which balancer

[ace1]

There are several size balancers for a sbc from 6"to 8". What determines which one to use?

[perfconn]

Big motor -big balancer.Small motor -small balancer

[stockcar5]

Big motor -big balancer.Small motor -small balancer

This is dead wrong.

Call a reputable company like ATI and have the experts help you out. There is no set size for a certain size motor. If anything it relates more to stroke not engine size. Would a big bore short stroke 360 need the same size balancer as a smaller bore longer stroke 360?? Dont think so.

[ace1]

I have a 327 with a small one. I had a 383 that had a smaller one. I looked at a 604 and it had the 8". Just wondering why the different sizes. I need one for a 358 and wondered which one to get.

[Lizardracing]

4.185 x 3.25=358

or

4.030 x 3.5=358

[ace1]

actually 357- 3.48, 4.040

[Rodz]

I use a 7" on nearly all of my sb builds.

[ace1]

Big motor -big balancer.Small motor -small balancer

I ran across this and thought it was interesting. I think I'm going with a 8".

6- or 8-Inch Balancer?
Is there a rule of thumb on whether to use a 6- or 8-inch balancer on different Chevy small-blocks? I believe the 6-inch-od balancer was used on the 283 and the 327s. Is this correct? I have used the 6-inch on my race motor for lighter weight and it seems to work fine. Phil Sprague St. Helens, OR

An engine's crankshaft may seem like it's a heavy, homogeneous, solid-metal mass, but in reality a crank bends and twists in response to the torque loads put on it by the pistons and rods. According to Chevrolet sources, "Tests have shown that the crankshaft in a 350ci racing engine can deflect 0.3 degrees at 8,000 rpm." Other authorities claim crank deflections of as much as 2 degrees. When the torque load on the crank is released and the crank untwists, vibrations occur. At certain engine speeds, the torque inputs become synchronized with the vibrations in the crank, resulting in what's known as resonance. Somewhat akin to a tuning fork, this resonance further amplifies the crank's vibrations, causing extreme stress that in a worst-case scenario can result in outright crankshaft failure.

Absent complete crank failure, torsional vibrations can also adversely affect cam timing by transferring the harmonics through the timing chain into the valvetrain. Since the distributor is driven off the cam, ignition performance and spark timing might also be degraded. Other problems include broken drivebelts, fractured oil pumps, and manual trans shift difficulties at certain rpm points. Rod and main journal bearing wear can be accelerated, which in itself can contribute to eventual crank failure.

A short, stiff crank is less affected by torsional vibrations than a long, flexible crank. Within a given crank length (a crank for the same engine family), stiffness is a function of stroke length, the amount of main and rod journal overlap, and crankshaft material. A 3.0-inch-stroke 283 or 302 crank is stiffer than a 3.25-inch-stroke 327 crank, which is stiffer than a 3.48-inch-stroke 350 crank. Forged steel generally makes for a stronger, stiffer crank compared with cast nodular iron.

An engine's rpm operating range also factors into the balancer equation. A drag race engine sees rapid rpm changes for only a quarter-mile at a time-there's not much time for the crank to vibrate. At the other extreme, oval-track engines can spend hours running at high rpm in a relatively narrow band close to the crank's critical resonance frequency. And even a street engine can spend hours cruising at a sustained rpm.

Chevy took all these factors into account when it spec'd harmonic dampers. A forged-crank 283 had little more than a harmonic balancer hub, while those 283s equipped with a nodular iron crank had a real 63/4-inch (not 6-inch) balancer. Low-performance 327s tended to use the smaller 63/4-inch balancers, while the high-performance or Corvette motors (because they were driven aggressively and at higher rpm) used 8-inch balancers. Nearly all 350s came with an 8-inch balancer because of their relatively long stroke. An 8-inch balancer was used on the short-stroke, 3.0-inch-stroke, 302 Z/28 engines because those mills were intended for road racing. The 400 small-block, with its 3.75-inch stroke, used a unique 8-inch balancer with external balance weights. Also, as a group, trucks tended to use the larger balancers because they were intended to carry cargo loads or be used for towing, operating cycles that over time impart extended stress on the engine's bottom end.

Also note that, at least as originally equipped, there were many different factory balancer variations, even on those with the same damper diameter. The original balancers were tuned to the specific engine, rpm range, and stroke they were designed for. Even fore and aft thickness may vary.

Bearing all this in mind, the theory behind using a small-diameter balancer (or even no balancer at all) on a drag-race-only motor is that the engine will accelerate quicker through its rpm range, just as it does by going to a lighter flywheel at the opposite end of the crank. It is thought this should translate into a better elapsed time. But better is a relative term. At best, a light balancer might be worth 0.02 second on a 9-second car.

On the other hand, tests conducted by David Vizard showed that the same 350 engine made more power on an engine dyno when equipped with an 8-inch factory balancer originally used on high-performance 302 and 327 engines (PN 3817173) than it did with just a lightweight aluminum hub. In his book How to Build & Modify Chevrolet Small-Block V-8 Pistons, Rods & Crankshafts (Motorbooks International, 1991, ISBN 0879385790, no longer in print), Vizard said the tests were conducted "under accelerating conditions-not static conditions-and took into account the additional weight of the damper." The large damper probably performed better because it reduced adverse torsional stresses transferred through the timing chain into the camshaft, valvetrain, and distributor.

I'd say the only way to know for sure if the superior power potential of a larger balancer offsets the quicker acceleration of a lightweight unit as installed in your car is by strip-testing them back to back. Assuming the lightweight unit in fact makes the car e.t. better, you still need to carefully inspect the bottom end when refreshing the engine for bearing cap walk, distortion, looseness, or fretting (material transfer). If the main-cap register's surface finish looks screwed up, you definitely have a problem. Cap fretting will cause uneven bearing loading, so a weird wear pattern on the bearings also is cause for concern.

If the engine is used in any long-duration closed-course racing, is installed in a boat, or is driven on the street, you should always use the biggest, stoutest balancer available. Assuming use of a factory balancer, the proper 8-inch unit designed for your performance engine offers the widest operating range and the best cancellation of unwanted resonant frequencies. As noted crank grinder Hank Bechtloff (aka Hank the Crank) puts it, "Even though it's more weight, the larger damper helps keep the frequencies that shake the crank apart from happening."

The preceding recommendations are for factory-style dampers. Most competition venues now require aftermarket SFI-certified dampers. The best professional-level, SFI-certified dampers-like ATI's Super Race dampers or TCI's Rattler series-come tuned for two different frequencies, as well as specific engine rpm operating ranges and strokes. They're high-buck, but the large-od versions are lighter than equivalent-size factory dampers, and the lighter and smaller-od versions are still suitable for nearly any application except the most extreme endurance usage.

Sources ATI Performance Products Baltimore, MD 877/298-5039 (orders) or 410/298-4343 (tech) ATIRacing.com

Hank The Crank Downey, CA 562/716-5900

TCI Automotive Ashland, MS 888/776-9824 or 662/224-8972 TCIAuto.com

[runyou-2]

This is dead wrong.

Call a reputable company like ATI and have the experts help you out. There is no set size for a certain size motor. If anything it relates more to stroke not engine size. Would a big bore short stroke 360 need the same size balancer as a smaller bore longer stroke 360?? Dont think so.

Although i usually don't always agree with this dude....He is right on with his answer,and if you don't listen to this theory ,then Good Luck!

[let-r-eat]

I took Perfconn to mean bigger stroke bigger balancer. However, the weight of the crank and how much bobweight matters too!