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Small Block Chevy and Ford Performance Cylinder Heads
Small block Chevy and Ford engines have been the mainstays of the aftermarket performance industry for nearly half a century. The original small block Chevy made its debut way back in 1955 as a 265 cubic inch V8, and the small block Ford appeared in 1962 as a 260 cubic inch motor.
By Larry Carley
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Over the years, the SB Chevy V8 evolved and grew in displacement from 265 to 283 to 302 to 327 to 350 to finally 400 cubic inches. Aftermarket blocks and stroker cranks have allowed even more displacement, with some “small blocks” now having as many cubic inches as a big block Chevy (427 and 454 cubic inches). As for Ford, the 289, 302 and 351 Windsor V8s have also enjoyed a long production run, with later renditions of these engines being relabeled the 5.0L and 5.8L from 1982 to 1995.
As these engines evolved, so did their cylinder heads. As displacements and power outputs grew, so did the size of the intake runners and valves. The best production performance head for the early 265/283 cid Chevy V8s was the Power Pack head with 1.72˝ intake valves and 1.50˝ exhaust valves (which are very puny by today’s standards).
A far better cylinder head was the “double-hump” production head for mid-1960s 327 engines that came with 2.02˝ intake valves, 1.60˝ exhaust valves, 160 cc intake ports and 64 cc combustion chambers. The best casting numbers for the double-hump head were 461 and 462, which had better flowing ports than the later 441 and 882 versions of this head.
Then Chevy came out with the famous Bow Tie performance heads for the small block, which were even better than the original double-hump heads. These were the hot set up until the mid 1980s, when aftermarket cylinder heads began to appear that out performed the original Chevy castings.
In 1986, Chevy upped the ante with their first aluminum cylinder heads on the L-98 Corvette engine. The aluminum heads were 40 lbs. lighter than cast iron heads, and offered a modest improvement in power. Better yet, the aluminum heads could be bolted onto older engine blocks. Even so, the new factory aluminum heads could not achieve the same performance level as many aftermarket heads that were available at the time.
The next major development was the introduction of the L31 Vortec cylinder heads in 1996. Though originally designed for truck engines, Vortec heads had 170 cc intake runners, and somewhat smaller 1.94˝ intake valves and 1.50˝ exhaust valves. But the flow characteristics of this cast iron head proved to be even better than anything before it thanks to the high velocity ports. Thus, Vortec heads became a good upgrade for 350 small block V8s making up to 400 horsepower. The Vortec heads differed from earlier designs by using center-bolt valve covers, and a different intake manifold configuration (8-bolt manifold instead of the previous 12-bolt manifold).
Later LT1 and LT4 engines (GEN II) featured a reverse-flow cooling system, so the cylinder heads on these engines are not interchangeable with those on the earlier GEN I blocks. And the LS1 and later heads that came out in 1997 are an entirely different design, though one aftermarket supplier of engine blocks (World Products) now offers a slightly modified SB Chevy engine block that allows late model LS Chevy heads to be bolted onto the block. The combination is said to be good for 30 horsepower.
As time goes on, it’s getting harder and harder to find stock performance heads for small block Chevy and Ford engines that are still in good usable condition. Many of these heads have been rebuilt more than once, and many have succumbed to cracks or wear that cannot be easily repaired.
That’s one of the reasons why aftermarket performance heads have prospered. They are brand new castings that are not in short supply. You can buy them anywhere and often at a cost that is less than what it would cost to completely rebuild an old cylinder head.
The other reason why aftermarket cylinder heads have proliferated so in recent years is that most of these heads offer much better out-of-the-box performance than even the best factory castings. Aftermarket heads are purpose-built heads designed for specific kinds of performance applications. That means port volumes, port locations, valve sizes, valve angles and combustion chambers can all be optimized for maximum performance.
Computer Numeric Controlled (CNC) machining is also readily available as an option with many aftermarket performance heads today. CNC heads are often touted as being better than as-cast heads. But that’s not always true. CNC machining is just another way to finish a casting. If the profile of an as-cast head has already been optimized to deliver the best flow possible, CNC machining it won’t make it any better. Likewise, if the CNC mapped profile that is machined into a head does not flow as many cubic feet per minute (CFM) of air as a well designed cast port, there’s nothing to be gained.
Where CNC machining does provide an advantage is when a stock casting needs to be opened up or reshaped to improve its flow characteristics. CNC machining is much faster, easier and less expensive than hand porting a cylinder head, and can easily replicate with a high degree of accuracy almost any port configuration that has been developed, tested and refined on a flow bench. But like any machining operation, the tooling tolerances must be tightly controlled to achieve consistent results.
Choosing The Right Cylinder Head
With so many different aftermarket performance cylinder heads available, how do you choose the best head for an engine you are building? Do you go for the head that delivers the highest flow numbers? That’s what some people do, and they often end up being disappointed because bigger is not necessarily better depending on the application.
Do you have to use heads with the same valve angle as the original, or heads that will bolt up to a stock intake manifold or a popular aftermarket intake manifold? That may limit your selection to heads with a 23 degree valve angle for a SB Chevy and heads without raised intake or exhaust ports (which improve flow but may also require special manifolds).
Is cost an issue? If you’re building a budget performance motor for a customer, $1,000 or less may be all he can afford for a pair of aftermarket heads. On the other hand, if your customer has deep pockets and can afford the best, it opens up more possibilities such as CNC-ported heads or even custom heads.
There are a LOT of variables to consider when choosing a cylinder head for a particular engine application. The “best” head will be the one that delivers the most torque, horsepower and throttle response within the target rpm range of the engine.
Street engines spend most of their time at low rpm, so a good street performance engine should be built to deliver maximum torque from 1,500 to 5,000 rpm. The engine should have good throttle response and high intake vacuum for everyday driveability.
A circle track engine, by comparison, also needs good throttle response but a higher torque curve. It depends on the weight of the race car, the gearing, tire size and length of the track.
With drag racing, it’s all about maximum power and acceleration. Bigger is usually better here, with large displacement engines sucking huge volumes of air through high flowing heads.
Aftermarket cylinder heads are available with different intake runner volumes, different port configurations and port heights, and different valve angles. The companies that make such heads typically label their heads according to their intake port volumes or their flow characteristics.
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i have been looking for old ford truck parts such as these cylinder heads. You are correct that it's hard to find
by: jackmoore 11/12/2009