Tech: GM 1st Gen Heads vs. Vortec Heads Reader Guide

Chevrolet’s small-block Chevy was manufactured beginning in 1955, and the first-generation engines (Gen I) can still be purchased as a replacement crate-engine through Chevrolet Performance. In 2005, General Motors estimated that 90 million SBC engines had been produced, which means there are still millions of the cast-iron blocks out there being rebuilt.

The Vortec heads are the best GM-type casting head. – Derek Ranney, L & R Engines

When it comes to rebuilding these Gen I SBC engines, selecting the right combination of cylinder head for the block becomes the ultimate performance question. With many different varieties of gen I cylinder heads, and the newer Vortec cylinder heads, it’s difficult to always know what is the right choice for home engine builders. We have some cylinder head information that might help clear the vision for your next Gen I SBC engine build.

sbc

The Gen I small-block Chevy cylinder heads feature a wedge shaped combustion chamber.

Gen I Cylinder Heads

The Gen I SBC engines were designed with economy in mind. Economical use of material and space were proven to also help streamline the production process. The Chevrolet engineers began designing the SBC engine by targeting a push rod type engine that could use lightweight overhead rocker arms and a wedge combustion chamber.

The lightweight rockers would allow higher RPM operation while the wedge combustion chambers would allow a broader power band. All of this would be based on 4.4-inch center bore spacing to keep the size of the engine smaller.

sbc

Casting markings on the end of the Gen I cylinder heads, which help identify which casting the individual heads are.

The first small-block with a 4-inch bore came in the 1962 Corvette 327ci, and it really opened eyes. Featuring the famous double hump marking on the ends of the heads, the #3782461 and #3782461X heads used 1.94/1.50 inch valves. These are commonly called “Fuelie” heads because they were released with Rochester mechanical fuel injection on stock engines. The only difference between the two casting numbers was that the #3782461 featured a 160cc intake port and 62cc exhaust port while the #3782461X had the larger 172cc intake port and 64cc exhaust port. As a result the double hump heads continue to be a favorite with performance minded enthusiasts.

In basic terms, enthusiasts separated the stock Gen I cylinder heads into open chamber and the more favored closed chamber cylinder heads. Closed chamber factory heads were preferred for making power due to the poor burn characteristics and no swirl inside the combustion chamber of the open chamber heads.

One thing to keep in mind with the Gen I cylinder heads is fuel. The Gen I heads that were capable of making decent power were developed when leaded fuel was used. The lead helped keep the valve seats lubricated. Unleaded fuel requires the valve seats be replaced with hardened valve seat inserts or they wear the valve seats very quickly.

Popular Gen I Cylinder Heads

#3767754 – 1.72-inch intake valves and 1.50-inch exhaust valves. 60cc combustion chamber

#3774692 – 1.72-inch intake valves and 1.50-inch exhaust valves. 60cc combustion chamber

#3795896 – 1.72-inch intake valves and 1.50-inch exhaust valves. 60cc combustion chamber

#3782461 – 161/62 cc port volumes and 62cc combustion chamber

#3782461X – 172/64 cc port volumes, 62cc combustion chamber

#3890462 – 64cc combustion chamber. No accessory mounting holes

#3917291 – 64cc combustion chamber

#3932441 – 161-165cc intake port. 76cc combustion chamber

#3932441X – 161/65cc ports. 80cc combustion chamber. 1.94-inch intake/1.5-inch exhaust valves

#333881 – 76cc combustion chamber. 2.02-inch intake/1.6-inch exhaust valves

#3991492 – Available on the LT1 engine and over the counter. 64cc combustion chamber

Other Details

Most machinists will agree that the Gen I style heads were built with a lot of metal. Many of those machinists and engine builders have taken advantage of that extra metal to manipulate and improve the flow of air through the heads. In the 60s and 70s, the factory didn’t pay much attention to how much metal was cast in each mold like they do today, so there were a lot more areas to clean up in the old style heads. Because of the extra metal, the old style heads seem to be less prone to heating problems. The newer Vortec heads have a lot cleaner castings and less material, but the cooling system needs to be adequate.

