Theo,
I'm surprised no one has questioned your formula. It raises the question of
what is really the definition of compression ratio. Of course, you can't
fool Mother Nature, and with her in mind I would agree that your definition
makes more sense. However, the rest of us like things a little simpler and
I believe the following definition is what is generally used. I quote from
the following source:
http://www.centralpaporsche.org/tech_sch/compress.htm
"Compression Ratio: The ratio is the product of the maximum displacement of
a cylinder (piston at bottom dead center) divided by the minimum
displacement (piston at top dead center). The compression ration is a
comparison of volumes during two different stages during the combustion
cycle. This ratio describes how much the air fuel mixture gets compressed
before ignition. Therefore, the higher the ratio the greater the
compression and the greater the explosion in the cylinder which yields more
power. As a result, high performance engines use very high compression
ratios to extract the maximum power possible. For a street engine the
average ratio today is about 10 or 11 to 1. However, in a race engine, it
is not unusual to use ratios as high as 14 to l. (On a side note, Diesel
Engines use ratios as high as 22 to l because the high compression is what
actually ignites the air/diesel fuel mixture which spontaneously ignites
when compressed enough."
Now, of course, to calculate pressures you need to use the total volume,
not swept volume. Now, do you agree, or is this just too early in the
morning for me to be thinking?
Here's a couple of other interesting links I stumbled on while
"researching" this topic. BTW, it's amazing how much wrong info is out
there too!!
http://www.baylor.edu/~rafdc/compression_ratio.html
http://www.comp.glam.ac.uk/~Minis/miniweb/gasoline.html
At 02:49 PM 1/21/00 -0700, Theo Smit wrote:
>Let's see...
>
>Assume it's a 302 with 11:1 compression. That makes the chamber volume,
>including all domes, head gaskets, valves, spark plugs, etc.
> ( 302 / (11-1) ) / 8 = 3.775 cid = 61.86 cc
>
>So going from 54 to 64 cc heads changes the chamber volume to 71.86 cc =
>4.385 cid
>The new CR is 1 + 302 / (4.385*8) = 9.608
>
>If it's a 289 with 11:1 the chamber volume is 59.198 cc
>Go from 54 to 64 cc and the chamber volume increases to 69.198 cc = 4.223
>cid
>The new CR is 1 + 289 / (4.223*8) = 9.554
>
>So the CR change depends on the displacement (but we knew that, right?)
>
>Theo
>
> > -----Original Message-----
> > From: Steve Laifman [SMTP:Laifman@Flash.Net]
> > Sent: Friday, January 21, 2000 6:16 AM
> > To: Alex MacLaurin
> > Cc: Ronak, TP (Timothy); 'Bob Palmer'; 'Tiger News Group List'
> > Subject: Re: Performance Tuning the Tiger
> >
> > Alex MacLaurin wrote:
> >
> > > With all this talk about compression,I have a question: if a motor has
> > 11:1
> > > compession with a 54cc chamber and all variables stay the same, how much
> > > compression does the engine have with a 64cc chamber?
> >
> > Assuming your numbers are accurate:
> >
> > 54 cc. x 11 = 594 original volume
> > 594 / 64 = 9.28:1
> >
> > Compression Ratio = Final Volume / Swept Volume
> >
> > I've assumed that the deck height and gasket thickness were contained in
> > your
> > "chamber" volume, but they usually are not, and need to be known for an
> > exactly
> > correct answer, but this one is close enough for 'Government Work"
> >
> > Steve
> >
> > Steve Laifman < Find out what is most >
> > B9472289 < important in your life >
> > < and don't let it get away!>
> >
> > _/_/_/_/_/_/_/
> > _/ _/_/_/ _/_/_/ _/
> > _/ _/ _/ _/ _/ _/ _/_/_/_/
> > _/ _/ _/ _/ _/ _/ _/
> > _/_/_/_/_/__/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
> > _/
> > _/_/_/
> >
Robert L. Palmer
UCSD, Dept. of AMES
619-822-1037 (o)
760-599-9927 (h)
rpalmer@ucsd.edu
rpalmer@cts.com
|