Blocks
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. 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.
Lifter Bores and Flat Followers
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 of a lifter diameter you can use depends on how accurately the lifter bores are located with respect to the camshaft. Also, big-block Chevys with flat-tappet cams have a reputation for eating cam lobes and followers. I have experienced that problem at least a couple of times. According to my friend Billy Godbold, Comp’s wiz kid cam profile designer, General Motors had a batch of several thousand big-blocks come off the production line in the early to mid-1970s that had miss-machined lifter bores. This made an already marginal situation worse, and these big-blocks had a propensity for destroying the cam lobes and followers.
Fig. 1.18. I tested this oil additive in the 1990s and was so impressed with how well it worked that I bought shares in the company. From that, I developed my own break-in lube. Use this in the oil and a steel-on-steel pin boss will last virtually forever. I have also proved to top pro engine builders on their dynos with their engines that my Oil Extreme break-in lube is the best there is—bar none.
Most cams are designed to utilize up to within 0.025 inch of the edge of the lifter. That being the case, a 0.842 lifter offers a diameter of 0.792, and delivers the performance it has to offer. This margin is a common standard within the aftermarket cam industry. However, if the lifter bores are both bored and positioned accurately and increased in diameter, the ability to utilize more aggressive flat-tappet cams is available. The most practical size to use here is the 0.904 Chrysler lifter diameter. By accurizing the lifter bore positioning, cam grinders (such as Comp Cams) push the diameter utilization envelope to within 0.010 of the edge of the follower. This means the working diameter has increased from 0.792 to 0.884 inch. That’s an increase of 12 percent in velocity capability, which translates into an 8-percent increase in opening area.
Lifter Bores and Roller Followers
Boring out cam follower bores to accept a larger size also is advantageous for a roller lifter but not for the same reasons as for a flat tappet.
When using a roller cam the bigger the base circle diameter and the larger the follower roller is, the better the dynamics and the more aggressive the opening event can be. The big problem with a roller cam (contrary to popular belief) is that the system is acceleration limited. In the initial off-the-seat acceleration phase, a flat tappet can easily beat a roller (typically used for a big-block). The roller lifter’s problem is side loading (see Chapter 9, Camshafts and Valvetrain Events, for an explanation). The bigger the roller diameter, the lower the side loading for a given acceleration rate. This means that a big diameter here can allow higher acceleration rates to be designed into the cam profile. This is exactly what you need for an under-valved engine that desperately needs high lift.
As you have probably guessed, not many machine shops are equipped to machine out lifter bores. The four shops I use are: The Checkered Flag in Desoto, Missouri; Terry Walters Precision Engines in Roanoke, Virginia; Blanks Machine in Clarksville, Virginia; and Jesel in Lakewood, New Jersey (which, of course, did much of the pioneer work in this area).
Complementing
