Hemi Rat

Sonny Leonard's 21st Century Chevy Engine

By Jeff Burk

For decades Chevy racers and engine builders have known in their hearts that the venerable Chrysler Hemi combustion chamber design with huge intake and exhaust ports and valves was more efficient, and under most conditions, capable of making more power than the Bow-Tie wedge-design cylinder head.

One of the main reasons the hemispherical design has the capability of making more power is the position of the intake and exhaust valves in the combustion chamber. The valves in a hemi head are opposed exactly 180 degrees, which facilitates better airflow characteristics than a wedge head where the valves configuration are more inline or side-by-side. That valve configuration inherently restricts airflow and, in most cases, results in an engine that makes less than optimum power.

Knowing that information, diehard Chevy and even Pontiac racers have tried to develop a hemispherical combustion chambered head for their Chevy or GM. In the early years, Chevy legend Zora Arkus-Duntov developed a hemi head that bolted onto the Ford and Mercury flatheads. In the late '70s, another legendary block and cylinder head manufacturer, Nick Arias, actually developed and built true hemispherical cylinder heads and purpose-built blocks based on the standard big-block Chevy design. Those engines ran very well burning alcohol or nitro, but not so good on gasoline.

The one problem that is common to all of the engine combinations mentioned above is that while a hemispherical combustion chambered engine that is either supercharged or turbocharged, burning alcohol or nitromethane, runs like stink, that isn't the case when you put gasoline in the tank.

It has been 20 years or more since a true hemi-headed gas-burning engine powered a competitive NHRA Pro class car. In fact, the last hemi-headed, competitive, Pro Stockers were those campaigned by John Hagen, Reid Whisnant and Bill Dempsy in the early '80s. Since that time the engine of choice has been a GM with a wedge head. In fact, when Chrysler returned to the NHRA Pro Stock wars it's generally accepted that they did so with a copy of a GM wedge head. That didn't set well with the Mopar folks, so the Chrysler engineers re-designed their wedge head by moving the valves around in the combustion chamber, putting them directly across from the intake and exhaust port, and called it a hemi, even though it didn't have a hemispherical combustion chamber.

But guess what they found out? They didn't have to have the hemi chamber and the wedge-style combustion chamber actually worked better than a hemi chamber when gasoline was burned for fuel. So, you could say that those engines were really the first of the new-generation Chevy "hemi".

Meanwhile, in the world of IHRA Pro Stock racing where engines are limited to a mere 815 ci, the Jon Kasse Ford engines using cylinder heads with the valves placed in the traditional hemi configuration have been the dominant engine for quite some time. Now, thanks to Sonny Leonard's "Next Generation" Chevy engine, that's about to change.

In the past, Sonny's big Chevy engines could win once in a great while, but the Ford-powered cars generally dominated the competition. To combat that, Chrysler introduced their new "hemi" head for NHRA competition. It was really a wedge except for the valve location that was, you guessed it, just like the traditional hemi. Engines with those heads immediately started winning in NHRA Pro Stock and it wasn't long before some IHRA Mopar Pro Stock racers had versions of those heads on 800-plus ci "hemis" and those engines proved to be more powerful than the GM-style head.

The "hemi" Ford and Chrysler powerplants were making the GM racers' lives miserable. Which brings us to the hero of this story: famed Chevy mountain motor builder Sonny Leonard.

Leonard, whose shop is located in Lynchburg, Virginia, has a justifiable reputation of being the premier builder of the 700-800 ci naturally aspirated Chevy engines. For nearly 20 years his engines have been the weapons of choice for Outlaw Street, Quick Eight, Pro Modified, and IHRA Pro Stock racers. Sonny's engines have won many IHRA Pro Stock World Championships, as well as championships in the sportsman ranks. Former Super Chevy Nitro Coupe champ Bill Kuhlmann used Sonny Leonard big-block Chevy components in the 615-inch Rat that powered him to the first 200-mph doorslammer pass.

Sonny was one of the very first engine builders to design and have cast his own 5-inch bore center Chevy blocks and cylinder heads to make the engines competitive with the Ford hemis of IHRA. Engines he has built using those components put IHRA mountain-motored Pro Stockers into the 6.50s at more than 210 mph. But the mountain-motored Fords and Mopar "hemi-fied" engines had a distinct advantage over Sonny's wedge headed engines, so he went to work and developed what he calls his "Next Generation" Chevy.

His design retains the 5-inch bore spacing that has been so successful in helping his wedge-head engines develop in excess of 1,100 hp, but in order to wring the maximum amount of power out of the engine he knew that he would have to redesign the cylinder heads. In fact, if he wanted his next-generation Chevy engine to go to the next horsepower level, the only answer was to design a Chevy cylinder head with valves that are opposed 180 degrees.

Sonny explains, "I wanted the 'Next Generation' engine to be more efficient than my wedge motors, so I designed the heads so that the intake runner is located 180 degrees from the exhaust runner. That provides a better path for the intake charge and a better one to move the exhaust gases out of the combustion chamber."

As you can see in the photos, the valves are not shrouded and the spark plug has been moved to encourage better flame travel in the combustion chamber. Better flame travel means more efficient burning of the fuel and air mixture, which results in more horsepower.

Just to make sure there is plenty of air moving through the heads, you can get an intake valve as big as 2.700 inches in diameter and an exhaust as big 2.000. This engine will move some air. With a good porting job and a cam with .900 Inches of lift, the intake will flow 590 cfm and the exhaust port 390 cfm. (I had a 390-cfm carb on my first hot rod. Each exhaust port on this engine will flow that much air!)

Sonny also must have figured that with those kinds of numbers some racer would be bolting a supercharger on the intake or installing four stages of nitrous because he built the heads with 10 studs and the block with 22. You're going to be hard pressed to push a gasket out of this engine.

While we were fortunate to be able to get a glimpse of the components, and thus have a lot of photos of cylinder heads, blocks and rotating assembly components, Sonny was unwilling to reveal the exact info such as bore, stroke, rod length, and other proprietary information. After all, this is an engine he spent a lot of time and money on and he's reluctant to give away information.

"I could tell you everything about it," he said, "but then I'd have to shoot you."

He added that if I really wanted to know what the specifics were, he would sell me a complete engine. He wouldn't give me a price, but knowing that one of Sonny's Pro Stock wedge motors goes for around $80,000, the price for the "Next Generation" motor might be considerably higher.

We stayed around the shop as Sonny put the engine on the dyno and made some pulls before sending this engine off to IHRA Pro Stock racer John Montecalvo. We did notice that the motor made in excess of 1,200 hp on the dyno and was turned less than 8,000 rpm. Just think what this baby might make with a full boogie nitrous system or a blower and fuel injection burning alky. Can you say 3,000 hp?

Towards the end of last year's IHRA season, Montecalvo and his "Next Generation" Chevy were qualifying at or near the top of the field at nearly every event, and that was with less than a half season of testing and racing.

This Rat is no warmed over Mopar or Ford in disguise. No, my friends, this is a brand new engine from the pan rails to the valve covers. Sonny had learned from testing that the engines with the intake and exhaust valves directly adjacent to the ports had a distinct horsepower advantage in the hot, humid, dog days of summer. So it was either fight 'em or join 'em for Mr. Leonard. He decided to join 'em.