Chevy Small Block Vortec Cylinder Heads Specifications - Heads Up!

In 1955 Chevrolet introduced the first generation small-block V-8 in the form of the 265ci engine. The original design has been so successful that 48 years later, the small-block is still in production. Although the first-generation small-block is no longer available in any new car or truck, it is still being manufactured for the marine industry and can be purchased new, in the form of a crate engine, at any General Motors parts department.

The second-generation small-block, used briefly only in Camaros and Corvettes, was similar to the first-generation, but with cooling and ignition system modifications. The third-generation is a new design and has nothing in common with the first- and second-generation engines. A cast-iron version of the third-generation engine has replaced the first-generation engines in General Motor's truck line.

The final chapter in the first-generation small-block was written in 1996. The engine, named the Vortec, was the last redesign for the traditional small-block Chevy. The evolution and development of the first-generation small-block Chevy will go down in history as a 42-year time span, between model years 1955 and 1996. Never again will large sums of money and research be invested by General Motors into the first-generation small-block.

To bring any older small-block up to the specifications of a Vortec engine is not difficult. The differences between the block, crankcase, rods, and pistons were minor between the '95-and-earlier engines and the Vortec engines. The main difference is in the new Vortec cylinder heads and intake manifold and the dramatic increase in power provided by them.

Although it has always been possible to increase the horsepower of any small-block engine with larger intake ports, low-speed torque has always suffered as a result. This is not a good idea for a production truck engine. The cylinder heads designed by GM engineers for the Vortec engines provide increased horsepower and increased torque.The name Vortec comes from the vortex that is created by the air/fuel mixture as it enters the cylinder. This refers to a swirl induced by the shape of the intake port and combustion chamber. The swirling air/fuel mixture increases burn efficiency by actively pushing itself into the flame, created by the spark plug, instead of waiting for the flame to travel across the combustion chamber to ignite a stagnant air/fuel mixture.

TBI Cylinder Heads

The first attempt to create a swirl with the air/fuel mixture came when GM designed a special head for the small-block engines that used the throttle body fuel injection (the TBI engine). These engines were manufactured between 1987 and 1995, and were option code LO3 for the 305 and LO5 for the 350.

The heads have one side of the intake port formed into the shape of a ramp to put a swirl into the air/fuel mixture as it is pulled into the combustion chamber. The design works well to increase low-speed torque and fuel economy but restricts high-rpm power.

Although the 305 TBI produced 170 hp and the 350 TBI produced 210 hp, these engines have a reputation for being under-performers. Because the new restrictive heads were installed along with the new fuel injection system, the disappointing performance was wrongly blamed on the change from carburetion to fuel injection.

The intake ports on the TBI heads look pretty strange to many of us that are familiar with the traditional small-block intake ports. Not many attempts have been made by engine builders to use these heads on performance engines. Although the intake ports are large -- they hold 185 cc's of liquid -- the TBI heads were never used on the high-performance 305 or 350 Tuned Port Injection Camaro or Corvette engines or the 305 and 350 marine engines. These engines used heads with the traditional intake port.

Installing traditional small-block cylinder heads on a 305 or 350 TBI makes them into better performing engines. GM has a replacement head (PN 10125377) for this engine that has the traditional intake port. Externally, it looks identical -- the combustion chambers and valves are the same size and shape, it uses the same valve covers, and has the same intake and exhaust manifold bolt patterns. Installing these heads will give the engine a stock appearance, maintain the durability of the stock engine, and, at a slight decrease in fuel economy and torque, increase horsepower at the top of the rpm range.

Vortec Cylinder Heads

To increase horsepower and torque, the GM engineers decided the swirl feature of the air/fuel mixture had to be accomplished without the ramp inside the intake port of the TBI head. The cylinder heads of the Vortec engines have a very sophisticated intake port and combustion chamber shape that induces swirl, maintains high air/fuel mixture velocity, and allows large amounts of air/fuel mixture flow.

