Engine Mechanics - Bottom-End Basics

The mechanical contrivance most responsible for converting the rising pressure from the combustion of air and fuel is the crankshaft, along with the rods and pistons contained within the block. What you choose to use here can make the difference between building an indifferent and potentially unreliable engine and building a real powerhouse. With that in mind, let's start the ball rolling at the centerline of the crank and work our way out from there.

Crank Know-HowCranks are essentially available in three grades: cast, forged, and billet. That is also about the order of their cost. Many production vehicles have cast-iron cranks. These won't take either the outright power or abuse a forged one will, but they are plenty strong enough for even a decent factory street performer. Also, cast iron has very good wear properties so the life of a cast crank can be very good. A forged crank is a step up the ladder and, for more money, provides greater strength

Although factory cranks are usually lesser grade steels, the steels most commonly used in aftermarket cranks are 4130 and 4340. Depending on the cost of the crank, these can be had with surface hardening which gives a very good wear life. Last on the list are billet cranks. These are made from a block of steel because there is no appropriate forging available from which to machine the desired design of crank. Needless to say, they can be very expensive

If an engine is stripped for a re-build, it is a cost-effective plan to consider all your crank options. These days the availability of quality cast steel cranks makes installing the original used crank only barley less money. Cast steel, which can be looked upon as a hybrid between forged steel and cast iron, allows far tougher cranks to be produced at near the cost of regular cast iron ones. Although it still falls short of a forged crank's ultimate strength, it is an extremely cost-effective halfway house between a regular OEM cast and an aftermarket forged crank. Also, cast steel opens the door to reliable low-cost stroker cranks. Because they can be so inexpensive, you can figure that a new quality stroker crank can be as little as about $75-$125 more than getting your existing, and almost certainly fatigued, crank re-ground.

ReliabilitySo how good are these cast steel cranks? Assuming a reputable company like SCAT, who also supplies an OEM market that calls for tight quality control, we find they are extremely good for all usage up to moderate race levels. Personal experience over a 15-year period with small-block engines up to 580 hp has shown none have failed or worn out. A road-race engine built in 1999 now has over 5,000 race miles on it at a power level of some 500 hp, and the last teardown revealed that it, and the bearings, was still in near-perfect condition.

Although potential reliability is built into a good stroker crank, it should not be taken for granted. Any time the stroke is increased, there is an increase in inertial loads. These and other associated factors need to be taken care of in an appropriate manner. Let's look at reciprocating loads first.

If the stroke is increased by 10 percent, the reciprocating loads will, at any given rpm, go up by 10 percent. Although reciprocating loads are proportional to the mass involved, they go up with the square of the rpm. What this means is that if the engine is turned at 10 percent higher rpm, the reciprocating forces go up by 21 percent (1.1x 1.1 = 1.21). To offset the inevitable combination of the greater stroke and the desire for more rpm, we need to look for a lighter-than-stock piston. Checking through various manufactures' catalogs looking for pistons that are toward the lighter side is time well spent. Here, ROSS, Mahle, JE and KB are worthwhile starting points. If the piston is offered with a lightweight pin upgrade, then, budget allowing, this is well worth considering

Piston SelectionWhen buying pistons you will be presented with some material options that you need to know about. Firstly, regular cast pistons are the cheapest option. To make them harder, the aluminum used for regular replacement and earlier-style OEM cast pistons is alloyed with silicon (which is in essence melted sand). The silicon content makes the piston harder and more wear-resistant, but set against this is the increased brittleness. This makes it the least desirable of the type of pistons available to us for performance use. Next on the list is a hypereutectic piston. These started gaining popularity for lower-cost performance applications in Europe back in the mid '70s. In the early '80s, Sealed Power started experimenting with hypereutectic pistons and subsequently introduced a successful line of them. This was followed by Silovolite, who introduced the Keith Black (KB) signature series performance-oriented hypereutectic piston.

Although the term "hypereutectic" is commonly used, it is often not understood. For "hyper - eutectic," read "super - saturated." An alloy that is at the point where no more of the principle alloying element will dissolve in the parent metal is said to be at the eutectic or saturated point.

A regular cast piston can have up to about 10 percent to 11 percent silicon in it. This is about the eutectic point. Any more silicone will put the alloy into the hypereutectic range and as a result, form free crystals of silicon. Normally, these free silicon crystals will cause a reduction in strength, but with suitable heat treatment a hypereutectic alloy can be toughened up to a level above that of a regular cast piston

In terms of outright strength and toughness, a hypereutectic piston still falls short of a forged piston, but not by as much as the difference in material strength might suggest. The reason for this is that the process of casting a piston, as opposed to forging it, allows the material to be put right where it is needed to support the stresses involved. With a forged piston, the underside has to be designed such that the forging punch can be extracted after the piston blank is formed. It is only by extensive (and costly) post forging machining that a forged piston can rival a cast piston in terms of lightness. By using up some of this low weight advantage by incorporating extra material at strategic points, the cast hypereutectic piston, in practice, comes out looking a lot stouter.

