It’s fair to assume that if you’re currently in the stages of building a high-performance engine, you’ve already pushed your engine to its limits or plan to in the near future. To accommodate your custom build, we’re going back to the basics with the help of our favorite question answerer, Eric Hsu, to help clear up many of the misconceptions that come with building an engine. Eric also gives some key pointers on building an engine, from machining the block and picking the right parts, to proper tuning for the most power.
There isn’t a universal set of rules that govern a proper engine build, as every builder has their own theories and methods, but the following are insightful tips we have acquired over the years that we are now going to pass on to you in the hope that you will consider them when you undertake building a performance engine.
Cylinder Boring/Honing
It’s unfortunate, but most automotive machine shops do not put enough attention to detail into honing the cylinder block. Reputable machine shops will go to great lengths when boring a block in order to simulate the same internal stresses the block is under while running. This includes bolting on components that stress the block (torque plate, water pump, head studs, etc.) in its assembled and torqued state.
Honing should always be done with a torque plate (usually a thick, heavy piece of precision-ground steel (minimum two inches) or thick piece of aluminum plate with holes cut into it for access to the cylinder bores) and with the studs and type of head gasket that will be used in the final assembly of the engine. This assures that the cylinder block is honed while in its assembled and torqued state. Torquing the head gasket in place during the boring/horning of the block will apply a certain amount of un-even pressure on the cylinder walls, distorting the final round dimension the machinist wishes to achieve. Some race engine machinists take it a step further and will even run heated coolant through the cylinder block during honing to further simulate the cylinder block’s expansion at operating temperature, states Eric.
How important is it to use a torque plate? We had a machine shop measure the bore distortion with and without the torque plate on a Nissan KA24DE block. Our machinist measured almost 0.001-inch difference, which may seem inconsequential, but in the world of racing that fraction of an inch means everything. Eric mentions it’s safer to run more piston-to-cylinder wall clearance than less; especially if your bores are not perfect.
Head Gaskets
Stock head gaskets maintain proper sealing under normal driving loads. But as compression ratios and combustion pressures go up, a stock head gasket may not be able to maintain a leak-free seal. But beware: not all multi-layered steel gaskets are created equal. Factory or generic MLS (Multi-Layer Steel) gaskets used in modern vehicles aren’t designed to seal under extreme conditions. What distinguishes performance head gaskets from ordinary head gaskets is their design and stronger combustion chamber armor. You’ll notice some of the generic MLS laser-cut gaskets have ripples in the layers after being riveted together, states Eric. This is due to poor dimensional consistency from attempting to laser cut a 0.15mm piece of stainless steel.
Over the years, performance engine builders have used a variety of tricks to keep their engines sealed, such as using copper O-rings around the combustion chambers. Copper O-rings are great in theory and can improve cylinder sealing in some cases, but they do cause additional cylinder head deflection, which is also detrimental to cylinder head sealing. Aside from the quality of the head gasket seal, it’s important to keep in mind the quality of the factory cylinder head castings as well. For example, the Mitsubishi 4G63 head castings were never designed to handle 600+ horsepower. They were designed for 300 horsepower and trying to seal double or triple the power for more than a quarter-mile is asking a lot from the cylinder head.
Some performance oriented head gaskets (e.g., Cosworth, HKS, Tomei, A’pexi, etc.) are designed with stopper layers that increase the sealing force immediately around the cylinders and are stamped from dies, which allow for very exact dimensional tolerances. Development of the stopper head gasket was brought about by those wanting to improve an MLS gaskets design for higher boost levels. The stopper head gasket’s folded layer design raises the gasket around the cylinders by 0.15mm to help increase the clamping load when torquing the cylinder head. Increasing gasket height by 0.15mm might seem inconsequential in terms of sealing, but those protrusions from the top and bottom layers significantly affect how well the gasket seals, as is shown on the ink imprint diagram comparing a Brand X gasket (left) versus a Cosworth (right). The side-by-side comparison shows that the Cosworth gasket produces a better impression around the combustion chambers than the Brand X unit. If your engine continues to blow gaskets even after using aftermarket gaskets and O-ringing, Eric has a simple solution: Turn the power down.