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-How
Cranks 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.
Reliability
So 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 Selection
When 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.
Stealth torque and power by means of a stroker crank stretched this 302 out to 360 inches. |
A stroker crank has the rod journal pin further from the center of rotation, thus causing the piston to sweep a greater length of the bore. |
A SCAT cast steel aero 3.75-inch stroker crank and a set of 6-inch 4340 I-beam rods can be had for around $450 and will stretch a small-block 350 Chevy to 383 inches. |
Journal overlap contributes toward the overall longitudinal and torsional stiffness of the crank. The longer the stroke gets, the less less journal overlap there is. |
Although the base material is not quite as strong, a cast piston can make up some of the difference between it and a forged piston by virtue of being able to place the material in structurally more favorable locations. Here, cast-in struts support the pin area and the skirt. In many other areas, this cast piston is usefully thinner than its forged counterpart. |
Here is a SCAT lightweight 4340 through-bolt small-block Chevy rod that has been smoothed, polished and Teflon coated. When used in a stroker application, the head of the rod bolt (arrowed) can hit even a reduced base circle cam. This necessitates grinding a little material off at the point indicated. |
This ROSS .125-inch overbore big-block Chevy piston features a high dome for high compression. Weight is kept to a minimum by the use of ROSS' hollow dome forging blank. |
Seen here is a CP cup car piston. It is structurally far more complex than most pistons because a great deal of effort has been made to put material where it is needed to keep weight down and strength up. |
If you have ever fought to install Spirolocs, here is the no-hassle installation technique. On this piston note that to accommodate the increase stroke and a longer rod, the pin bore invades the oil ring groove. A steel support rail ring is used to bridge the pin bore/ring groove gap so the oil ring can function as normal. |
A better plan than the previously-mentioned through-bolt rod is to use a stroker-clearanced version of this SCAT rod as seen here. Complete with ARP bolts these 4340 rods are light, strong, and very cost effective. |
These entry-level priced H-beam stroker rods from SCAT are among the lightest available, short of upgrading to significantly more expensive super-light rods. |
Here we can see that for any given stroke, the longer the rod is, the less the friction-inducing rod angularity is. Much of the friction is caused by gas pressure on top of the piston translating into two distinct components, one that turns the crank via the rod, and the other that tries to turn the piston and rod assembly about the rod journal, thus rotating it into the cylinder wall. The side of the cylinder that sees these loads is known as the major thrust side. |