Re: Compression questions

["I.Mannix" <mannix@privatei.com>]

At 6:33 PM -0600 05/28/1997, Jay Docherty wrote:
>> From:          trnsfrma@ix.netcom.com (Shawn C. Meze)
>> Subject:       Re: Compression questions
>> To:            scirocco-l@privatei.com
>
>> (My turn) Wrong. Changing from the 8-1 or 8.5 -1 CR pistons to the 10-1 CR
>>
>OK, sorry to interfear on the "nit picking" guys:), The compression
>ratio is two numbers representing the piston at its highest state and
>lowest state, correct?
>
>What excactly do the numbers mean though, does the "1" represent the
>lowest point of the piston and "8.5" (for example) represent the
>highest point.  Do they represent distences or pressures? therefor
>the 1 would be the bottom of the stoke (no pressure) and the "8.5"
>represent the amount of pressure at the top of the stroke?  If so,
>what messurement are the numbers in, psi?

Well, basically, a 10:1 engine is taking 10 units of air/fuel and
compressing it into the space one unit occupies, hence the phrase "Ten to
one."  Basically.  It is not possible, really, to extrapolate a pounds per
square inch value from a compression ratio.  Compression ratios are
determined by bore and stroke vs. combustion chamber.  B&S determines
volume, the amount of air/fuel, like a can of beer - if you took the bore
and stroke, or the diameter and height of a beer can, you could figure out
mathematically how much air is inside the can.  This is basically what bore
and stroke tells you, among other things.  Then, the volume of the
combustion chamber, where all the unburnt gases are stuffed pre-ignition,
is the other side of the equation.  The piston moves downward, sucking in
air/fuel, then back up, compressing it into the CC - 10 units of air into
the space of 1.  Basically.  So, to corrolate the CR with PSI is impossible
- an 8.5:1 engine is always 8.5:1, no matter how well the valves/seals are
holding the pressure.  8.5:1 is a mathematical figure, whereas the
PSI/compression test is a physical test - the engine is still taking in 8.5
units of air/fuel, still putting that air/fuel into the CC, but it does not
know if the a/f is rushing out past a burnt, cracked exhaust valve, or
leaking out past a ring.  Hope this makes sense, it is one of those things
I simply understand, but have never tried to put into words, although I
think I got it (basically) right & comprehendable.


Mannix(there's WAY more to it than that, but the basic jist is there)


>
>                                      Jay
>'80 Scirocco 1.8 8v
>
>Check out the Watercooled VW gallery...
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