As stroke length is increased,...
As stroke length is increased, the amount of journal overlap is decreased. This reduces the torsional stiffness of the crank, thus making the job of the crank damper (often incorrectly termed a harmonic balancer) that much more important.
Dealing With Bigger Torsionals
Assuming no changes in journal size or crank material, we find that as the stroke is increased, the journal overlap decreases and the crank's stiffness is reduced. In practice, we find that most stroker cranks are made from significantly better material than stock, so overall crank stiffness is not a real issue; however, that doesn't mean you should ignore the fact that a longer stroke crank is more prone to the negative effects of torsional vibrations. If these vibrations are not adequately damped, two things will happen. First, the crank will break prematurely. Crank breakage brought about by an inadequate damper, though, is relatively common knowledge. What's not quite as obvious is that an ineffectual damper (such as one of those lightweight hubs that has no damping effect) also causes a loss of power.
Take a look at the torsional tests in the nearby graph. What you see here are the vibrations that exist in a crank when there is 1) zero damping, 2) a reasonable damper, and 3) a good high-end aftermarket damper. If the damper is not doing its job, then all those unwanted crank torsionals are fed, via the timing chain and gears, straight to the cam. If the cam experiences any significant torsionals, all those carefully calculated cam dynamics and the precision manufacturing of such will be to no avail. In other words, the dynamics will go down the toilet real fast, and the result is inaccurate valve timing and spurious valve bounce at multiple points as the engine rpm climbs higher.
At about this point, you may be wondering just how much power can be lost by having inadequate crank damping. A number of years ago, your author asked just that question. I built a special engine to test dampers and then teamed up with a big commercial damper manufacturer with all the gear to measure torsionals. I was ready to test torsionals versus power down to the single horsepower in terms of accuracy. I even brewed my own fuel for added repeatability. About six weeks later, I had the answer. On a 420hp engine, the difference between the best and the worst damping scenario, under accelerated dragstrip conditions, was a solid 14 hp. The moral here is, don't dismiss the damper as inconsequential. If you do, you will pay in terms of reliability and power. In other words, a stiffer crank will keep all the moving parts where they should be. The cheaper crank might not break in your application, but it may cost you power.
The blue line in this graph...
The blue line in this graph is the intended motion of the valve as it opens and closes. If crank torsionals are transmitted to the cam, the motion is substantially modified as per the red line.
To the budget-constrained hot rodder, a crank damper looks like a nuisance expense. Ford Motor Company always installs one on the front of the stock crank, and by the time you get to rebuild that engine, the damper usually has better than 100,000 miles on it. The temptation is to just use the original stock damper, but before you do, consider that the best part of its life is already used up, and you're about to give it some real work to do.
My advice is to go with a unit like the Professional Products damper shown nearby. It's surprisingly cheap and degreed, to boot. Of course, there are many dampers on the market, and most brand names will do a great job.
D.S.S. has one of the less-expensive, high-end dampers that also has a good reliability record. But for the racer who's looking for the best damping money can buy, ATI and BHJ seem to be at the top of the pile. The worst scenario that any damper has to deal with is a NASCAR Nextel Cup engine. These have to run at pro-longed periods at rpm between 7,000 and 9,500. That's hard on any crank. In addition, these engines have a valvetrain designed literally to "'toss" the intake valve at high rpm for the length of the race. As such, the controlling-or should we say, managing vibration dynamics-are not just important, but super critical. On such appli-cations, the damper is worth a big chunk of power.
In the end, getting educated and sweating the details will make your stroker a winner whether it's in your street machine or track superstar. Ford designed its small-block engines with the factory displacements for a reason, and when you modify them, it's important to understand what you'll get and what you won't.
 The Professional Products...  The Professional Products damper represents a cost-effective solution to a stock replacement damper. The degree markings are a worthwhile bonus. |  This BHJ damper is the choice...  This BHJ damper is the choice of many pro race-engine builders. A custom damper is built for the job, and to do this, BHJ is pioneering a computer simulation system that models the engine concerned. Torsional tests are showing a good correlation between computed and measured. |  Here's a Nextel Cup engine...  Here's a Nextel Cup engine I rebuilt a while back. The ATI damper behind all those pulleys was specifically built for the job. The crew at ATI will send a technician to the dyno facility where the engine is being tested and set up their equipment to measure torsionals. From there, they build a custom damper tuned specifically for that engine. |