PCV Vacuum with different HW 5200 Spacers

[BIGREDRASA]

I am just a bit neurotic about paying more for less. There seem to be two main choices in 2 into 1 adapters for the Webers (Holley, 32/36, and 38s). The cheapest adapter seems to be the one from Stovebolt, at $15. The Clifford unit is 3 or 4 times the price. The main advantage to clifford seems to be the ability to connect the PCV hose in a nearly stock position.

Stovebolt's, on the other hand, has no provision for connecting the PCV. I intially connected my PCV to the single port in the manifold, using a "Tee". The idle was horrrible, and the vacuum reading was around 5" Hg. I then drilled and tapped the adapter for a barb fitting. It runs much better with the PCV hooked to the carb adapter, instead of the manifold. I'll take vacuum readings at the two spots later. Just something to ponder for those who have hooked their PCVs directly to manifold vacuum, specially if mthey are also using that port for the dizzy's vacuum advance. I don't even want to talk hypothetical issues. Just, as Nike says, "Do It!" Or the old Alka Seltzer commercial, "try it. You'll like it!"

I have the "J" shaped tube that goes around the front hooked up as originally, and "T" in for the Cruise Control. The Power Brake Booster hose goes toward the rear using an elbow into the "T".

 

[Harte3]

FWIW, when I changed carbs from a Holley 4360 4v 450 cfm to a Holley 4160/1848 4v 465 cfm I lost 2+ hg of vacuum as I had to plumb directly to the manifold. When I changed back to the 4360 with all the vacuum fittings in the carb base I got that 2+ hg back. I know what the results are but I have no explanation as to why the difference.

 

[66 Fastback]

Lots of things could contribute to the 2" of vacuum variance. The carb itself may have more idle air coming through, the fuel mixture and jetting could have been different enough to cause a change in idle quality, the ignition timing could have been slighlty different, the idle speed may have been different or required more or less throttle plate openning in order to acheive the desired idle.
Doug

 

[Bort62]

Tony,

It sounds like you have an incorrectly sized PCV valve.

 

[BIGREDRASA]

Tony,

It sounds like you have an incorrectly sized PCV valve.

I used the one for the stock engine. After all, my internals are unchanged. It did make a difference whan I changed the vacuum source point for the PCV. That was all I changed.

 

[Bort62]

Tony,

It sounds like you have an incorrectly sized PCV valve.

I used the one for the stock engine. After all, my internals are unchanged. It did make a difference whan I changed the vacuum source point for the PCV. That was all I changed.

Try going to a smaller PCV. While your engine internals are the same, your vacuum source (and the flow rate resulting) is different.

 

[BIGREDRASA]

I actually used a smaller connector at one point:

vs.:

The difference was in the placement. The fitting is close to the stock location. Right now, it runs without hesitation. I'm curious if that's part of the problem for others who have experienced similar vacuum problems after doing a carb swap. I get one "knock" if I nail the gas on takeoff. Otherwise, no pinging. I don't think that's too bad, after going from 185/70-14s to 215/60-15s. I'll probably back off 1* in the timing.

 

[Ronbo]

I'm asking because I don't know, decreasing the size of the PCV will raise vacuum? I've had vacuum problems with two engines now and like Tony I'm using the "book" PCV. I'm using the Clifford adapter so I guess its set up the way Tony has his now. Changing carbs had no effect on it but had a big effect on how the car ran with the new motor (better). I attribute that to the worn out throttle shafts (like a Q-Jet) on the old one.

 

[Ronbo]

I was also suspect of opening the air breather to the atmosphere vice going into the air cleaner? I am wondering if it might have an effect?

 

[66 Fastback]

Well that line looks like it would reduce the PCV flow as it is smaller than the 3/8" hose. Another thing that may be increasing your vaccum is the placement of the tap just down stream of the venturi's and the swedging down of the 2bbl down to 1 bb. You might have some localized increase in a vaccum signal with the placement of the tap where you did on the adapter.
Doug

 

[Bort62]

Well that line looks like it would reduce the PCV flow as it is smaller than the 3/8" hose. Another thing that may be increasing your vaccum is the placement of the tap just down stream of the venturi's and the swedging down of the 2bbl down to 1 bb. You might have some localized increase in a vaccum signal with the placement of the tap where you did on the adapter.
Doug

I can't see the pictures, but certainly measuring vacuum (absolute pressure, really) in a different location may change the results.

The PCV is just a flow control valve. If the one you have is flowing too much, it will act like a vacuum leak.

 

[BIGREDRASA]

I really dislike our weather here. It's either too hot or too cold. I plan to take vacuum readings at the different points when time, weather, and disposition permit.

I just don't know how these PCV valves can be identified as "large" or "small." My concern with the smaller tube was that it would be too small. I also can't justify going out and buying an AFR meter right now.

 

[Ronbo]

I'm may be wrong here, but if you decrease the size of the PCV won't pressure start building in the crankcase? Or will the breather compensate for it?

 

[Bort62]

I'm may be wrong here, but if you decrease the size of the PCV won't pressure start building in the crankcase? Or will the breather compensate for it?

If you went too small, yes.

It would have to be pretty small, however.

The PCV can become an interesting tuning obstacle.

 

[BIGREDRASA]

I believe we have two separate and distint--though related--tuning ussues. Where to connect the PCV, and which PCV. How do we identify which PCV is which? Does someone have flow ratings?

 

[Bort62]

I believe we have two separate and distint--though related--tuning ussues. Where to connect the PCV, and which PCV. How do we identify which PCV is which? Does someone have flow ratings?

Well, the PCV is a controlled vacuum leak, plain and simple. You need to tune your carb to account for this vacuum leak.

That is typically going to mean, at idle, the throttle plates will be more closed than they would sans PCV to attain the same idle speed. Also, your idle mixture adjustment will need to be richer so as to flow the same amount of fuel given the weaker signal across the idle transfer slot.

Not sure if flow ratings are really the right way to go about it. PCV valves are more likely to be matched to carburetors than engines, but still will vary.

It is totally possible to have a carb that has insufficient adjustability @ idle to accommodate the additional air from the PCV, and can lead to high-idle and other problems.

Locating the PCV should not change things significantly provided it is attached to manifold vacuum. However, as you have noted - different locations in the manifold can potentially have different vacuum signals (due to venturi effects, etc) and as a result will draw more or less air through the same PCV valve.

I'm afraid that, once you venture too far away from a stock setup, it's going to be a "figure it out on your own" type of situation.

I would suggest using the smallest PCV valve possible, as the crankcase does not require a whole lot of suction to evacuate.

 

[BIGREDRASA]

How do we know what is a "smaller" PCV? Parts counter people surely don't know.

 

[Bort62]

How do we know what is a "smaller" PCV? Parts counter people surely don't know.

One that is more restrictive? haha.

Honestly, hard to know w/o kinda looking at it and making a best guess (short of setting up a rig to measure pressure drop across it at a constant flow).

You can get a real back of the envelope feel by sucking through one and seeing which feels more restrictive.