Engine and Transmission Tech

Hi all,

My TR7v8 now has an ex-Disco, rebuilt 3.9L has 10.1 compr., Erson mild street cam, Edelbrock intake and 1404 carb, and headers leading into a 2-1 exhaust.

For the past several weeks, off and on, I've been sorting out electric fuel pump wiring issues. I wired it through a 3 prong oil pressure sensor and had nothing but issues. Changed my wiring a dozen times trying to sort it out, only to discover that I actually had wired it correct the first time, but had but bad sensors, 3 new ones in a row! Finally took my electric multi tool to the auto parts place and opened a half dozen boxes, testing them to find one that worked as advertized. Grrrrr. No quality control these days!

Anyhow, while timing it and setting up the carb yesterday (engine sounds great!), at a fast idle (1100rpm) I took off the oil filler cap to see if oil was reaching the rockers. I was surprised by the amount of negative pressure, sucking the cap back down. Is this normal?

FYI - I have the stock small Disco air inlet at the drivers side, rear of the rocker cover and have fed the large oil separating breather from the front pass side rocker cover to the timed vacuum port on the carb. Perhaps this is creating too much vacuum?

Regards,

Martin

Martin,
Check out the archives.
I think it was Jim Blackwood that wrote a very good article on crankcase breathing for the rover engine.
You shouldn't have any vacuum, or pressure for that matter.
The way it is right now will destroy your seals very quickly.
Cheers
Fred

OK, so I need to stop de-pressurizing the crankcase. But I am still confused. If anything, wouldn't there be some positive crankcase pressure from blow by, however little? Negative pressure indicates to me that my breather plumbing to the carb port is causing too much suction. No?

Here are two pics showing the plumbing:


You can see I have fed the large breather from the front pass side rocker cover to the timed vacuum port on the carb. I've also T'd in the breather from the charcoal canister (upper right corner). The small red filter (lower right) should allow air in, although it's only about a 1/8" dia tube.



Guess I will try disconnecting the large breather and see if that makes a difference. Will keep you posted.

M.



Edited 1 time(s). Last edit at 11/14/2011 08:39PM by bsa_m21.

Martin, I think you actually do have it plumbed correctly. The Brits use a completely different system from American practice and you can't mix-n-match, it has to be all one or all the other. The way the British system works is that it draws a vacuum on the engine crankcase to suck out any blowby using a large diameter tube (to allow the large volume of blowby gasses at max power to escape) and then introduces a small metered amount of fresh air through the breather, which has a restricting orifice. Having a strong vacuum at idle would be normal operation and the seals are made to take it. HTH

JB

Hey Jim,
I never disagree with you, well maybe just this once.....
The way this breathing system is set up is a problem.
Using ported vacuum to vent the engine leaves the engine unvented at idle, over evacuated at part throttle and under vented at full throttle. Any significant blowby would blow off the hoses or blow out the engine seals.
For the European style system to work the large hose from the large breather needs to go directly to the air cleaner.
This large hose needs a tee in it nearer to the valve cover. This tee would feed a smaller hose with a metered orifice in it of approx 1.5 mm. this hose then gets connected to manifold vacuum, not ported.
The breather on the left side manifold is of no use what so ever. It's connected to a 5mm tube with a .25 mm orifice.
To run a more familiar north american style system the big breather would still connect directly to the air cleaner with a large diameter hose with out a tee in it. The left valve cover would get drilled out for a pcv valve where the red filter is. the pcv valve would then connect to the large manifold vacuum port on the rear of the carb.
The last problem with the system is tying the canister vent into the crankcase breathing system. A single backfire could be catastrophic.
Better to use that ported vacuum for the canister only.
For what it's worth
Fred

Interesting. Well, not having a British system here at the moment to refer to, (and illustrations are hard to find it seems) but here is one somewhat useful reference: [en.wikipedia.org]
It only has one very short reference to the British system though, mentioning the constant depression SU carb. I don't claim to be an authority, PCV systems have been done in a surprising variety. So many configurations are possible. However some requirements should be observed in any case. First, the use of flame traps anywhere the system connects to the intake tract, such as the large canister screwed into the valve cover. Second, whatever fresh air is introduced into the system must be accounted for in the fuel calibration, whether by carb or EFI and this is the single biggest problem in using the British system with an American 4bbl carb and intake as the calibration is thrown off.

