Chevy Big Blocks - David Vizard

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      Brodix and Pro-Filer offer heads to suit 4.9 bore spacing blocks. These heads differ from others because the chambers are slightly repositioned and some head bolts are repositioned. Other than that, they look much like heads for a regular 4.840-pitch block. These blocks are available with deck heights up to 11.1 inches, and they accept strokes up to 5.5 inches. With a maximum bore of 4.7 inches and a 5.5-inch stroke, a short-block assembly of 763 inches can be built without undue hassle.

      With the right heads, an “all engine” build can produce right around the 1,400-hp mark with torque in excess of 1,180 ft-lbs. So far the biggest Dart block build I have been involved with was a 712-inch unit that made 1,098 ft-lbs and 1,346 hp on nonoxygenated 116-octane race gas. I built this engine with Terry Walters at TWPE. There is no doubt in my mind that oxygenated fuel would have added about 40 hp.

      Maximizing Bore Size

      The entry-level Sportsman blocks from Blueprint, Dart, and World Products represent a good return on investment in either 9.8 or 10.2 tall-deck configurations. The cylinder walls on these blocks are much thicker than stock. Often, the bore limit is 4.6 inches, but a sonic tester can often find a block that can go significantly bigger. I have gone to as much as 4.67 inches with a Dart block and have had no subsequent problems. A 4.5 stroke in a short-deck block produces 616 ci. If engine bay space accommodates the block, the tall-deck version of this block can, with a 4.75-inch stroke and a 6.7-inch rod, go to 652 ci, and that many inches has some serious torque and horsepower potential.

      My last word on Dart blocks here is that you should visit their website and check out all the variants they offer. However, don’t let the name “Sportsman” in any way make you think you are buying an “also ran” member of the Dart performance line-up.

      GM Performance Blocks

      I have used GM’s one-piece-seal tall-deck block for a 572 (4.375-inch stroke) and a 588 (4.5-inch stroke). Both instances used a 4.56-inch bore; they were street/strip pump-gas builds and produced very satisfying results. In round numbers, these were 850 ft-lbs and 904 hp for the 588 and 828 ft-lbs and 883 hp for the 572. That was with some super-ported 24-degree heads and a 10.8:1 CR. You should be aware of these Bow Tie blocks. They are available in short- and tall-deck versions, and most can bore to 4.6 inches. Compared to stock blocks, the Bow Tie blocks have revised oil passages and much stronger upgraded mains caps.

      World Products Blocks

      World Products has gone through some big changes during 2011–2012. First, the production of blocks has been split. The “trade only” parts manufacturer/distributor PBM of Louisville, Kentucky, manufactures the iron Merlin blocks. BMP, formally World Products, exclusively produces the World aluminum blocks. What does this mean? Trade customers, such as pro engine and big speed shop outlets, buy from PBM. As a retail buyer, you can purchase your Merlin block either from an engine shop, a speed shop, or directly from BMP. Regardless, you should go to the BMP site to review the lineup of blocks, details, and prices.

      The iron Merlin blocks have gone through a series of important updates. Without the need for any additional clearance, the short-deck block is good for 582 ci and the tall-deck, 632. With a little grinding for additional rod clearance some extra stroke increases these figures by about 12 ci. I have not used the latest block but I used the previous version for a street/strip nitrous build that made more than 1,500 hp. That was some years back and the block is still in one piece.

      Most aluminum blocks cost a pretty penny but sometimes you can find one advertised on eBay, Racing Junk, or at a swap meet. Usually they go for comparatively little money. However, aluminum blocks are susceptible to far higher rates of corrosion than iron blocks, so you need to diligently inspect a prospective purchase.

      Almost without exception aluminum blocks have a lesser bore capability than their iron counterparts. The smaller bore means less displacement, but they more than make up for this by weight reduction. A 9.8-inch-deck aluminum block weighs about 90 pounds less, and this difference is substantially more when considering taller-deck blocks.

      If you can afford to pay a little more than twice the cost of an iron block, you should consider what Brodix, Dart, and BMP have to offer.

      Although you need bearing housings, bores, and the like precisely sized, an often-overlooked dimension can cause a substantial power loss, and that is an incorrect crank-to-cam centerline distance. When a new set of main caps have been installed, the main caps and journals have been line bored or honed and sometimes the cam tunnel needs to be cleaned up because the centerline between the crank and cam closes up slightly. As a result, the timing chain is sloppier than would otherwise be the case. You can install a timing set with a slightly larger cam gear to fix this. Although it’s only by a couple of thousandths, it can take a considerable amount of slack out of the timing chain.

      At the other end of the scale, a timing setup can be too tight, which can be worse than a loose timing chain. A simple check is to notice how much the timing chain can be moved back and forth at the midpoint between the two gears. I typically expect 1/8 inch or so of play in the chain. The problem here is that this test is a little on the subjective side.

      The best simple check is to install just the crank and use light oil on the main bearings and the cam bearing. Install the cam first without the timing chain and verify that it rotates freely. Now install the timing chain and recheck. This procedure allows you to feel how freely, or not, the crank and cam rotate. With the timing chain installed, the amount of effort it takes to rotate the crankshaft, cam, and timing gear should be barely perceptible.

      The reason I have gone into detail here is that I had a significantly tighter than normal timing chain on one engine due to an incorrectly packed timing chain set. The gears were slightly oversize to compensate for a crank align hone job that moved the centers closer. With the timing chain installed, the assembly took about 5 ft-lbs more to turn. I dyno’d a 468 build with this setup with the intention of swapping out the timing gear for one that gave the proper tension.

Fig. 1.17

       Fig. 1.17. Guarding against failure of flat followers is very important and that is why I am emphasizing the need to take steps against it. Using Comp Cams’ lifter grooving tool, a groove such as seen here can be cut. This allows a stream of oil to spray onto the cam face just before it contacts the lifter. This is a very effective move and only takes seconds per lifter bore.

      I was expecting to see about 5 ft-lbs and about 6 hp difference. Surprisingly it was much more than that. The torque, with the correct timing chain tension, increased by an average of 9 ft-lbs, and power went up by 11 hp. I am also sure that cam bearing life also increased somewhat. When a well-used timing chain replaced the new one, there was quite a bit of slack in the chain. The power dropped only minimally, but at part throttle, the ignition timing danced around far more than before. This indicated that the cam was oscillating back and forth as much as 2 to 3 degrees.

      A big-block Chevy has the same lifter diameter as a small-block Chevy; this size is too small for a small-block and way too small for a big-block. (Refer to Chapter 9, Camshafts and Valvetrain Events, to see the effects of size and geometry on the opening envelop of a lifter, whether it is a roller or flat tappet.) For a flat-tappet lifter, the peak lifter velocity is dictated solely by how far off center the cam-to-lifter face line of contact is. As a result, diameter of the lifter totally dictates maximum velocity. A bigger-diameter lifter means that more velocity as well as more lift can be designed into the cam profile. As previously stated, one of the factors to take care of is building a valvetrain that has high-lift capability.

      Just how large

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