Chevy Big Blocks. David Vizard
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 the bigger-diameter lifters equipped with larger rollers is the big cam journal move. Here, a larger-diameter cam allows for a larger base circle to be used and (like the larger lifter roller) it reduces the side loading and allows for greater lifter accelerations before once more reaching the limit. At the time of this writing, a typical cost for boring lifters and cam tunnels larger is about $900. Just how much bigger can you go on the cam and lifter bores? It is best to consult the block manufacturer. At the time of this writing, the typical Dart blocks accept a 55-mm cam core and 0.937-diameter lifters.
Before selecting a block and building an engine, be sure to review and understand the information in Chapter 3, Lubrication Systems; Chapter 8, Electronic Fuel Injection; Chapter 9, Camshafts and Valvetrain Events.
PISTONS, CONNECTING RODS AND CRANKSHAFTS
In this chapter, I go into more depth on power-producing factors that may have only been touched on briefly in previous books, and you may not have seen the piston tech I provide.
You could easily buy pistons that needed to be modified for clearance somewhere on the crown. The biggest difference you are likely to find between one piston and another is in the plug positioning. The piston’s spark trough, or flame ditch, can be up to 1/2 inch off from where the plug is. This needs to be checked, and in some cases redone so the flame path has a clear run to the rest of the chamber. Make sure no part of the piston crown inhibits the flame front passage; cutting the spark ditch is always a move in the right direction. However, the passage of the flame through the charge is complex and understanding what goes on takes a lot of testing to find what is needed.
Fig. 2.1. Short of a supercharger or nitrous oxide, tapping in to the big-block Chevy’s displacement potential via a stroker crank and bigger bores is the best route to high-torque output. However, there are many issues to be dealt with. Although most are minor, those that are not have to be dealt with in an appropriate manner for best results.
An area where much work is currently being done is in the design of the cylinder heads’ quench pad and its interaction with the piston crown. For years, it was assumed that the quench pad should be just a flat surface that the piston closely approaches. Now, this is proving to be not the case. But it is not only the pistons’ quench area that interacts with the head; some less than obvious aspects of the piston design also affect the airflow.
Due to the fact that a performance Chevy big-block piston needs to generate a high compression and accommodate a high valve lift, especially on the intake, it presents some issues related to the breathing ability of the cylinder as a whole. The issue with the valve cutouts is easy to identify, but there is another flow-inhibiting issue that is rarely appreciated. What you read in the following paragraphs may be your first introduction to flow-inhibiting factors for pistons.
Extra Cubes: How Effective Are They?
Cube utilization is of such importance to the success of a big-inch build that it is worth a serious (if short) mention here. The point is this: Unless the rest of the engine spec is reevaluated to reflect the needs of the increased displacement, the time and effort involved in garnering those extra inches are largely wasted.
When displacement has been increased the biggest and most influential change in the engine’s optimal spec to utilize those extra cubes will be in the cam and valvetrain department. (See Chapter 9, Camshafts and Valvetrain Events, and Chapter 10, Valvetrain Optimization, which detail how increased displacement affects optimal cam event timing and lift.)
Fig. 2.2. At first glance, these appear to be trick pistons that are ready to be installed. In reality, these need some crown reworking. With this work, a 10-hp gain is achieved.
The edge of the intake valve pocket needs attention and it’s the easiest area to take care of. As can be seen in Figure