[DonMaximo]

A question of idle curiosity...

The PCV valve at the top/rear of a Ford block is typically routed to the intake manifold. Some people/companies have created air/oil separators so that the oil mist from the PCV/Crankcase (CC) is prevented from puddling up within the intake.

My question is this and please correct any incorrect assertions I make...

Upon a piston's power stroke, spent air/fuel can pass the cylinder rings and is vented into the Crankcase. This air has presumably already been metered... accounted for even before it entered the cylinder.

If this same air passes through the PCV and back into the intake, shouldn't we be calling this "unmetered" as it did not pass through the MAF ?!?!

With this line of thinking, what would prevent me from plumbing in an air/oil separator and have a filtered breather to allow CC gases to excape freely -and- NOT back into the intake ?

If this is so, why is it not a problem ? Does the environmental shortcoming outweigh the compromise of tainting the air/fuel mixture ??

Thank you,

[cgrey8]

Your question has a few explanations.

The fuzzy feel-good answer is that with PCV, you gather those unburned and partially burned hydrocarbons that slip past the rings and route them right back where they came from to be burned. It has the effect of keeping them out of the atmosphere which makes the EPA happy. It also has the virtue of being more efficient and thus a good idea to do even if it wasn't an emissions issue.

The technical reason for using a PCV is two fold. The first one being that 1/2 burned fuel can collect in the crank case, merge with the oil, and eventually contaminate the oil. There's also acids and other nasty things that get produced as part of the combustion process that you don't want becoming part of the oil supply. So keeping a flow of fresh air through the crank case coerces as much of the fumes and unburned fuel back into the engine and minimizes the amount that gets into the oil. And what does get into the oil hopefully is evaporating out of the oil back into the fresh air to further minimize the concentrations of chemical contaminants in the oil.

The other benefit of PCV is that it reduces the amount of air in the crank case by keeping the case under slight vacuum. This thins the air and means there's less windage going on on the crankcase. Just like putting your hand out the window of a car at high speed, the fast moving parts inside the engine are having to beat their way through the air. Ideally, you would like the crank case under tight vacuum to minimize the atmosphere those parts have to displace as they move, but that's just not possible or practical. The next-best thing is to have the PCV pull down the crankcase pressure as much as it can at cruising speeds. This thins the air and adds some efficiency. It's evidently enough to measure over long distances, but I've never disconnected my PCV to test that. I have heard of people connecting vacuum pumps to their crankcase to gain a few HP at WOT. Since WOT doesn't have any vacuum to pull the case down with, you have VERY high speed parts moving through full atmosphere. Pulling a vacuum down there evidently is worth a few HP.

All-n-all, PCV is not a bad thing and is worth maintaining in a daily driven vehicle if possible. It certainly doesn't reduce performance as long as it isn't gumming up the intake with oil. Synthetic oil can help reduce the amount of gum that builds up in the intake. But ultimately an oil separator in-line between the PCV and intake is best.

[Chucko]

Remember that PCV is not just the PCV valve - there is also a fresh air inlet after the MAF, but upstream of the throttle. So what comes through the PCV valve is metered. Anything other than fresh air coming through the PCV has already been burned once (even if only partially).

If you're going to vent the crankcase to the atmosphere, you'll need to plug the feed from upstream of the throttle as well. Doing this is fine for a race car, but any car that has to pass a smog test will need a functioning PCV.

Back in the "bad old days", Detroit iron came with unsealed breathers and draft tubes venting the crankcase to the underside of the chassis. PCV was the first emissions control, added to capture the blowby and burn it up.

The serious racers use a vacuum pump to lower the crankcase pressure for a decent increase in HP - definitely more than it takes to turn the pump! Where you vent the output of the vacuum pump on a smog legal street car is a good question though.

[DonMaximo]

Chris :: Killer detail man - thank you

Chucko: Regarding those vacuum pumps, it was my understanding that the vacuum pumps were used in racing so engine builders could use lower tensile strength piston rings. The rings would generate less friction, the vacuum pump would compensate by improving the ring seal between piston and cylinder walls.

A similar method was ventilating/plumbing both valve covers directly to the exhaust header collectors. At an elevated RPM level, the connection at the header would create a vacuum that was transmitted back to the crankcase via the valve cover connection.