Many performance engine builders have attempted to reconfigure the original small-block intake port and many have come close to a design that increases both horsepower and torque. The difficulty in attaining this goal has always been the physical size of the port. The width of the port is restricted by the necessity of pairing the intake ports between the pushrods. Increasing the height of the intake port is difficult because of the low valve cover flange.

Aftermarket cylinder head manufacturers could design and sell a head with taller intake ports, but such a head would also require designing and selling a matching intake manifold. Convincing the average high-performance guy to step up and purchase new heads is difficult enough, but then to inform him that he also needs to buy a matching intake manifold might be a little too much.

With the installation of two heads on every new truck sold, GM has the ability to sell hundreds of thousands of cylinder heads. Building and selling in such high quantities allows the cost of research, tooling, and manufacturing to pay for itself very quickly. This allows GM to make replacement Vortec heads available for purchase at a very reasonable prices at any GM parts department. The aftermarket intake manifold manufactures quickly seized upon this and started manufacturing a variety of Vortec-compatible intake manifolds for carburetors. Since then, the installation of the Vortec heads on '95-and-earlier small-block carbureted engines has become very common.

Vortec Intake Ports

Intake port size is measured in liquid volume. The cylinder head is positioned with the port inlet facing up, and the amount of liquid it requires to fill the port determines the size. The traditional '86-and-earlier cylinder head will hold approximately 160 cc's of liquid. The Vortec head has a much larger intake port; it holds 170 cc's of liquid.

Increasing the size of the Vortec intake port was not accomplished by making it wider, but by making it taller. The valve cover flange height was increased, allowing the roof of the port to be considerably raised.

The primary job of the intake port is to change the flow of the air/fuel mixture from a horizontal direction into a vertical direction. A sharp 90-degree turn in the intake port will kill the air/fuel mixture velocity. Raising the roof of the intake port allows the cylinder head engineer to design a port with a larger radius. A larger radius helps the air/fuel mixture maintain velocity while making the turn. The Vortec intake port is based on the principle that a large port with a large turning radius will have the same port velocity as a smaller port with a smaller turning radius. Flowing more of an air/fuel mixture because the port is larger and maintaining air/fuel mixture velocity because the port has a larger turning radius, is the key to the increase of high-rpm horsepower and low-rpm torque found with the installation of the Vortec head.

The intake port of the Vortec head doesn't have a ramp to induce air/fuel mixture swirl like the TBI head. The design of the port is much more subtle and it is integrated into a sophisticated heart-shaped combustion chamber. The new intake port and combustion chamber allows a higher compression ratio with less detonation than the traditional intake port/combustion chamber arrangement.

Valve Sizes

The intake and exhaust valves have the same size head diameter as most small-blocks. The intake measures 1.94 inches and the exhaust measures 1.50 inches in diameter. Most small-block heads since the late '70s have an exhaust valve with a 3/8-inch-diameter valve stem. Prior to this, the intake and exhaust valve stems both measured 11/32 inches. The Vortec heads have returned to 11/32-inch-diameter valve stems.

ValveSprings

The valvesprings are a new design. They are single springs wound from a special spring wire with an oval cross section. This helps the spring self dampen without the use of a separate damper. The springs have a 1.241-inch outside diameter, 80 pounds of spring pressure at 1.70 inches of installed height, and have an average spring rate of 256 pounds per inch.

Combustion Chamber Size

Depending on the engine application, the small combustion chamber on the Vortec heads can be an advantage or disadvantage. The combustion chamber size is measured in liquid volume. The Vortec heads have a small 64cc combustion chamber. Most small-block 350 engines will have a 9.0:1 compression ratio using flat-top pistons and 76cc heads. Installing Vortec heads on a 350 with flat-top pistons will give it a 10.0:1 compression ratio. Even with the efficient combustion chambers, the engine might detonate while using high-octane pump gas in a heavy vehicle, such as a truck. To be able to run the engine on 87-octane regular fuel, the engine should have low-compression pistons to drop the compression ratio down to 9.1:0 or less.