As far as materials for forged pistons are concerned, there are three of note. Probably the most widely used, because it is also used for OE applications, is the Federal Mogul (TRW/Sealed Power division) VMS75 alloy. This was developed probably as far back as the 1960s as a good all-around high-performance alloy for pistons. Most of the rest of the piston industry uses one or both of two alloys, these being 2618 and 4032. The 2618 is most commonly used for outright race pistons as toughness at elevated temperatures (up to about 575 F) is good. On the down side, it lacks the hardness of 4032, which is a high-silicon alloy. Though slightly less tough, the 4032 alloy is a better material for high performance street and race use and would, in most cases, be the alloy of choice for such. At the end of the day, consulting your piston manufacturers about the choice is the way to go, as they are the experts here.

Let's now consider a few key dimensional aspects of the piston design. Probably the most important is the piston's compression height (see the sidebar for definition) as this controls the rod length that can be used with any given block height and stroke length. For most normal situations, a compression height of 1 inch is about the lower limit, but it can, by using thinner, and usually more expensive rings, be made less. Also, a reduction in pin diameter (which should not be contemplated unless you know it will be a functional combination for your particular application) can allow another incremental reduction in compression height. Once we know the compression height that can be used, we can, using half the stroke length (the crank's throw) and the block's deck height, work out the longest rod that can be accommodated within the block. By adding the compression height to the crank's throw dimension and subtracting this from the block height, we end up with the longest rod that will fit. For example, the Ford 5.0 engine stretched to 347 cubes looks like this: crank throw (1.700 inches) + piston compression height (1.000 inches) = 2.7 inches. 2.700 inches subtracted from the block height (8.200 inches) = 5.5 inches. As it happens, a 5.5-inch rod for this application is not an off-the-shelf deal, but companies such as Crower can easily custom produce such. In this instance, a 5.4-inch rod is commonly available, so if the budget does not allow a custom rod, this is what we would go with and use a piston with a hundred thousandths (0.100) more compression height.

The last important deal to look at is the selection of the rings. Increase the side loads and the inevitable greater block flexure means that sealing the cylinder will get a little more difficult. Not only will the use of thinner compression rings help reduce the compression height but it will also better seal the cylinder. For rings, 1/16 inch (0.062 inch) and 1.5mm (0.059 inch) are common these days and are thinner than were normally used just a few years ago. But even thinner rings are becoming more popular. Both 3/64-inch (0.046-inch) and 1mm (0.039-inch) rings are an everyday deal for many up-market hi-po vehicles being built today. If you want to see what is available, check out Total Seal's Web site.

Oil ring widths are also coming around with down sizing. For most V-8s, a 3/16-inch (0.187-inch) wide ring has been the most commonly used size. But efforts to effectively reduce size and weight have brought the 1/8-inch (0.125-inch) and 3mm (0.118-inch) rings into vogue

To accommodate the longest rod possible, piston manufactures are pushing the pin bore up to the point it cuts across the oil ring grove. To allow this to be successfully run, a steel ring support rail is located on the bottom face of the oil ring groove after the piston has been assembled onto the rod. This bridges the break in the bottom of the groove and also allows the oil ring to function as intended.

Rod SelectionThe same weight goals we apply to pistons also need to be applied to the selection of rods. This becomes significantly more important when a stroker crank is involved in the engine build. The rule here is, select a rod stout enough for the job, but no more. By keeping a close tab on the weight of the pistons and rods, we can simplify the balancing job needed for the crank. The goal here is to have an internally balanced crank assembly. An internally balanced crank is one were all the counterbalance mass necessary to obtain full balance resides between the crank's front and rear main bearings. A stroker crank needs more counterweight, meaning either a heavier counterweight, or one at a larger radius. Often there is room to accommodate a longer stroke within the confines of the crankcase, but not enough room for sufficient counterweight. The fix is to put counter balance mass into both the flywheel and the damper on the nose of the crank. Unfortunately, an external balance deal is a bandage fix for insufficient internal counterweight. However, when crank stroke is stretched to the limit, an external balance job may be the only option. The problem with an externally balanced crank assembly is that it applies bending loads to the snout of the crank, which will ultimately cause an earlier demise of the crank if it is pushed to the limit on a regular basis.

We have already touched on the subject of the rod center-to-center length in the discussion on piston compression height. Now let's look at why we should go for all the rod length possible within the confines allowed by the other key dimensions involved. Firstly, a longer rod is mechanically more efficient. The reduced angularity given by a longer rod means less force acting into the cylinder wall on what we call the major thrust side of the cylinder. Also, a longer rod runs quieter.

Because overall engine dimensions can vary so much, from something like a Honda 50 to a navy destroyer diesel with a displacement measured in cubic feet, we cannot categorize rod length just by quoting its center-to-center length. What dictates the angularity experienced by the rod as the engine rotates is the ratio of rod length to stroke, that is, the length of the rod divided by the crank stroke. Anything above about 1.85 can be considered a long-rod engine. Between 1.85 and 1.6 is the most commonly used range for production engines. Rod-to-stroke ratios below about 1.6 will put the engine in the realm of a short-rod design.

As for the power advantage a longer rod/stroke ratio will give, tests on a 3.75-inch stroke, 383 small-block Chevy have shown that, for rods between 5.5 inches and 6 inches, each tenth of an inch longer rod was worth about a 2 lb-ft and 2hp increase. These figures, though, cannot be universally applied across the board. In practice, we find that the more low-lift breathing capability the head has, the more the engine likes a shorter rod/stroke ratio. A two-valve V-8 may well be about optimum with a 1.8 to 2.0:1 rod stroke ratio, but a four-valve engine may well produce best results at 1.55-1.6:1.