OE blocks are not the only candidates for sonic testing. While demonstrating the StressTel's sweep feature, which shows the thinnest section within that sweep, on an aftermarket block we discovered a thin patch. In a highly localized area, the cylinder wall (normally about 400 thousandths thick) was down to less than 100. The block was intended for a Pro-Mod type application and was going out of the country, so reliability was a major factor. Had we not found this anomaly, the block, under the pressures of 1600 -1800 hp, would have certainly failed almost right away. Not only would it be embarrassing to have a $30,000 motor fail on the first pass but also shipping it back from a quarter of the way around the world and dealing with all the associated customs/shipping hassles would have made it a financial nightmare.
During the years we have had the sonic tester, two points stand out above all else related to this device: First, we cannot do business without it, and secondly, those engine builders who claim they don't need one are probably right, simply because they aren't building any serious horsepower.
Fixing A Marginal CastingWhat is marginal? This is not easily answered as many factors influence the situation. Not only is the thickness of the cylinder wall a factor but also its unsupported length. If you are building a 302 small-block Ford, you can get away with thinner cylinder walls because the water jacket is only about half the length of that used for a small-block Chevy. For a small-block Chevy, when cylinder wall thicknesses get down to 5/64ths of an inch (140 thousandths), then things are getting a little on the thin side. The fact that the cores shift during the block casting can mean that the cylinder walls are generally thicker in most places but have some areas that are thin. The way to counter the negative effect of these thinner sections is to partially fill the block. Filling the bottom half of the water jacket can substantially increase support for the cylinder walls while having zero effect on water temperature, although the oil will run hotter. Race Engineering in Lake Worth, FL (561) 533-5500 sells an easily poured block filler.
What Do Big Bores Deliver?More cubes from a bigger bore means more torque and power without an increase in piston speed. Bigger bores can accommodate larger valves or help unshroud the existing valves more. Either way, breathing potential is increased. A higher compression ratio can be achieved before a raised crown is needed. If a longer stroke crank is used, a bigger bore can help restore the bore/stroke ratio to a more favorable figure. Also the increase in bore size with a longer stroke crank means a greater number of extra cubes is contributed by the overbore operation. If the full potential of an overbore is taken advantage of, it is possible to get a bigger percentage increase in output than the percentage increase in displacement. Some of the more positive instances of this have been a 5 percent increase in displacement delivering a 7 percent increase in output.