Extracting maximum power from racing engines requires extraordinary attention to small details that may be forgotten or easily overlooked. Among these seldom noted areas of attention, matching piston crown shape to the combustion chamber shape is not a practice widely employed by many engine builders. Nonetheless, it is a proven source of hidden power through which hardcore builders can influence or manipulate burn characteristics to increase cylinder pressure for greater power output. This is particularly effective in normally aspirated applications where the careful manipulation of atmospheric pressure is applied to generating additional ram filling after BDC to further increase charge density. For customers requiring a very precise match between the piston crown and the combustion chamber, JE Pistons offers precision chamber digitizing so the piston designer can optimize the piston to chamber relationship and extract hidden power.

JE Pistons engineer Connor Morris has set up this big block Chevy Dart Big Chief head to digitize the combustion chambers so they can be mated to the optimum piston crown shape for maximum compression and combustion efficiency.

The vast majority of high performance and racing engine builds rely heavily on off-the-shelf racing pistons with dome shapes developed by manufacturers who have groomed their products over time to match commonly used cylinder heads with well-defined combustion spaces. The fact that they work so well is sound testament to the effective R&D efforts and technical savvy of modern piston designers. Moving beyond the norm, tech savvy builders are very concerned with the specifics of charge motion as the mixture exits the valve into the cylinder. Effective mixture conditioning and charge quality are strongly influenced by the flow characteristics entering the cylinder. While the piston is still far down the bore at intake valve closing, the effect of dome shape and valve reliefs becomes increasingly important as the piston compresses the mixture into the final combustion space.

This relates closely to piston crown shape because the piston top essentially forms the floor of the combustion chamber as it approaches TDC. A primary concern of professional engine builders is the pressure recovery effectiveness of high-speed air (low pressure) entering the cylinder on the intake stroke. In a normally aspirated engine, it is essential that incoming airflow recover to atmospheric pressure and more with the addition of flow dynamics that promote ram filling of the cylinder after the piston has passed BDC and begins rising on the compression stroke.

Shaping the piston crown to match the combustion chamber is an effective means of increasing compression ratio while directing mixture flow patterns to improve combustion and maximize cylinder pressure.

Interestingly, compression does not initiate until the intake valve closes with the piston already part way up the cylinder bore. If for example (simplified version) the piston has risen 30 crankshaft degrees up the bore at intake valve closing and you light the mixture off 30 degrees before TDC, that leaves an actual compression stroke of only 30 degrees. But the actual length of the compression stroke is less important than the mixture compression and charge motion that occurs as the piston approaches the combustion chamber immediately prior to firing the spark plug. To ensure optimum charge density and combustion efficiency, the mixture must be composed of very fine droplets with similar surface areas. Swirl and tumble are flow characteristics that seek to support this by swirling and tumbling the mixture as the cylinder fills. As the piston approaches the chamber mixture activity must remain active to ensure even combustion and optimum flame travel across the combustion chamber. A well-matched piston crown that mirrors the combustion chamber helps promote this important action.

Once the matching piston crown design has been formalized the designer can go back in and perform the necessary tweaks for a perfect fit. He can use your camshaft specs and head gasket dimensions to ensure that all of these components work together for maximum effect.

Normally aspirated applications react favorably to increased compression combined with appropriately matched fuel quality to avoid detonation. To achieve very high compression it is important to not only pack the mixture tightly into the combustion chamber, but to squash it even further by creating tightly matched valve reliefs, the minimum required fire slot in the dome and a dome shape conducive to fast flame travel and a rapid pressure rise to push the piston down the bore with maximum effect.The quench area where the flat part of the piston crown approaches the closest to the cylinder head deck is employed to impart turbulence or active mixture motion to maintain high mixture quality.

All of these areas come into play when the piston designer strives to achieve the minimum possible clearance without physical contact between the piston and the head when the piston rocks over at TDC. Hence, accurately modeling the chamber shape and pinpointing the location of the valves and spark plug becomes an effective means of achieving the ideal match between the piston top and the combustion chamber.

At JE, they use a Romer articulating arm equipped with a 3-dimensional non-contact laser scanner capable of capturing complex geometries at 30,000 points per second. This enables the scanning engineer to capture a detailed 3D image of the combustion chamber that can be fully modeled and rotated via the computer software. The captured image is transferred almost in real time to the computer and appears immediate on screen. The engineer is able to examine the scan for anomalies and determine if it is suitable for the follow-on piston modeling procedure. To prevent scanning errors, the chamber is first sprayed with a dulling agent that eliminates reflections. The hand scanner is then passed over the chamber. Multiple passes are often used to ensure accuracy. The software remembers every data point and compares it to new data points to pinpoint the exact chamber shape with the valves, spark plug and quench area perfectly located.

It's noteworthy that valves can also experience some radial movement as well so adequate clearance must be maintained to account for it. To accommodate valve angle and placement the Roamer scanner can read the location and angle of the valves in the head and translate that to the software rendering with great precision. This allows JE to place valve reliefs in precisely the right location at the right angle and with the proper surface finish already machine onto the piston. The scanning head can be placed against any exposed valve stem as seen below. It then renders precise valve placement in the captured screen images.

To ensure optimum design features, the scanner is also used to pinpoint the location and center points of head bolt holes, water passages and head gasket material. As seen here Morris carefully checks the actual head gasket dimensions to match them with the scanner's digital profile of the cylinder location in relation to the combustion chamber. Remember when you used to install the gasket and cylinder head on a bare block and reach in the bore to scribe a line on the bottom of the head to indicate cylinder placement. That's all done automatically now and it's more precise.

This procedure is also employed to ensure proper piston to head clearance and to verify that domes and valves don't get cozy with piston tops. It accounts for predicted piston rock based on clearance and ensures the best possible piston crown to combustion chamber interface with all critical variables accounted for. This technology is particularly useful for custom applications outside the mainstream. Vintage combustion chambers and one off race engines are just a few of the applications where digital scanning shines a brighter light on performance enhancements you will never see once you torque that last head bolt. But you will certainly feel the difference and appreciate its worth. Now go build something fast.

To see this procedure in action watch the accompanying video that illustrates the process from beginning to end. It's how pistons get designed and further it's how high-end builders extract mega-power with the aid of complementary components that match and function perfectly with each other.

SOURCE:

JE Pistons
10800 Valley View St.
Cypress, CA 90630
(714) 898-9763
www.jepistons.com