* Specific year and casting number of the heads: Different heads have different valve spring designs and strengths. Older heads generally have weaker springs than later ones.
* Valve spring condition: Springs weaken over time and with use. Worn springs will be unable to handle as much lift.
* Cam profile: A high-lift camshaft needs stronger springs than a low-lift cam.
* Valve train geometry: Proper geometry is crucial to prevent valve float and interference.
While some sources might suggest a general guideline (e.g., .450"-.500"), it's dangerous to rely on these general numbers. Exceeding the safe lift for your specific heads can lead to:
* Valve float: The valve doesn't close completely, resulting in loss of power and potentially engine damage.
* Valve spring breakage: This can cause catastrophic engine damage.
* Valve-to-piston contact: The valve hits the piston, causing serious damage.
To determine the maximum safe lift for your specific heads, you need to:
1. Identify the casting number of your heads. This will allow you to find information about their design and specifications.
2. Inspect the valve springs. If they're worn or showing signs of fatigue, they need to be replaced.
3. Consult a valve spring pressure chart or a professional engine builder. This will help determine the appropriate spring pressure and maximum lift for your chosen camshaft and head casting.
In short, relying on a general number is risky. Proper research and consultation are vital to avoid damaging your engine.
