Dick,
I believe your technique for calculating compression ratio is a bit
inaccurate. When my TF engine (XPEG) was torn down, the head volume,
compressed gasket volume and other volumes were carefully
measured/computed. The compression ratio was determined to be 9.6.
Compression readings averaged 175 psi. We live very close to sea level
(Seattle), where standard pressure is 14.7 psi. Using your technique,
dividing the former by the latter results in 11.9. This is significantly
higher than the actual 9.6.
I believe the problem with your method is that the change in temperature is
not considered. If you had the equipment to measure the temperatures when
air is compressed during a compression test, I suspect you would get a
pretty good figure.
>From basic physics, the formula here is:
PV = nrT where T is in Kelvin
Suppose that the starting temperature is T, and the final temperature is T'.
Then going through a little algebra shows that
compression ratio = (your calculation) X (T/T')
Ben
54 TF 1500
Message -----
From: Dick Dell <rddell@mindspring.com>
To: Bill Loubiere <loubiere@earthlink.net>; <mg-t@autox.team.net>
Sent: Thursday, August 31, 2000 10:04 AM
Subject: Re: Compression Ratio Calculation
> At 11:01 AM 08/31/2000 -0500, you wrote:
> >Is there a calculation which you can use the compression test data to
> >calculate the compression ratio on your car?
> >
>
>
> Bill
>
> Theoretically, you can calculate the actual compression ratio by simply
> taking a compression test reading with a basic compression tester.
>
> Atmospheric pressure at sea level is about 14.7 lbs/sq inch. This is known
> as 'one atmosphere'. If you live in higher altitudes (like Colorado, for
> instance) your normal atmospheric pressure might be 0.85 atmospheres or
> about 12.5 lbs/sq in. If you life in Colorado and you do a compression
test
> and it shows a cylinder pressure of 125 lbs, then you have a compression
> ratin of 10:1. If you live close to sea level and you have a 10:1
> compression ratio then your compression reading would be 147 lbs.
>
> If atmospheric pressure at your location is 13.5 lbs/sq in, and your
> compression test reads 115 lbs, then your compression ratio is about 8.5:1
> (which is 115 divided by 13.5) I have found this to be a pretty accurate
> means of finding your true compression ratio.
>
> If you need to find out what the true atmospheric pressure is at your
> location call your local airport weather service. They routinely give that
> information to pilots who need to check their gauges.
>
> I'm not a scientist, and my process might not be correct, but it's worked
> for me! If I'm wrong, someone please correct me.
>
> Dick Dell
> Raleigh NC
>
>
>
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