Engine Build - The Outer Limits

Build Your Next Engine Big, But Build It To Last. Here Are Seven Simple Rules To Help You Do Just That.

By David Vizard, Johnny Hunkins

Photography by David Vizard

Make one crucial mistake, and big inches can spell big disaster. Here I've paid my dues the hard way, and for that matter, the expensive way when it comes to pulling all the inches possible from a block and crank assembly. Most of these lessons were learned while I lived in England, and the engines I built were mostly small four-cylinder units.

My most enduring disaster was a Ford Lotus twin-cam build. The engine, as it comes in a Ford Lotus Cortina, displaces 1,498 cc (91.4 inches). It had a deep-breathing Hemi twin-cam head, and if you knew what you were doing, could punch out about 200 hp. That, for 1965, was not too shabby. With as much flow as the head could deliver, it looked like it could handle much more displacement under it. Without going overly into detail, a block bored to what was normally considered safe was acquired along with a stroker crank and the appropriate rods and pistons.

For an underpaid aerospace engineer, this was a considerable investment. What the money and effort bought was a bottom end that displaced almost 1,800 cc. That represented a displacement increase of almost 20 percent. Putting this into perspective, that was like stretching a 327 small-block Chevy (at this time the 350 was still in the future) to an unheard of 390 inches. On the dyno, things looked good. As the fuel injection and timing were dialing in, the torque came up. At about 145 lb-ft, this twin-cam had already surpassed the 122 hp we had seen from a 1,498 unit by a good margin. At 7,000 rpm the engine was approaching the magic 200hp mark when it stopped in an instant. No warning, no apparent slowing down, just a seemingly instant stop. I stared in a state of disbelief. What, I asked myself, could stop an engine from 7,000 rpm to zero in no time flat?

About that time something caught my eye on the dyno window, so I refocused from the engine to the window. There, I could see a horizontal thin brown line across the window. I then noticed this brown line went entirely around the dyno cell and, as I watched, it slowly got wider. The reality of the situation then hit me--the engine had split a cylinder and hydraulically locked on the water that had become trapped above the cylinder-wall split. It then lifted the head and shot the water out horizontally at very high speed. I actually manage to remove about 90 percent of the head gasket without removing the head.

What I salvaged from the hydraulic mess amounted to three rods and pistons, the pan, the fuel injection, and the cam cover. The rest, including the head, (which was bent about an eighth of an inch) was junk. The crank, amazingly enough, did not break, but the big end journal of the offending cylinder was realigned by about 45 degrees. One thing is for sure about disasters: they teach you what not to do really fast. What I learned here was not to so readily accept that the overbored cylinders are still thick enough to hold out. It was some years before I once more attempted overboring one of these small Fords above the normally accepted plus-40 or -60 sizes. This time I took the block for what would be a maximum-effort engine to Cosworth for sonic testing to establish just how thick (or otherwise) those cylinder walls were. And, should you be wondering, the moral of this story is that the block is the case of the engine. Due to both cylinder pressure and reciprocating loads it has to contain all the forces. In a high-output engine, these are considerable. The fact of the matter, especially when inches are achieved with an overbore, is that power and block strength are going in opposite directions. The price for trying too hard to maximize an engine build on a minimal block is much higher than starting with a block of known capability.

For most hot rodders, big inches are almost a fatal fascination, but to make big numbers and retain reliability it is important to stick to a few important rules. Let's start with the most important.

Retain Block Integrity
If you are looking for additional cubes, then the first source should always be from an increase in bore size. Why? Because other than the possible loss of sufficient cylinder-wall integrity, there is no down side. Bigger bores increase displacement without a significant increase in bore friction and they allow the valves to be unshrouded a little more for a small improvement in breathing. Buy a good set of pistons, and the reciprocating weight is unlikely to change much. Also, when a stroker crank is used with a big bore it adds more inches than with a smaller bore.

An example here shows the difference. Assuming a 3.48-inch stroke as a starting point, installing a 3.85-inch stroke crank in a small-block Chevy with a 4.03-inch bore results in 37.8 extra cubes. Making the same crank change but with a 4.155 bore (400 block) delivers 40.1 extra cubes.

When you are looking for all the bore size possible, it is the sonic tester, not the boring bar, that is your best friend. If time is spent with a sonic tester measuring thickness of production factory blocks (especially small-block Fords) it won't take long for you to realize the limitations being dealt with. Unless you own a sonic tester and test every block that passes your way, you will quickly realize the benefits of going straight to a good aftermarket block. Of late I have been working up experience with Dart's blocks, especially its Ford blocks. Factory-produced Ford blocks are cast using what Ford calls a thin wall casting technique. Sounds good if you want a lightweight engine, but it spells death to big overbores. What partly saves Ford's thin walls on its small blocks is that the unsupported water-jacketed length is considerably shorter than its Chevy counterpart. This means it can get away with a slightly thinner wall thickness.

