Fixed Orifice PCV
 

In the Facebook Sniper EFI forum I read that a fixed orifice style PCV is preferred when using EFI - apparently it helps with idle. Has anyone experimented with this and also is anyone aware of a fixed orifice style PCV that looks and fits in old Pontiacs like ours.

I went from a standard PCV system to a draft style system that is close to a fixed oriface PCV on my FiTech. I didn't notice any change in idle characteristics, nor did I have to make any tuning changes.

The item that made the biggest difference in idle quality was moving to timing control. Being able to dump in idle timing and letting the system idle stabilize by adjusting advance relative to fuel made a fairly dramatic difference in both idle quality, temperature control and idle vacuum. I gained over 2" Hg at idle by going to timing control and commanding about 24 degrees of initial advance.

This worked so well, that my once "somewhat rowdy" 455 with a 232/238 112 cam behaves and sounds as if it has more of a factory type cam in it.

I've tuned mine with a pcv system and with a draft style open breather set up. They each react differently because of the vacuum differences, but I can attain a great idle with either.

I switched the Chevelle over to fixed orifice PCV, after I had run the Sniper setup for more than a year with a regular PCV.

On mine, going to a fixed orifice, basically made the vacuum leak much smaller and caused the IAC counts to rise a bit to bring the idle back up where it was commanding. Once warmed up it's not a big deal but when it's cold it really tightened up the window I had for a high speed cold idle, basically pegging the IAC at 100 when cold to barely maintain an idle.

What it boils down to for me, and something I complained about very early on when we bought these Snipers, is that the IAC could stand to be a bit bigger for these larger engines with big camshafts that have a bunch of overlap.

I've played around with drilling the fixed orifice, I think I'm up to 3/32" or so and it hasn't really made a bunch of difference. Of course that's not even close to the near 1/4" or 5/16 hole the regular PCV had, LOL

Funny this all came up because I was going to switch it back over to the regular PCV today. I never had any drivability or running issues with it so not sure why they preach that so much over on the Holley forum.

The GTO got switched over to a vacuum pump so the engine is totally sealed, eliminating the PCV. But that was done for other reasons.

Which fixed orifice PVC did you use?

Oooof, honestly I'm not sure if there are a bunch of choices or not. I grabbed one off the shelf at the local store with a straight up 3/8 hose nipple that would work on my Chevelle. Same thing you would see in a Pontiac valley pan.

From what I understand though, they all pretty much have just a 2mm or a 3mm hole size in them. Not very big at all. Basically they are just a gutted PCV with a tiny hole in the bottom. A controlled vacuum leak....

I removed mine yesterday and reinstalled the standard PCV. But I'm just sort of playing around with the differences to understand more how it affects the fuel table.

Thanks- I’ve been searching on line but will take a look at AA and AZ…

Here is the link to the one I used: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

I used it form the get go so I don't have anything to compare it against. I just had to put a piece of electrical shrink wrap around the bottom to fit a bit snugger in my stock PCV gasket.

I have on many big cammed engines used a fixed orifice PCV. On street cars here, a working PCV is reqd for registration. I calculated the orifice area of a stock PCV at idle & came up with a round hole of 7/64" diam.
I cut open the PCV, discard the innards & braze in a disc, in which I drill a 7/64" hole. Then braze the PCV back together.

Have done many over the years & work well.

I've had good luck with the ME Wagner dual flow pcv valve for my Pontiac, it's adjustable and or fixed orfice for efi.

Thanks all

Now that I've had a few days to drive the car and actually work on the tune today, for sure a regular pcv has changed things over the fixed orifice.
I had that fixed orifice drilled to 3/32 and it apparently still wasn't enough.

The new pcv specified for the engine has the closed loop now adding 3-4 percent more fuel thanks to the larger "vacuum leak", the iac counts are lower to compensate thanks to the slightly higher idle. This has my cold start rpm back to a more reasonable area (IAC isn't really big enough for a big engine with a rowdy cam)
Overall after mild tweaks in the tune a regular pcv seems to help this particular engine deal with an inadequate IAC. It's actually now idling with a pinch more vacuum
(Lower map)

Similar IAC issues I've dealt with on dad's engine since his engine is completely sealed and using a vacuum pump, so no pcv with a controlled vacuum leak.

Just thought I'd share