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Tech / General EngineIs your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
Regarding the valve specifically.
With the new top end completed, I'm finding (understandably) that my idle vacuum is now less than 10" whereas before it was closer to 13-14".
How does one go about determining the correct crankcase valve with respect to vacuum?
For the record, the PCV system I'm using consists of:
Right side valve cover breather.
Left side valve cover PCV valve.
PCV valve to catch can.
Catch can to intake manifold.
PVC operation, in as much as I can determine should be as described in the attached picture.
What I think I'm experiencing now is that the valve is permitting too much flow at idle.
Does anyone have any PN's for valves that are better suited to bigger cammed engines?
As the engine is freshly assembled (new heads and new to me cam) I'm only in the preliminary stages of tuning. Base timing, basic fuel mixtures at WOT, cruise and idle are more or less set. Fine tuning will take time but that's where I'm getting into my idle vacuum.
For the record, I have to say that I'm impressed with smoothness and lack of drama this bigger cammed engine has presented. Idle RPM is rock steady at 850 RPM (park) with vacuum at or around 9-10". Also steady. AFRs are hovering in the high 13's to low 14's although I know that can be misleading. Initial spark lead is 16 degrees. Ported vacuum advance until I can dial in all of the adjustable aspects so I can use full manifold vacuum advance.
Thing is, it seems to me that I should be able to both decrease idle speed AND increase idle vacuum.
That's what's drawn me to the PVC valve. I recall having worked with several different PVC valve PNs on the previous engine (13-14" idle vacuum at 750 RPM). I have no record of what I ended up with or what went through and into the parts pile.
If the low manifold vacuum is making the valve open up fully (as described in the above picture) and that vacuum happens to be my idle vacuum, you can see where there would be an issue.
My concern is to be overlapped somewhere in the low to mid vacuum valve setting.
An interesting test you propose via the curb idle screw. I'll have to work through this Barry Grant VS carb and their "Idle-Eze' feature, which effectively by-passes the butterflies.
That said, it is possible to be drawing excessively via the valve. Something I'll investigate further. Pinching the line closed between the valve and the manifold will certainly eliminate it from the picture.
EDIT: This isn't to say I have an issue here but it's something that needs to be addressed with the new build regardless.
Something I didn't know about PCV valves (which I'm learning is quite a lot actually).
This from a 1977 GM training book.........
"Problems can also occur if a customer installs the wrong PCV valve for the application. As we said earlier, the flow rate of the PCV valve is calibrated for a specific stock only GM engine application. Two valves that appear to be identical on the outside (same diameter and hose fittings) may have different size and flow pintle valves and springs inside, giving them very different flow rates even at the same level vacuum. This flow rate is a result of taking a dozen parameters into consideration. A PCV valve that flows too much air will lean the air/fuel mixture, while one that flows too little will richen the mixture and increase the risk of sludge buildup in the crankcase. On a non stock application, the technician is forced into a trial and error situation which may or may not yield good results."
Or so a post on the internet said...
The more I dig into this, the more I'm seeing familiar part numbers. I had experimented with the PCV in general. Different engines, different vacuum characteristics. I may have notes somewhere.
Right: the Idle-Ease feature doesn't really change the concept, only the methodology. You would have to use both the throttle stop screw and the other to verify that the PCV isn't allowing so much air to bypass the carb that the carb no longer has the proper "authority" over it.
Although in the end, I'd expect that you'll end up with a PCV that bypasses MORE air than is typical, since with a bigger cam, the engine will most likely want MORE air at idle than a "typical" stock setup anyway, at a lower vacuum besides.
Look up an M/E Wagner adjustable pcv valve. There are steps to take to ensure the valve setting is correct. I have a couple of them on cars - I have no affiliation with them. Just use them - I believe the cost is somewhere in the $125 area - haven't purchased one in a few year. Byron
I've read about those. Also read the price. That's pretty steep for a PCV valve. I'm going to check them out again.
In the meantime, I'm finding the interweb is full of discussions along these lines. What I'm also finding is that there isn't a lot of factual information regarding the valves themselves. One is left to sort through a selection of valves and determine from their application, whether or not they would be suitable.
Here are a few examples.
FV178 This is the Corvette application.
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FV181 Also the Corvette application.
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FV184 Corvette.
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Looks like, at least to me, that the likelihood of a low vacuum installation would be something like the mid-late 60's 427. Solid roller engines IIRC. Low idle vacuum. That pairs the FV178 and FV181. The FV184 looks to be a smogger engine application. Not sure how they fared with engine vacuum. I believe it was around this time that GM was using ridiculously retarded initial ignition timing and port vacuum advance in an attempt to raise EGT and reduce idle emissions.
