This T&L-built 5.0 was stretched...
This T&L-built 5.0 was stretched from 302 to 382 inches by means of a Dart block and a 3.47-inch stroke crank. For a moderately priced engine, the return was more than 500 lb-ft of torque and over 600 hp on pump gas.
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.
Here's a budget Scat crank...
Here's a budget Scat crank and rods plus a set of D.S.S. pistons to build 347 inches from a stock 302. This is about the easiest and cheapest stroker deal to do. Because the crank and the longer rods are stronger, everything gets better, which means there's no downside.
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.
To produce a longer stroke,...
To produce a longer stroke, the offset of the rod journal is increased. The stroke increase is twice the offset increase.
From the diagram on the right,...
From the diagram on the right, you can see that for any given stroke length, the rod angularity quickly increases as the rod length is shortened. As the piston travels down the bore, the gas pressure not only acts on the crank to turn it, but a component of that force also pushes the piston into the side of the bore. The greater the rod angularity, the greater the side loading and the frictional loss stemming from it.
Shown here are the dimensions...
Shown here are the dimensions involved in determining the engine's rod/stroke ratio. The ratio is equal to B divided by A.