Positive Crankcase Ventilation valve testing

[rcodecj]

Test engine was a fresh 302 with 14” of vacuum at idle which is reasonable for the E series cam and 9.0 compression. Carb is a new 750hp Holley with holes in the primary throttle blades. Vacuum advance source was manifold vacuum that I wanted to keep despite the several hundred increase in idle rpm.

Purpose of the tests was initially to chase down why the engine needed to have the throttle blades closed below the transfer slot to get the idle down.. Engine was first tested for vacuum leaks and an intake manifold gasket leak was found and repaired but made little difference. Both valve cover openings were temporarily blocked and no vacuum was measured at the dipstick. In fact there was slight pressure not vacuum as it should be.

After that the test was to block the breather on one valve cover while leaving the PCV valve in the other side and measuring vacuum at the dip stick. I found information from a college mechanics test that it should read 1-3” of vacuum. With the hose pinched off the rpm should drop 50-80 rpm.

 With an unknown Fram PCV valve with the number 2286 on it, the vacuum at the dip stick measured 7.5” vacuum - too high despite being checked for vacuum leaks. My 460 measures 2.9” with the same PCV valve.

#2286 PCV valve hose was restricted to 3/32-.094 and dipstick vacuum reduced to 3.5”.

Next an AC Delco #CV774C PCV valve was tested and dip stick vacuum measured 5.9”.
PCV hose was restricted to 5/64-.078 and dip stick vacuum measured 2.2” but the PCV valve would not suck up to the carb side. I switched the restriction to the valve cover side and it still would not suck up.

Next up was an AC Delco #CV789C that measured at 1.9” dip stick vacuum.

While this helped a bit with the idle it was not major so I temporarily JB Welded the Holley throttle blade holes shut and was able to reduce the idle speed by 300-400 rpm. This allowed the carb to be set with the primary throttle blades uncovering the transfer slot instead of well below.

In looking at catalogs I found that the same PCV valve is listed for big blocks as well as small blocks and of course if you have a big cam you are out of luck to find a match. Because of this I have always just pulled one off the shelf that physically fit but it is obvious to me that the internal pintle valve makes all the difference in pan vacuum.

Just so we are clear at idle with vacuum the internal pintle should get sucked towards the carb side and closes (but not entirely) and at cruise (lower vacuum) it sits in the center and is more open.

I also discovered an adjustable PCV valve which would be perfect for setting the PCV to work at idle with a big cam but there is no adjustment for the cruise setting, they just say it is formulated to be correct by calculations based on the idle vacuum setting.

http://mewagner.com/?page_id=444

So, what do you guys think? Am I over thinking, under thinking?

[Barry_R]

I've seen some of this in practice - chasing idle situations like your's.  I have stuck drilled orifices in PCV hoses as a restriction, but its been trial & error.  A $129 PCV valve seems crazy though, no matter how cool it is.

I would LOVE to have a flow reference chart for a bunch of common PCV valves by part number.

They are pretty cheap - so I suppose we could empty the PCV rack at Autozone for under a hundred bucks.

Just need to put a flow rig of some sort together.  Might even be able to do it with a big shop vacuum and a vacuum gauge.  Set a vacuum "standard" (10", 12", ?) and flow a range of orifices (Holley jets come to mind) first to establish a reference - then flow each valve at the same level for comparison.

Anybody have some extra time on their hands?

[frankenfords]

Many of the original Autolite and FoMoCo PCV valves from the 60's could be disassembled. Some had a body composed of two machined pieces that screw together, and some use a snap ring. On those PCV valves, you can take them apart and tune them with washers to change the orifice size and also by stacking washers to change the spring rate. I can't say I've ever gone so far as to measure crankcase vacuum, but I have tuned them to fine tune idle speed and throttle tip in response. You can find them in junkyards, at swap meets, and on that auction site from time to time. Sometimes they're still installed in used valve covers too.

Regarding the excessive idle speed and the need to set the throttle plates so low relative to the transfer slots, sounds like you figured out that the drilled throttle plates were the culprit. I had a used carb once that I rebuilt and couldn't get to idle down. Once I realized it wasn't originally manufactured with the drilled butterflies and swapped in some that weren't drilled, it idled just fine. That backyard attempt at tuning with a Milwaukee was probably why the carb was sold to me in the first place. (For what it's worth, I've had other carbs that were supposed to have drilled throttle plates that worked just fine with them.)


Brett

[427Fastback]

Interesting thoughts on the drilled butterflys..I have been drilling primary butterflys for years.I do it for our emission testing or to get the idle screws to respond better..
I have never suffered from the high idle problem but I can see it happening.I generally drill 1/8 holes..It is actually listed in Holleys tuning book..
Every engine is different tho.....
Cory

[frankenfords]

Another thought, if the secondary throttle plates are open too far, the primaries become unresponsive. There is a worm screw that is accessed from the bottom that establishes the closing point. If the worm screw is turned in too far, holding the secondary butterflies open too much, that can cause fat idle problems too. I've experienced out of the box carbs that needed to have that point adjusted as well to work right.


