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 2.4 the air entering the cylinder around the short-side turn during the overlap period runs into the wall of the valve cutout. This needs to be rectified as shown in Figure 2.5. This “piston porting” exercise is easy enough to appreciate if you spend a little time studying these figures. And it definitely delivers performance benefits.
However, one aspect that is mostly peculiar to 24-degree heads is far from intuitive. A pressure/flow distribution plot (Figure 2.6) of the intake port reveals that the busiest exit area around the valve is not, as is normally the case with most other two valve heads, toward the cylinder’s center. Measuring seat exit velocities with a vented valve shows that the busiest section of the port is often toward the shrouded side. Once this becomes known it is easier to see why, when the block is chamfered in this area, the response is a sizable amount of extra output. The block chamfering, however, starts to aid flow after the valve is around 0.150 inch and more off the seat.
Fig. 2.3. The high dome and deep valve pocket design of a Chevy big-block piston can inhibit the flow of gases into and out of the cylinder. Air travels into the cylinder following the path of the blue arrows. During the overlap period, air runs into the ridge on the plug side (left) and the edge of the valve pocket (right). Exhaust gases exit in the direction of the red arrow, and the ridge (adjacent to the spark trough) impedes this exhaust gas flow. The black shaded areas are the first locations that need attention.
Fig. 2.4. This computational fluid dynamics illustration shows air entering the cylinder during the overlap period. At this stage of the intake event, the air that exits the short side turn runs into the wall of the valve cutout. This reduces the effectiveness of the overlap period and typically reduces the volumetric efficiency more than it might be supposed.
Less obvious is that the flow pattern on the cylinder wall side of the port is spiraling past the edge of the bore shrouded intake valve. In effect, air is corkscrewing past the edge of the intake valve at about the 10 o’clock position, and during the overlap, the dome of a high-compression piston can block this flow. This suggests that you not only need to cut the top of the bore as discussed in Chapter 1, Displacement Decisions, but you should also find out if the piston dome can have any negative influence on the flow into the cylinder other than the effects of valve shrouding from the aspects indicated in Figure 2.3. From