All engines have some degree of blowby. Even my race engine has about 2% leakdown past the rings. This means there will always be pressure in the crankcase when the engine is running and the pressure will increase as rpm increases.
The purpose of the PCV is to allow manifold vaccum, usually at the base of the carb, to pull this pressure back into the manifold. This will also pull oil vapor out of the crankcase to be burnt in the cylinders. For a street car, this is an optimum solution however from a performance view, it's a step backwards.
Along with oil vapor being pulled into the intake to be reburnt, it also pulls condensation from cool engines. Condensation is water and water doesn't burn.
Many performance engines do away with a PCV system and run open breathers. The open breather system allows any crankcase pressure to vent to the atmosphere. EPA regulations don't like this so you need to check your own local regulations to see if it's allowed if you need to be tested. The downside to an open breather system is a messy oil flow leaking from the breathers and onto the valve covers.
Racers have always looked for other ways to control crankcase pressure. They originally used a header evac system to let the exhaust gasses produce a vaccum to pull crankcase pressure from the breathers on the valve covers. This does work but the vaccum produced is very low.
Modern technology has snuck in and many faster race cars now use a belt driven vaccum pump to pull the pressure from the crankcase. This has proved to be very efficient and there are now many different types of pumps to use. Vaccum can be pulled so low that it can actually pull oil from the bearings so vacuum relief valves are install in the system to limit how much vaccum the pumps can pull. Smaller electric pumps have been tried but they just don't pull enough vaccum at high rpms to make them work.
A big advantage of having your crankcase in a vaccum, even with a good working PCV system is that when the crankcase isn't pressurized, oil leaks are zero to none.
When my engine is running, if I pull the dipstick, I can hear and feel the vaccum in the crankcase through the dipstick tube.
Belt driven pumps are not an option for a street vehicle and although they work, I really don't like PCV systems. In all the details in the above posts, you can see why they do and don't work since they rely on manifold vaccum. At WOT or any other type of hard acceleration where you're making the most crankcase pressure, there's is little to no manifold vaccum to make a PCV system work even if the PCV is wide open. Start installing bigger camshafts that lower manifold vaccum and PCV systems work even less.
The PVC system and performance is exactly why I'm exploring the types of valves out there and how they function.
With the new engine package, my idle vacuum is down about 3-4" from the previous engine. I had experimented with a few types of valves with that engine too, seeing as it had lower vacuum than stock and the valve I was using was standard fare for an 80's era EFI small block or V6. Ultimately, I ended up with a valve that operated at a lower vacuum than general. I also installed an oil separator. The mileage was getting up there on the old engine, the guides were shot and oil consumption was an issue. The separator kept most of the crap out of the intake.
Now that everything is fresh, I want a valve that will help me to optimize my idle vacuum. That part is easy as I can measure it. The difficult part is ensuring that the valve is fully open at cruise RPM and load. The old engine was beginning to sweat around the valve cover grommets and the pan gaskets and seals started looking a little weepy as well. I want to ensure that there is always adequate ventilation.
The link to the adjustable PVC valve had some excellent reading attached to it. If nothing else, it gave a real world view as to what's going on in the function of the valve. A little pricey at 125$ (US) a pop but I suppose if you have a lot of money invested in your ride and still aren't broke, it would solve a few problems.
Skinnyz - your point is well taken on trying to figure out the pvc needs for an engine. There is a lot more to it than meets the eye. I went with a pvc vale (originally) that was for a 427 chev engine. Part CV789C - ACDelco - I took into account all of the things that I had read - I still have 4 or 5 of them sitting on the shelf - (have 4 old muscle cars - modded engines) thought if they worked for one of them - they should work for all. They didn't - anyway, went with the adjustable and picked up a little more than 2 pts. on vacuum. I found it interesting when you can't adjust the carb - or minimal adjustment how the adjustable pvc valve brings full adjustability back by working the pvc and carb together. Best of luck - byron
Thanks for that Byron.
I still have good adjustability with the 4-corner idle. Capping the PCV valve only yields about a 1" improvement in idle vacuum so I'm thinking that may be just about right. Of course what happens at cruise is another story altogether. Cruise vacuum is about 13-14" at 2600 RPM. I would think that the valve would be fully deployed. Something I'll have to work on.
Well I have to see what the problem is before buying and adding parts. So IIWM I use a combo vac/press gauge to monitor the crankcase pressure and then the pressure downstream of the PCV or both simultaneously. Combo gauges are cheap and you can attach one to dip stick tube with the other spliced into the PCV line. Then tape the gauges to the windshield and go for a drive. That way I could see how the PCV is performing and even more important if any changes I make are a improvement or make the problem worse.
