Calculate Valve Overlap

Guide to Calculating Valve Overlap

Essential Tools and Resources

To accurately calculate valve overlap, a cam card is essential. This card should list crucial details such as when the intake valve opens (IO) and when the exhaust valve closes (EC). To determine the overlap, you simply add these two values together, as indicated by the formula: IO + EC = Overlap.

Understanding the Calculation Process

The primary formula for calculating valve overlap is straightforward: add the numbers from the cam card indicating when the intake valve opens to when the exhaust valve closes. For instance, if the values are IO=20BTDC and EC=18ATDC, the overlap at a lift of 0.006" is 38 crank degrees.

Advanced Calculation Considerations

For more precise calculations, especially relevant at different valve lifts like 0.050", the process involves additional steps. First, add half the intake duration to half the exhaust duration, then divide the result by 2 to find the overlap in cam degrees. From there, subtract the Lobe Separation Angle (LSA) from the obtained number to finalize the overlap degree. This method offers a refined approach to customizing engine performance based on specific engine builds.

Utilizing Valuable References

David Vizard's "How to Build Horsepower, Volume 3, Smallblocks on a Budget" provides an invaluable overlap requirement chart based on a 350 model with modified head flow specs. Such resources assist in adapting the calculation based on differing engine characteristics, like building a 400 with better flowing heads, which requires adjustments in the overlap value for optimized performance.

Practical Steps for Calculation

The procedural approach ensures correctness in the calculation. Here is a step-by-step guide:

1. List the intake opening and exhaust closing points from the cam card.
2. Add these values together as per the formula: Exhaust Closing Point + Intake Opening Point = Overlap.
3. Apply specific adjustments as required by engine specifications or performance goals.

1. List the intake opening and exhaust closing points from the cam card.
2. Add these values together as per the formula: Exhaust Closing Point + Intake Opening Point = Overlap.
3. Apply specific adjustments as required by engine specifications or performance goals.

By following these steps and utilizing the correct resources, anyone can accurately calculate valve overlap, leading to better-engineered engine performance.

How to Calculate Valve Overlap

Valve overlap is crucial for optimizing engine performance by ensuring that intake and exhaust valves operate effectively. Valve overlap, quantified in degrees of crankshaft rotation, describes the period when both intake and exhaust valves are open simultaneously, at the transition from the exhaust stroke to the intake stroke.

Step-by-Step Calculation

To calculate valve overlap, you can either use durations or cam card listings. If using durations, first add the intake duration and the exhaust duration. Then, divide this sum by two and subtract the lobe separation angle (LSA). Finally, multiply the result by two. The formula can be expressed as: ((Intake Duration + Exhaust Duration) / 2 - LSA) * 2.

If the cam card specifies points when valves open and close (IO and EC), add these points directly: Intake Open Point + Exhaust Close Point. This method is generally accepted as the most straightforward way to determine the overlap.

For engines with the cam card not listing IO and EC, perform the overlap calculation by adding half of the intake duration to half of the exhaust duration. Then, subtract the lobe separation angle. This alternative method is useful when specific opening and closing points are unavailable.

Practical Example

Suppose an engine's cam card gives an intake open at 20° before top dead center (BTDC) and an exhaust closes at 30° after top dead center (ATDC). The valve overlap would be calculated as: 20 + 30 = 50°.

For a more complex example using durations and LSA, consider an intake duration of 260° and an exhaust duration of 270° with an LSA of 115°. Using our duration-based method, the calculation would look like this: ((260 + 270) / 2 - 115) * 2 = 40° overlap.

Understanding and calculating valve overlap helps in tuning engines for specific performance outcomes, particularly beneficial in racing applications where maximizing airflow is essential.

Calculating Valve Overlap in Engines

Valve overlap refers to the duration when both the intake and exhaust valves are open simultaneously at the end of the exhaust stroke and the beginning of the intake stroke. This process is critical for engine efficiency and performance. Below are examples illustrating how to calculate valve overlap.

Example 1: Basic Calculation

Determine the exhaust valve closing (EVC) and intake valve opening (IVO) points. If EVC is at 10° after top dead center (ATDC) and IVO is at 20° before top dead center (BTDC), calculate overlap as follows: Overlap = 20 + 10 = 30°.

Example 2: Including Camshaft Adjustments

If adjustments are made to advance the intake cam by 5°, and the original IVO is 15° BTDC and EVC is 5° ATDC, the new IVO becomes 20° BTDC. Calculate the new overlap: Overlap = 20 + 5 = 25°.

Example 3: Impact of Exhaust Cam Retardation

For an exhaust cam retardation of 5° where the original EVC is 10° ATDC and the IVO is 25° BTDC, the new EVC is 15° ATDC. Calculate the overlap: Overlap = 25 + 15 = 40°.

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