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Crankcase Breather Mod


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All engines get some blowby of pressurised combustion gasses past the piston rings and into the crankcase. This must be allowed out or pressure will build up inside and force oil out past gaskets and seals and up the dipstick tube.

 

Some engines have more blowby than others, but all will tend to increase as they wear, and all will have more blowby when worked hard.

 

Up to the 50s and early 60s the problem was dealt with using a simple relief pipe to the outside world, usually with some kind of trap to try and catch entrained oil. This then became unacceptable from the emissions perspective, so the tube was connected to the engine inlet upstream of the throttles. Some even used engine vacuum to scavenge blowby gasses from the sump and create a negative pressure inside, Something which is done on every production engine today.

 

This has a number of advantages:

- water vapour and exhaust gasses get drawn off and have less chance to contaminate the oil

- Lower pressure inside than out greatly reduces leaks from gaskets and seals

- Lower pressure differential across the rings on the intake stroke reduces oil passing the piston rings

 

Disadvantages:

- Contaminates intake charge with non-flamable products.

- Some systems have poor oil separation leading to oil mist contamination of inlet charge - possible detonation risk increaser.

- General increased complexity - clogging can easily lead to system failure.

 

From the mid 60s, Triumphs all used either a simple connection to the atmospheric side of the throttle, usually via some kind of flametrap, or a proper PCV valve as described here.

http://mgaguru.com/mgtech/engine/cv103.htm

 

In general they work ok, but can suffer from oil contamination issues due to minimal baffling or oil trapping measures in their designs.

 

Triumph were also prone to using the "one size fits all" approach so systems that were fine on a 1300 were a bit stretched on a 2.5.

 

My 2.5 PI was born with a simple pipe between rocker cover and inlet plenum. With just a single 1/2" outlet from the whole engine, this is too small leading to quite high gas velocities at the exit, encouraging oil carry-over,which the minimal baffling in the rocker cover does little to stop. Also, the outlet slopes down into the plenum, so oil collecting in the plenum is always an issue. There is a rudimentary flametrap in line which is prone to clogging with slimey oil/water emulsion.

 

My particular engine is also somewhat past its youth and was blowing alot of oil out of the breather and anywhere else with the slightest path out - messy. Also has a marked tendency to burn oil.

 

Some reading here suggested it might be possible to improve matters

http://www.teglerizer.com/triumphstuff/emissions/index.htm

 

I decided to use the redundant fuelpump location as an additional outlet. Some kind of pot is needed to reduce outlet velocity so any entrained oil drops out. Ideally this is able to drain back to the sump.

 

This sketch shows the basic idea:

separator pot section.JPG

 

Which after some rummaging in the scrap bin, some hammering, sawing and welding became this

separator pot.JPG

 

I also had to invest in a couple of stainless steel scouring pads. These are held between two sections of perforated plate.

My actual pot has a second inlet connection into the lower chamber as I had considered connecting the rocker cover outlet there to separate the oil. Currently being use as a takeoff to measure crankcase pressure (larger white nylon tube).

 

Current layout is shown here

P1050090s.jpg

 

Green line - indicates full manifold vacuum

Yellow line - indicates scavenge vacuum to the separator pot, fed from full engine vacuum via a restricting orifice

Yellow circle - indicates location of restriction orifice

Blue line - filtered air at atmospheric pressure feeding the cold idle valve and idle adjusting screw

Red line - original breather line to added PCV (Nissan Almera donated one)

Red circle - PCV valve location

Orange line - atmospheric outlet from PCV valve

Purple line - to vac gauge monitoring crankcase pressure

 

Those used to the normal PI cars should note that mine is EFI converted and the 6 throttle butterflies are removed, with a single large throttle fitted at the plenum entrance. This means that the plenum and link tubes see engine vacuum.

 

Orifice 3mm.jpg

 

Orifice is a cut down 5/16" bolt with a small amount of the head left on to stop it pulling through into the T. It's about 6mm long in total. The orifice is simply the hole drilled through it. I started at 1.5mm and worked up to 3mm (so far). The size needed will depend on the actual engine and condition.

