Air Flow Explained

[PAC-MAN]

Performance and Air Flow


Components that influence airflow into the engine are the:
1.air filter
2. intake air piping
3. mass air sensor (if applicable)
4. throttle body or carburetor
5. intake manifold
6. camshaft
7. intake port and valve of cylinder heads
8. turbo's, compression and supercharger (if applicable)
Components that influence airflow out of the engine are the:
1.exhaust valve and exhaust ports of the cylinder heads
2. camshafts
3. exhaust manifolds
4. turbo's turbine (if applicable)
5. exhaust tubing
6. catalytic converters
7. muffler

When these components are modified to increase flow out of the engine, pumping losses are reduced. Pumping losses refer to the amount of HP used to push the exhaust gases out of the cylinders on the engine's exhaust stroke. Since less HP is used to get the exhaust out of the engine, more horsepower is available at the flywheel. An added benefit of reducing pumping losses is that fuel mileage will also increase.
No matter how much additional air is forced into the engine, no additional HP will be made unless additional fuel is also added. The energy that makes HP in an engine comes from the fuel, not the air. In general, every two HP produced requires one pound of fuel per hour. When mods are performed that increase airflow into the engine, more air is available for the combustion of fuel. The combustion of the additional fuel is what translates into additional HP.
Air flow is not just influenced by the size (area) of the paths it takes into and out of the engine. It is also influenced by the speed at which it moves.
Specific Port Flow(cfm) = Flow Velocity(ft/min) x Average Path Area (ft^2)
Whenever an engine mod increases the average area of the airflow paths into and out of an engine, there is a chance the velocity of the flow will decrease. Most of the time the velocity decrease is very small compared to the area increased, so flow is increased(example - most aftermarket exhaust systems). If things are taken too far, the velocity will decrease more than the area increases, so flow is adversely affected (example - four inch exhaust system on a 1.6 liter engine) From Technical Correspondence of Road and Track
The following was asked: How can I improve low to midrange performance?
I have an '89 Honda CRX Si with 4mm larger throttle body, Super Power-Flow air filter, HKS sport exhaust and MSD ignition. Performance above 4500 is fine, but below that leaves something to be desired.
The answer from Oscar Jackson of Jackson Racing is:
Some of these changes could hurt low RPM performance. An engine is essentially an air pump, and your stock cylinder head, with its stock valves, intake and exhaust ports and cam timing, is really the bottleneck to that pump. Because that cylinder head will draw in only so much air, the larger throttle body you've fitted lowers the air velocity into the intake manifold, hampering throttle response under 4500 RPM. Now if you had a ported cylinder head, big valves and more radical cam timing, you could maintain that intake velocity with the larger throttle body, better preserving low speed performance. You might try bolting the stock throttle body back on while leaving the other high performance pieces in place; it might restore some performance down low and take some away at the top.
If you are willing to invest more time and money for healthier low-speed operation you can:
» increase the compression ratio with different pistons
» increase the displacement by boring the cylinders and fitting larger piston
» wait until spring for Jackson Racing to introduce its supercharger kit

All options are bound to increase the engine's octane sensitivity, so premium pump gas or better will be a necessity.