pcv valve
- Thread starter trvguy
- Start date
The road draft on that engine is in the front corner of the sump. Little hangy triangle chap with a sort of wide slit tucked around the back. On the passenger side.
To fit the pcv, this needs to be blocked. I have a stock pcv setup on my truck (some military issue flatheads had them) they have the pcv plumbed thus, front passenger side of the inlet manifold on the part that rises up to the generator mount. Here is a through hole tapped 1/8" pipe, this is the engine to pcv point. The pcv itself sat passenger side slightly behind the carb base. So pipe to pcv, then pipe from pcv to the inlet manifold just below the carb base. The manifold that I have is the military one so has the right vacuum boss for the pcv. Truck ones also have the extra vacuum point, that was used with the boosted brake system. If you don't have this extra vacuum boss, you can use the southwind heater carb spacer (fairly easy to find) for the vacuum port.
Hope this makes sence.
Martin.
To fit the pcv, this needs to be blocked. I have a stock pcv setup on my truck (some military issue flatheads had them) they have the pcv plumbed thus, front passenger side of the inlet manifold on the part that rises up to the generator mount. Here is a through hole tapped 1/8" pipe, this is the engine to pcv point. The pcv itself sat passenger side slightly behind the carb base. So pipe to pcv, then pipe from pcv to the inlet manifold just below the carb base. The manifold that I have is the military one so has the right vacuum boss for the pcv. Truck ones also have the extra vacuum point, that was used with the boosted brake system. If you don't have this extra vacuum boss, you can use the southwind heater carb spacer (fairly easy to find) for the vacuum port.
Hope this makes sence.
Martin.
38 coupe
Well-known member
The only person I am aware of who has installed a pcv setup and checked the results with a fuel ratio exhaust analysis found that the stock carburetor cannot be adjusted to compensate. The flathead carbs simply are not capable of running decently with a pcv system. If you are switching to a modern four barrel you may have better luck.
This is one of the modifications that simply isn't worth it in my opinion.
This is one of the modifications that simply isn't worth it in my opinion.
V8 Bob
Well-known member
The only person I am aware of who has installed a pcv setup and checked the results with a fuel ratio exhaust analysis found that the stock carburetor cannot be adjusted to compensate. The flathead carbs simply are not capable of running decently with a pcv system. If you are switching to a modern four barrel you may have better luck.
There are too many unknowns above to make a final (and wrong) decision on the merits of a PCV system.
1-How was the F/A data created? (Location of the A/F probe, tune and condition of the engine, basic knowledge, etc)
2-How was the PCV installed? (Many are installed wrong, and with the wrong valve)
I installed a PCV on my stock '51 239" with the stock Holley 94 and saw NO difference in mileage, vacuum levels and plug readings, except no more fumes under the hood, and cleaner oil. Adding a second 94 also made NO difference in mileage, vac levels, plugs, just better performance.
This is one of the modifications that simply isn't worth it in my opinion.
Why is it not worth it when the environment, oil and internal engine stays much cleaner with a properly operating PCV system? I, along with many others, know first hand adding a PCV is well worth it, fact not opinion.
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Mike51Merc
Well-known member
Is there any real data to report here? I'd like to know more than you heard it from somebody once as the basis for making decisions.
In my opinion Ron there are two things of note in your statement.
"wrong pcv valve" could well have been, too large an orifice in the valve could present problems with carb meetering. I've heard a rule of thum being "use a valve from similar breathing ability engine" with stock flatheads, that may mean lifting a valve from something in the 180-200 ci ohv engine.
The other note, plumbing to the end carb on a 3 carb setup. The pcv valve really needs to be placed under the middle carb to distribute the "vacuum leak" evenly to all cylinders. If the valve is tapped in at one end, those cylinders at the end will get different air/fuel ratio, and a smooth idle can be elusive. Like an actual vacuum leak at one point of the inlet to block gasket shows a more noticeable "miss" in the closest cylinder.
Could these two things be the idle problem you had?
Martin.
Ross F-1
Well-known member
I did the AFR testing that 38Coupe refers to. I was using a Toyota PCV valve I saw mentioned on here, a Fram FV306. Interestingly, Toyota specs the same valve for 2.2L thru 3.5L engines.
What I saw on the AFR was that to get a good smooth idle, the idle mix screws were too rich to give a good AFR at cruise. I was really surprised how drastically the idle screw setting affected cruise AFR up to 55 mph (2300 RPM). If I set the idle screws to a decent idle AFR (high 12's), cruise AFR was mid-15's, about where I wanted to be. But it flat idled like crap unless I jacked up the idle speed to 750 RPM. If I set the AFR at idle to where it idled reasonably smooth and at 550 - 600 RPM, I had idle AFRs of mid-10's, my cruise AFR's were mid-14's; not "bad" but leaving a lot on the table. The difference at the idle mix screws was less than 1/8 of a turn between the two extremes.
