There is probably no V-8 pushrod engine in the entire world that has more dedicated cylinder heads designed specifically for racing than the first-generation small-block Chevy. A scant few decades ago, there was basically the iron Bow Tie head cast by GM and a few class-specific aluminum castings, and that was the entire race head buffet menu. Now well into the 21st century, you can’t swing a dead camshaft without hitting a new small-block race head. You would think that Brodix damn near owns the race head market with its incredible array of 18, 16, 15, 13.5, and 12-degree valve angle heads aimed at the wild world of drag racing, sprint cars, and dirt track slammers. And as it turns out, valve angles are really the whole point of the race head market.
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Rather than get into the details of each cylinder head as we have in the previous chapters, we’re instead going to touch the wave tops, if you will, in order to keep this section brief. Frankly, if building race engines occupies your every waking moment, then most of what this book has to offer is of little application. Instead, we look into what makes up a race head and why intake valve angle is such a big deal.
As we’ve stated before in various chapters in this book, the stock smallblock Chevy valve angle is 23 degrees, but it didn’t take engineers long to figure out that a taller, more vertical valve angle offered several advantages to improved flow. In addition to this rather steep valve angle, the stock valves were not located on the cylinder bore centerline. Instead, the OEM valves (and all subsequent versions of the 23-degree small-block head) are located over a quarter of an inch (0.275 inch) away from the bore centerline. At maximum valve lift, this off-center location places the valves closer to the combustion chamber and the cylinder wall, restricting potential airflow.
For the small-block Chevy, the original race heads accepted for use in NASCAR stock car racing that first addressed this valve angle issue were the GM 18-degree heads. This move was quickly followed by a set of 15- degree heads, both of which are still available through GM Performance Parts (GMPP) in semi-finished configurations without seats or guides. Since then, several other GMPP heads have appeared that further complicate the issue, requiring us to go much deeper in detail than just a change in valve angles.
Way back in the 1960s when GM engineers began designing the “Mystery” big-block 427 that would eventually evolve into the Mark IV 396/427/454, cylinder head port designers realized that if they tipped or canted the valves at an angle from off-center, the intake flow improved. This created the canted valve head design that made the big-block Chevy famous. This is also the concept that Ford engineers integrated into the Cleveland head design that became famous (or infamous) with the Boss 302 head. The Cleveland and Boss 302 engines may be considered infamous because of their huge port cross-sectional areas. But the concept of the canted valve has not been lost on cylinder head designers. The idea is to cant or tip the valve toward the center of the cylinder bore, which means the valve moves away from the cylinder wall as it opens, unshrouding the air path to move more efficiently around the valve. This not only creates a more efficient air path, but it also allows the engine builder to use a larger intake valve within the same bore size.
As an example, the GM SB 2.2 heads (which will probably be replaced in NASCAR for GM cars by the time you read this) are a symmetrical intake port head that not only stand the valve angle more upright at 11 degrees, but also cant or tilt the intake valve 4 degrees off vertical to improve airflow. To complete the design, the engineers also created an exhaust valve that is not canted, but is angled at a mere 8 degrees. All this contributes to substantial flow improvements over a typical 23-degree valve angle head with inline valves all at the same angle. But there is even more to the story than just a little tilt to the valve.
The moment the designer begins to alter valve angles, this affects all sorts of other cylinder head requirements. For example, the whole idea of moving to a more vertical valve angle is to improve airflow. However, this means the intake port must also move in order to take full advantage of this angle. The port entry at the head must be raised relative to the stock port entry angle. This requires a new intake manifold port entry angle, which means that the old 23-degree intake manifold designs will no longer work, requiring a new intake manifold. But that’s just the beginning.
