Sonic Testing - Sonic Treasure

Finding cubes without a blowup is easier and cheaper than you might think.

By David Vizard

Seven hundred and ten fuel-injected, street-drivable normally-aspirated horsepower from a 441-inch small-block Chevy hardly sounds like a budget deal, but the truth of the matter is the block, at only $75, was a steal. If you think that's good, how about a 302 Ford block that, when punched out .60-over and equipped with a Scat 3.40-inch stroke crank, delivered 352 inches. The block for this project motor was from a salvage yard and set us back a whole $50. This street driver, without blower or nitrous, went on to produce 525hp and 465 lbs.-ft. of torque. Normally, for engines of this stature, one would expect that heavy duty (and usually expensive) factory blocks would be mandatory. So how, you may ask, especially in the case of the usually fragile 302 Ford, was it possible to get away with such big cubic-inches generating huge overbores and still have the engines live? Was this simply dumb luck or smarts? Well, if it were not for bad luck we would not have any luck at all so, biased though it may be, there is only one option left.

In our world, building project engines is a way of life and building powerful ones, a passion. Each one, regardless of inches, has cubic effort put into it and having it grenade on the dyno is a major disaster. Such an event not only impacts magazine copy dates but also has a serious negative effect on the thickness of your wallet! So how do we minimize such disasters? Two words--sonic tester. This is a piece of equipment that allows the measurement of the thickness of a homogenous material from one side only. Before we go into the implications and cost analysis of using such a device, let us give you a little history and also a basic explanation of its function.

This author's first experience with sonic testing was back in the days when Cosworth first started to dominate Formula One. Back then, Formula two and three were feeder classes for F1 and as such relied on power from stock block race engines. I was running a derivative of an F3 engine in a one-liter Ford Anglia road racer. This 61-inch short-stroke big-bore pushrod Ford engine made an ear-piercing 109hp at the rear wheels at some 10,000-rpm. I remember getting the block sonic tested at Cosworth and it cost what then seemed an arm and a leg. Since then, the value of sonic testing has become vastly more appreciated and as a result their use has proliferated. During 1997, this author found that one manufacture of Sonic testers, StressTel, had the price down to almost $1000, so it had to go into the tool box. Currently, the same unit from StressTel is now under $1000. Granted, that is still far from peanuts but, in our experience, the payback is so good not buying one would have proved far more expensive.

The mode of operation of a sonic tester is easy to understand. Essentially it comprises a sending/receiving probe and a box of electronics to analyze a signal return time. The diagram shows the principle of the probe's mode of operation in conjunction with the data processing box. When sound encounters a sudden change of material density in its path, it is reflected. If the speed of sound through the material/medium is known, then the distance to that change in density can be accurately determined by measuring the time it takes for the sound to return to its source. As the diagram shows, the probe emits pulses of high frequency sound that travel through the part being tested. These sound pulses are then reflected by the opposite surface and picked up by the receiving side of the probe. Given the right calibration, it is possible to measure the thickness of a section of cast iron or aluminum to less than five-thousandths accuracy. This has great implications toward producing relatively cheap power and torque increases while retaining reliability. Cubes = Cheap Torque

Acceleration that pins you to the back of your seat is the direct result of the engine's torque output. Torque is a function of the charge weight drawn into the cylinder. The greater the charge weight, the higher the torque and the greater the acceleration. It's that simple. Inducing greater charge weight is achieved by means of a supercharger of some kind, nitrous or more cubic inches. By far, the cheapest route to torque is more cubes via an increase in bore size, especially if it is done at the time of a rebuild. Big over bores are about the cheapest inches you can get. Stroker cranks--and there are some good inexpensive ones around these days--are great, but be aware they are better yet when paired with big over bores.

The days when bores were almost universally half an inch thick are gone. Weight reductions and economy of material have seen to that. This means before considering a big over bore it is paramount that we know whether or not the block will deal with it. Cylinder walls that get too thin will suffer excessive blow by and eventually fail (and inflict a degree of financial ruin). The sonic tester allows you to determine exactly what you are dealing with in advance. It not only ensures you do not waste time and money machining up a block that is doomed to failure, but also it can reveal some very low-cost performance gems. The first year of owning the sonic tester, we found two 400 Chevy blocks that were as thick as Bow Tie race blocks. Both of these were bored plus sixty and still had cylinder walls that, other than at the siamesed section, were around the 300 thousandths mark. One of these blocks came from a salvage yard, the other from a swap meet. One of these went on to be a 441-inch small-block, the other with a shorter stroke crank, a 427. These blocks each cost $75 a piece. By comparison, a pair of Bow Tie blocks would have set us back well over $2,000.

Staying with small-block Chevys a little longer, it's generally believed that for a performance 350, any bore over +.030-inch is out for a high-performance application. Because of this belief, there are a lot of unwanted +.030-inch blocks around. The bottom line here is that 2 or 3 blocks out of ten will go to +.060-inch and still leave the cylinder walls at or above the average block selected on the basis of minimal visual core shift. We have picked up a couple of very clean +.030-inch blocks that tested out okay for as little as $20! Bored to .060-over, they gave 5 cubes more and subsequently proved fine for 500-plus horsepower build-ups.

Also in the first year of ownership of a sonic tester we found a 302 block that, at .060-over, was still thicker than 2-out-of-5 standard blocks. Knowing how delicate small-block Ford blocks can be, we made good use of the sonic tester to weed out thin blocks. For a reasonably reliable 500hp small-block Ford, probably half the blocks out there are non-starters. Another notable find the first year was a 454 big-block Chevy block that had cylinder walls over about 400-thousandths thick. This made a perfect candidate for a .125-overbore using KB hypereutectic pistons and boosting inches to 482. That block cost $60. The money saved by being able to utilize stout production blocks instead of having to buy factory/aftermarket race blocks was about five times the cost of the Sonic Tester--and that was just the first year. Along with that saving was the near certain grief we avoided by detecting blocks totally unsuitable for the rigors of high output.

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 CASTING
What 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.