A lot of owners have what they think is a PCV problem after replacing the valve covers only to find the new valve covers don't have as an effective oil separator as the stock covers had.
Finally a vacuum pump is a good idea as the increased ring seal really adds power - enough to justify the added load of the vacuum pump.
If you look on U-tube you can find demonstrations of the exh evacuation system pulling like 4"Hg vac. Not really to bad as that is at higher RPM where it's needed. The trouble is to get it to work with the PCV and not against it and pull from the intake.
I've been examining vacuum pumps and had a look at the "old school" pan evacuation systems. Given my car is what it is, I don't think I need anything too exotic to be able to dial in the crankcase valve.
The issue I'm seeing, and it's not unique to me, is that the typical PCV valve is designed to have high vacuum at idle and somewhat less so at cruise. A performance engine, or at least one with a cam that reduces idle vacuum, functions slightly the other way around. Idle vacuum is low and cruise vacuum somewhat higher. In my case it's 9" at idle and 14" at cruise. Following a typical valves function curve, low vacuum/fully open (accelerating), moderate vacuum/ "properly open" (normal operation) and high vacuum/ slightly open (idle or decelerating), you can see where if the wrong valve is utilised, it could be fully open at idle. It a question of how much vacuum for what degree of opening.
Adjustability would be great here but for the money spent on an adjustable PCV valve, I'll experiment with the selection of valves I have. Now that I have my vacuum advance dialed in and my idle is nice and stable, I can run a few tests.
Interesting thought about examining the crankcase pressure. I'm not entirely sure how or if that could be pulled off.
I've been examining vacuum pumps and had a look at the "old school" pan evacuation systems. Given my car is what it is, I don't think I need anything too exotic to be able to dial in the crankcase valve.
The issue I'm seeing, and it's not unique to me, is that the typical PCV valve is designed to have high vacuum at idle and somewhat less so at cruise. A performance engine, or at least one with a cam that reduces idle vacuum, functions slightly the other way around. Idle vacuum is low and cruise vacuum somewhat higher. In my case it's 9" at idle and 14" at cruise. Following a typical valves function curve, low vacuum/fully open (accelerating), moderate vacuum/ "properly open" (normal operation) and high vacuum/ slightly open (idle or decelerating), you can see where if the wrong valve is utilised, it could be fully open at idle. It a question of how much vacuum for what degree of opening.
Adjustability would be great here but for the money spent on an adjustable PCV valve, I'll experiment with the selection of valves I have. Now that I have my vacuum advance dialed in and my idle is nice and stable, I can run a few tests.
Interesting thought about examining the crankcase pressure. I'm not entirely sure how or if that could be pulled off.
Cruise vacuum shouldn't be higher than idle vacuum Skinny. I think you are measuring vac at "ported vacuum port" on the carb as this measures inside the carb throat where vacuum signal increases with airflow increases though the carb.
Try measuring vacuum directly off the manifold if you can.
To measure crankcase pressure/vacuum all you need is a combo fuel pressure/vacuum gauge which more likely you already have and 30 minutes at the auto parts store to select plastic fittings to match the gauge hose to the dip stick tube. Measure the tube size before you go for parts.. I would measure vacuum at manifold first then on the tube. It helps to write what you read and how you measured it as we can forget numbers fairly fast with a load engine noise.
Idle vacuum: 9". Measured from full manifold vacuum. This is also where I run my vacuum advance from.
Cruise vacuum at 2600 RPM. 14". Measured from the same source. I run an in-dash vacuum gauge full time along with my AFR gauge.
Decel vacuum is in excess of 20".
This situation isn't unique with a high overlap cam. Typically, guys will struggle with idle vacuum and larger cams. Cruising around town or on the highway, when the engine speeds are higher, 2600 compared to 850, vacuum will naturally go up for a given throttle position. Yes, throttle opening will be wider at cruise than at idle however, the RPMs more than make up for this. With the advance curve built the way I have it, and that's with about 40 degrees of advance at cruise, it takes very little throttle opening to maintain my cruising speeds.
As for the measurements, I'm not sure my handheld gauge has a lot of accuracy in the lowest range. What would crankcase pressure be. A couple of PSI? I suppose a completely sealed crankcase, that is no breather or PCV valve connected, it could get quite high. When my valve is removed from the cover (and the breather installed), I can see the pressure puffing out of the grommet as oil smoke, mist and vapour.
Something I'll consider as I continue to test and observe.
