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 Figure 2.6, you can see that the flow into the cylinder does not follow a pattern that is by any means intuitive.
Fig. 2.5. Here are the two ways to relieve shrouding, which is caused by the valve pocket wall. The simple way is shown on the left and the more effective but tedious method is shown on the right.
Fig. 2.6. To appreciate what is going on here, first locate the ghosted image of the intake port and chamber and orientate that with the pressure differential contour lines around the valve. These contour lines show the pressure differential between the valve and valveseat on a 24-degree head as the valve progresses through its lift.
Although this appears to be a subject for Chapter 4, Cylinder Heads, the flow pattern developed has a strong influence on how the top of the bore and piston should be shaped. You need to recognize that the busiest area with the highest velocities occurs between the 9:00 and the 10:30 o’clock position. The edge of the bore and the piston dome can block flow in this region unless steps are taken to prevent it. The arrow indicates airflow through the port into the cylinder.
Fig. 2.7. Indicated here are the areas of a typical high-compression piston that need attention as far as valve pocket shrouding is concerned.
This flow test and the port probing just prior to the intake valve show an important flow pattern. As unlikely as it may seem, the flow corkscrews off the edge of the valve on the cylinder wall side of the port and then proceeds over the edge of the intake valve and into the cylinder. At least that is the way it would go if there were no obstructions. Because this flow pattern is generally unknown, piston domes rarely have a form that makes allowance for it. Depending on the height of the dome there is a potential 10 to 15 hp to be had by some subtle and some less than subtle reshaping.
The best piston crown shape to have is a flat one or one with a shallow dish in it. Unfortunately that usually results in a really undesirably low compression ratio unless the short-block has a lot of cubic inches. The first move is to address the edge of the piston’s intake valve pocket as per Figure 2.5, which shows the previously mentioned piston mod. From here on out the valve shrouding reduction moves are a little more subtle.
Fig. 2.8. This piston came out of a 900-hp bracket engine built by Throttle’s Performance. Although this engine ran very well, I knew there was more in it if the pistons were suitably reworked.
Fig. 2.9. A trough cut in the piston accommodates the spiral-flow pattern on the cylinder wall side of the port. The top edge of the trough needs to go under the valve head by about 0.100 inch and extend to the deck of the piston at the lower edge.
Fig. 2.10. This piston is nearing completion. The yellow arrow indicates the trough to accommodate the spiral flow seen at low and mid lift. The blue arrows to the right show the laid-back edges that inhibit flow during overlap. The blue arrows to the left show areas that have been lowered, so flow is improved to and from the area around the spark plug. The red arrow indicates the reworking location when bore chamfers on the block are used.
Fig. 2.11. These Mahle pistons have a crown shape that is on the way to emulating the recommended form. As such, they are a very effective piston right out of the box. In addition to a good crown and valve cutout form, these pistons come with 1.5-mm-wide compression rings, which typically have less bore drag than the 1/16-inch-wide rings.
Fig. 2.12. This JE piston is a classic example of why your big-block Chevy project should be focusing on as many cubes as possible. This piston, in a 572 with heads only minimally milled, delivered a 10:1 CR. The valve cutout illustrates just how little work there is to be done when a flat top or even a dished piston is used.
Fig. 2.13. For a drag-race-only application, gas porting through the piston crown is most often the preferred method