'The performance world is full of cool stroker engine combinations these days that fill more pages in crankshaft catalogs than ever before. We're talkin' about long-arm 460 Fords, wild Cleveland combos, oddball inline-six cylinders, and stroke bumps for the new generation of GM Gen III engines. All of this is in the service of the "mine is bigger than yours" approach to horsepower heroism. But all this stroker hype can also cause confusion.
Car crafters are famous for buying parts through nontraditional means such as swap meets, shop closeouts, and clandestine good-guy deals. While the price is usually right, this can also lead to hidden costs when it comes time to balance the rotating assembly. Balancing sounds simple, but as we found, there are many shops out there still drilling holes the traditional way when applying a few simple tricks may make life easier and less expensive. We ran across a typical weight issue when it came time to balance a small-block 331ci stroker Ford. The gruff old guy at the local balance shop said, "I'm not gonna balance this. I'd have to turn it into a piece of Swiss cheese. Take it someplace else. . . . " So we did and learned a little about balancing engines in the process.
A properly balanced high-performance crankshaft and rotating assembly should come in at le
Crankshaft counterweights are positioned to offset the weight of the combined upper portio
This is a stock, elastomeric big-block Chevy balancer that has been subjected to multiple
A Question of Balance
Before digging into the custom stuff, we figured we'd better brush up on exactly how OE engines are balanced and look into the internal/external-balance issue. To start us on our journey, we decided to talk to Scat Crankshaft's Tom Lieb, who has a strong background in the area of balancing. Lieb is very opinionated about this issue because he has seen just about every imaginable variation on crankshaft failure. Most of these are not due to poor quality, as many think. Usually, the engine has either been abused with excessive rpm, balanced improperly, or often suffered some parts-abusing combination of the two.
Let's start with a short lesson on crankshaft design. Crankshaft counterweights are designed to offset (or balance, if you will) the inertia effect of a relatively heavy piston and connecting rod moving in both a rotational and reciprocating (up-and-down) fashion at speed. The weight of the piston-and-rod combination affects the size and placement of the counterweight. A longer stroke combined with a heavy piston, pin, and ring package requires a larger counterweight (more mass) to balance the greater reciprocating weight. Most V-8 engines use large counterweights toward the front and rear of the crankshaft, leaving the center portion without counterweights. That splits the engine into front and rear halves. The positions of the counterweights on all V-8 90-degree crankshafts are the same. The height of the counterweight as measured outward from the crankshaft centerline is limited by both the cylinder block and by the placement of the bottom of the cylinders. A counterweight placed farther away from the crank centerline has more balance effect, but it is limited by the width of the block crankcase. Weights placed toward both ends of the crank also have a greater effect and therefore don't need to be as large to effectively balance the engine. This makes the overall crank lighter.
Internal vs. External Balance
Packaging is also an important issue. During the design of the 400ci small-block, a major engineering hurdle was insufficient real estate inside the small-block crankcase for the larger counterweights demanded by the 400's supersized 4.125-inch piston. This was especially difficult in the rear of the engine because the rear crankcase area on a small-block Chevy is restricted by the placement of the oil filter. The solution called for external balance weights placed on the harmonic balancer and flywheel/flexplate. One advantage to external weights is that they are generally lighter because they are positioned at the extreme ends of the crankshaft. The disadvantage is that these offset weights impart their own twisting forces back into the crankshaft, which is not good. This same situation occurs with the 454ci big-block Chevy, which is also the only production big-block that uses external weights to balance the engine.
It is easy to spot an externally balanced engine by looking for either an offset weight in
Ford small-blocks have always been externally balanced, but the amount of the offset weigh
Small-block Fords have always been externally balanced, but because Ford is a name synonymous with change, the Blue Oval engineers altered the amount of the external balance when the engines morphed to a one-piece rear main seal. Early small-block Fords used 28 ounce-inches as the external weight amount, changing in 1981 to 50 ounce-inches. Like small-block Chevys, parts can interchange between early and late engines, but to guarantee smooth engine operation, the crankshaft, balancer, and flexplate/flywheel must all be kept within the same balance family.
Bob Weight Blues
When it comes to any kind of crankshaft purchase or if you are considering adding a set of new, lightweight pistons to an existing engine, balancing that rotating assembly can be challenging. One of the references you will run across is something called bob weight. This spec refers to the mass located on a single crankshaft journal that includes both the reciprocating and rotating weight portions of the entire assembly. Rotating weight refers to the mass of the bottom half of the connecting rod that is attached to the crankshaft. The reciprocating weight is basically everything from the middle of the connecting rod upward, including the piston, wristpin, and rings.
Before the entire crankshaft assembly can be properly balanced, the shop first weighs all
The balancer and flexplate or flywheel must also be used to ensure that the entire rotatin
Every high-performance crankshaft has a specific bob weight value, such as 1,800 grams. A gram is a metric measurement equivalent to 11/428 ounce. An 1,800-gram bob weight is basically the amount of weight in each of the counterweights. However, you don't just add all the values up to determine bob weight. The formula is 100 percent of the rotating weight plus 50 percent of the reciprocating weight. Let's use an example to see how this breaks down:
| ||Rotating weight (gm.) ||Reciprocating weight (gm.) |
|Rod bearing ||50 || |
|Rod big end ||420 || |
|Rod small end || || 180 |
|Piston || || 450 |
|Wristpin || || 80 |
|Rings || || 38 |
|Oil || || 2 |
|Subtotal ||470 ||750 |
Before we go further, remember we have a pair of reciprocating weights (two pistons) per crank journal. The math looks like this:
Half of reciprocating weight is 750/2 = 375 x 2 pistons per journal = 750 grams, while total rotating weight is 470 x 2 = 940 grams. Thus 750 + 940 = 1,690 grams.
