ENGINE BUILDING with KB PISTONS
The high performance race engine by definition indicates that limits are going to be pushed. As far as pistons are concerned that limit is peak operating cylinder pressure. Maximizing cylinder pressure benefits Hp and fuel economy. Considering the potential benefit, owners of non-race engines from motor homes to street rods also look to increasing cylinder pressure. Increasing the compression ratio is one sure way of increasing cylinder pressure, but camshaft selection, carburetion, and supercharging can also alter cylinder pressures dramatically.
Excessive cylinder pressure will encourage engine destroying detonation, and no piston is immune to its effects. An important first step is to set the assembled quench ("squish") distance to .040". The quench distance is the compressed thickness of the head gasket plus the deck clearance (the distance your piston is down in the bore). If your piston compression height (not dome height) is above the block deck, subtract the overage from the gasket thickness to get a true assembled quench distance. The quench area is the flat part of this piston that would contact a similar flat area on the cylinder head if you have zero assembled quench height. In a running engine the .040" quench usually decreases with RPM to a close collision between the piston and cylinder head. The shock wave from the close collision drives air at high velocity across the combustion chamber. This movement tends to cool hot spots, average the chamber temperature, and speeds flame travel after TDC to increase power. On the exhaust cycle, some cooling of this piston occurs due to the closeness of the hopefully cooler cylinder head. The power increase occurs because the shock wave occurs at exactly TDC on all cylinders, every time. It tends to make all cylinders alike and receive more identical flame travel speed. Spark scatter tends to be averaged with the TDC kick received from a tight quench.
Some non-quench engines, such as '68 and later Chrysler V-8's, can be converted to quench type with pistons such as the KB278, KB280, KB372, and KB373. Most Mopar cylinder heads recess the quench area into the head, so a raised area on the piston is necessary to get the close collision. If you are building an engine with steel rods, tight bearings and pistons, modest RPM, and automatic transmission, a .035" quench is the minimum practical to run without engine damage. The closer the piston comes to the cylinder head at operating speed, the more turbulence is generated. Unfortunately, the operating quench height varies in an engine as RPM and temperatures change. If aluminum rods, loose pistons (they rock and hit the head), and over 6000 RPM operation is anticipated, a static clearance of .055" could be required. A running quench height in excess of .060" will forfeit most of the benefits of the quench head design and can push the engine into severe detonation.
The suggested .040" static quench height is recommended as a good average dimension for stock rod engines up to 6500 RPM. Above 6500 RPM, rod selection becomes important. Since it is the close collision between the piston and the cylinder head that reduces the prospect of detonation, never add a shim or thick head gasket to lower compression on a quench head engine. If you have 10:1 with a proper quench and then add an extra .040" gasket to give 9.5:1 and .080" quench, you will likely create more ping at 9.5:l than you had at 10:1. One way to cheat the system is to make sure the piston of choice is light on quench side and to make sure the piston of choice is light on quench side and heavy on spark plug side. As RPM increases the piston tries to cock away from quench surface, allowing a tighter quench at most all RPM. The suitable way to lower the compression is to use a KB Dish Piston. KB Dish Pistons (reverse combustion chamber) are desinged for maximum quench area. Having part of the combustion chamber in the piston can improve the shape of the chamber and flame travel. The Step Dish is sort of an upscale version of our reqular configuration. It allows some piston weight reduction and allows the quench action to travel further across the chamber. It is especially favored when large dish cc's are required.
When detonation occurs, stock type pistons usually break the 2nd and 3rd ring lands. The massive strength of the KB 2nd land prevents this from happening. Detonation destroys by heat, and aluminum does melt. By providing extra-strength ring lands, we postpone piston failure due to detonation. We say postpone rather than eliminate because continued detonation creates hot parts that act as glow plugs. The out-of-control combustion creates welding temperature. Melt down is only seconds away!
For a successful performance engine use a compression ratio and cam combination to keep your cylinder pressure in line with the fuel you are going to use. Drop compression for continuous load operation, such as motor homes and heavy trucks, to around 8.3:1. Keep a cool engine with lots of radiator capacity. Reduce total ignition advance 2? to 4?. A setting that gives a good Hp reading on a 5-second dyno run is usually too advanced for continuous load applications. Normally aspirated drag race engines have been built with high RPM spark retard. The retard is used to counter the effect of increased flame travel speed with increased combustion chamber heat. "Seat of the pants" spark adjustment at low RPM will almost always cause detonation in mid-to-high compression engines once the combustion chamber and pistons are at full operating temperature.
Accumulator Groove is the groove between the 1st and 2nd compression ring. It does make the piston lighter, but the real purpose is more abstract. Pressure spikes that get trapped between the 1st and 2nd compression rings tend to unseat to top ring. This action encourages ring flutter and loss of piston ring seal. The void created by this groove between the rings tends to average the spike pressure of combustion, keeping the pressure low enough to prevent lifting the top ring while maintaing some pre-load on the 2nd (oil scraping) ring.
Top Ring End Gap is often a major player when it comes to piston problems. Most top land damage on race pistons appears to lift the land into the combustion chamber. The reason is that the top ring ends butt and lock the piston at TDC. Crank rotation pulls the piston down the cylinder while leaving at least part of the ring and top land at TDC. Actual running end gap will vary depending on the engine heat load. Piston alloy, fuel mixture, spark advance, compression, cooling system capacity, duty cycle, and Hp per c.i. all combine to determine an engine's heat load.
Most new generation pistons incorporate the top compression ring high on the piston. The high ring location cools the piston top more effectively, reduces detonation, smog, and increases Hp. If detonation or other excess heat situations develop, a top ring end gap set toward the tight side will quickly butt, with piston and cylinder damage to follow immediately. High location rings require extra end gap because they stop at a higher temperature portion of the cylinder at TDC and they have less shielding from the heat of combustion. At TDC the ring is usually above the cylinder water jacket. The current design KB Pistons do a better job of keeping the rings cool.
If a ring end gap is measured on the high side, you improve detonation tolerance in two ways. One, the engine will run longer under detonation before ring butt. Two, some leak down appears to benefit oil control by clearing the rings from oil loading. A small amount of chamber oil will cause detonation and significant Hp loss. The correct top ring end gap with KB Pistons can be 50% to 100% more than manufacturer's specs. Design changes have been made that reduce top land problems dramatically. Read more detail on this in the "New Piston Improvement" article.
Ring Options of 1/16" or stock 5/64" are offered in many KB applications. The 1/16" option reduces friction slightly and seals better above 6500 RPM while being considerably more exspensive. Stock (usually 5/64" compression rings) work well and help with the budget. Metric ring options are becoming more common.
Piston to Bore Clearance of .0015", .0020", .0035", and .0045" were wide-open throttle dyno tested. After 8 hours of maximum torque and 7? hours at maximum Hp, the pistons were examined and all looked new, except the tops had normal deposit color. Even with 320?f oil temperature, the inside of the piston remained shiny and completely clean. Excess clearance has been shown to be safe with KB Pistons (no reported skirt cracks in 13 years). The added skirt stiffness of all KB Pistons reduces piston rock, even if it is set up loose. Less rock allows you to run a tighter quench. KB Hypereutectic Pistons with over .002" clearance may make noise. As they get up to temperature they may still make noise because they have a very low expansion rate. Our hypereutectic alloy not only expands 15% less, it insulates the skirts from combustion chamber heat -- when the skirts stay cool they don't grow. Running additional piston clearance because friction is reduced can sometimes have a short-term Hp improvement.