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From: John De Armond
X-Source: The Hotrod Mailing list
Date: Aug 1992
Subject: Re: measuring power/performance

>is there an easy and accurate was to find the compression ratio of an engine?
>This sounds easy, but how do you get/find the final volume when the piston
>is at the top of its stroke?? is this really needed or can you calculate CR
>from a standard compression check?  Help!!

Easy and accurate tend to be mutually exclusive.  With the head on, one
can bring the piston to top dead center and use a burette to measure
the volume of the chamber by how much water/oil/etc it holds.  This
assumes the engine can be tilted so the plug hole is the highest point.
Head off, the chambers are CC'd conventionally using a plexiglass plate
across the chamber and a burette to fill it through a hole.  If there
is any popup on the piston (or dish, for that matter), the residual
volume of the block at TDC must be measured.  If the piston does not
extend above the deck, the same method as used on the head can be used.
If it does stick up, it gets more complicated.  The method I use is to
seal with wax or grease, a one piece ring compressor on the deck.
This device is a short cylinder the same diameter as the cylinder.  The
assembly is then filled from a burette until the liquid just reaches the
top of the deck.  This is the volume of the block part of the combustion
chamber.  Then fill the assembly until the top of the piston is just covered.
Note how much extra it takes.  Measure how tall the cylinder of fluid is above
the deck.  You can carefully prick the meniscus (sp) and then measure the
distance from the bottom of the ring compressor to the prick with a depth
mike.  Use this distance to calculate the volume of the cylinder you
just filled.  Subtract the actual added liquid.  What remains is
the volume of the part of the piston that extends into the combustion
chamber.  After you compute the head gasket volume, you can compute
the compression ratio.  Fun, huh?

IFF your engine has a low leakdown rate AND you can spin it
fairly rapidlya and you can pop the rockers off, AND you can
measure the pressure rapidly (pressure transducer and O scope),
you can actually measure the theoretical compression ratio.   The actual
compression ratio is simply calculated from the peak pressure recorded
on a compression gauge.  Note that with an ordinary compression gauge,
the checkvalve in the end requires several PSI to open so that will
skew the pressure.  The _effective_ compression ratio - that CR
that determines torque and when detonation, varies with engine speed,
ram tuning, exhaust performance and so on.

>One other question that is kind of a bonehead type question...what is it
>that defines a car as a "Z" car??  I have seen many posts about "Z" cars
>Is it just the fact that the manufacturer put a "Z" in the car's name??

Us Datsun 2x0Z afficionados owned the marque until the Z28 crowd stole
it :-)  (It's a joke, son, Ah say, it's a joke.)


From: John De Armond
X-Source: The Hotrod Mailing list
Date: Aug 1992
Subject: RE: Compression Ratio Stuff

>  You wrote something about computing compression ratio using a
>compression tester recently.  You said IF you can keep the valves
>closed and IF you can spin it fast enough etc etc.

>so when you say "spin it
>fast enough" do you mean fast enough that it can be considered
>to be instantaneous for the purposes of heat generation?

Not for heat generation but for heat loss to be considered neglegable.
Adiabatic compression as it's known as.  The second consideration is
to minimize leakdown.

>   When I compression test my Z, I get 210psi with the valves
>opening(for quite a while I might add) and all the other losses.
>Now, its not a 15:1 compression motor, its more like 11:1.  Is there
>some other cause of the increased pressure?  I had always assumed that
>the heat generation was the cause.

Are you speaking of a 240Z?  Never seen a street motor with that kind of
compression.  Is is modified?


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