Short-stroke, big-bore pistons have a thicker ring land on the piston to provide more space between rings. This additional space is good for strength and power, as rings need volume between them to work properly. It also acts as a reservoir so pressure does not build up between rings, which can result in ring flutter. Ring flutter is when ring seal is compromised because the ring is unseated from the bottom of the ring groove. The ring is pushed against the cylinder and sealed by compression or combustion pressure getting on top and behind the ring, not underneath. With the big-bore, 347 pistons, there is an additional groove between top and second, offering more volume. There's simply not enough room on the piston for that groove on the conventional 347.
"Almost all aftermarket cylinder...
"Almost all aftermarket cylinder heads measure 4.100 inches across the chamber," Naegele says. "Most gasket bores measure 4.100, and a standard 302 bore is 4.00. A decent rebuild is 4.030. You can see in the illustration that you end up with a lip at the top of the cylinder that really causes havoc with flow. The shrouding effect of the cylinder and disruption of smooth flow into the cylinder is detrimental to power. If you want to maximize your investment in high-flow cylinder heads, put them on a big bore or use a Level 20 block with the elliptical chamfer."
A big-bore 347ci combination offers a better rod ratio than with a conventionally designed 347 (1.63 compared to 1.58). This improved rod ratio combined with more efficient piston design means better ring seal, less friction rubbing the cylinder wall, reduced piston rock, better cylinder-head breathing, and improved longevity. All D.S.S. pistons feature forced pin oiling, from the Pro-X series pieces we'll be using down to the $349 economy pistons.
"Many competitors' pistons use the OEM method of splash oiling and forged-in oil-return windows," Naegele says. "This method is OK on low-horsepower applications, but it does not support big power or power adders." D.S.S.' Pro-X pistons are aluminum forgings that are CNC-lightened to remove weight in areas where material is not required for strength.
"Most piston companies are mainly concerned with how many pistons per day they can produce," Naegele says. "We feel the extra time spent on details like CNC-lightening, drilled oil returns, full CNC valve reliefs and profile, and forced-pin oiling set our pistons apart and ultimately benefits our customers and our reputation in this business."
The D.S.S. Pro-X forged pistons can take whatever you want to throw at them: blower, turbo, nitrous, or a combination of these induction modifiers. These pistons are also light too, as a single D.S.S. 302 forged piston (PN 4000) is about 160 grams lighter than a stock piston, which translates into more power and higher rpm capability.
There are two designs available: a flat-top, single-fly-cut design (pushrod) that raises compression for naturally aspirated and nitrous cars. Optional is the D.S.S. Max Quench reverse-dome piston that lowers compression for boosted applications. All D.S.S. pistons feature tight piston-to-wall clearance for long-lasting performance, standard full-floating wristpins, and deep CNC-machined valve reliefs (pushrod applications) in the Pro-X series pieces to allow for even bigger valves and higher lift cams. With the cylinder heads we chose and a 0.040-inch gasket thickness, our Boss 347 should come in at a 10.5:1 compression ratio.
Making the connection between the pistons and the crankshaft are the D.S.S. H-beam connecting rods. Naegele specified the H-beam design for added strength, given our intended road-course use and extended amount of time at high rpm. The H-beams are made from 4340 forged steel and feature ARP 8740 cap screws.
The base D.S.S. Pro-X stroker...
The base D.S.S. Pro-X stroker kit retails for $1,295 sans block and main support system. This includes a nodular-iron crank, I-beam rods, and forged Pro-X pistons rings and bearings. Our big-bore kit features Twisted Wedge-spec forged big-bore pistons, forged H-beam rods, severe-duty ductile iron rings, and a 4340 forged steel crankshaft, which bumps the cost to $2,165.95.
For our application, D.S.S....
For our application, D.S.S. chose to upgrade our Pro-X rotating assembly to include these forged steel H-beam connecting rods. Since Stolen Goods will see high rpm for an extended amount of time, such as on the road course, the H-beams are a more reliable choice.
Using sophisticated equipment...
Using sophisticated equipment such as this TR250 Tallyrond machine allows development of piston skirt profiles that hold the piston square in the bore. Maximizing skirt contact spreads the load, minimizing drag and extreme pressure points, a key issue with small-block Ford stroker engines. "Flat ring grooves are essential in making horsepower," says Naegele. Measurements taken here result in a six-decimal place (0.000001) number. That's about as accurate as it gets.
The whole rotating assembly relies on the D.S.S. Pro-X Series big-bore rotator-a forged-steel crankshaft made from premium aircraft-quality steel using a vacuum-degassed forging process to ensure a clean and pure crankshaft. Each crankshaft is visually inspected, X-rayed, magna fluxed, ultrasonic-tested, and finished with a multistage heat-induction process hardened to HRC60 and micropolished for superior wear resistance. D.S.S. utilized Speed-Pro heavy-duty Tri metal bearings to support the crankshaft inside our Boss block.
Now that we have our short-block assembled, all of D.S.S.' hard work on our project comes to a close. We have to thank Tom Naegele and his team for getting our Boss 347 done in time to meet our deadlines-a feat not easily accomplished given that D.S.S. was in its busiest time of the year, when racers are banging down the doors and blowing up the phones trying to get their engines back and in their cars for the start of the racing season.
For Project Stolen Goods, it's time to button up the engine and get this Pony burning rubber and banging corners. Next month, we'll give you the skinny on our induction setup-which includes Anderson Ford Motorsport-modified Twisted Wedge cylinder heads, an AFM camshaft, and a Trick Flow intake manifold-and drop this bullet between the framerails.