href="#ulink_e7736209-1147-5838-89c6-48d951d80bd1">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 to boost cylinder sealing.
The principle job of the top compression ring is to seal against the pressures experienced above it. To do this, it must have some radial load pressing it outward onto the cylinder walls. A relatively high-compressive preload can achieve this, but that means excess frictional losses on the induction or exhaust stroke. You need to increase the rings’ radial cylinder wall loading as the cylinder pressure increases. Gas porting the top ring groove is an attempt to do just that.
If maximum output for a given piston is the goal, gas porting is the way to go and is offered by most piston manufacturers. For drag racing, using a vertical style of gas port through the piston crown is the preferred method. However, in time, these can clog up, so for use other than drag racing, a horizontal gas port in the top ring groove is preferred.
Fig. 2.14. The two notable aspects of this performance JE piston are: the horizontal gas ports (arrows) and the 0.043/0.043/3-mm ring grooves.
Fig. 2.15. Many off-the-shelf budget-oriented pistons do not have gas porting to the top ring. This Goodson tool allows you to gas port your own pistons with little more than a drill.
Fig. 2.16. I typically use Total Seal rings because my dyno tells me I should. The use of thin cross-section rings also pays off. There can be as much as 8 to 11 hp difference between 0.062 and 0.043 compression rings.
Thinner rings are better than thick ones due to reduced friction; Total Seal rings are about as good as it gets because they seal tight. This is not just my opinion but the result of a lot of tests with rings at various gaps all the way down to the zero gap given with a Total Seal ring. Like the bores the piston rings need to have a low-friction prep. The first move is to use a very fine stone to remove the sharp corners from the edges of the rings’ outside diameter (OD). Then polish the rings with a Scotch-Brite pad until they feel really slick.
