The Lunati forged, 40cc reverse-dish piston offers the 542ci Indy engine ample combustion chamber size, despite a zero piston deck height and the 78cc chamber volumes of the Indy 440SR heads. This is important. In order to eliminate any chance of detonation with the moderate 9.5:1 compression ratio, the temperature throughout the chamber must be consistent. As a result, combustion occurs predictably across the piston surface. As bore diameters increase, this "squish effect" is harder to control, and when not considered, detonation is a likely result.
The series of narrow grooves just below the piston surface are anti-detonation grooves. They also help reduce the chance of detonation as the grooves disperse raw fuel along the cylinder wall through piston travel rather than allowing it to reservoir along the ring and piston top surface.
While on the topic of rings, the set is made up of 1/16, 1/16, and a 3/16 standard tension oil ring. The Holley Roadrunner will primarily be a street machine, yet we intend to do a little racing. We made it clear to Ken that we wanted both power and seal without the oil contamination that looser, high-horsepower race engines are known for.
The cylinder head for the 542ci Holley Roadrunner would be Indy's 440SR head. Using 906 castings as a baseline, the 440SR is Indy's first step in terms of a performance gain with its extensive line of Mopar-specific B/RB cylinder heads. Designated a street/strip head, the 440SR is the closest thing to a stock replacement that Indy offers. It doesn't require an offset rocker arm-any 906 style rocker will work, as will any intake manifold in the aftermarket.
According to Ken, "Our goal with this head in this application was to produce a peak intake airflow rate of 320cfm while maintaining intake port runner volume at a moderate 270ccs compared to a ported 906 head's volume of 190cc."
As for flow balance, the exhaust port is 75 percent of the intake and deemed a high-velocity head, perfect for the moderate airflow goal we were interested in obtaining. "Will the 542 make the most horsepower with this head?" you ask. "No," according to Lazzeri. "This engine has far more potential-it is basically a detuned race engine specifically designed for a street machine. With a much larger port volume and greater airflow volume rates, the 542 could easily produce 800hp with a roller cam, high compression, and fully ported heads-maybe even with the 440SR head."
Ken is quick to note that Mopars have always been well known for torque. "Recognize that the 320cfm intake airflow rate for a 440 may have seemed large for a torque-only engine, but remember Chevrolet offered airflow rates of 270cfm right from the factory with 396 Chevrolets." As airflow goes up proportionately with port size, so does horsepower, as long as the engine can effectively pump the air available.