The Drawbacks to Using Vortec Heads

It would be great if all we had to do was bolt on Vortec heads, and the engine produced extra power. But, it is not that simple. Because the intake ports are much taller, a standard intake manifold will not cover the intake port inlet. When the cylinder heads were first introduced, many people thought that a standard intake manifold could be modified to work but were soon convinced that it would require extensive welding and reshaping to fit correctly. An aftermarket manifold needs to be purchased along with the heads.

Not only are the intake ports much taller, but the intake-mounting-bolts are also a different size. The traditional small-block heads use 3/8-16 course thread bolts, while the Vortec heads use 5/16-18 course thread bolts. There are also fewer bolts, with 16 on the traditional intake manifolds and only eight bolts on the Vortec intake manifolds.

When installing the Vortec heads on an '86-and-earlier engine, the '87-and-later style valve covers need to be installed. All '87-and-later heads have valve covers that fasten through the center, while valve covers for '86-and-earlier heads have valve covers that fasten around the perimeter.

The Vortec heads are the first cast-iron head designed for individual-port fuel injection, which means that the exhaust heat crossover passage has been eliminated. The heated crossover passage helps vaporize the fuel that puddles on the floor of the intake manifold. Most performance-engine builders will block off the passage for a cooler running intake manifold, so this is not a serious setback for engines built for performance. The elimination of the passage has made the plumbing of an EGR valve more difficult.

The most troubling part of the Vortec head installation is the limitation of the valve lift. A '95-and-earlier head has the valve lift limited by the distance between the valvespring retainer and the valveguide when the valve is at its maximum lift. The Vortec head has the same valvespring retainer to valveguide clearance, but also has a valveguide seal installed between the retainer and the valveguide. The valveguide seal will be pinched between the retainer and guide if valve lift exceeds 0.470 inches.

The '95-and-earlier heads use a tin shield over the spring that keeps excess oil from running into the valveguide and an O-ring installed on the valve stem that keeps oil from running down the valve stem and into the guide. The tin shield and O-ring allow up to 0.500 inches of valve lift.

The tin shield and the O-ring system was intentionally designed not to be oil-tight because the GM engineers wanted a small amount of oil flowing into the valveguide for lubrication. If a little oil seeped past the valveguide and valve stem, entered the combustion chamber, and burned on engine startup, it was a small price to be paid for extended valveguide life. After the Federal Clean Air Act was passed in 1970, burning a little oil in the combustion chamber was not acceptable, so to further limit the oil from seeping past the intake valveguide, the GM engineers added a small plastic umbrella seal that floated on the intake valve stem. With emissions regulations becoming more stringent, the GM engineers installed the large, oil-tight valveguide seals in the Vortec heads, limiting almost all of the oil from seeping past the valveguide.

To increase valve lift, the '95-and-earlier tin shield and O-ring can be used on the Vortec heads. The problem is that the Vortec valves do not have an O-ring groove on the valve stems. Using valves from an older head, and installing the tin shields and O-rings, is a good way to increase valve lift clearance to 0.500 inches. Although the tin shield and O-ring look a little ineffective, the amount of oil allowed to lubricate the valveguide seems to workout just right for a high-performance engine.

Installing Vortec Heads on a TBI Engine

The topic of increasing the performance of the TBI engine often comes up around here. We would guess it comes up often with the people at GM Performance Parts as well, because they have introduced a special aluminum TBI intake manifold (PN 12496821) that adapts the TBI fuel-injection system to the Vortec heads. The manifold has a special water passage under the plenum to keep the fuel vaporized -- remember, the Vortec heads don't have a provision for an exhaust heat crossover passage. The drawback to installing the Vortec heads and the new manifold on the TBI engine is the need to plumb exhaust gas from the exhaust manifold to the EGR valve. GM recommends using the '96 exhaust manifold (PN 12557828), EGR pipe (PN 10220275), and EGR valve (PN 17052693).

Vortec Value

The best part about the Vortec heads is that they are reasonably priced and available at your local GM parts department. The GM part number is 12558060, and this includes the valves and springs assembled on the head. CCT