Well a pressure gauge is normally a bourdon tube gauge and the most accurate part of the gauge is the mid range. That would be near the zero vacuum/zero pressure mark. I think what you are worried about is the resolution is not small enough and it's difficult to read less than 1 psi. I would not worry how much accuracy it provides but whether the crankcase is pressurized or seeing some amount of vacuum no matter how small. There are more accurate gauge or instruments but I don't think it's worth it for what you are looking for.
I also have a vacuum gauge mounted on my camaro's dash but I'm running an awfully short cam that's giving me 18.5"Hg vacuum at idle. Increasing vacuum only happens when the throttles close on my car. I really like that vacuum gauge as I can see the engines health all the time from the driver's seat. I can see any change in the motors compression as a loss of vacuum or a burnt valve as a irratic needle. I can see the load/vacuum change as the grade of the road changes. They used to install vacuum gauges in production cars but the old timers say get tired of watching it - I haven't yet by any means.
Anyways let us know what you find for crankcase pressure.
You've created an interesting series of tests with monitoring the crankcase pressure in real time.
My concern initially was measuring the crankcase pressure with the PCV blocked. I'm certain that there is enough pressure produced to blow out a gasket or seal. Of course this would have meant that the crankcase would have to be completely sealed (breather, PCV valve and fittings, etc). Naturally, there will be pressure so that part is not worth investigating so I need not be concerned.
But doing a real time test with a gauge hooked up (to the crank case) and running through a series of valves at various throttle positions might provide some useful information.
First off, I'll do some simple valve swaps and observe the idle vacuum and RPM. Comparing my selection of valves to what I get with the system wide open (that is with the valve removed and the vacuum source exposed to atmosphere) and with the system closed (with the vacuum source plugged) should give a range to work with. I suppose ideally I'm looking for the valve which provides the smallest vacuum "leak" under idle conditions.
It will take a few road tests to determine the compatibility of valves vs cruise vacuum.
It'll be necessary to revisit the idle mixture and curb idle speed after each substitution as well.
Hmm...
Just wanted to add here that you may see more pulsing/fluctuating reading from the dipstick where up at the valve cover/PCV valve it maybe dampened quite a bit by the turns in the flow path as where down at the dipstick end it's gonna see the pressures from the pistons stroking in the bores. Just a heads up.
Last edited by cardo0; 11-01-2016 at 01:14 PM.
Reason: Heads up.
Another thing to consider is altitude. Barometric pressure at sea level is considered 14.7:1. That's a standard pressure and will fluctuate as high and low pressure cells move in. That's also considered 1 BAR.
I'm not sure about Edmonton because it's lower than Calgary, but here in Calgary, good barometric pressure is only 13:1. So figure we're 2 psi lower in pressure than at sea level. Hard to make naturally aspirated HP at altitude. Placed like Denver are even worse.
As for vacuum, very deep vacuum is around 28" of mercury. Even when I'm pulling an AC system down into a vacuum for a recharge, I rarely see anything deeper that 26".
Now with manifold vacuum, books always say to tune a carb to get the lowest manifold vacuum and they expect it to be around 17". Here at altitude, 15-16" is normally the best you can get no matter what cam is in the engine.
Now you 9" of vacuum at idle is low but not as low as you would expect. At sea level, it may be closer to 11". This however means everything you do to make things better means using components for what could be considered a high altitude tune. Something adjustable as mentioned above would be better than some OEM type component.
One thing never considered is actually how much vacuum is measured. Pressure sensors on cars such a MAP and BAR sensors actually measure absolute pressure. They'll never say you have "15 inches of manifold vacuum", they measure pressure with the engine off then measure pressure with the engine running and use the difference. They have no idea how much zero pressure there is because they're measuring air pressure with the engine off so they still see pressure. As barometric pressure changes, so does the absolute pressure.
I'm learning the altitude difference here is significant. Agreed, it's not Denver but my recent relocation has demonstrated about a 1" - 1.5" drop in idle vacuum. Cranking pressure was down about 10 PSI as well. That's going from about 700' in central Ontario to over 2100' near Edmonton.
You raise an interesting point regarding the tune. Yes, I've had to make (and am continuing to make) choices regarding the elevation and the parts selected to make this new top end work here. As it is, this engine, with only 165 PSI cranking pressure isn't likely to suffer from spark knock even with an aggressive timing curve and a trip to the ocean. Mission Raceway, B.C. in particular. (Nice dragstrip at sea level).
10-15-2016, 11:17 AM
. I would measure vacuum at manifold first then on the tube. It helps to write what you read and how you measured it as we can forget numbers fairly fast with a load engine noise.