Some engine builders say they prefer to use horizontal gas ports or small horizontal “D-shaped” cutouts machined into the top ring groove land around the circumference of the piston to get more even pressure behind the top ring.ģ. Vertical gas ports tend to clog with carbon, which may result in uneven gas pressure behind various areas of the top ring. Gas ports in the pistons can help route combustion gas behind the top ring for a better seal, but it’s a trick that’s used primary on the race track, not the street. Another recommendation is to use a shallower crosshatch in the cylinder bores if you are building a high revving engine. Gas trapped in the accumulator groove will help maintain ring seal at high RPM. Adding gas ports can allow you to use somewhat tighter ring side clearances.īryce also recommends using pistons that have an accumulator groove between the first and second ring. He recommend using the lightest rings possible to reduce weight, and adding gas ports (vertical or horizontal) to the pistons to help keep the rings sealed. He said most engines will start to lose horsepower at high RPM because of ring flutter long before they experience valve float. However, if the rings fit too tightly in the grooves, there may not be enough gas flow to get behind the rings to push them out against the cylinder wall, resulting in ring flutter, a poor seal and loss of power.Īt the recent Advanced Engine Technology Conference that preceded last year’s Performance Racing Industry show in Indianapolis, George Bryce of STAR Racing gave a seminar on big bore high RPM engine building. 0004˝ of side clearance with a good set of rings! Some piston manufacturers make pistons that are machined with such a high degree of accuracy that they say you can run as little as. 0012˝ of side clearance for its diamond-lapped second compression ring. 0008˝ of clearance for its diamond-lapped top compression ring, and.
It all depends on the application.ĭiamond-lapped rings, which are smoother and flatter than most standard rings, can often be installed with somewhat tighter clearances. 0025˝ of side clearance for the rings in their grooves. Some original equipment engines may allow as much as. Most rings for performance applications require. Side clearance between the ring and piston groove is easy to measure with a feeler gauge. If gas pressure can’t get behind the ring to push it out against the cylinder wall the ring will seal poorly and leak compression. The top compression ring needs a certain amount of gas pressure behind it to seal the ring face tightly against the cylinder bore surface. The piston supplier can then determine how deep and wide the ring grooves should be to match the rings you will be using.Ģ.
Once you’ve determined which rings will work best in your motor, you can then go to your piston supplier to work out the details of your custom pistons. They know which materials and which ring designs and facings will best match your application. Like your favorite jewelry store, manufacturers and suppliers are the ring experts. Most importantly, listen to their advice. Domed pistons are great for increasing compression, but the valve pockets may have to be made deeper depending on cam lift and clearances (courtesy of Ross Pistons). All of these factors will influence the type of rings that will perform best in your application. In addition to obvious things like bore diameter and desired ring thickness and ring tension, they may also want to know what type of fuel the engine will burn (gas, alcohol, nitrous or nitromethane), if the engine is blown or turbocharged, and what type of racing the engine will be used for. If the application will require custom rings, your ring supplier will need as much information about the engine as you can give them. Depending on the diameter of the pistons and bores, off-the-shelf rings may or may not be available for your application. If you are ordering custom pistons for an engine, get the rings first and then order your pistons.
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