July: Raptor III Piston Tech
Recently, a karter who builds his own engines asked for some help setting up his Raptor III piston shortly after they arrived on the karting scene. The following is a reply to his e-mail asking what cylinder clearances etc. for use on the Raptor III piston.
Honestly I think that will be up to builder preference as always. Here's what I can tell you though...I have tried the new pistons at .0002 to .006 and everywhere in between. Federal Mogul, makers of the new R3 piston, reccommends that you run as tight as .0005 to .0015. The one thing that I think they fail to realize is that most of us run methanol fuel which has a tendancy to wash our synthetic 0-weight oils off the cylinder. Coupled with the fact that we don't necessarily run these engines in the cleanest environment, ie air filtering is not as important to racers as air flow. Racers also have a tendancy to run these engines on the warm side if there's only a few laps to go, and they are leading a race. Thermal expansion and contraction has always been a problem with the cylinders and pistons. This is made even more critical consider we're using an air-cooled engine with relatively thin cylinder walls. Although I saw no problem with running as tight as .002 clearance, I also saw no gain. I think that in a high rpm application there might well be. With tighter clearances, say, .0005 (1/2 thou.), the rings will obviously be mroe stable in the cylinder and retain good wall tension even at high rpm as compared to much wider tolerances. Even at .006 I saw no scuffing of the skirt or top ring land as was typical on the older style pistons. Also no chatter marks at the top of the cylinder. This is probably due to the new ovality design that the piston has. The piston is considered "barrel shaped", think of an old wooden wine barrel. So far I have really liked the new piston. I have seen slight power gains, specifically on top end, and I am sure that there is some increase in acceleration due to the reduction of weight, although I have no accurate way to measure acceleration on our dyno at this time. Biggest problem that I found when the pistons first became available, was the unavailability of rings, specifically over sized top rings or aftermarket top rings. I feel that the low tension bottom and second ring are a welcomed addition, however, I was never really a fan of low tension compression rings. Sure they'll rpm quicker, but I bet we'll see some ring flutter after just a couple races, the same as with the current aftermarket soft top rings. With no ring sets available when the pistons were first introduced, you couldn't play around with ring end gap at all. We're kinda stuck with what comes with the piston, or steal rings out of another piston box, as builders have done for years. Briggs worked very hard filling back orders as we I first wrote this article, I assure you.
I will continue building our customer engines with R3 pistons at .002 clc. until I find something that works better. But the true test will be when guys get a few races on these and they come back for rebuilds. We've magnafluxed some pistons and saw what they looked like after some hard miles. It's difficult to duplicate racing conditions and strains on a dyno alone. In engines with larger than .004 clearance piston to cylinder wall, the skirt of the piston was really taking a beating, end small cracks developed around the oil hole area of the bottom ring land. I would, at this time, recommend against clearances greater than .004. The engines also consumed larger amoutns of oil, and had more oil residue on top of the piston in larger clearance set-ups. For what little they may have possibly gained in freeing up the cylinder drag, I have to believe that performance was offset by the amount of oil needing to be fired by the spark plug.