How to choose a camshaft
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===Overlap=== | ===Overlap=== | ||
| − | This number | + | This number represents the amount of duration in camshaft degrees when both the exhaust and intake valves are open at the same time. This factor is ground into the cam and can't be changed without physically altering the camshaft lobe profiles. Increasing duration at the same LSA will increase overlap. Decreasing LSA at the same duration will also increase overlap. |
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| + | Overlap is usually not found printed out on the cam card, but it's easy to calculate. *Add the intake opening point BTDC to the exhaust closing point ATDC. | ||
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| + | If the intake opening and exhaust closing points aren't known, you can estimate the overlap by using the '''advertised duration''' (or duration @ 0.050" lift, etc.) and the '''lobe separation angle'''. | ||
| + | *Add the intake and exhaust durations, | ||
| + | *Then divide the results by 4, | ||
| + | *Then subtract the lobe separation angle, | ||
| + | *Then multiply the results by 2. The result is the overlap. | ||
Overlap and LSA are closely tied together. Increasing overlap contributes to a race cam's choppy idle, along with the intake valve closing point and the exhaust valve opening points. The extra time the valves are open at the same time causes what is called ''reversion'', which is a situation in which the exiting exhaust gasses are partially pushed back up into the intake runner at low speeds. This causes big fluctuations in vacuum and uneven fuel metering if a carb is used (EFI metering isn't affected but reversion can still be a problem). Once the engine reaches higher RPM, the overlap is helpful since it adds to the time the cylinder can be filled with air/fuel mixture. Also, a tuning effect can come into play where the fast-moving exhaust gasses create a slight vacuum which helps to pull in more air/fuel mixture and remove more spent exhaust gasses from the cylinder, which is called ''scavenging''. Overlap also has a large impact on the amount of intake manifold vacuum an engine makes. Less overlap allows more idle vacuum, and vice versa. | Overlap and LSA are closely tied together. Increasing overlap contributes to a race cam's choppy idle, along with the intake valve closing point and the exhaust valve opening points. The extra time the valves are open at the same time causes what is called ''reversion'', which is a situation in which the exiting exhaust gasses are partially pushed back up into the intake runner at low speeds. This causes big fluctuations in vacuum and uneven fuel metering if a carb is used (EFI metering isn't affected but reversion can still be a problem). Once the engine reaches higher RPM, the overlap is helpful since it adds to the time the cylinder can be filled with air/fuel mixture. Also, a tuning effect can come into play where the fast-moving exhaust gasses create a slight vacuum which helps to pull in more air/fuel mixture and remove more spent exhaust gasses from the cylinder, which is called ''scavenging''. Overlap also has a large impact on the amount of intake manifold vacuum an engine makes. Less overlap allows more idle vacuum, and vice versa. | ||
| − | More overlap (less vacuum) can cause tuning headaches with modern OEM engine management electronics and EFI. It can also make tuning a carburetor more difficult. More overlap makes a choppy idle and tends to make peakier power for the same reason as a narrow LSA does. More overlap and the subsequent lower intake manifold vacuum might mean giving up vacuum-driven accessories like power brakes. Some cars even use vacuum to operate the | + | More overlap (less vacuum) can cause tuning headaches with modern OEM engine management electronics and EFI. It can also make tuning a carburetor more difficult. More overlap makes a choppy idle and tends to make peakier power for the same reason as a narrow LSA does. More overlap and the subsequent lower intake manifold vacuum might mean giving up vacuum-driven accessories like power brakes. Some cars even use vacuum to operate the HVAC, headlight covers, door locks, and windshield wipers, so consideration for those devices has to be given if choosing a cam for a vehicle so equipped. If the vacuum produced is insufficient, a vacuum pump can be installed. |
===Lobe intensity=== | ===Lobe intensity=== | ||
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