Engine Building with KB Pistons

    The high performance race engine">



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 horsepower 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 alter cylinder pressures dramatically, also.

    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 (a.k.a. "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 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 the piston that would contact a similar flat area on the cylinder head if you had .000" assembled quench height. In a running engine, the .040" quench decreases to a close collision between the piston and cylinder head. The shock wave from the close collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, averages the chamber temperature, reduces detonation and increases power. Take note, on the exhaust cycle, some cooling of the piston occurs due to the closeness of the water- cooled head.

    Some non-quench engines, such as '68 and later Chrysler V8, A, B or RB's can be converted to quench type with pistons such asKB232, KB190, KB191, KB184, KB278, KB146 or KB236, KB215 and KB280. 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 the benefits of the quench head design and can cause severe detonation. The suggested .040" static quench height is recommended as a good usable 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 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 create more ping at 9.5:1 than you had at 10:1. The suitable way to lower the compression is to use a KB dish piston. KB dish (reverse combustion chamber) pistons are designed for maximum quench area. Having part of the combustion chamber in the piston improves the shape of the chamber and flame travel.

    In the past, detonation would break the 2nd and 3rd ring lands. The massive strength of the KB 2nd land now prevents this; however, all aluminum pistons will fail if heated excessively. By providing extra-strength ring lands we postpone piston failure due to detonation. We say postpone, rather than eliminate, because continued detonation leads into pre-ignition. Detonation occurs at 5 to 10 degrees after TDC (top dead center) . Pre-ignition occurs before TDC. Detonation damages your engine with impact loads and excessive heat. The excessive heat of detonation is what causes pre-ignition. Overheated combustion chamber parts start reacting as glow plugs. Pre-ignition induces extremely rapid combustion and welding temperatures. 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. Run a cool engine with lots of radiator capacity. Consider propylene glycol coolant and low temperature thermostats where legal. Reduce total ignition advance 2 to 4 degrees. 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 engine heat. "Seat of the pants" spark adjustment at low RPM will almost always cause detonation in mid- to high compression engines once they are rung out and start making serious Hp.

    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 the top ring. This action encourages ring flutter and loss of piston ring seal. Past efforts to reduce ring unseating pressure have included increasing the second ring end gap. Now, with the addition of the accumulator groove, ring flutter can be controlled in all engines. The void created by this groove between the rings tends to average the normal pressure present, keeping the pressure low enough to prevent lifting the top ring while maintaining some preload 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 into the combustion chamber. The reason is that the top ring ends butt and stick tight at TDC. Crank rotation pulls the piston down the cylinder while leaving at least part of the ring and top land at TDC.

    Actual end gap will vary depending on the engine heat load. Lean mixture, excessive spark advance, high compression, low capacity cooling system, detonation and high Hp per c.i. all combine to increase an engines' 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 and smog, and increases Hp. If detonation or other excess heat situations develop, a top ring end gap set towards 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 above the cylinder water jacket.

    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 of oil build up. Clean, open oil rings are necessary to prevent oil from reaching the combustion chamber. 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 specifications. The Special Clearance Requirements for KB Pistons chart gives minimum recommended top ring end gaps based on expected operating combustion chamber temperatures. For 2nd ring end gap follow ring manufacturer's specs.

    Ring Options of 1/16" or stock 5/64" are offered on most of the KB Series. The 1/16" option reduces friction slightly and seals better above 6500 RPM, while being considerably more expensive. Stock (usually 5/64" compression rings) work well and help with the budget. Please see the Numerical/Ring Reference Listing.

    Piston to Bore Clearance for KB Performance Pistons were dyno tested at wide open throttle with .0015", .0020", .0035" and .0045" piston-to-bore clearance. After 7-1/2 hours, the pistons were examined and all looked as new, except the tops had normal deposit color. Even with 320 degrees 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 cracks in four years). The added skirt stiffness of the KB pistons reduces piston rock, even if it is set up loose. Loose KB pistons over .0020" do make noise. As they get up to temperature they still make noise because they have very restricted expansion rate and do not swell up in the bore. Our hypereutectic alloy not only expands 15% less, it insulates the skirts from combustion chamber heat. A short term Hp improvement can be had by running additional piston clearance because friction is reduced. To obtain actual piston diameter, measure the piston from skirt to skirt level with the balance pad. See The Special Clearance Requirements for KB Pistons.

    Pin Oiling should be done at pin installation. Either pressed or full floating, prelube the piston pin hole with oil or liquid prelube, never use a grease (if you are using a pressed pin rod be sure to discard the spiral pin retainers). All KB Performance Piston sets supplied from the factory include a tube of Torco/MPZ engine assembly lube. A smooth honed pin bore surface with a reliable oil supply is necessary to control piston expansion. A dry pin bore will add heat to the piston rather than remove heat. Pistons are designed to run with a hot top surface, and cool skirts and pin bores. High temperature at the pin bore will quickly cause a piston to grow to the point of seizure in the cylinder.

    Marine Applications generally require an extra .001"-.003" clearance because of the possible combination of high load operation and cold water to the block. A cold block with hot pistons is what dictates the need for extra marine clearance see our clearance chart on The Special Clearance Requirements for KB Pistons.

John Erb Chief Engineer, KB Pistons

Previous Page / Table of Contents / Next Page