So, I've done my best to understand the operation of both systems but may have made some mistakes so it wouldn't hurt to go through it again, starting with what the system is intended to do.

First, blowby. A very minor problem at idle, blowby continues to increase in volume as engine output increases, reaching maximum along with the torque and horsepower curves. At max the volume of flow can be considerable.

Next, fresh air management. Enough fresh air must be introduced into the crankcase to flush out the combustion by-products. Any fresh air introduced must be accounted for in the fuel metering. This can be a more or less constant supply or it can be variable. In both systems the volume generally varies relative to engine load but in different ways.

Finally, the selection of a vacuum source for evacuation. Both systems make use of vacuum to move fresh air through the system, and both systems can reverse flow through the inlet during maximum blowby.

It is crucial to proper operation to have the system plumbed in such a way as to account for the changing relative pressures in the system. If you think in terms of absolute pressure rather than vacuum the picture may be a little clearer. The intake at idle is the lowest pressure in the system, while the crankcase during max blowby (WOT) is the highest, followed by atmospheric.

Fred, I don't entirely understand your plumbing scheme, so I'll present what I see as workable and ask you to elaborate if you would.

I'll start with the domestic system as it is more familiar. The air intake is typically inside the air filter housing after the air filter and is fitted with a fiber type flame trap at only very slightly below atmospheric due to the restriction of the filter element. This is an open tube to the valve cover and during WOT it acts as a vent and allows blowby to escape into the carb inlet. Usually 5/8 or 3/4" diameter it will allow a great deal of unrestricted flow, thereby preventing pressurized seals, and incidentally, the type of seals used has some relevance, as a rope seal or a labyrinth seal are not designed to resist positive pressure and will leak in one direction or the other, depending on which is the high pressure side. So the seal can also act as both an air inlet and a vent. Neoprene lip seals OTOH are quite good at resisting small pressure differences such as those generated by a vacuum or light crankcase pressure, but can also allow flow if the pressure differential becomes too great. More on that later.

The suction side is fitted with the ubiquitous pcv valve and the above link has a good description of its basic operation, bearing in mind that increasing blowby has the same effect on the valve as increasing vacuum, i.e. decreasing flow, so that at WOT almost all of the blowby is venting through the inlet, passing through the carb a second or third time, and since it is either incombustible or uncombusted mixture, causes the carb to enrich the a/f mixture further. The port for the pcv is at manifold vacuum so it follows the throttle opening closely and flow is varied by vacuum and by blowby.

The British system may come in more than one variety. As Fred mentions there is a port on the side of the SU carb and I'd have to look but it's probably pretty safe to assume this port operates at a constant vacuum most of the time, though what it does at idle is open to conjecture. However, somewhere in the system is an orifice to meter airflow. If you have a fixed orifice and a fixed pressure drop you will have a uniform and consistent volume of flow. However this can be (and is in some cases) affected by blowby volume as well. Two possibilities exist. The orifice can be at the outlet from the crankcase or it can be at the inlet. If at the outlet (MGB) it connects directly to the vacuum source and the inlet (lifter box) goes to the air filter and operates very much like the one on the domestic version. I think this is the system Fred was referencing. The other system (Rover) uses an orifice fitted with an inlet filter on the off side valve cover, (the small breather) and then runs the large diameter tube from the canister type flame trap directly to vacuum. If the engine is fitted with SU carbs this will be the constant vacuum port. I don't believe this port is blocked off at idle, but I could be wrong about that. At any rate, the blowby at WOT is almost entirely directed through these ports with only a small volume discharged through the oriflce. If a 4bbl carb is fitted the plumbing gets a little trickier. The question then becomes, do you want to have a positive flow through the engine, or to just vent blowby? Because you basically have two options. You can plumb the large line to the inside of the air filter element or to the intake manifold. If to the intake manifold you will positively evacuate the crankcase, draw a vacuum on it, and even possibly draw in a small volume of air past the seals. (This may help eliminate oil leaks, however in the case of a labyrinth seal, the seal becomes the metering orifice and may bring in some clutch dust. Neoprene seals will resist the vacuum reasonably well.) Note that the volume of blowby has a minimal effect on the air/fuel mixture as it is not recycled through the carb.

