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454 Chevy Big Block Build - Ask Anything

Rat Power
Michael Rasmussen; Massaprqua, NY: I have a 454 big-block from a Chevy truck in my Chevelle. I just purchased the Edelbrock Performer RPM 60459 heads, an 800-cfm carb, and an Air-Gap manifold. I wanted to step up to a hydraulic roller cam, and I heard a roller cam from a Mark VI block can be used as long as my block has provisions for the thrust plate and a Mark VI timing chain is used. I read the online story on your test of the oval-port big-block heads. I'm wondering if I should build that engine or go with a 496-or-larger short-block? I noticed the cam part number you listed was for a Ford. Everything I read about the XE274HR-12 cam says it's for a Ford. Any suggestions would be great. I love the mag and look forward to it every month.

Jeff Smith: The story on our 496ci big-block oval-port head test in the Mar. '08 issue was a very successful evaluation of all the oval-port big-block heads that were available at the time, including a stock set of peanut-port iron castings, Brodix, Dart, Edelbrock, and TFS oval-port heads. We were surprised by the power we made with these heads, and the Brodix castings made the most peak power at 597 hp at 5,600 rpm and a solid 626 lb-ft of torque at 4,000 rpm. The Dart and TFS heads were right there as well, within 2 hp at the peak with the same torque, so the results were a push among the Brodix, Dart, and TFS castings. The Edelbrock heads were down slightly at 582 hp at 5,600 through 6,000 and torque at 618 lb-ft at 3,900 rpm. As we pointed out in the story, the Edelbrock heads were within 2.5 percent of the peak power, which isn't very much, but more important, they were the least expensive of all the heads we tested with the exception of the stock iron castings. As for the incorrect cam card information, that was a weird deal where the wrong cam card ended up in our cam box and we just copied the information without double-checking. The actual cam we used was an XR276HR with 224/230 and 0.510 lift with a 110-degree lobe-separation angle that ironically has less lift than the Ford cam. When you consider that even with the softest aftermarket heads we made 582 hp and 618 lb-ft of torque with a cast-crank, 10:1-compression 496 Rat motor with a very mild hydraulic roller cam, that's pretty stout.

To get to your question, Michael, the hydraulic roller cams for the Mark VI blocks will interchange with the earlier blocks, but you will need to prevent the cam from moving. Flat-tappet cams employ tapered lobes to prevent the cam from moving in the block. By design, roller cams cannot use this feature, which means you need to use some kind of limiter to prevent the cam from moving forward in the block as engine rpm increases. The Mark VI blocks use a camshaft thrust plate to locate the cam, while your older Mark IV engine does not. The least expensive approach would be to add a cam thrust plate as you suggested-along with the Mark VI-style single roller timing set. This allows plenty of room for the timing set under the cover. The plate will require minor drilling and tapping and is best done with the engine disassembled, although if you are careful, it could be accomplished if you take measures to keep all the drill and tap shavings out of the engine.

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