About one in ten 5.0 blocks have cylinder walls consistently in the 180 thousandths range. At the other end of the scale, a not-so-good casting can be down in places to under 90 thousandths. A thick block will stand a 60 overbore; a thin one isn't even safe at stock size!

Windsor 351 blocks, especially the older ones, are usually thicker. By spending time with a sonic tester I have found blocks with cylinder walls in the 200 to 250 range. But, time is money, and at the end of the day, if I want a serious big-inch Ford, I pick up the phone and order a Dart Ford block. These consistently have cylinder walls in the 375 thousandths-plus range and are cast using a much better grade of cast iron than the stock factory blocks. With a 4.125 stroke crank and a little re-pitching of the bores, these blocks will go to a comfortable 460-plus inches and still have the strength to contain over 1,500hp output. For more details, I suggest you hit its Web site at www.dartheads.com.

With Chevy blocks we are in somewhat of a better situation as far as block integrity goes. For the most part, factory Chevy blocks are good--at least for anything other than endurance racing (Cup car and the like) to power levels in the 700-800hp range. Find a thick 400 block or a regular Bow Tie block and bore sizes up to 4.185 are no problem. Once in a while a really thick one comes along that will go to as much as 4.250 on the bore. With a 4-inch stroke, that makes 454 inches, but that is almost certainly the limit for a stock-configuration small-block Chevy. To get even a 4-inch stroke into the block it is necessary to run a cam with a smaller base circle in order to provide clearance for the shoulders of the con rods. The solution here is the Rocket-style block. This was originally made exclusively for GM by Dart, but has been available for some years now direct from Dart. These beefy blocks have the pan rails wider apart for longer stroke cranks, and the camshaft centerline has been moved up some 390 thousandths to give rod clearance. Also, it is now practical to go to a 4.25-inch bore using the big bore (up from 4.185 to 4.250 bore) Fel-Pro 1036 head gasket. To date, the biggest I have worked on with these blocks is 468 inches (4.185 bore x 4.25 stroke) and I am currently working on a 482-inch one (4.25x 4.25). These engines are what I call regular big motors. The next category above that are the super-big motors that take a lot of time and effort to piece together. Sonny Leonard, who is most likely to collect the award for crown prince of super-big-inch motors, has built a 502-inch small-block using the Dart block, but be aware, he is not in business to tell you how it was done.

As far as big-blocks go, we find that given a sonic tester, many factory blocks will bore 100 over and quite a few at 125 over. Past that, you are back to aftermarket blocks from Dart, Bill Mitchell, and the like. If you have a good big-block Chevy-block that is capable of going to a 4.375-inch bore (125 thousandths over) then a relatively inexpensive way to bump that to 511 inches is to install a 4.25 stroke crank. Scat is a good source here, but a new name on the block for budget cranks at a good price is SYSC. If you find a really thick big-block Chevy, there can be enough cylinder wall material to bore as much as 250 thousandths over (540 inches), but these are not that common. Even if you do find a good stock block, there can be a lot of work required to transform it into a good candidate for a high-output big-inch unit.

Accept Realistic Geometric Block Limitation
To sum up the initial problems arising when stretching production factory blocks, we can say that bore size is limited by casting thickness, and crank stroke by crank case clearance problems (including hitting the cam). So for cylinder walls, how thin is too thin? For engines producing 3 hp-per-cube, I personally do not like to have less than 200 thousandths at the thinnest part. On real street engines, even with a moderate dose of nitrous, 180 thousandths will work.

What is marginal? This is not easily answered because 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-wall values 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 gets down to about 5/64-inch (140 thousandths), then things are getting a little on the thin side. The fact that the cores shift during the block casting can also mean that the cylinder walls are generally thicker in most places, but could have some areas that are dangerously 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, Florida (561-533-5500), sells easily poured block filler.

The forgoing are not the only factors weighing against success with big inches. Remember, the idea is to make big output, not just be able to brag about cubes. When the stroke is increased, the con rod length really needs to be increased in proportion, or maybe more. A short rod-to-stroke ratio brings about more piston-to-bore friction, and that's not good for power. Unfortunately, many production blocks are just too short to be able to get a decent rod/stroke ratio. This makes a tall-deck aftermarket block just that much more desirable.