Sonny Leonard's "HEMI" Chevy head

Sonny Leonard's 5 inch bore cylinder

One reason for the opposed valve Hemi head's superiority to the wedge is that in wedge heads the valves are side-by-side or inline, if you will. According to the engine/cylinder head builders I've talked to, that valve configuration inherently restricts air flow. The Hemi head usually has the intake and exhaust valve directly in front of the ports and, as a result, generally but not always make power. For decade's engine builders, manufacturers and designers like Richard Maskin at Dart, Reher-Morrison, Sonny Leonard and others have spent insane amounts of time and money modifying the basic wedge cylinder design to improve combustion and airflow. They've succeeded, especially in the NHRA Pro Stock class.

Ford guru Jon Kaase

In the world of IHRA Pro Stock racing, where engines are limited to a mere 815 cubic inches, Jon Kaase-built Ford and Chrysler engines using cylinder heads with the valves placed in the traditional Hemi configuration have been dominating the class for some years and making the GM racer's lives miserable.

Which brings us to Sonny Leonard's new generation "Mystery Motor" -- an engine built using the first truly new Chevy cylinder heads developed in about 40 years. Leonard, whose shop is located in Lynchburg, VA, has a justifiable reputation of being one the premier builders of 700-800 cubic inch, naturally aspirated GM engines.

Seven years ago he designed and built the first Chevy engines that featured a five-inch bore center. That innovation improved the flow characteristics of the GM-style heads and made Sonny's 815 cid IHRA Pro Stock engines competitive with the Fords and Chryslers...most of the time. His engines made similar power to the Hemi's -- most of the time, but not all.

Under certain atmospheric conditions, the Sonny Leonard Chevy just wasn't competitive with the Hemi. So, Sonny went back to the design board and came up with a completely new cylinder head and block that he could use to build a Chevy that would run with the Hemi under any atmospheric conditions or applications.

The basic problem with Sonny's five-inch bore center engines was the combustion chamber and exhaust air-flow characteristics. They simply weren't as efficient in hot weather as the Fords were.