Racing Engine Break-In Procedure - Break-In Basics

How you break-in your newly built race engine can greatly affect its subsequent life and power output, as well as the amount of cash left in the bank account for racing. There are many facets to an optimal break-in. The first aspect we should appreciate is that no matter how well or accurately the engine and its components have been machined and assembled, the engine will never be broken-in as built.

I have seen a number of really good engines that have had the edge taken off their potential power because the necessity for an adequately extensive break-in was not appreciated. I remember a dyno session with one well-known West Coast engine builder (and a good one at that) who announced that the engine would be broken-in by the time it had warmed up. Prior to firing the motor, I took the opportunity to use a borescope to look at the bores while the plugs were being gapped and installed. The block was machined at the same machine shop I used, and as expected, all the bores appeared to be in pristine condition.

After about a 10-minute warm-up at 2,500 rpm or so, this roller-cammed engine was declared ready to do some serious power pulls. Well, our engine builder friend had done several engines similar to this one, so ignition and fuel calibrations were close. The results of the first pull really impressed me. I had built a similar motor to the same basic rules the previous year, and this one topped mine by over 30 hp. After some calibration, that figure went to about 40 hp.

Obviously, this guy knew what combinations worked. Having announced we were all done and it was lunchtime, I took the opportunity to pull the plugs to see what the mixture spread looked like. I also looked at the bores while doing this, and they did not look good! There were fine scores in every cylinder and deeper scores in some. In less than an hour's running, this engine had completely bypassed the broken-in phase of its life and had gone straight to the wearing or worn-out phase. This meant its competitive life on the track would be shorter and less effective and a new high-dollar engine, or at least a rebuild, would be needed that much sooner. Had the need for a break-in been more clearly understood, this engine would have made more power and lasted longer.

The two prime factors we attend to during break-in are friction and ring/bore seal. Although important, components such as timing chains and bearings are very much secondary considerations. The whole purpose of paying extra attention to bores, pistons, and rings is to improve their combined ability to seal high-pressure gases above the piston crown with a minimum amount of frictional losses. The higher the intended engine rpm, the greater the horsepower loss is for a given frictional torque. Let's say we have a set of pistons and rings that are of a production style and have a combined 5 lb-ft more friction than a more race-orientated design. At 5,000 rpm, that extra friction will cost 4.7 hp, but the loss caused by that same 5 lb-ft of extra friction will be 8.1 hp at 8,500 rpm.

Like it or not, the last machining operation done on all the moving parts within your engine occurs after it is built. Those of you who have machining experience realize that machining a piece of steel can produce anything from an ugly finish to a smooth shine. It all depends on the tool shape, cutting speed, depth of cut, and the cutting fluid used. Using that analogy, what we are trying to achieve here is an assembled machining process (normally called break-in) that produces the smoothest surfaces within the engine. Since ring and bore friction typically accounts for half the friction within the engine, it makes sense to gear a break-in to more specifically address these surfaces.

Various phrases such as "the rings need to seat-in," or "the rings have not seated," are pretty common expressions, and I bring this up for one reason. In 44 years of building engines, I have not had one case in which the rings did not seat-in. The ring manufacturers I spoke with about this were very surprised, as rings not seating properly was one of the biggest break-in issues they had to face. At the end of each discussion, the conclusion was that I was one of those freaks of nature-a racer who actually read the instructions pertaining to bore finish and such for the rings in question. I am sure that using new rings on old glazed bores could be a problem, but I am also equally sure that the rings will seat-in just fine if everything is done right. The question is, what is right?