The air box system doesn't provide much vacuum since it's on the filter side of the carbs. Vacuum is controlled by the restriction of the filter. That's the big difference between the old system and the "positive" system.

I don't see the need to add an AN fitting to a welch plug, as the machined AN fitting can fit. I found the fitting, it was machined too much and hence given to me. Its steel, but my friend has an aluminium AN fitting. The black AN is a speedflow one, which I think is Australian brand, and I am unsure where the steel one came from. The threads one side are just machined off the large fitting. Obviously its a reducer fitting to aid your search.

Here's an interesting catch can take on a 3.4 L stroker. From a guy billing himself as "Orginal Datsun parts for sale" on Facebook. Allegedly in Phoenix, AZ. Anyone know of him?

Edited May 7May 7 by Seppi72
More detail added.

So much for AC I guess. Great idea if you’re not running air or a track car.

I see fire. Oil-soaked filter, lots of surface area, hot headers.

Another thought - as far as I know, none of today's modern systems have a vent in the block. They're all vented from the valve cover, typically just one. With a fresh air inlet on the other valve cover.

Edited May 7May 7 by Zed Head

@RIP260Z So if I understand correctly, The fitting in the pic was originally the same size as the”Welch”plug. Forgive me if I misunderstood. Seems extremely large. Thanks for the input.

Well, regardless of how I eventually do the plumbing from the block and the valve cover, I would love to vent both into a cold air box feeding my Webers so the blow-by vapors would be distributed equitably.

On searching, the only source I seem to find is EFI Hardware in Australia. They offer a fiberglass airbox that would seem to handle triple carbs but the price is about $450. So adding shipping and tariffs to that probably puts it over well $600. Does anyone have a better source?

Some folks will argue that by doing that you introduce the oily vapors back into the engine. There are catch cans although pricy can return the air to the air box, but you need vacuum for that. Watch the videos on you tube and there are some on “ Motion Raceworks” Website. Will give you a better idea of the initial purpose was not to introduce the vapors back into the engine to avoid valve train issues and they claim it creates a hp increase. Nominal I’m sure. That’s what I’ve found in the past couple of days. Ton of information out there.

6 hours ago, Zed Head said:

as far as I know, none of today's modern systems have a vent in the block. They're all vented from the valve cover, typically just one. With a fresh air inlet on the other valve cover.

I would expect that pretty much all V-block engines run ports on both valve covers. One to PCV vacuum and the other to atmospheric at the air cleaner. Flow through from one valve cover, down into the block, and out the other valve cover.

And I don't know about all modern inline engines, but I can tell you from hands on experience that early 2000's Civics and late 2000s Hyundais have a system that is conceptually identical to the Z. Vent on the valve cover and PCV connected to a port on the block. Even though those examples aren't really new, I would consider them to be "modern". Ish.

Good point. I assume that engineers considered the flow path of the fresh air in each system. I've been surprised at how many different types of valve there are. They look similar but the springs and orifice sizes must be different, they have different part numbers. One of those finely engineered simple-looking devices.

Fresh filtered metered air from somewhere and an exit through a valve. I browsed around the internet and see that many people are unaware of or just ignore the fresh air part of the system. In the 280Z EFI engine the hole in the valve cover is actually where the fresh air is supposed to come from, unless there's high blowby and it gets overwhelmed.

Sorry for the over-do, I can't help it. It's an interestng topic.

Just for fun:

1995 Pathfinder

https://www.rockauto.com/en/moreinfo.php?pk=438093&cc=1211993&pt=5052&jsn=916

1976280Z

https://www.rockauto.com/en/moreinfo.php?pk=438072&cc=1209226&pt=5052&jsn=966

2010 Pathfinder V6

https://www.rockauto.com/en/moreinfo.php?pk=14825361&cc=1444653&pt=5052&jsn=1044

Yeah, I think the PCV system is much misunderstood. I guess it's one of those seemingly simple things that "just is" and many people don't really know that it's more sophisticated than it seems at simple quick glance.

But I have to agree.... I don't see the need to have 1000 different PCV valves. It's like oil filters... Couldn't engine designers spend a little more effort on reusing sizes and shapes that already existed??

On 5/7/2026 at 8:16 PM, Yarb said:

@RIP260Z So if I understand correctly, The fitting in the pic was originally the same size as the”Welch”plug. Forgive me if I misunderstood. Seems extremely large. Thanks for the input.

The machined back part, behind the hex, is a tolerance push fit into the block. Yes its quite large, but its alot less protruding the standard pipe. If wanted, the hex could be machined down as well to become just oversize of the block hole, for a slimmer looking fit.