“Vortecs are so much more improved and better,” said Derek Ranney of L & R Engines. “Everyone wants power these days and the Vortec heads are the best GM type casting head out there.”

In the 80s, Chevrolet Performance came out with their famous Bowtie performance heads for the small-block. Introducing the Phase I Bowtie small-block Chevy cylinder head in 1981 for racing applications, Chevrolet Performance subsequently released variations such as Phase II and Phase VI, as well as the NASCAR approved SB2.2 series. The Phase II was the most popular for hot rodders and featured a 184cc intake port volume and 64cc combustion chamber. These worked well into the mid 80s when after market cylinder heads became the hot ticket.

“The only negative thing that I have to say about the Vortec, especially in the carbureted 350 type heads, is when they cast the heads, they weren’t cast in the United States,” added Ranney. “The US castings are far superior. If you don’t get them hot, you are probably going to be ok.”

Vortec Heads

Starting in 1996, on several GM Trucks and Vans, the L31 Vortec heads came on the scene. Not just a modification of existing heads, but complete redesign using the 1996 Caprice/Impala SS LT1 cast-iron head castings as a base.

The biggest change GM made in the new design was revising the water jacket so the new Vortec heads could be used on conventionally cooled small-blocks. The idea of using the 1996 LT1 cast iron head as a starting point for a new performance stock head came from the fact that it was the highest flowing LT head used by GM.

“As long as you have a good cooling system, you’ll probably be ok but if that thing ever gets hot, it’s gonna crack,” explained Ranney.

sbc

The Vortec head (top) does not have the heat crossover that the Gen I cylinder heads have (bottom). Heat can be an issue with used Vortec heads. When upgrading to the newer Vortec heads, Ranney recommends upgrading your cooling system too.

The 1996 Caprice/Impala heads outflowed the Corvette Aluminum LT1 heads by as much as 20 cfm on the intake side. The cast iron Vortec head was in development six months longer than the aluminum head.  During that time, GM engineers tweaked the intake and exhaust ports for additional flow. The cast iron Vortec heads was one of the first to purposely integrate tumble instead of large swirl numbers in the design.

What got the performance enthusiast’s attention was the increase in horsepower from 200 to 255 solely based on the power generated by these heads. By the time you purchased a used double hump cylinder head and had it reworked, the cost was the same as buying a new Vortec head that produced more power.

Types of GM Vortec Heads

The L31 Vortec heads have larger ports than the older Gen I cast steel heads, so they outflow the older heads. The Vortec heads are a little bit lighter than the older heads and have a “kidney” or “heart” shaped combustion chamber that is more efficient than the double hump chamber.

L31 Vortecs comes in two different casting numbers, 10239906 (#906) or 12558062 (#062). Originally, the stock #906 casting head was available in two versions. One version had an Inconel exhaust seat with single angle valve grind and was available on one ton trucks. The other version was the traditional three angle valve grind. Other than that, the #906 is the same as the #062 head.

Vortec head casting numbers are also in the rocker arm valley but usually cast width-wise instead of length-wise.

Chevrolet Performance Parts’ Bowtie Vortecs are offered in “small port” (#25534351), which has 185cc intake ports/65cc exhaust ports, or the “large port” (#25534445), which features 225cc intake ports/77cc exhaust ports.

Both GM factory and Chevrolet Performance Parts’ Vortecs come with 1.94-inch intake/1.50-inch exhaust valves. According to the engineers at GM, “Stepping them up to 2.02-inch valves doesn’t help them any, so it’s not recommended. The port was designed to match the 1.94-inch valves. With the Vortec flow velocity, you need less spark advance to make power which is a clear indication of a more efficient burn.”

Air-flow in the Vortec heads begin to decrease between .500-inch and .550-inch valve lift. Their true strength is low lift flow which gives more area under the total flow curve which is where valves spend most of their time during engine operation.