If the bob weight figure for your new crankshaft is more than the bob weight of your new rods, pistons, and rings, then weight must be removed from the crankshaft in order to balance the assembly. This is a relatively simple task of precision-drillng a few holes in the crankshaft counterweights. If the bob weight figure for the crankshaft is lighter than the piston-and-rod combo, Mallory metal must be added to the crankshaft. Mallory is a very dense metal, more than twice the density of steel. A 11/42-inch-diameter, 31/44-inch-long slug of Mallory weighs 43 grams. The problem is that drilling a hole to place the Mallory in the crank removes roughly 24 grams, so the net gain is only about 19 grams. Thus adding 50 grams can be expensive because Mallory metal isn't cheap. It also means that researching your stroker-engine package carefully to avoid major balancing headaches and expenses is well worth the effort.
Many car crafters prefer to assemble parts for a stroker package from several different sources to save money. Where this can cause major difficulties is when mismatched components are assembled haphazardly and the balance shop must make major changes to the crankshaft in order to balance the engine properly. A simple way to avoid this problem is to purchase a complete rotating assembly from a single source. Most of these stroker kits have already been properly matched so that only minor machine work such as drilling the crank must be done to complete the final balancing.
Here is the Professional Products balancer's external weight for a small-block Ford. To reduce the amount of drilling necessary to balance this combination, Ishigo removed some of the weight from the balancer.>
Custom Balance Shortcut
Oftentimes, high-performance stroker applications can get a little complicated. As an example, we're working on a 331ci small-block Ford stroker engine buildup that includes a Scat forged steel crank, Scat I-beam rods, and a set of Mahle forged pistons. When we first assembled this package, we chose the Mahle pistons because of their high quality and light weight. Unfortunately, this caused a problem when we took the assembly to the balance shop because an excessive amount of weight would have to be removed from the counterweights to balance the engine. That's when we asked Tom Lieb at Scat for some guidance. His suggestion was simple: "Take the weight off the balancer and the flywheel." That sounded easy, and he offered to show us how it's done.
At Scat, the company's lead technician, Craig Ishigo, showed us the procedure. Our situation was made more difficult because the custom flywheel required for the engine could not be used on the engine dyno. This meant that Ishigo could not reduce weight by drilling the flywheel because a different flywheel with the standard offset weight would be used on the dyno when we tested the engine. After weighing all the components, it appeared that more than 560 grams (20 ounces) had to be removed from the entire assembly. That's quite a bit of metal.
Ishigo says that while the electronic balancer can read each counterweight individually, he generally treats the engine as front-half and back-half setups. Because this Ford is an externally balanced engine and so much weight had to be removed, Ishigo's first step was to remove 77 grams from the bolt-on offset weight in the Professional Products balancer. But keeping the amount of weight removed from the front and rear symmetrical required removing metal from the crank counterweights on both ends. So Ishigo set the crank in a large lathe and removed more than 400 grams from the front and rear of the crankshaft. The amount of weight removed was limited by the height of the rod journals on the opposite side of the counterweights. When the lathe just barely touched the rod throws, the cutting stopped. Next, he removed additional weight by drilling multiple holes in all the counterweights. Ishigo decided to drill three shallow holes in each counterweight rather than two deep holes, removing another 82 grams from the rear, roughly similar to the 78 grams removed from the balancer. Ishigo says that drilling multiple holes is safer because it's shallower, but drilling holes alongside the direct path of imbalance is also less effective than removing weight directly in line. It comes down to how much weight must be removed. We added up the total weight removed from the crank, and it came to just under 1 pound, 4 ounces. While not a huge amount compared to the overall weight of the entire rotating assembly, it still equates to a lighter package that will rev a little quicker and help acceleration.
Most balance shops are not equipped with a large lathe like this one at Scat, but this is
The farther the holes are drilled from the opposite side of the rod journal, the less effe
Here is a screen shot of the electronic readout of a Hines balancer after Ishigo finished
Another area of potential confusion lies with the extremely popular 383ci small-block Chevy stroker packages. The 383 originated using a 400ci engine crankshaft in a 350 block. As mentioned earlier, the 400 was the only small-block that was externally balanced, and most 383 kits retain this external balance feature. But for higher-end applications using a 4340 forged steel crankshaft, the ideal path is to internally balance the rotating assembly to reduce crankshaft flex. Now let's make this more complex by using a one-piece rear main seal block and crank. With a Lunati Sledgehammer crank and rotating assembly, this is exactly the combination used in the 383 small-block that was our test engine for the giant parts-test story "Are Premium Parts Worth the Price?" in the Sept. '07 issue.
It's easy to spot the weight for the flexplate on a one-piece rear main seal flexplate (ar
Here is what Mallory metal looks like in a crankshaft. Note that the weight is added horiz
This is what happens when too much weight on the end of the crankshaft is combined with lo
At first this would seem simple, since you don't have to use an offset weight balancer or flexplate/flywheel. However, a one-piece rear main seal rear flange does not allow for the offset weight that is used on a two-piece rear main seal engine. To account for that weight difference, all one-piece rear main seal flexplates (and flywheels) require an offset external weight. But with an internally balanced crank, this weight is not necessary. Thus in the case of a flexplate, the external weight must be carefully removed. For flywheels it's possible to have a zero-balance flywheel custom made. If you already have a flywheel, additional holes can be drilled to bring it back to a zero-balance condition. If this is not done, the engine will experience a vibration that will eventually tear up the main bearings and cause damage that could easily be avoided.