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Thread: Engine breather hose - is it necessary?

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    Default Engine breather hose - is it necessary?

    The breather hose on my 2.25 petrol has perished and fallen off.

    Rather than feeding crankcase fumes into the inlet, is it acceptable to block off the inlet end, readjust the carb and put a small filter on the exit from the crankcase? I've seen small K&N style filters for this job.

    It strike me that the engine should be more efficient with just air and fuel going in rather than part burnt combustion products and oil vapour.

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    Forum Supporter. DarrenH's Avatar
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    Default Re: Engine breather hose - is it necessary?

    there are pros and cons of having the crank case sucked. you are right, some of whats coming out the breather is inert. in a small way that works like exhaust gas recirculation on modern engines, i.e dilutes the air fuel mixture with non flammable gas, which reduces combustion temps, and directly reduces poisonous nitrogen oxide exhaust emmisions

    on the flip side, having the crank case "sucked" helps reduce windage, which is mechanical drag on the crank from airborn oil particles. another benefit is that whats coming out of the crank case breather is noxious and corrosive in its own right, at best will give you nausia and stomach ache if it makes its way into the cabin

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    Default Re: Engine breather hose - is it necessary?

    Perhaps I'll just get some new hose then...

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    Default Re: Engine breather hose - is it necessary?

    I fitted one of those filters to mine, raised near the top of the engine and turned upside-down on the end of a hose to make sure water can't drip onto the filter and fill the sump over time.
    1970 Series IIA 2.25 petrol - Many things done, many more to do, even more waiting to leap out of the shadows and break.
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    Default Re: Engine breather hose - is it necessary?

    The gas vented through the breather is NOT incombustible - it is regular atmospheric air with oil vapour (so is quite combustible), with a trace of exhaust gas that has blown past the rings into the crank case.

    Venting it to the induction system is done because ventin it to atmosphere is illegal. Fitting the vent with filter will just end with an oil soaked filter and a mess. Just replace the pipe and keep the standard spec and you can be confident that the MoT emissions and engine performance should be as intended, and you won't need to waste money on filters or mess about with catch tanks.

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    Default Re: Engine breather hose - is it necessary?

    apologies if i have confused anyone, the tone of your reply is as if to correct me, but then you reworded what i said as being true.

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    Default Re: Engine breather hose - is it necessary?

    Venting to the inlet is only required on vehicles with emission control which started to come in in the mid 70's prior to this it was quite common just to vent to atmosphere, That said most of the breather components are sized to work together, so the best option is to fix what you have, As most of what comes out is exhaust gas it's not ideal to breathe it either.

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    Default Re: Engine breather hose - is it necessary?

    I guess the legalities of the issue depend on who/where you are. The early 2.25s simply let those fumes escape through a breather on the valve cover. Other engines of the period also allowed fumes from the crank case to exit via a road draft tube. There are advantages and disadvantages to these different systems but if the law requires you to keep the breather tube you'd best replace it.
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    Default Re: Engine breather hose - is it necessary?

    Quote Originally Posted by DarrenH View Post
    apologies if i have confused anyone, the tone of your reply is as if to correct me, but then you reworded what i said as being true.
    If you're referring to my post, I didn't reword or edit anything. The crank case gasses are combustible and are not comprised only or even mostly of exhaust gasses - air from the compression stroke gets past the rings just like the exhaust gasses during the combustion stroke. You also have the air from any time the dipstick, oil filler, rocker cover or any other access to the engine is opened. The breather is to allow for the adiabatic expansion of the gasses and oil and to allow those gasses, both combusted and clean, than get past the rings or valve guides. So, your comment that breather gasses are incombustible is untrue.

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    Default Re: Engine breather hose - is it necessary?

    please quote where i stated most or all of the crank case gases are incombustible.

    i said some of it is inert, you said a fraction of it is exhaust gas. six of one half a dozen of the other. (although thats entirely conjecture, since i doubt even the best rebuilt engine of this design has anywhere near 100% cylinder seal, and it certainly doesnt get better after 30 years worth of miles)
    i said youll get stomach ache and nausea from the stench as its poisonous and corrosive, you said its illegal to vent to atmosphere. (which is the reason its illegal) six of one, half a dozen of the other.

    one also needs to consider what the oil vapour is, because making bits of sharp metal more slippery is not engine oils only function. it also does the lions share of engine cooling, front line, but more importantly here its the actual media for cleaning and suspending the abbrasive, caustic, poisonous byproducts of the combustion process itself, from the cylinder walls.

    bottom line is, its diluting the air/fuel mixture. but my advice to the thread starter doesnt change whatever the science is. i'd leave it plumbed in.

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