Compression ratios and cranking pressure

[RangeR BoB <samurai@xxxxxxxxxxxx>]

All:

        One factor in determining the cranking pressure is the speed at
which
you are cranking.  At the speeds that the engine is cranking, I don't
think you will get a whole lot of inertia based charging.  Rather the
rise in pressure is the reverse case:  It is lack of proper scavenging
of the air OUT of the cylinder thru the exhaust valves.  

        Watch the gauge as you crank, you will see it rise quickly with
a few
cranks and then hold more or less steady.  That is because you are
trapping air in the cylinder and it is not entirely scavenged on the way
out.

        Exhaust works on pressure:  Hot gasses and piston pressure from
the
exhaust stroke.  With cold air in a compression test, at low speeds (a
few hundred revs) you can't get it all out, so it builds up.

        Compression checks are a relative measure, and are useful to
determine
if your engine is losing compression somewhere.  It is related to the
mechanical compression ratio (already described by another poster)but
all of the variables in intake and exhaust keep compression checks from
being a good measure of much else.

        As for ram air, that does use the principle of moving air to
pressurize
the intake.  It nets you 3% at 150 mph.  The real advantage of ram air
is it gets you cool air from outside of the car, which is why I have no
faith in cone filters that breath engine compartment air.  your losses
to hot air will far exceed any gains you get from a "less restrictive"
filter. I think most models of BMWs (correct me if I am wrong) have
decent amounts of filter area such that it doesn't form too much of a
restriction.  Perhaps there are some baffles and such that can be
removed, but I don't know if you can actually feel the difference.

        Your mileage (and airflow) may vary.
- -- 

 RangeR
   BoB
 Hembrook
 
 '89 BMW 750iL                   	"Sputnik"	92k
 '89 Suzuki Samurai 4x4          	"Sammy"   	94k
 '88 Kawasaki ZX-10					28k

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End of bmw-digest V9 #60
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