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Re: engine breathing modification

To: "Evangelos G. Makris" <egmakris@otenet.gr>, <mgs@autox.team.net>
Subject: Re: engine breathing modification
From: Barney Gaylord <barneymg@ntsource.com>
Date: Thu, 09 May 2002 16:06:02 -0500
At 11:03 PM 5/9/02 +0300, Evangelos G. Makris wrote:
>Consider this early B-series engine breathing system: non-vented oil cap, 
>a hose linking the rocker cover to the air filter and a metal tube from 
>the rear chest cover pointing downwards.

Same as my MGA

>Now, consider this simple modification:
>a. throw away the old air filter and put a pancake-type in its place
>b. use a long rubber hose from the rocker cover downwards, alongside the 
>chest cover metal tube

I wouldn't recommend that (with heavy emphasis).

>Will preformance improve at all?

No, unless the piston rings are in such bad shape as to be allowing lots of 
combustion bypass to blow oil out of the crankcase into the air filter, in 
which case it needs an overhaul anyway.

>Breathing?

No improvement, except to the extent that it may avoid clogging the air 
filter an a badly worn engine.

>Emissions measured at the exhaust tailpipe?

For an engine in good condition, no.  For an engine in sad condition, 
maybe   But any improvement in the bad engine will still leave it in far 
worse state than optimum.  Engines with this type of vent system are pre 
1968 design and were not subject to any mandated emissions standards (in 
the USA).

The first emisions standards here were applied to the 1968 models, which 
usually resulted in the addition of an air pump and a sealed 
crankcase.  After that the crankcase vent outlet was connected to the 
intake manifold to intentionally route the crankcase emissions through the 
engine to be burned with the normal combustion.

>I've seen it done and the owner told me his car pulled a little bit better 
>and gave lower emissions readings.

That engine is likely in bad condition, and either improvement is probably 
minimal.  Higher emissions at the tailpipe would be caused by crankcase 
emissions getting into the carburetor intake, which is not supposed to be 
happening with an engine in good condition.  An engine overhaul would 
likely improve both power and emissions readings considerably more.  But if 
you're on a shoe string budget and not going to pay for the engine repair, 
then maybe keeping the oil out of the air cleaner would be of some benefit 
to performance and/or emissions (but still not good for the engine).

>I'm not sure if the oil cap should remain a non-vented item or be replaced 
>with a vented type though...

In the case of a badly worn engine, a vented oil cap would vent outwards 
and dump oil on top of the valve cover.  For an engine in good condition a 
vented cap would allow unfiltered air into the crankcase.

To get a handle on your choice, you should understand how the original (pre 
1968) vent system is supposed to work.  The hose connection on the air 
cleaner is inside of the filter element.  The metal tube extending downward 
from the tappet cover terminates at the bottom of the chassis and has a 
angled cut at the bottom end of the pipe.  This is properly called a "draft 
tube".  When exposed to the air moving underneath the car there is a slight 
vacuum created in the draft tube which draws air through the crankcase to 
ventilate it.  The incoming air is fiiltered at the air cleaner, and 
nothing should ever pass back in the other direction from the crankcase to 
the air cleaner.

This "forced" venting of the crankcase removes combustion bypass vapors to 
reduce contamination of the engine oil and build up of sludge in the oil 
and oil pan and to reduce solid deposits on the parts inside of the engine, 
all of which reduces engine wear.  This service includes helping to keep 
the valve gear on top of the cylinder head clean.  If you were to re-route 
the valve cover vent hose to a point near the bottom termination of the 
draft tube you would defeat/disable most of the intended forced ventilation 
of the crankcase.  To the extent that some ventilating flow might still 
occur, the unfiltered air entering the crankcase would be introducing 
contaminants, like lots of dirt being thrown up by the tires.

Now understanding how the vent system works, you can figure out the rest of 
the consequences of modifying it.

Barney Gaylord
1958 MGA with an attitude (and all original crankcase vent system)
http://www.ntsource.com/~barneymg

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