The Role of Cylinder Heads
Internal combustion engines are, simply said, air pumps. The more air you pump into an engine, the more air/fuel you can burn, and the more power you can make. Building horsepower is not quite as simple as that, but it's the basic concept. The cylinder head is the main component that flows air/fuel in and exhaust gases out. Needless to say, the cylinder head is a major component in building horsepower and torque. Because the way the shape and size of the combustion chamber affects so much of an engine's operation, you have to choose the appropriate selection, not just the one with the best peak numbers. Understanding cylinder head terminology and ultimate affects, will lead to a better choice for your application. Your goal should be to choose the best heads for your application. Going with the BIGGEST heads, is not always best. Read on to learn how to choose the right cylinder head for you.
Cast Iron Cylinder Heads
OEM cylinder heads are often cast iron which offers durability and a lower cost to produce and manufacture. Cast Iron heads, as you would expect, are also heavy. For almost all applications, lighter is better. Drag cars in particular benefit to help transfer at launch and road course/ circle track cars benefit from better weight distribution. Another downfall of cast iron is it lacks the ability to dissipate heat. Aluminum dissipates heat better, which will allow for a higher compression ratio without detonation. Cast iron will provide a good option for the budget minded builder. Performance options are available for cast iron heads that include larger valves and increased flow designs, but should be limited to lower compression street cars for the most part.
Aluminum Cylinder Heads
Aluminum, as stated above provide a sizable weight advantage over cast iron heads. Since most performance vehicles benefit from the weight savings, many upgrade to aluminum heads for this reason alone. There are many companies casting high performance aluminum cylinder heads for street and track performance. Aluminum alloys are now much more durable and resistant to warping than they were 20 years ago, and material problems are minimal. Most manufacturers offer different configurations of the same head, depending on your need. Most offer from a raw or bare casting to allow the end user to finish completion to a complete, or assembled, head built to your specs to simply take out of the box and bolt on.
Cylinder Head Terms You Should Know
To address cylinder heads in an overview we need to speak in relative terms. Smaller cubic inch engines will typically require smaller measurements on all components when compared to a larger cubic inch engine. Valves, for example, are usually smaller for small blocks vs. big blocks. Engine displacement works hand in hand with the cylinder head specifications and individual component size. Simply said, as the capacity of the engine increases, the need for higher volumes of air in and out of the engine will increase proportionally.
Combustion Chamber - The size of the combustion chamber partially determines engine compression and displacement. A larger combustion chamber makes the cylinder displacement (or volume) larger. Assuming you don't change anything else in your engine setup, swapping cylinder heads with a smaller combustion chamber will increase compression. Because you're compressing the same amount of air into a smaller place, the overall compression ratio goes up. Adding a head with a larger cylinder head combustion chamber would yield the opposite - less compression.
Intake Port Shape & Size - Cylinder heads have intake port runners where fuel and air flows through to get to the cylinder. On a 16 valve V8 engine, each cylinder head has 4 intake and 4 exhaust ports. Each port matches up to an intake or exhaust valve. The size and shape of the intake ports affect the torque and horsepower curve. Larger ports can flow more. Plus, you usually try to match your intake port type on your cylinder head to an intake manifold with similarly shaped ports. Air and fuel flow through the intake into the intake ports on the heads. Restrictions, or misaligned ports, at this joint will alter the flow of air and fuel and may affect performance. One of the major concerns is that you get a good seal between the heads and intake manifold to avoid coolant leaks. There are basically 2 types of ports. Oval port heads are basically oval shaped. The shape is not an exact oval. Rectangular shaped heads are also not an exact rectangle shape. They are mostly rectangular with the corners usually rounded off.
Intake Runner Volume - A larger intake runner volume allows more air and fuel to flow through it. However, the larger "tunnel" will slow the velocity of gases moving through it. Smaller intake runners would speed up the flow of gases through the cylinder head and increase throttle response and torque. If you want the magic combination: A head with good flow and air velocity would give you the best of both worlds.
The bigger your engine displacement, the larger your cylinder head intake runners generally need to be. The bigger your intake runners are, the higher the power band is pushed. If you want to build power for the street, huge intake runners will build big horsepower at NON-streetable RPMs. You're better off with smaller intake runners which help build power at lower RPM's.
Exhaust Port Shape & Size - Exhaust port shape is less important than intake port shape. As long as your headers or exhaust manifolds are bigger than the port on the head, and the gasket seals well, the gas will exit as it should. Where intake runners on heads have to line up more precisely with the intake manifold for good flow and coolant openings must seal, the exhaust port shape is less critical.
Intake Valves - Intake valves are typically larger than exhaust valves. 2.02 inch diameter intake valves are common small block valves. As you would expect, the larger the valve, the more air and fuel can be flowed through the valve (valve lift will also affect the volume of air/fuel flow). Intake valves are larger than exhaust valves because the piston is moving away from it, creating a vacuum to pull air and fuel into the cylinder. Pulling air and fuel in is not as efficient as pushing air out of the engine, as is the case with the exhaust stroke pushing exhaust gases out of the cylinder.
Exhaust Valves - Exhaust valves are smaller than intake valves. 1.60 inch diameter exhaust valves are common on SBC engines. Like the intake valve, the larger it is the more it will flow. Cylinders are only so wide and you have to fit an intake and exhaust valve plus a spark plug in each cylinder. There is a point at which a 2 valve engine maxes out the area to fit larger valves. They won't physically fit in the head over the cylinder.
Choosing the Right Cylinder Heads For Your Set-up
Reading this page and understanding the above terms will hopefully help lead you in the right direction. The best suggestion overall is to stay conservative. For example, a cylinder head with a large intake runner has no business on a low RPM street application This will result in a sizable torque loss, thus not meeting the needs of your intended purpose nor of the engine. Be realistic, choose the head that matches what you are doing and if you are selecting an assembled head, make sure the springs included match your camshaft selection. If you read and understand the topics included in this article, we are confident that your selection will be at least an informed decision. If you still have trouble deciding which cylinder heads to buy, the JEGS tech line is available 24/7. You can also reach us by our JEGS Ask A Tech Email service where we will be glad to help you choose.