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Ford Small-Block Stroker Engine Build - Stroker Strategies
Building A Stroker For More Cubes Is Easy Enough, But With The Right Strategy You Can Get 50 More Horses.
When it comes to cubes, this is how I look at it: If some are good, more are better-so too much must be just right. Correct?
Get behind the wheel of a car powered by a combination of well-selected parts with big displacement and you'll rapidly come to the conclusion that more cubes equate to more adrenaline. But while big is always good, the key is having the correct combination of parts to extract max power. What we're talking about are strokers, but a stroker built without knowing what it takes to correctly spec the rest of the engine can easily fall short by over 50 lb-ft of torque and 50 hp.
These days, the popularity of stroker motors is at an all-time high, and there appears to be no slowing of the trend. But there's a problem here. At the end of the day, the goal is to build more cubes into your engine and get the maximum return in terms of output. For me, finding maximum power boiled down to years of work and several thousand dyno runs on about 35 different stroker motors. Each of these was run with various combinations of major parts such as heads, cams, intakes, and so on. All of this was made possible by the patience and willingness to supply the necessary parts by such companies as D.S.S. Racing, Scat, Ross, J&E, Comp Cams, Dart, and more than a few others. It has been a long, hard road, but MM&FF wants to thank all who helped. The benefit to our readers is that we've done most of the hard work for you; now you can read on and learn what it takes to make big power from your 302 or 351-err, 347 or 408 bullet.
Why a Stroker?
Before we delve into detail, we need to ask why would we, or anyone, build a stroker? The obvious reason is that it increases the engine's displacement, and we know there's no replacement for that. But there's much more. In stroking an engine, it may look like we're building a motor with an excessive stroke length in relation to the bore. Not so. Until the advent of the modern pushrod V-8, engines such as the good, old, flat-head Ford/Mercury V-8 had a stroke/bore ratio of about 1.13. That means the stroke was 13 percent greater than the bore. With the develop-ment of lower hood lines and the quest for more rpm, engines went on a different route. One engine is the 302 with a 4-inch bore and a 3-inch stroke. For a while, bore dimensions got bigger and strokes got smaller.
With the 302 and the 351 (which has a 4.00-inch bore and a 3.5-inch stroke), we ended up with stroke/bore ratios of 0.750 and 0.875, respectively. In terms of a workable configuration, these are more than just OK. This means Ford left us in a strong position to boost cubes without having an engine with an excessively long stroke in relation to the bore size. The goal is cubes, and one of the positive side effects of a stroker crank is that it makes the SBF engine that much more effective at producing cubes. If there's an opportunity to increase both the stroke and bore, then every effort should be made to do so.
How about a stroker's downsides? Assuming the stroke increase has not been so big as to compromise block and piston integrity, there are essentially only three significant disadvantages to a stroker: increased piston side loading, increased crank torsionals, and that valve size constraints by the bore have an increased negative impact.
As the cubes increase, the valve size (and the overall induction system) can become a limiting factor faster than the advantage of the increasing cubes. Generally, increasing the size of the carb/throttle body, intake manifold, cylinder head ports, and camshaft is needed to go with the larger displacement. At the end of the day, that's the difference between a great stroker build and an indifferent one. Since it greatly affects reliability and longevity, let's start with piston side loading and crank torsionals.
Piston Side Loading
The single biggest factor affecting side loading is the rod length to stroke ratio (rod/stoke ratio). The shorter the rod is in relation to the stroke, the greater the angularity it goes through. The negative consequence of this is that the piston experiences greater side loading as it approaches its maximum angularity. The bad news is that when rod/stroke ratios approach the 1.6 mark, you really need to do something about it. The good news is that some of the power-producing moves we need to do help offset the negative issues here. A prime one is that as the compression ratio is raised, the negative effect of a short rod becomes less in terms of overall percentage of power lost to additional side load friction. In a nutshell, this means your 10.5:1 (or more) fairs better with the shorter rod than does an 8.5:1 motor. With higher compression ratios, it's possible to successfully utilize rod/stroke ratios down to the 1.55 mark. However, those lower rod/stroke ratios are not so good in supercharged engines, as the frictional losses escalate with lower compression ratios and big boost numbers.
Allowing that it's not that practical to have pin heights (the distance from the center of the wristpin to the deck of the piston that comes flush with the top of the block at TDC) less than about 1 inch puts a limit on the stroke that will fit into a given block. For instance, if we assume a 1.55:1 minimum rod/stroke ratio and a 1-inch piston height, the maximum stroke that will fit (assuming no other clearance problems) in a 5.0 block having an 8.2-inch deck height is a few thousandths over 3.5 inches. For typical street motors, most of the top engine builders attempt to keep the rod/stroke ratio's lower limit to about 1.58:1. With a small margin, this allows for a stroke of 3.4 inches and gives a workable 1.1-inch pin height with a 5.4-inch rod. It's this combination that delivers the popular 347 inches in a 0.030-inch-over 5.0 block.
We've looked at one half of the equation in terms of minimizing the potential to lose power through the use of a long stroke and the consequence of a short rod. Now let's consider the other half-friction. If we could eliminate friction between the skirt and the cylinder wall, then a short rod actually begins to have an advantage over a longer rod.; but we can't, so the next best thing is to minimize it. Fortunately, piston manufacturers are always looking for ways to cut skirt friction. Millions of dollars have been spent on researching the best skirt shape to cut friction, and recently we've seen off-the-shelf pistons with low-friction coatings applied to the skirts. There's not a lot you can do to a piston's skirt, but you can get it coated. In addition, you can buy pistons with the lightest and thinnest rings your budget will allow. Another factor is to have the machine shop size the bore to give the biggest clearance recommended by the piston manufacturer.
Also, watch the weight at the pin end of the rod. Anything to lessen this, such as a lighter-than-normal piston and wristpin, is a help. When it comes to lubrication, run the best oils out there at the lowest viscosity your engine builder may recommend.
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.
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.