The Vortec head’s combustion chamber is much smaller than the Gen I head and can be easily identified by its heart shape.

Valves spend much more time at .400-inch lift and below, which is where the Vortec outperforms most other heads. Combine this with high velocity, lack of turbulence, and superior combustion chamber design, and this is where the Vortecs really stand out. Even the GM Fast Burn heads can’t touch the Vortecs at low lift because their ports are too big which makes their air flow similar but with less swirl.

The “bolt-on performance” crowd immediately loved the Vortec’s advancements. Gaining 35-40 horses by simply bolting on a new set of heads is definitely a plus.

How are Vortec Heads Different from Other GM Cast Heads?

•The intake port is designed with a cast “ski jump” on the port roof which is there to increase port flow velocity.

•Port flow was designed to be high in the .300-inch to .500-inch valve lift area to make power with relatively low-lift truck camshafts, whereas the old school Gen I heads liked higher-lift camshafts to make their power.

Vortec heads have the “sawtooth” pattern casting mark on the end of the heads.

•The bowl area is wide around the valve guide – much wider than the old GM cast iron ‘camel-hump’ heads.

•The intake and exhaust valves and valve seats have a three-angle grind from the factory.

•The intake and exhaust valves are back-cut from the factory.

•The combustion chamber has a “heart-shaped” design (‘double-quench’ design), whereas the Gen I cast heads were either open or closed chamber designs.

•Best power is made with 32-degrees total timing, although these heads can make power with timing reduced to 29-degrees when used with short-duration camshafts. Older heads needed 32-degrees and up to ensure a more complete burn.

•Stock out-of-the-box Vortec heads have approx. 480 horsepower potential naturally-aspirated. Out of the box double hump cast heads produced from 25 to 40 horsepower less, depending on valve size.

•Maximum valve lift on Vortec cylinder heads is .460-inch to .480-inch. This range is due to production line machining and casting variance. It is highly recommended to check for clearance on anything over .460-inch lift.

•Vortec heads require a Vortec style 8-bolt intake. They should not be modified for older 12-bolt style intakes because there is not enough section thickness for proper bolt retention at the location of the center bolts. Besides, most intakes can’t be ported enough to match the raised intake ports on the Vortecs.

The intake ports on the Vortec heads are slightly narrower and include a steeper ramp to encourage air to tumble into the combustion chamber. Porting these heads is discouraged.

•When porting the Vortec heads, a little clean-up on the Vortecs is alright. However, if you dramatically open up the ports you will lose the venturi effect in the intakes that makes them flow and perform as well as they do.

•Use flat top or dished pistons to enhance flame travel and intake swirl.

•Larger valves will increase flow but chamber modification is not needed. The trade off between shrouded vs unshrouded valves not worth the decrease in laminar flow and swirl.

Conclusion

We found the Vortec heads are very economical and easy to get your hands on, and out performed the old style heads based on our information gathering, discussions with Chevrolet Performance and engine builders.

A side-by-side visual comparison of a SBC Gen I cylinder head (left) and the newer Vortec head (right).

Ranney warns, “If you pull a set of Vortec heads from the junkyard, the first order of business is to get them cleaned and magna fluxed.” Ranney went on to explain; “I have an El Camino with a 383ci and I put the Vortec heads on it. It makes over 400 horsepower naturally aspirated. It’s a very good head but it’s a thin casting and they crack easily. That’s one of the very few issues that they have.”

The Vortec heads reigned supreme in every category and when paired with a decent cooling system, the Vortec heads will be as dependable as any cast iron head that came before it while increasing the over-all performance of your engine.

Article Sources

About the author

Bobby Kimbrough

Bobby grew up in the heart of Illinois, becoming an avid dirt track race fan which has developed into a life long passion. Taking a break from the Midwest dirt tracks to fight evil doers in the world, he completed a full 21 year career in the Marine Corps.
Read My Articles

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