I tried this with both my stock 94 and with the Rochester 2G I am running now, same difference. The Rochester is off a '63 truck, so it wouldn't have been calibrated for PCV. Both carbs were in very good shape, and I had a 6.5 PV in the Holley with #51 jets.
With the draft tube back in place, I can get the idle AFR's I want and get the cruise AFR's I want, with a smooth idle at 550 - 600 RPM. My engine is fresh, the amount of smoke coming out of the draft tube after a good run is less than what you'd see off a cigarette. No drips off the tube.
Note that I am at 5300', and run an MSD with 8* (manifold) vacuum advance and 18* mechanical advance, all in at 1800, total advance w/o vac of 22*. If you are running some other kind of setup, your mileage may vary. This is also a truck, so I'm into the throttle a little more than a car would be at any given speed. I couldn't run a test with the Holley and a L-o-M distributor, but I would guess a PCV would have even worse effects on them.
What I saw on the AFR was that to get a good smooth idle, the idle mix screws were too rich to give a good AFR at cruise. I was really surprised how drastically the idle screw setting affected cruise AFR up to 55 mph (2300 RPM). If I set the idle screws to a decent idle AFR (high 12's), cruise AFR was mid-15's, about where I wanted to be. But it flat idled like crap unless I jacked up the idle speed to 750 RPM. If I set the AFR at idle to where it idled reasonably smooth and at 550 - 600 RPM, I had idle AFRs of mid-10's, my cruise AFR's were mid-14's; not "bad" but leaving a lot on the table. The difference at the idle mix screws was less than 1/8 of a turn between the two extremes.
I tried this with both my stock 94 and with the Rochester 2G I am running now, same difference. The Rochester is off a '63 truck, so it wouldn't have been calibrated for PCV. Both carbs were in very good shape, and I had a 6.5 PV in the Holley with #51 jets.
With the draft tube back in place, I can get the idle AFR's I want and get the cruise AFR's I want, with a smooth idle at 550 - 600 RPM. My engine is fresh, the amount of smoke coming out of the draft tube after a good run is less than what you'd see off a cigarette. No drips off the tube.
Note that I am at 5300', and run an MSD with 8* (manifold) vacuum advance and 18* mechanical advance, all in at 1800, total advance w/o vac of 22*. If you are running some other kind of setup, your mileage may vary. This is also a truck, so I'm into the throttle a little more than a car would be at any given speed. I couldn't run a test with the Holley and a L-o-M distributor, but I would guess a PCV would have even worse effects on them.
Ol' Ron
Well-known member
I'm pretty sure you're right. The valve we're using is the one recomended by Richard as it works well on his engine and it' close to the same specs asn tne 294. Because od other problems I haven putttz with it yet. It would be easy to add to the center carb as I've made 1" risers for all three carbs. This was necessary to clear the banjo fittings on the carbs and the progresive linkage. It also allowed me to add a vacuum port for the dist and PVC and an extras one for the heater/defrost system The truck is back in the road now and after we work out the bugs of a NEW build, I'll give it another go.
Mike51Merc
Well-known member
Ross,
Any idea if a Stromberg (or two, or three) might bring different results?
In theory, the PCV should only be open at a low-load cruise situation. Perhaps an experiment with different PCV valves would bring better results?
Any idea if a Stromberg (or two, or three) might bring different results?
In theory, the PCV should only be open at a low-load cruise situation. Perhaps an experiment with different PCV valves would bring better results?
Bruce Lancaster
Member Emeritus
The Ford built PCV used on military stuff, starting I think in 1942-3, was throttled down where it connected under the carb. It connects there to a plate very like the ones used to add a vac port for some Columbias and South Winds...
The PCV itself has a pretty large hole but the hole in the plate connects to each bore (drillings V out from the threaded hole) via very small passages. I'll try to remember to measure those tonight.
I would think a too-large PCV could be throttled at any point between valve and the vacuum hookup by using a plug with a small hole. I strongly suspect that pulling from both sides on manifold is also important.
Nobody in 1943 gave a flying %$#$$$ about air quality in the Libyan desert...they wanted to pull water, acids, and other crud out of the oil in Fords pulling around heavy trucks and armored Bren carriers in hundred degree heat.
The PCV itself has a pretty large hole but the hole in the plate connects to each bore (drillings V out from the threaded hole) via very small passages. I'll try to remember to measure those tonight.
I would think a too-large PCV could be throttled at any point between valve and the vacuum hookup by using a plug with a small hole. I strongly suspect that pulling from both sides on manifold is also important.
Nobody in 1943 gave a flying %$#$$$ about air quality in the Libyan desert...they wanted to pull water, acids, and other crud out of the oil in Fords pulling around heavy trucks and armored Bren carriers in hundred degree heat.
Bruce Lancaster
Member Emeritus
The Ford built PCV used on military stuff, starting I think in 1942-3, was throttled down where it connected under the carb. It connects there to a plate very like the ones used to add a vac port for some Columbias and South Winds...