If you connect it to the air filter housing the fuel metering situation with the 4bbl carb is greatly simplified as it acts very much like the domestic system, close enough to probably work right out of the box. However the only pressure differential causing fresh air to move through the system is that caused by the air filter and if this is a K&N type that isn't much at all. Plus there is a filter on the orifice as well, more or less equalizing the system. Then you also have the orifice, so it's pretty safe to say nothing moves here except blowby. With this scheme it would probably be best to remove the orifice entirely, except that this would then allow blowby and oil vapors to exit and collect on the valve cover.

That's pretty much the rundown and explains why I said I thought the system was plumbed correctly, but if you wanted to make it more consistent with domestic, (which may simplify carb tuning) you could just run the large line to the filter housing, and either plumb the orifice to manifold vacuum or install a pcv valve in place of it. I think Fred mentioned that.

JB

OK, I think I understand these replies.

In the British system, a metered amount of fresh air flows into the crankcase via the small inlet, and out of the crankcase with any blow by, into the intake manifold via the large outlet. There is no PCV valve.

The American system works in reverse. The fresh air flows relatively unrestricted via the air filter into the crankcase using the large hose and out from the crankcase with any blow by, into the intake manifold via the small hose connection through a PCV valve.

OK, but some more info on my engine. The air inlet pipe, the small red filter is on, is not the original 5mm tube with a .25 mm orifice. It was broken off when I got the engine, so I removed the stub and replaced it with a 1/8" ID pipe that the filter is clamped to (I'm a newbee, what do I know...). I'm not sure what impact this will have, as it lets more air in than before. Guess I could always fit a restriction in it if need be.

Questions -
Could I just insert a PCV valve into the large diameter line? Then the flow is still in from the small orifice and out via the large hose.

Or, should I go the American route, hook up a PCV and reverse the flow?

What still confuses me are all the pics on the site that show a lot of engines with plumbing similar to what I currently have. I wonder how they are performing?

Jim and Fred, thanks for all your words of help! Truly appreciated.

M.

Remember that the large line allows excess blowby at WOT to vent. You do not want to restrict this line. The result of doing so is an abundance of optional self-oiling features which Carl can tell you all about.

JB

I agree that excess vacuum applied to the crankcase is most likely the cause of your issues, however I just came across something a little strange that might apply to your situation. A buddy of mine just tore down a Rover 4.9 stroker that had negative crankcase pressure even when no vacuum was applied. First thought was an ill fitting intake gasket. That turned out to not be the issue. It seems that two of the cylinders had bad oil rings on the pistons. The pistons were fitted with total seal rings. The vacuum in the crank case was being caused by the bad oil rings. I find it a little hard to believe, but this is according to the engine builder who is removing the engine from the car and completely rebuilding the engine and reinstalling the engine at his expense. He plans to overbore the engine and install new forged pistons. The builder blames the bad oil rings on gas wash from the carb during the startup and warm up period. Plan is to fit a fuel injection system to cure the overfueling issue, and to go back to gapped rings. Does this make sense to any of you that are more knowledgeable engine builders?

I would be more likely to suspect the total seal rings. Bear in mind they were developed for racing applications where the crankcase is evacuated either with a header scavenge system or with a vacuum pump in the effort to increase power output. The removal of air from the crankcase is thought to eliminate windage. Of course racing systems are not designed to draw vacuum at idle.

No ring is going to seal perfectly, which is why the top ring is backed up by a second one in the first place. Under certain circumstances they are going to leak a little and conditions for the top and second ring will be a little different. Personally, I just suspect that one of the rings may have been installed upside down. This would cause the two rings to fight each other and could result in the conditions you describe I suspect. As for the oil control rings, I doubt they would have any effect on either combustion pressure or crankcase pressure, as they are usually vented through the piston at the separator, though the total seal set could be a little different.

JB