Gen III/IV Small Block Engines - Big Displacement Gen III/IV Engines

This is a shot of an LSX-block... 

   

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This is a shot of an LSX-block engine with a 4.00-inch stroke Callies crank, a 6.125-inch-long Oliver rod, and a JE piston. Note how the piston skirt extends only minimally past the bottom of the cylinder due to a shorter skirt piston.

This is a Mahle piston for... 

   

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This is a Mahle piston for a similar 4.00-inch stroke iron block 408ci engine we are currently building. Note the exceptionally short skirt that will only minimally extend past the bottom of the bore.

Bore Length
One major concern with any stroker package on any engine has to be how far the piston extends out the bottom of the bore. We dug a little deeper and learned all production Gen III/IV blocks have a bore length of 5.500 inches with the exception of the LS7 engine, which has a cylinder bore length of around 5.900 inches. GM extended the length of the cylinder liners in the LS7 because of the longer 4.00-inch stroke using a shorter 6.067-inch-long rod compared with the normal production rod length of 6.098 inches. Most aftermarket connecting rods are longer, with the most popular being the 6.125-inch version. When increasing stroke, a longer rod not only improves the rod-length-to-stroke ratio (rod length divided by stroke) but also moves the piston farther up the bore, which reduces the amount of piston skirt that can extend out of the bottom. Even the GM LSX iron race block has an LS1 bore length.

Another variable here is the overall length of the piston skirt. In the photos supplied by Schwartz Extreme Performance, these pistons were used in a 402ci engine using a 4.00-inch-long stroke crank. The photo shows a skirt with scoring from 2,000 miles of street driving. While part of this may be attributed to a skirt that extends farther out of the bottom of the bore, scoring like this can also be a result of not adequately chamfering the bottom of the bores after honing. Schwartz says the bores on this engine were chamfered. Our local shop, Jim Grubbs Motorsports, also uses abrasive points from McMaster-Carr or Standard Abrasives to polish the chamfer. This prevents damage to the piston skirt.

We also spoke to Judson Massengill, owner of the School of Automotive Machinists in Houston. Massengill says he dislikes the 4.00-inch stroke crank packages for a street engine because they have difficulty with oil control. Apparently, the excessive piston rock at bottom dead center (BDC) causes oil control problems. Massengill prefers a 3.75-inch stroke crank, and he says while you might lose a little bit of torque compared with a 4.00-inch stroke displacement, the shorter stroke is cleaner and will make almost as much power, although slightly higher in the rpm range. Massengill did say that for a bracket engine or very limited street use engine, the 4.00-inch stroke systems do work. He just prefers not to run them in a street application. If the displacement is too tempting to avoid, Massengill suggests going to a Darton sleeve conversion on an aluminum block engine because the sleeves extend the length of the bore to accommodate the longer stroke. As you can imagine, this is not exactly a budget approach, but it does solve the piston skirt problem.