Performance Exhaust Systems: How to Design, Fabricate, and Install. Mike Mavrigian
volume and cross-section area than intake ports because exhaust gases are pushed out by the pistons and pulled out by exhaust system scavenging.
Intake and Exhaust Valve Diameters
Intake and exhaust valve diameters are a function of the design of the intake and exhaust ports. Generally, exhaust valve diameters are 70 to 80 percent of the intake valve diameter because of the difference in flow requirements of the intake side versus the exhaust side.
These are typical valve diameter combinations of aftermarket small-block cylinder heads:
| Intake (inches) | Exhaust (inches) |
| 1.940 | 1.500 |
| 2.000 | 1.575 |
| 2.020 | 1.600 |
| 2.055 | 1.600 |
| 2.165 | 1.600 |
Combustion Chambers and Valve Angles
Combustion chambers in cylinder heads come in a variety of different shapes and sizes depending on the engine application. The combustion chamber’s main functions in a cylinder head is to aide in the intake cylinder filling, compressing the air/fuel mixture, controlling flame travel during ignition, and processing exhaust gases into the exhaust port.
Combustion chamber volume and shape design has a major effect on the static compression ratio in an engine, the amount of ignition timing advance that can be added, and the output emissions through the exhaust system. Some modern engines use two spark plugs per cylinder to achieve maximum efficiency, while reducing exhaust emissions by more complete combustion.
Valve angles and combustion chamber volume are directly related to one another. Generally, valve angle is measured 90 degrees (perpendicular) from the cylinder head deck. In the case of OEM GM LS cathedral-port cylinder heads, the valve angle is 15 degrees. For the sake of comparison, early small-block Chevrolet cylinder heads feature 23-degree valve angles.
Numerically lower valve angles allow for a shallower combustion chamber, resulting in lower combustion chamber volume. Numerically higher valve angles require a deeper chamber, resulting in a larger combustion chamber volume.
The combustion chamber aids in the intake cylinder filling, compressing the air/fuel mixture, controlling flame travel during ignition, and processing exhaust gases into the exhaust port.
Citing the GM LS cathedral-port cylinder heads as an example, valve angle is established at 15 degrees, compared to 23-degree valve angles featured in early small-block Chevrolet cylinder heads. Numerically lower valve angles allow for a shallower combustion chamber, resulting in lower combustion chamber volume.
It is important to note that valve angles affect the amount of clearance between the valves and pistons. Choosing the right combination of camshaft, piston, and cylinder head is crucial to the reliability of the engine build. A general rule of thumb for valve-to-piston clearance is .080-inch intake and .120-inch exhaust, measured +/- 15 crankshaft degrees from intake top dead center.
Throughout this book, I talk about exhaust gas scavenging. This refers to the engine’s system being able to pull the exhaust gases out of the engine. The more exhaust volume pulled out, and the faster it’s pulled out, the more the air/fuel mixture is allowed to be drawn in. Efficiently burning more air and fuel means more power.
Positive exhaust pressure runs from the exhaust ports to the end of the exhaust pipe. The pressure wave collapses at the exit, and a negative pressure wave is created that tries to return to the cylinder head’s exhaust port. Ideally, you want the negative pressure wave of the exhaust gas to hit the exhaust valve just before the valve closes.
Due to valve overlap, the intake valve starts to open while the exhaust valve is still open (with the exhaust valve off its seat before it closes). This helps reduce cylinder pressure, allowing a more efficient intake stroke. As the piston moves up during the exhaust stroke, exhaust gas is pushed out. When the intake valve starts to open just before the piston hits TDC, and with the exhaust valve still open, the exhaust gas helps to pull the air and fuel charge into the cylinder.
Particularly in a naturally aspirated engine, where forced induction isn’t a factor, this valve overlap aids in both exhaust push and intake charge entry. The higher the planned engine RPM, the more intake and exhaust valve overlap is needed.
Camshaft Timing
When the camshaft is advanced the intake valve opens sooner and the engine delivers more low-end torque. Advancing the camshaft also decreases intake valve-to-piston clearance and increases exhaust valve-to-piston clearance.
When choosing your camshaft, pay attention to lobe separation angle (LSA). A tighter (smaller number) LSA tends to move torque to a lower RPM range, while increasing maximum torque. A wider (larger number) LSA tends to move the torque to a higher RPM range.
Retarding the camshaft keeps the intake valve open later and thus delays the intake closing. This helps generate power at higher engine RPM. Retarding the camshaft also increases intake valve-to-piston clearance, while decreasing exhaust valve-to-piston clearance.
Lobe Separation Angle
The lobe separation angle (LSA) refers to the number of degrees between the centerlines of the camshaft’s intake lobe and the exhaust lobe. Differences in LSA have an effect on engine performance. For example, a camshaft may feature a 108-, 110-, 114-, or 118-degree LSA.
The tighter the LSA is, the smaller the LSA number. Tightening the LSA tends to move engine torque to a lower RPM and increases maximum torque with a narrower powerband. A tighter LSA also builds higher cylinder pressure and increases the engine’s effective compression. The increase in compression also increases the possibility of detonation/knock, which may require the use of higher-octane fuel. Engine vacuum at idle is decreased, with a degradation of idle quality, and valve-to-piston clearance tightens up. A tighter LSA moves torque at a lower engine RPM, increases maximum torque, and provides a narrower powerband. In addition, it increases cranking pressure, reduces idle vacuum, increases valve overlap, and decreases valve-to-piston clearance.
A wider LSA typically provides a broader powerband and improves vacuum at idle and idle quality. Engine torque is slightly reduced and moved to a higher RPM range. Cylinder pressure and effective compression is reduced and the chance of detonation is lowered, making a wider LSA camshaft slightly more accommodating for today’s fuels. A wider LSA moves the torque band to a higher engine RPM range, reduces cranking pressure, increases idle vacuum, creates a wider powerband, decreases valve overlap, and increases valve-to-piston clearance.
Here’s a comparison of lobe separation angles (LSAs). A tighter