PCV valves are more complicated than they appear despite just being a spring loaded plunger in a climped together housing. What is complicated are all the different calibrations they are available in. Though first here is a chart that explains PCV valve operation.
As can be seen in the chart above, PCV valves reach max airflow when the pintle is about mid stroke. The chart skips the backfire mode of PCV valves where a backfire creates a pressure front in the vaccum hose to slam the pintle shut to prevent the backfire from reaching the crankcase. PCV valves are available in many combinations of different spring rates and plunger weights to match the vacuum curve of your factory engine. This is why PCV valves commonly have either numbers or letters stamped on the plunger to indicate their specs, this is called the weighting number. A PCV valve's tune is called weighting.
If you modify your engine then the stock spec PCV valve may no longer work correctly. For example, put in a new cam that reduces idle vacuum will require a PCV valve with a lower spring rate and lighter plunger, ie a weight load to prevent a idle vacuum leak due to lower idle vacuum. However, charts of the weighting specs for PCV valves are purposely kept hidden from the public so its difficult to cross reference PCV loading for modified or custom engines. A PCV that has a mismatched weight for your vacuum curve can cause all sorts of bizarre tuning issues. Even if your engine is bone stock, you still might have PCV tuning issues as manufacturers have consolidated PCV valve applications. So the replacment valve listed for a stock engine could still be off spec.
As can be seen in the chart above, PCV valves reach max airflow when the pintle is about mid stroke. The chart skips the backfire mode of PCV valves where a backfire creates a pressure front in the vaccum hose to slam the pintle shut to prevent the backfire from reaching the crankcase. PCV valves are available in many combinations of different spring rates and plunger weights to match the vacuum curve of your factory engine. This is why PCV valves commonly have either numbers or letters stamped on the plunger to indicate their specs, this is called the weighting number. A PCV valve's tune is called weighting.
If you modify your engine then the stock spec PCV valve may no longer work correctly. For example, put in a new cam that reduces idle vacuum will require a PCV valve with a lower spring rate and lighter plunger, ie a weight load to prevent a idle vacuum leak due to lower idle vacuum. However, charts of the weighting specs for PCV valves are purposely kept hidden from the public so its difficult to cross reference PCV loading for modified or custom engines. A PCV that has a mismatched weight for your vacuum curve can cause all sorts of bizarre tuning issues. Even if your engine is bone stock, you still might have PCV tuning issues as manufacturers have consolidated PCV valve applications. So the replacment valve listed for a stock engine could still be off spec.
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