 

There is no active control on this system - it's just a balance between airflow through the orifice (itself dependent on the level of manifold vacuum) and blowby. The PCV valve in the old line is configured to allow positive pressure out but prevent air being drawn into the engine through it. I also had to block the small hole in the oil filler cap (another token breather). If your oil seals are old and shot they may too much air through to get a decent vacuum. Dip stick is another possible lead path - there should be a leather/felt sealing ring at the top of it.

 

The system will work most effectively at idle and light cruise where manifold vacuum is low and blowby low. As you open the throttle, manifold vacuum will decrease and blowby increase. The balancing act is to get enough flow to give a reasonable vacuum in the cruise regions without overdoing it under idle/over-run conditions. I was aiming to hold a slight vacuum in the crankcase at least up to fast cruise conditions, with my rather tired engine this needs a 3mm orifice giving about 4psi negative and it remains just negative cruising at 70 mph on the flat.

 

There are valves out there, including the one in the first link, and as originally used by Triumph on some models, which active control the pressures. I'll being trying one of these when I can lay hands on one.

 

Meanwhile, the car did 320 fast motorway miles last night (longest run by far while in my ownership), blew no oil and "only" used about 1/2 pint. The recently added can of Wynns may have helped the latter!

 

Cheers

 

Nick

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Edited to re-instate missing pics and also to say that the system was later modified to use a proper PCV valve to maintain a more even crankcase pressure.  I actually used an AC "CV770C" valve imported from the USA and originally intended for some large V8 or other.  It was fitted in place of the orifice and worked alot better.  Ultimately the full cure was an engine rebuild to cure the rust-pitted bores and knackered piston rings.

Nick

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  • 5 weeks later...

I feel sure that I read this before, and replied, but there is no page 2.  Strange.  Deja vu Syndrome?   A glitch in the Matrix?

Anyhow, I would have answered with a pic of my crank venting system.   NOT to disagree with Nick, but to say that a large bore, longish hose, angled to as to drain any condensate back into the engine, can make droplet catching simple, and easily clogged wire wool/scouring pads redundant.

The central white plastic tank is the coolant header.   The tank to your right of it is my catch tank - Its a Renault or other French item, another header, I think.    Not very visible but seen curving between and behind the ignition leads is a half inch hose that goes down to the fuel pump opening.     Another, smaller hose connects it to the rocker cover with a tiny K&N filter on a T, seen between the two tanks.    This the gas vent.

The big French catch tank could hold 3 litres, but it never does.  All condensate runs back into the crank case, while some gas and steam escape though the filter, which catches anything left.    Wide bore, no resistance, save for the filter, and never gets clogged!

JOhn

 

 

 

 

P1030445.JPG

Edited by JohnD
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  • 2 weeks later...

Hi,

 nice one John. Looks like you have no windows?

 

To all,

I believe:

The PCV valve cannot function alone. To work correctly, the intake air to the crankcase/head/rocker-box must be controlled. Most are passive.

With the PCV in place, without control over the flow of filtered air into the engine, I will have the Devils job trying to get idle?

Upsetting airflow by blanking/freeing air to rocker box on a PCV car will change idle speed if not electronically controlled?

 

Getting confused,

Iain.

 

Edited by spitfire6
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Au contraire, mon vieux!   It's just that the camera angle that has beheaded the rest of the car!

In an unmodifed car, the Pressure Control Valve IS what controls gas flow from the crankcase/rocker box to the inlet manifold.      It works inversely to the vacuum in the inlet manifold, so as to be near closed at idle, wide open at WOT.   If you like in parallel to the throttle butterfly!

Under normal running, gas flow into the crankcase etc. is exclusively by piston blow by.   There is no other inlet.    But you are right in that openeing the oil filler port in the rocker will change idle speed, esecially if you have excess crankcase pressure, as that will be released and less gas reaches the inlet manifold.

John

 

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Hi,

 I could not see a wiper motor? It's a shame it's not an easy job to rotate the thermostat housing 90 degrees CCW. Would save you around 200g of hose clamps. :biggrin:

I will need to put a restrictor on rocker box inlet set to allow maximum flow that does not affect idle speed, yet needs to flow more at high RPM.

Waiting for an ICV to arrive. 2 & 3 pin.

The idle control thread is 9/16" UNF?

Cheers,

Iain.

EDIT: My eyes are bad. I think I see the end of it now.

Edited by spitfire6
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Wiper motor???? In the usual place.

And if you look carefully at the radiator, you will notice that it's not OE, but a Honda CR one.  With the top hose tube in a different place to Triumph.  Damn right it's not easy to rotate the thermostat housing.  That one, rotated through 180 degrees, took me AGES to get right.     Brazing would have been good, fills holes better, when Mig doesn't, but I don't have a hot enough torch.   It took a long time, making up test clamps to pressurise the new cover, repeated grind backs and rewelds, to make it air/water tight!    I should have farmed it out to Bad Obsession - Nick'll get it right first time!

As for your restrictor, what you describe is exactly what the OE PCV does.

John

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Hi,

 Would love to see a higher res photo if possible?

I see Nick utilises a 3mm orifice. I have a couple of flow control valves somewhere. Have to find a way to connect one between the air filter and the rocker box.

I hope that taking filtered air in at the top and out at the bottom (Mech fuel-pump location)?

One of the flow control valves, ICV. Two pin type. Going to bung some voltage up it to see if it can seal!

Cheers,

Iain.

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Thing about a simple orifice is that it’s only right for a specific set of conditions and wrong everywhere else. A PCV valve is a self adjusting orifice which will be right over a wider range of conditions. There are many different valves out there in various shapes and calibrated for different ranges of conditions according to the application (engine) they were designed for. Picking the right one involves witchcraft, casting of runes...... or at least a bit of trial and error. But pretty much any one will be better than a simple orifice.

Nick

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  • 1 year later...

Apologies for thread revival but this came to my attention via the TSSC forum.

If I put a catch tank on the bulkhead, connected in between the rocker cover breather pipe and PCV, would you expect much oil to be deposited and avoid being sucked into the intake? I'm not sure if it would make any difference but the extra pipe-work and catch tank might make some oil precipitate. 

My Spitfire is back on the road after some head work. I was hoping that new valves and guides would incrementally reduce its greedy oil consumption. However it seems that upping the compression ratio and/or clearing the rocker oil feed, have undermined any possible benefit. When I put my foot down now it's smoking more than before ! This looks bad when driving through the Ultra Low Emissions Zone...

Taking the oil filler cap off when the engine is idling, there is quite a breeze coming out and spattering oil around, so I wondered about the PCV. Took it apart and the diaphragm is in one piece and there was a lot of oil all over it as you'd expect. I guess it's working as designed but my worn bores are producing a lot of blow-by. 

Wondered what others' experiences were - whether catch tanks actually catch anything, and it it needs to be large or small, closed or open, etc. 

I know the real cure is the same as Nick's - a rebore! But I haven't got time & money for that just now. I've got a can of Wynne's 'Stop Smoke' but am a bit suspicious of putting magic potions in the engine. Maybe that's silly. 

Pete

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Wynns oil treatment (the stuff that looks like clear treacle) kept the old Vitesse engine alive 5 years and 2 10CRs beyond it's natural life.  10CR 2009 used 6L over 3000 miles - plain oil, 10CR 2011 used 1L over 2500 miles with Wynns with most of the use being towards the end when the Wynns got tired.  Cold oil pressure can be a bit scary.....

As a quick and dirty experiment you could just try plugging off the PCV system and running a hose into a container.  This will show what collects (probably a certain amount of emulsified "snot") and whether it makes any difference to smoke issue.  My guess is that unless there is actually something wrong with your PCV system it will make not difference or be slightly worse as my experience suggests that the system works well for normal driving - just gets overwhelmed when giving it stick.

You do appear to following the same denial path as I was :smile:

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2 hours ago, Nick Jones said:

You do appear to following the same denial path as I was :smile:

:biggrin: Thanks Nick, it's not denial - it's expediency! 

I'll experiment with the breather but with low expectations. Then give Wynnes a try. If the RBRR is postponed to next year I'll think about doing the re-bore this winter

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Pete, 

Might I also suggest checking that whatever breather you have is free flowing?

Two years ago on the RBRR myself and Nuala (in her Yellow 1500 Spitty) had issues with sudden huge oil consumption, and plenty smoke (& oil) coming out the exhaust. 

We were alerted to this by a rather irate Stag owner whom Nuala had rather spiritedly chased and overtaken on the A9 before JoG, who was not so happy that we covered his pride and joy in a large amount of oil whilst doing so!

After thinking the worst, we eventually traced the issue down to the breather pot/filter that was fitted that was getting blocked by the oily "snot" and thereby not letting the pressure out the rocker space. Was quite surprised at the time as to how big an effect that had!

It does sound like you have covered this base, but I thought I would mention it in the off chance you hadn't checked it.

Phil

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Haha I wonder if that was the same Stag I pulled over to help in Wales, who were laying a massive smoke screen after filling up with diesel?!

Thanks Phil, interesting - does Nuala have a non-standard arrangement on that 1500? Mine is just the breather from the rocker cover into PCV on the inlet manifold. Good point though I'll check it's not restricted.

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11 minutes ago, thebrookster said:

Was quite surprised at the time as to how big an effect that had!

I'm surprised a blocked breather would have that effect as well. It suggests that crankcase pressure results in more oil in the cylinders. Therefore my worn bores, creating lots of blow-by, might benefit from more breathing. Hmmm.

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6 minutes ago, PeteStupps said:

 

I'm surprised a blocked breather would have that effect as well. It suggests that crankcase pressure results in more oil in the cylinders. Therefore my worn bores, creating lots of blow-by, might benefit from more breathing. Hmmm.

 

11 minutes ago, PeteStupps said:

Haha I wonder if that was the same Stag I pulled over to help in Wales, who were laying a massive smoke screen after filling up with diesel?!

Thanks Phil, interesting - does Nuala have a non-standard arrangement on that 1500? Mine is just the breather from the rocker cover into PCV on the inlet manifold. Good point though I'll check it's not restricted.

Unfortunately I don't have any photos, but yes her setup is not completely standard. It goes from rocker cover to a modified dashpot from an SU carb mounted by the radiator.

To test yours, I would simply remove the pipe from the manifold, replace it with a longer pipe and fasten it down by the chassis rail. Run like this for a day or so and see if you are still making smoke.

Phil

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Limited testing last night: I blanked off the PCV inlet and ran a length of hose from the rocker cover breather to a suitably bodged bottle (tied to radiator support).

Was quite surprised to have collected about 2 teaspoons of fluid in 4 miles of urban stop-start, with maybe two moments up to 5k rpm in 2nd but the rest just pottering along 20 to 30 mph. Dipping a paper wick into the fluid proved it to be oily, although it crackled slightly when set alight so contained water - as you'd expect for blow-by gases.

The catch bottle isn't even a short-term arrangement though because I don't want my engine bay coated in the oil mist blowing out of the top. So for the time being the rocker cover hose is venting to atmosphere beneath the chassis, as Phil suggested. May well leave it like that, not really sure. I think it's smoking less, but only slightly less.

One thing I did notice, I had to lean off the carbs by about 2 flats. Which suggests the PCV is slightly open at 800rpm idle, but now I've blanked it off it made the mixture go rich. Either that or it was oiling the K&N filters that did it. 

By the by, found quite a good vid on Youtube where some Subaru tuners experimented with catch tanks and sent the contents off for analysis. It was mostly oil but with notable fuel and water content. The analysis company strongly recommended not pouring this back into the engine. https://youtu.be/vcohpGMFwI8

Pete

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Which I why I think my arrangement has merit.

The hose from the crankcase breather (was the fuel pump hole) rises to meet the hose from the rocker cover, then is connected to a catch tank on the bulkhead.    This allows the higher evaporation point oil vapour and droplets to condense and run back into the engine, while water vapour stays in the exhaust gas flow and escapes via  small filter between the T and the catch tank.    The result is, automatic separation - distillation, even! - of oil and water, and NOTHING in the catch tank, ever.      On cold days, I can see fine  wisps of water vapour condensing as it leaves the filter.

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I agree John, if I swap to an electric fuel pump I would probably copy your layout. As it is, i've looped my temporary breather/drain pipe upwards a bit in the hope that the bigger oil droplets run back down. The open end down at the chassis isn't oily at all after a few more miles around town, which I regard as promising. Still smokes from the exhaust but you can't expect miracles. There must be less oil drawn in through my new inlet valves now, in any case.

Pete

DSC_0070.JPG

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