Vented vs Non-Vented Breather

[Bob B.]

Hi all,

At least for C1s, oil caps are vented on low-HP engines and non-vented on high-HP. This seems backwards or maybe both should be vented? High-revving drag engines have big breathers added to the valve covers. High-revving Corvette engines go the opposite way and eliminate even the tiny vents on the oil cap. Anyone know why this is?

Thanks,

Bob

[Duke W.]

Re: Vented vs Non-Vented Breather

Drag racing engines and road engines are totally different animals. A drag racing engine doesn't have to meet emission requirements, last for decades and 100K+ miles, or go month/thousands of miles between oil changes.

This has been discussed before and I believe some C1 engines that didn't require a California closed crankcase ventilation system were equipped with non-vented oil fill caps due to "oil misting" in the engine compartment, so the crankcase vent system was merely the road draft tube. I recall GM wrote a TSB and if a customer complained about oil misting in the engine compartment, the "fix" was to install a non-vented oil fill cap.

Theoretically, a road draft tube has slight negative pressure relative to the engine compartment, so a vented oil cap allows some fresh air into the crankcase for better purging of blowby gases.

It's a lousy system that allows much quicker contamination of the oil than later PCV systems, which were initially installed on '61-'62 cars that were destined for sale in California and required on all cars sold in the US beginning in 1963.

I recommnd retrofitting early engines without PCV systems with the RPO PCV components used on '61-'62 engines though all may not accept them without modifications. The components are available from the various Corvette parts suppliers.

Duke

[Christopher R.]

Re: Vented vs Non-Vented Breather

[QUOTE=Duke Williams (22045);720462

I recommnd retrofitting early engines without PCV systems with the RPO PCV components used on '61-'62 engines though all may not accept them without modifications. The components are available from the various Corvette parts suppliers.

Duke[/QUOTE]

With the PCV system, would you use a vented or non- vented cap?

[Duke W.]

Re: Vented vs Non-Vented Breather

A fresh air inlet is absolutely required for a PCV system.

Duke