The PCV itself has a pretty large hole but the hole in the plate connects to each bore (drillings V out from the threaded hole) via very small passages. I'll try to remember to measure those tonight.
I would think a too-large PCV could be throttled at any point between valve and the vacuum hookup by using a plug with a small hole. I strongly suspect that pulling from both sides on manifold is also important.
Nobody in 1943 gave a flying %$#$$$ about air quality in the Libyan desert...they wanted to pull water, acids, and other crud out of the oil in Fords pulling around heavy trucks and armored Bren carriers in hundred degree heat.
The PCV itself has a pretty large hole but the hole in the plate connects to each bore (drillings V out from the threaded hole) via very small passages. I'll try to remember to measure those tonight.
I would think a too-large PCV could be throttled at any point between valve and the vacuum hookup by using a plug with a small hole. I strongly suspect that pulling from both sides on manifold is also important.
Nobody in 1943 gave a flying %$#$$$ about air quality in the Libyan desert...they wanted to pull water, acids, and other crud out of the oil in Fords pulling around heavy trucks and armored Bren carriers in hundred degree heat.
Ross F-1
Well-known member
Can't say, and anyone running a tri-carb likely has higher compression, hotter cam, etc. so there would be other variables. A Stromberg wouldn't be calibrated for the air bypassing the carb thru the PCV, so I doubt it would be different. Both the Holley and Stromberg have very crude idle systems by modern standards.
PCV's do draw at high vacuum (idle), that is where their metering function comes in. It's much less than at cruise for sure.
Bruce, if you forget I can measure my manifolds tomorrow. I have two pcv type manifolds from military engines I've tugged. Only one had the actual pcv valve, the Donaldson crankcase ventilator. Both these manifolds have a rectangle carb mounting platform, for four bolt carbs, but only actually drilled for the normal three. The four bolt was I believe for a "backdraft" holley, not the 49 Merc one, an odd military thing with hundreds of screws holding it together.
You mentioned that the pcv went into a plate, southwind style. Both mine tapped into the manifold proper, using an extra port on the back of the carb mounting area.
You got one of the plates?
Thanks
Martin.
You mentioned that the pcv went into a plate, southwind style. Both mine tapped into the manifold proper, using an extra port on the back of the carb mounting area.
You got one of the plates?
Thanks
Martin.
Bruce Lancaster
Member Emeritus
The extra port on manifold, I think, is a big one (3/8 pipe??) with two tiny drillings into the actual passages...is that what you have?? That would be a port for truck vac brakes, and a logical connection. I've seen those plugged on some manifolds, on others just a flat area to allow the drilling.
Mine matches drawings that are in 1943 editions of Canadian military Ford manuals, with the plate under carb hooked to the line from PCV mounted vertically below front of carb and connected to the fitting on side of gen stand.
I discovered recently that WWII Jeeps used a VERY similar PCV that is now available in repro. I'm going to get around to buying one to compare.
My setup, the most complete I have found, is on the same Canadian aluminum manifold with 3-4 bolt capable carb stand. I think the 4 bolt may have been for some British Solex rig, pure guesswork on my part. There was considerable effort to keep spare part interchangeability between Canadian, US, and British flatheads.
The plate is a thin die cast piece that lives below carb, and at the front it has a threaded hole for adaptor to the steel line. This fairly large opening dead ends and is drilled with two little holes Veed out to meet both sides of intake.
Mine matches drawings that are in 1943 editions of Canadian military Ford manuals, with the plate under carb hooked to the line from PCV mounted vertically below front of carb and connected to the fitting on side of gen stand.
I discovered recently that WWII Jeeps used a VERY similar PCV that is now available in repro. I'm going to get around to buying one to compare.
My setup, the most complete I have found, is on the same Canadian aluminum manifold with 3-4 bolt capable carb stand. I think the 4 bolt may have been for some British Solex rig, pure guesswork on my part. There was considerable effort to keep spare part interchangeability between Canadian, US, and British flatheads.
The plate is a thin die cast piece that lives below carb, and at the front it has a threaded hole for adaptor to the steel line. This fairly large opening dead ends and is drilled with two little holes Veed out to meet both sides of intake.
Yep, the port that was used for truck brakes, is where mine went in. My manifolds are iron, same (1/8" pipe?) fitting in the side of the generator stand. Both mine also have blocked off oil fill holes near the pcv fitting in the generator stand area, an oval plate with two bolts.
I'm fairly sure the four bolt was a holley carb, I'll see if I can find a pic of one.
Martin.
I'm fairly sure the four bolt was a holley carb, I'll see if I can find a pic of one.
Martin.
Bruce Lancaster
Member Emeritus
That oval took a curved pipe that put an oil filler up next to the generator...no more pouring oil all over the back of the engine! That piece is harder to find than the manifold.