Pregapped RingsWhile theory and conjecture make for nice bench-racing conversations, what better or more ambitious way to test the effects of ring endgap on power output than to run the various configurations on the dyno and let the numbers do the talking? Our test engine was a Vortec-headed 350 small-block we built several months ago and have used for a few dyno-tests since. The bottom end in this mild 350 featured conventional pregapped rings in the budget-priced mail-order short-block. We had recently run this engine in a camshaft lobe separation angle test session establishing a baseline with the pregapped rings. The engine was making a respectable 434 hp. So good, in fact, that the crew at our dyno facility was skeptical that there was much room to the upside with a ring change.
After the teardown, we noted that the engine's ring gaps were very much on the wide side, swallowing a 0.030-inch feeler gauge with ease. Some of the clearance was no doubt gained in the nearly 100 dyno pulls this engine has been subjected to. Despite the wide ring gap, this little Chevy was delivering strong power without any visible signs of excessive blow-by, so we had to wonder how this test would turn out.
Peak hp: 434.7 hp @ 5,700 rpmPeak torque: 450 lb-ft @ 4,400 rpmAverage hp (3,000-6,000 rpm): 363.3 hpAverage torque (3,000-6,000 rpm): 425.6 lb-ftCranking compression: 160 psiAverage leakdown: Not recordedPeak volumetric efficiency: Not recorded
File-Fit RingsThe pregapped rings in our test engine were much wider than any custom-engine builder would let out the door, so we broke out our trusty hand-crank ring file (from Summit Racing Equipment) to custom-fit a fresh set. The rings we chose were Total Seal's Classic set, a conventional file-fit ductile-iron/moly ring package. In accordance with the recommendations from Total Seal, the top rings were gapped with 0.0045 inch of clearance per 1 inch of cylinder bore diameter, for a total of 0.018 inch, and the second ring was gapped to 0.014 inch, per the recommendation of 0.0035 inch per inch of bore diameter. The rings were changed without any additional cylinder-wall prep, and the engine was reassembled for another turn at the dyno. After a short cycle to seat the rings, the numbers were in, and they were impressive, eclipsing the power turned in by the pregapped rings. Most notable were the higher average power and torque numbers showing the engine was stronger across the board. So there's something to this ring-gapping stuff after all.
We spun the engine over for a cranking compression check, and found an average gain of 5 psi per cylinder, indicating the cylinders were sealing measurably better and making use of the increased seal to produce more power. We also hooked up a leakdown tester and ran a leakage check of every cylinder. The average leakage was 12 percent, which was higher than we expected, but air escaping the ports indicated that the valve sealing on our Vortec heads was less than optimal. Unfortunately, we did not perform a leakdown test with the original ring set, but the amount getting past the valves remains a constant compared to the amount escaping past the rings.
Peak hp: 441 hp @ 5,700 rpmPeak torque: 459.2 lb-ft @ 4,500 rpmAverage hp (3,000-6,000 rpm): 370 hpAverage torque (3,000-6,000 rpm): 433.9 lb-ftCranking compression: 165 psiAverage leakdown: 12 percentPeak volumetric efficiency: 94.8 percent