used on the aluminum block instead. Since aluminum blocks expand and contract with temperature changes a lot more than iron blocks do, Petralia used copper spray gasket sealer on the aluminum block’s head gaskets for added security.
The main requirement for each build was to tailor the power-band of each engine to work best for each application. Since everything else in these engines except the oil pans was practically identical, he was able to customize the camshafts to suit each need. And since it was only the difference of a few degrees and a bit of lift separating them, he didn’t expect huge to see huge differences in peak horsepower between them.
Left - The Dart aluminum block had several spots of crank and rod interference that had to be addressed. Here is the rear main where the counterweight just barely nicked the block. Right - The crank's counterweight nicked the block on both sides at the rear main of the Dart aluminum block.
Dyno Comparison
Here’s a dyno graph comparing both engines(click to enlarge). The bold lines are the iron block. It appears that if the iron engine had been taken higher in the RPM range, it would have made more horsepower. Keep in mind that these engines were tested several months apart, and the goal was never meant to compare each of them identically.
“The cam in the iron engine really did run out and stop making power above 6,200rpm. Power dropped off rapidly and the engine would not even continue to accelerate past 6,400. We used that information to our benefit when we designed a new cam for the aluminum engine,” says Petralia.
“And both engines made practically the same horsepower around 6,100 rpm, which was where the iron engine peaked making 582hp. The aluminum engine made 583 horsepower at 6,100 rpm, but its cam kept pulling harder and higher; the peak for this engine was at 6,500 rpm with 594 horsepower, and it revved decently to almost 7,000 rpm, still holding onto almost 570 horsepower up there,” says Petralia.
He’s always been a bigger fan of torque in the mid-power band, where it can be felt driving around in a street car or truck.
“I’d bet that even with its weight deficiency, the iron block would’ve actually gotten down the quarter mile quicker than the alloy version if they were both dropped into the same car with good traction capabilities. My old friend, and former boss, Jeff Smith, and I proved this theory true when we built identical cubic inch small- and big-block Chevy engines, and then dropped them into my ’70 Camaro for some runs down the old LACR drag strip in Palmdale, CA. The big-block dyno’d out with just a bit more hp and torque, and it got to the finish line quicker too, even though it outweighed the small-block by something like 170 pounds,” he notes.
Left - The Hardcore Horsepower dyno cell has a very powerful exhaust fan to exchange all the air inside the cell 16 times per minute. This means it moves a lot of air under the engine to pull heat off the headers and oil pan too. Combine that with aluminum's rapid heat-dissipation capability, and this created one of the issues during the dyno testing of the all-aluminum engine - they couldn't keep enough temperature in the oil in-between dyno pulls. A heating blanket was placed on the oil pan to keep temps consistent. Right - Valve springs specifications were played with on the higher-revving aluminum engine too, enabling it to run smoothly past 7,000 RPM.
Every engine has its proper place, and no two engines are exactly alike.
“We feel each customer wants something to call his own, not just the same engine that two other guys at the car show bought online. We feel pretty well that we built each one of these engines correctly for their due duties. No one needs an engine that makes peak power past 6,500 rpm to climb rocks. And an engine that tops out at 6,200 rpm won’t be as much fun to powerslide for a mile either,” laughs Petralia.