Brett

[machoneman]

Great topic as I've wondered about the effectiveness of PCV valves. The hot set-up for Fords with racy cams was the old Boss 302 unit, C7AZ-6892 used also, and only, on the GT390 and 428. This valve did seem to work best on all kinds of hot engines beyond even Fords.

As to Holleys, most miss the fact that the secondary adjustment screw must be pretty close to factory specs before one starts drilling primary side throttle blade holes. One good trick is to eyeball the machined slot on the rear wall of the secondary body. A correctly set adjustment will make the slot look like a perfectly square box, not oblong. JDC and I had an extended discussion recently with a SBF Forum member just last month:

http://www.network54.com/Forum/119417/thread/1411511382

[rcodecj]

Another thought, if the secondary throttle plates are open too far, the primaries become unresponsive. There is a worm screw that is accessed from the bottom that establishes the closing point. If the worm screw is turned in too far, holding the secondary butterflies open too much, that can cause fat idle problems too. I've experienced out of the box carbs that needed to have that point adjusted as well to work right.


Brett

This carb has the adjustable secondary throttle plate and was turned down also. I think it did not help that it is a little 302 and the 750 throttle plates even when turned down a lot still pass more air than say a 600 or 650. This same carb had to be turned down more on the 460 too so I'm not sure what's going on. I did check for carb leaks and did not find any. The throttle plate holes were somewhere around 3/32 so it's not like they were huge for that size carb. This will continue to bug me but the engine is in a 65 mustang with 3.00 gears and it runs great while using the pink #330 accelerator pump cam #3 hole in the #2 throttle lever and stock 30cc pumps. In theory the carb may be too large but in real life it loves it. Probably because the car is so light.

edit: meant to say 3/32

[rcodecj]

Interesting thoughts on the drilled butterflys..I have been drilling primary butterflys for years.I do it for our emission testing or to get the idle screws to respond better..
I have never suffered from the high idle problem but I can see it happening.I generally drill 1/8 holes..It is actually listed in Holleys tuning book..
Every engine is different tho.....
Cory

Drilling throtle blade holes definitely works if you have to open the throttle blades too far to get enough air causing the transfer slots to be uncovered too much.
I had the opposite problem.

[frankenfords]

Interesting thoughts on the drilled butterflys..I have been drilling primary butterflys for years.I do it for our emission testing or to get the idle screws to respond better..
I have never suffered from the high idle problem but I can see it happening.I generally drill 1/8 holes..It is actually listed in Holleys tuning book..
Every engine is different tho.....
Cory

Drilling throtle blade holes definitely works if you have to open the throttle blades too far to get enough air causing the transfer slots to be uncovered too much.
I had the opposite problem.

In my experience, if the throttle blades have to be opened up too far to keep it running at idle, the initial advance needs to be cranked way up.

The frankenwagon is currently running a 306 with about 9.5:1 compression, a cam with 230-236 duration at .050, and a 650DP which has the same size throttle plates as a 750. Other than jets, sticking in a bigger primary accelerator pump squirter and playing with different pump cams, the carb is out of the box. With 24 degrees initial, 40 total in by 2500rpm, idles strong at around 750 rpm with about 8-inches of manifold vacuum in gear (auto), runs real snappy on 87 octane California gas, and never pings, never diesels, fires right up hot or cold.

[rcodecj]

So back to the PCV discussion, has anyone else measured their crankcase vacuum at the dipstick as I did?
I would love to see what everyone else has.
It's a simple test if you have a pcv on one valve cover and the breather on the other.
Just plug the breather side while measuring vacuum at the dipstick.
Just make sure the engine is well warmed up.

[babybolt]

About 10 years ago I asked a Ford emissions engineer about this very question - interchangeability of PCV valves and how it affects engine function.  He said that smaller engines like 4 cylinders were very sensitive to the PCV size.  As the engine size increases, the importance of matching the PCV to the engine decreases.  He didn't think much of the aftermarket generic one size fits all PCV's. 

[bsprowl]

I've seen some of this in practice - chasing idle situations like your's.  I have stuck drilled orifices in PCV hoses as a restriction, but its been trial & error.  A $129 PCV valve seems crazy though, no matter how cool it is.

I would LOVE to have a flow reference chart for a bunch of common PCV valves by part number.

They are pretty cheap - so I suppose we could empty the PCV rack at Autozone for under a hundred bucks.

Just need to put a flow rig of some sort together.  Might even be able to do it with a big shop vacuum and a vacuum gauge.  Set a vacuum "standard" (10", 12", ?) and flow a range of orifices (Holley jets come to mind) first to establish a reference - then flow each valve at the same level for comparison.

Anybody have some extra time on their hands?

First a simple ball valve can be placed inline in he PVC hose so you can adjust the flow.  Sure don't need to spend $129.

I'm available to do this.  I can probably borrow them from a local store at no cost.  What numbers do you what to test?

[babybolt]

I've got lots of original Ford PCV valves, mostly of a few numbers which I can't remember at the moment.