The crank determines stroke and changing stroke changes the actual distance the piston travels int he bore from Bottom Dead Center (BDC) to Top Dead Center (TDC). If you change only rod center-to-center length, that changes only the location of where the piston starts and stops in the bore, but the distance between BDC and TDC would remain the same. To package the piston within the block deck height, the connecting rod length or piston compression height has to be changed, or both.

Since stroke determines how far the piston travels in the bore, you have to also consider component clearance at TDC and BDC. While you don't want the piston protruding past the block deck surface, neither do you want the piston skirt to interfere with the crank counterweights.

Due to increased stroke, the rod big end will rotate in a larger circumference, so you need to make sure tha rod bolts don't interfere with the bottom of the cylinders or the block oil pan rails. So when designing a stroker combination, you have to make sure interference doesn't occur. If it does, then notching will be required!

Piston Speed

One central theme to keep in mind in all aspects of engine component selection is rpm, which is related to piston speed. Summed up, the hgiher the rpm, the more strain that's placed on engine components. Expressed in feet per minute, piston speed describes how fast the piston moves between BDC and TDC at a given rpm.

Since the crank and piston are linked together, the piston has to move at a speed dictated by the crank. When stroke is increased, the piston has to move faster in order to cover the additional distance. The formula for piston speed in feet per minute is:

2 x stroke (in inches) x rpm/12

Piston speed for a 3" stroke at 2000 rmp is:

Piston speed = 2 x 3 2000/12
Piston speed = 12000/12
Piston speed = 1000 feet per minute.

Added component strain results from the piston's reciprocating motion. Since it's stopping and starting at TDCand BDC, faster piston speed increases the force the block main webs, crank, rods, and main caps have to cope with. Because Force is defined as Mass x Acceleration, piston weight plays a major role in stroker applications. It's important that lighter pistons be used, since less weight reduces the Mass in the force equation.

lBlock Considerations

Finally, you can't talk about stroker engines without discussing the block. While you can use dedicated performance ones, like a Ford Racing casting, most strokers are built using stock castings. Essentially, all years can be used, as long as you follow some sound basics.

While the 289 block can be a stroker candidate, you shouldn't use it involving strokes longer than 3.10". This is because when Ford changed over to the 302, they extended the cyldiner bores to help icnrease piston stability. The 289 doesn't have this upgrade, so you don't want the piston moving too far out of the bore at BDC. Otherwise, use a 302/5.0L block, which will be more plentiful anyway.

Based on experience, the 302 blocks are good for about 600 horsepower. A 351w stroker is usually good for up to 700 horsepower. Picking a block core for a stroker kit is fairly basic. You want a block that's been Magnaflux-inspected for cracks. A 1974 or earlier 302 and 351w blocks with a D4 or earlier casting prefix are superior in terms of main web strength, because the webs are thicker and the pan rails aren't as thin.

The thicker webs and main cap rails on these blocks are stronger and they can better accommodate installation of a four-bolt main cap. If you're planning more power, or want unquestioned durabitlity from the outset, then you need to check the Ford Racing block offerings.