[cgrey8]

Yeah, I forgot to circle back around to that point about the blow-by air being metered or unmetered. And both of you are correct.

Yes, PCV air coming into the crankcase should be metered as it should be pulled from the pipe AFTER the MAF.

Yes, the blow-by you get from the pistons is technically metered because it was measured once. But it is being recycled for a 2nd pass. So for the 2nd pass, it is actually unmetered and will add fuel that the EEC didn't have to compensate for. But the amount of extra fuel supplied by the PCV is minuscule compared to the amount the injectors are dumping. So the fact that there's extra fuel entering the engine than was expected is not a concern. That ranks right up there with the extra hydrocarbons and oxygen that come back into the engine from EGR gas. Technically that's 2nd pass unmetered fuel/air (depending on whether the engine was rich or lean at the moment). But the amount EGR throws things rich or lean is easily ignored particularly if you are holding the EEC very near stoic. However if you are running forced OL and are purposely running lean, the further away from stoic the engine is, the more significant the amount of "extra" oxygen is getting back into the engine that the EEC didn't account for. That's going to throw things and cause you to be even leaner than expected. However I don't think the extra oxygen from EGR becomes significant until you are running AFRs upwards of 1.10 lambda (10% excess air). Most cars don't cruise well that lean anyway. They start lean surging or bucking.

[Chucko]

Chucko: Regarding those vacuum pumps, it was my understanding that the vacuum pumps were used in racing so engine builders could use lower tensile strength piston rings. The rings would generate less friction, the vacuum pump would compensate by improving the ring seal between piston and cylinder walls.

There are other benefits, two of the biggies being increased pressure differential across the piston for the power stroke, and less aero drag for the reciprocating assembly.

BTW one of the big advantages of a dry sump, in addition to much improved oil control and reduced windage drag, is also the crankcase vacuum it generates.

[cgrey8]

At 1st, I thought that greater differential pressure across the piston on power stroke was a benefit. But then I got to thinking about it. Anything you gain on the power stroke due to the vacuum, you lost on the compression stroke literally fighting the vacuum to move the piston up. So I believe the power increase is purely due to less wind resistance in the crank case. There might be some wiggle-room for debate related to when the intake valve closes on the compression stroke and when the exhaust valve opens on the power stroke. But that argument would change from cam-to-cam.

[Chucko]

If you think about it, crankcase vacuum helps on the 'down' strokes (intake and power), and works against you on the 'up' strokes (compression and exhaust). So yeah, probably a wash overall. Whatever the cause of the HP increase, it's definitely enough that a max-effort engine is going to run a vacuum, whether specifically via a vacuum pump or as a side effect of a dry sump oil system.

[cgrey8]

That's why I think the realized gains are due to less air density.

[dualdj1]

I was going to mention the dry sump systems as well, but Chucko hit that. They are obviously preferential if you can afford them, and then you don't want to run a PCV system due to dry sump handling your vacuum and circulation.

Now I'm not sure how emissions controls are adjusted for dry sump, but there must be some method as vehicles like Corvettes use dry sump. Anyone have any insight on commercial dry sump systems? Only application I've used them on are off road race engines.

[bender460]

What can a PCV valve do at WOT when manifold vacuum drops to near zero??

[liljoe07]

That's when its supposed vent through the filler neck to throttle body tube.

[dualdj1]

Joe yes it'll vent, but the point he was making is that there is little to no vacuum on the crankcase at that point, so your air volume in the case is normal. This is why many will use a smog pump or something similar to evacuate the crankcase, as at WOT you're still getting vacuum.

Now, the real question that raises is, is there merit to running an external vacuum pump and redirecting the output back into the intake stream instead of open air, from an emissions standpoint (IE not a manifold vacuum driven PCV). I would think the answer to be yes, and not just from less windage...

[liljoe07]

I agree. At WOT the volume of the crankcase gases would be high, and probably so high that the filler neck the TB tube pressure differential would not be great enough to expell it. You would just have crankcase pressure trying to force itself out.

My take is we could have even better results at stages where the crankcase is under vacuum if we increase the volume the PCV system could carry. Like larger diameter PCV valves and lines.

I would also like to know just how much vacuum(if any, but I assume there is just by the nature of air being sucked by the TB) is present at the oil filler neck at WOT.