The reduction to compression height was only going to be obtainable if the ring belt region could be appreciably shortened. This necessitated a change to the ring widths, and a reduction in height of all lands. In all a reduction of 6 mm was needed. However the proximity of the valve recesses to the top ring imposed a limit on the reduction possible on this land because of the danger that breakage would occur at this point so this meant that the second land also had to be shortened to less than the ideal.
To obtain a robust design capable of withstanding combustion pressures that would occur at an elevated compression ratio (say 10.0:1) meant that the radial depth of the top ring had to be reduced to less than standard for the diameter. This allows the groove depth to be reduced with a consequence that the bending moment on this land is smaller. Larger than normal groove root radii were specified to improve fatigue resistance at this point. The reduction of groove width coupled with minimised back clearance lead to an improvement in strength of the second land of some 25% which compensates for the reduction in land width.
(An alternative option was to dispense with the second ring. This would have allowed much more room for the rings and lands. However this option was tested in ACL Technical Centre and while blowby was, as expected, much improved, the oil controlling function which is the primary purpose of the second ring was lost and oil consumption was considered to be excessive. It could have been restored with further development of the remaining rings but this would have delayed introduction of the piston).
A 351 Cleveland engine was obtained and built for the planned test. This incorporated modified 302 closed chamber heads with 57 cc combustion chambers, larger valves, a high lift camshaft, and four into one extractor exhausts. A 750 Holley carburettor was fitted to the standard inlet manifold. The compression ratio was measured at 11.3:1, much higher than would be recommended for eventual customer use of these pistons. Avgas was used throughout the test to enable the engine to operate without detonation occurring.
The engine developed 250 kilowatts at 5,000 RPM and 502 Newton metres of torque at 3,500 RPM.
After run in and power checking the engine was subjected to 94 hours of running at the maximum torque condition. This imposed greatest combustion loading on the narrow lands and the thin section between the valve recesses and the upper side of the top ring groove.
The engine ran well for 67 hours but the crankshaft broke at this point. Fortunately only one piston was damaged so the remaining 7 and a new one were installed in a replacement block assembly and continued the test. A valve broke after 94 hours and terminated the test. The piston in the affected cylinder suffered some crown damage but did not break. Throughout the interrupted test the blowby was monitored and was fairly constant at 50 litres/minute. This is a good figure considering amount of bore distortion that is usual for this engine type. The lower than usual radial depth of the top rings no doubt contributed to this good result as they would have been more conformable than deeper radial rings. Oil consumption averaged between 80 and 100 cc/hour, also a reasonable result for a performance engine.
On inspection of the piston assemblies at the end of the test there were no signs of excessive wear on the skirts, the pin bores or ring grooves and all rings were in excellent condition. The pistons were crack tested with dye penetrant and found to have no cracks.
Shortened compression height pistons have been developed for use in Ford Cleveland 351 engines using longer 302 connecting rods. The shorter lands and special narrow top rings with lower than standard radial depth have proven satisfactory in an engine running at much greater compression ratio (11.3:1) than would be recommended for the pistons in service (i.e.10.0:1). The weight saving (after adjusting for the 12 gram increase in weight of the 302 rod) is 60 grams.
The compression ratio will be 8.8:1 if the combined volume of the combustion chamber, deck volume and gasket volume is 85 cc. (The valve recess volume is 2 cc as per 9351).
The test was the equivalent of a round Australia trip towing a heavy van at full load and proved that the piston is very durable despite the compromises that had to be made to land and ring dimensions. However obviously detonation would have to be avoided as this would most certainly lead to piston failure as it would with any piston.
As a result of this project the piston has been released for production and will be added to the existing range of Duratech® performance pistons produced by ACL. Duratech® is the name assigned to second generation Duralite® ACL pistons.
Oversizes will be limited, at least initially, to 020 and 030. The part numbers are as follows: