24 replies to this topic

[dennismini93]

iv been doing a lot of reading into engine breathing today for a series and and other engines and i just wanted to know what was right for the 1275 a series engine.

 

as far as im aware an engine breathing system is designed to take oil vapors away from the crankcase etc to reduce internal pressures created by evaporating oil due to heat (and i guess exhaust blow by through the rings??) and then fed back through the intake system to burn off in the cylinders as the fumes are harmful and carcinogenic. oil vapors arnt the best fuel for combustion so iv been trying to read into the best way to resolve this.

 

i have a couple of queries and i wondered if you knowledgeable people could help

 

A 1275 has an engine breather on the crankcase and the timing cover that as standard get fed into the carb. A 998/1100 just has the breather on the tappet chest cover and early cars have a breather on the rocker cover.

firstly why are the big bore and small bore engines different, i know a 1275 doesnt have a cover on the back of the engine but why doesnt the 998 take the 1275 locations.

 

secondly i read that having a rocker cover breather was bad but i cant remember why and now cant find the post.

 

some people put filters on their breathers and i know this is bad for mr environment but what are the engineering pros and cons for this?

 

finally is an oil catch tank a good road to go down? as far as i can see it still allows for the standard breathing system to stay as a closed system but it appears to remove the oil from the vacuumed gasses so creating a cleaner gas going back to the carb.

med sell one with a rocker cover take off so im curious as to the pros and cons and for the best way to allow a 1275 to breath 

 

thank you in advance 

[nicklouse]

A plumbed in catch tank will offer little over any other plumbed in system.

Your generalisation as to breather locations is just that a generalisation. And incomplete and not correct.

A closed system does not really need that many breathers as the gasses are sucked out.

An open system needs more so as to allow the gas in there to move as freely as possible.

The filters are people's ideas of keeping dirt out of the engine. The drums on the breathers are filters. They have wire wool in them (did when new).


So what are you doing? What are you wanting or trying to achieve?

[dennismini93]

just wanted someone to explain why the engines were different, what is best for a 1275 (even if that is just to leave it alone) and if possible the reasons for the different ways of running the system on a 1275 with pros and cons.

 

there are loads of bit threads on here with a lot of advise and no reasoning from pub technicians for want of a better phrase. i just wondered if someone could tell me an actual engineering reason why one method is better than another or if they were all similar etc

 

im not looking to change my car as its just a boggo 1275 and i have no current plans to develop on from this but i would like to learn more about this side of the car.

 

at work when they introduce a new car we get told why each bit is there, why its different or the same and how it works and i would like to have this knowledge with minis. i just want to understand the system and its evolution really.

[Spider]

I think the 1275 engines had an extra breather over the smaller engines due to the possibility of shorter runs and oil temps remaining low. Unless it's a restoration, I only fit the one these days, on the transfer housing, more than enough. I have found it's pretty easy to have liquid oil surging up the one fitted to the timing cover on RH corners, resulting in that big oil smoke screen.

 

A secondary function of Crankcase Ventilation via positive means is reduced Oil leaks due to usually lower crankcase pressures, so for a lot of reasons, including this, I don't like Open venting them (even via a filter) or Oil Catch Tanks. Not needed or warranted on most street cars and if they are found 'necessary' then maybe a revisit of the piston / ring / bore honing is in order. Fix the problem rather than clean up the mess. I also don't like open vents or catch tanks as they stink and are messy.

[nicklouse]

at work when they introduce a new car we get told why each bit is there, why its different or the same and how it works and i would like to have this knowledge with minis. i just want to understand the system and its evolution really.

There is no evolution and no reason why with the companies that made the Mini.

They used to have the machined breather fitting on the transfer case but then it was cheaper not to machine it and just use the existing one on the tapet chest.

As Mr Spider says having negative pressure in the drank case is desirable. Does not really matter how you get it you can only get so much as you only have one pump. Adding more ports does not mean less pressure.

Now on to catch tanks. Here the result is different. The racer does not want anything that might reduce the power output of the engine by polluting the charge going into the combustion chamber.

But you still have the issue of crankcase pressure. So there you have as many breathers as you can going into catch tanks. I have 4 breathers on mine to two catch tanks. But it is not a road car.

[Spider]

A brief history of them;-

 

The first 850 Minis (and maybe even the 997 Cooper?) just had a pipe off one of the Tappet Covers leading down to just below the sump rail, facing the road. I think you can see why this was short lived.

 

Then these were crimped off and a breather from the Rocker Cover to either the Air Filter Case and then a little later an adapter between the air filter elbow and the carb.

 

Around this time, the Cooper S came on the scene and these had a true PCV System (which was invented by GMC and made available to all manufacturers Royalty free) I think the early autos had a similar set up. The 'problem' with the PCV System was that at low loads it drew a high air volume through the system, however little blow-by occurs (normally) at low loads, however at higher load, which blow by is more likely, less Air (gas) is drawn through the system. This set up also draws fresh air (as required) to the crank case via either the Oil Filler Cap (which has a filter in it) or via the Charcoal Canister (if fitted).

 

Later again, the SU Carb Company came up with a better System that vents in to the depression area of the Carb. At low loads, there is a lower air flow from here, however at higher loads, there is more air flow and this is more likely when blow by will occur. This set up also draws in fresh in the same way as the PCV System. IMO, this really is a good system.

 

Oil Separators (as such) are also part of the systems since the open pipe set up to supposedly prevent (reduce?) liquid Oil being drawn through these systems, and these do work well until spirited driving is done.

 

This is a twin carb set up of this later type

 

[dennismini93]

ahh thank you Moke Spider this was exactly what i was after

 

There is no evolution and no reason why with the companies that made the Mini.

 

They used to have the machined breather fitting on the transfer case but then it was cheaper not to machine it and just use the existing one on the tapet chest.

 

There was evolution as is described above as well as into the tuning methods people now adopt today. i just meant any evolution of the system not necessarily a production change.

 

it was on the later cars they moved from the tapet chest to the transfer case and surely just because it was cheaper wouldnt be a reason even austin would do?

[Wim Fournier]

I read this all. It's allright. But, one more thing.

When you start your enige on a cold morning you see 'white' smoke from the exhaust. That is condensing watervapour.

When it is not that cold, you don't see the white smoke, because the watervapour stays 'damp', does not condensate in the open air.

Thare is always (a lot of) water in the exhaust gas and acid.

The piston rings try to seal the piston against the cilinderwall. But 100% they don't get. So, there is 'blow by'. Exhaust gas with watervapour and acid in it going down into the crankcase.

When the engine is at the right operating temperature, the watervapour does not condensate.

As long as the operating temp is not reached, the vapour condensis against the coldest sufraces. For instance the inside of the rocker cover. You find 'mayonaise' inside.

So crankcase ventilation has the purpose to take the watervapour out of the crankcase and -with that- the other exhaust gases and acid.

With that there is a light vacuüm in the crankcase. As a result the oil does not come so easy through the seals on the crankshaft. The engine stays better dry on the outside.

Racing people put airfilters on the oil separators, because they want to suck in as much as possible air into the crankcase for cooling purposes. The pumping is done on a catchtank with oil separator and the fumes are not inhaled in the cilinders because of the bad burning caracteristics (octane is going down).

Edited by Wim Fournier, 10 December 2015 - 02:02 PM.

[1984mini25]

No matter how well the piston rings seal you will always get blow by witch then pressurises the crank case, the sump/gearbox in the case of a mini. If this pressures isn't vented it will lead to blowing oil out of the seals and even back into the cylinders and out through the exhaust. You could, as a test try completely blocking off the breathers and going for a drive, then noticing the huge cloud of blue some trailing behind you.

 

As stated above the reasons for the breathers to be fed back into the engine, is so the oil fumes and excess pressure can be released, drawn out and burnt off, rather than behind dumped all over the road. The only issue with that is the breather fumes completely mess with the fuel air ratio, fine on a road car but not on a race engine where every last bhp counts.

 

The main reason for the 1275s not having tapped chest covers is probably down to cost. With the extra machining, the separate tappet chest covers/breathers and that the tappet chest covers do have a habit of leaking oil on the 998.

 

Although your also wrong on assuming that the 998 only has one breather, it actually has two. As the standard rocker cover cap is also vented/filtered.

Edited by 1984mini25, 08 December 2015 - 08:54 PM.

[Spider]

 

The main reason for the 127's not having tapped chest covers is probably down to cost. With the extra machining, the separate tappet chest covers/breathers and that the tappet chest covers do have a habit of leaking oil on the 998.

 

 

Sorry for going OT here, but you have raised a good point.

 

I agree that but not having tappet covers would likely be much cheaper from a production point of view, as well as making the blocks stiffer and it's two less places for oil leaks. It's often had me curious when they developed the A+ range that they didn't do away with the covers on the small bore blocks, especially considering that they re-tooled to cast the new ribbed blocks that also had changes around the distributor drive as well.

 

Just going back to the two breathers on the 1275 engines, the Cooper S Engines only had one breather and that was from the tappet cover, however when the non-S blocks came about, they had no tappet covers and reading the old technical bulletins, it's clear BMC had Ring Sealing Problems around this time too, so at a guess I'd say the second breather was included at that time as a just in case. I'd also guess that it wasn't fully reviewed for am change at any time during production and / or they wanted to retain it rather than get a reputation (imagine that!) for having engines / cars that spat oil on the roadway after blowing out oil seals.

 

I really can't see any advantage to retaining two breathers from the engine. They 'breath' internally from a common and well flowing space (from the timing chain cover through to the transfer housing) and I know for certain that the series from 1977 through to 1981 (and likely later) there was a restrictor (P/N 12G3436) fitted at the Carb end that only had a 1/8" hole in it (so it wouldn't suck up liquid oil). Just looking at some info for this post, I see that the 1275 GT Engines from 69 to 71 only had the one breather on the Transfer Housing. I would agree that more is better than less, but only if plumbed the right way. I do run less, but only to de-clutter the engine bay and only after having found that I only need one.

 

If customising the breathers and running Oil Separators (IMO a good idea) some careful consideration is needed that the lead pipe to the Separator is roughly double or bigger, the size of the outlet, this is to ensure that the gas flow in to the separator is slower that what's been drawn out of it. This helps with not drawing liquid oil through the system.

[Spider]

Here's some more general info on engine breathers;-

 

http://www.engineact...case-breathers/

[nicklouse]

Here's some more general info on engine breathers;-
 
http://www.engineact...case-breathers/

They need a proof reader.

[Spider]

 

Here's some more general info on engine breathers;-
 
http://www.engineact...case-breathers/

They need a proof reader.

 

 

Yes, even I picked up on that!

[sixtyeight]

Sorry for the thread bump, less than a year old so should be ok! lots of good info in here also, no need to bring it all up again in a new post.

 

I'm running an 1100S 1275cc with only one breather on the transfer housing. @Moke Spider - is this what you have run previously? It's a mild tune, nothing over the top. Have driven it for years this way with no major oil leaks. Only concerned as I just started reading about the ventilation system and read that 1275s should have the timing cover vent also.. I prefer a tidy engine bay so with no leaks I'm happy to keep it this way...

[Spider]

Sorry for the thread bump, less than a year old so should be ok! lots of good info in here also, no need to bring it all up again in a new post.

 

I'm running an 1100S 1275cc with only one breather on the transfer housing. @Moke Spider - is this what you have run previously? It's a mild tune, nothing over the top. Have driven it for years this way with no major oil leaks. Only concerned as I just started reading about the ventilation system and read that 1275s should have the timing cover vent also.. I prefer a tidy engine bay so with no leaks I'm happy to keep it this way...

 

Tim, is that the advice someone suggested on your forum? Is it still running?

 

998's and 1100's - particularly the latter - are more prone to blow by, ring flutter etc and therefore would logically need more breathers than a 1275. Likewise, the Cooper S only had one breather.

 

The earlier 1275's did have an issue with poor Ring Performance / Sealing and that's why they mundane 1275's ended up with two breathers from quite early on, this was covered in one of the BMC Tech Bulletins, but, likewise the Ring issue was resolved about 1969 - 1970 off hand, but, for what ever reason, the two breathers have stayed.

 

No draw back I can see from running two breathers. I only run one, as that's all that's needed and the one down on the timing chain cover is just in a bad location, for other things!