Generally, the exhaust ports also need to be raised to take advantage of the new exhaust valve angle. This also requires a new header design since stock exhaust port location headers won’t work. Plus, when it comes to the new symmetrical-port race heads, the entire exhaust flange changes because now the exhaust ports are symmetrical rather than having the two center exhaust valves next to each other as in the stock 23-degree head configuration. This symmetrical port arrangement is far better for endurance engines since this no longer concentrates heat in the middle of the head with the two exhaust valves next to each other. Of course, this new exhaust layout also means custombuilt headers.
This different valve angle also means that the OEM valvetrain arrangement with regard to rocker arms, springs, and the entire valvetrain will also be different. Most race heads rely on using an aftermarket rocker shaft system rather than individual rocker arm studs and rockers. This is because most race heads are intended for high-RPM use where the shaft systems excel. With different valve spacing and angles, each new cylinder head then requires its own specific valvetrain, which comes at a much higher cost than the typical 23- degree small-block Chevy stuff. Part of this is a move to offset intake rocker arms in order to clear the larger intake port. This also requires high-quality pushrods to be able to withstand the rigors of high-RPM use.
Taller valve angles also mean using taller valves, and generally, smaller stem diameters in an attempt to keep weight to a minimum. And don’t forget, as soon as we change the valve angle, we also need new pistons to be able to accommodate that different valve angle. We need custom valve covers for these heads as well. That old 4-bolt perimeter valve cover stuff for stock 23-degree heads just won’t do. And let’s not even get into titanium valves and their cost. Finally, many of these heads require custom-length head studs or bolts to keep the head glued to the block.
As you can see, there’s an incredible pile of details that must be accounted for when swapping to an 18- or 15-degree race head. Chamber size is another variable. Generally, taller valve angles also reduce the depth of the chamber, allowing the chamber to be smaller for higher compression ratios without having to resort to large piston domes to create compression. The GM splayed-valve race head offers as small as a 40-cc combustion chamber. To put that tiny chamber into perspective, on a 420-ci small-block with a 4.155-inch bore and 3.875-inch stroke, a 40-cc combustion chamber would allow the engine builder to create a 15.5:1 compression ratio with a 0.041- inch thick gasket and 10-cc worth of valve reliefs in an otherwise flat-top piston. With this smaller chamber, the engine builder eliminates piston domes that are notorious for reducing combustion efficiency and power.
If you intend to use a set of 18 or 15-degree heads, it doesn’t make much sense to bolt them onto a stock production block. If serious horsepower is your goal, then an aftermarket block is also in your future. Besides the inherent strength of these overthe-counter castings, there’s also the added bonus of employing a larger bore to take full advantage of the larger valves and higher flow potential. As we’ve mentioned in Chapter 2 on flow benches, almost any head will deliver better flow numbers when tested on a larger bore. Unless limited by rules on bore size, the goal would be to bolt a set of these high-flow heads on as large a bore as possible.
Basically, you can expect to spend a lot of money making a set of 15 or 18-degree heads work on a small-block Chevy. You can also expect to have to perform quite a bit of custom machinThe race-head flow numbers are a result of a full CNC custom porting, which isn’t really fair to the off-the-shelf Pro 1 head with its cast ports, but it does point out how much potential there is in these heads if you’re willing to spend the money to get there.
The race-head flow numbers are a result of a full CNC custom porting, which isn’t really fair to the off-theshelf Pro 1 head with its cast ports, but it does point out how much potential there is in these heads if you’re willing to spend the money to get there.
We hope that this cursory look at race cylinder heads and the exotic world of 15 and 18-degree heads and the baby Rat heads has been interesting. We just touched the surface of this subject mainly because this book is aimed directly at street-oriented engines. The main reason for including this chapter was to show you what is out there and also to take a closer look at what is required of the rest of the engine, should you find a set of these race heads sitting on your doorstep one fine summer morning. While tempting to use, these heads require a significant investment in other parts in order to make them work. The wise engine builder will construct an entire engine around these specialty heads, rather than try to make these heads work on an existing engine. The latter approach can be both unrewarding and frustrating.
Written by Chris Petris and Posted with Permission of CarTechBooks