Compression For DOHC

Hi,

I wanted to know what everyone else is getting for their compression numbers
for the DOHC engine? I've got a '97 SC2 and the numbers are 141 and 142 PSI
for all four cylinders.

Saturn indicates the compression should be 172-205 PSI, this brings up the
next question. I live at 8,300 ft. so should I adjust my compression
numbers accordingly or do compression numbers stay static no matter where
you are? I read in one book to add 2 PSI for every 1,000 ft. of altitude.

Thanks,

Mike

 

I wanted to know what everyone else is getting for their compression numbers

When I had oil burning trouble, I had all cylinders but #1 at 200-205. #1 was
140.

I would think that your high altitude will alter compression. The air is less
dense so in each stroke of the piston it is compressing less air.

I don't know how much per 1000' feet, but since your numbers are all the same I
would say that you don't have any problems there.

Must be crazy living at 8300' !!

 

http://www.geospectra.net/kite/weather/h_altit.htm

gives a table of different atmospheric densities under standard atmospheric
conditions for different altitudes. I've reproduced it here, adding some
columms:

Altitude (ft) Pressure (mm hg) Temp (deg F) Density (%) Compensated
141 psi
Density Pressure
0 29.92 59.0 100.0 141.0 141.0
2,000 27.82 51.9 94.3 149.5 151.6
4,000 25.84 44.7 88.8 158.8 163.3
6,000 23.98 37.6 83.6 168.7 175.9
8,000 22.22 30.5 78.6 179.4 189.9
8,300 21.97 29.4 77.9 181.0 192.0
10,000 20.57 23.3 73.8 191.1 205.1
12,000 19.02 16.2 69.3 203.5 221.8
14,000 17.57 9.1 65.0 216.9 240.1
16,000 16.21 1.9 60.9 231.5 260.3


I also took the liberty of inserting an 8,300 ft point in the table by
linearly interpolating the 8,000 and the 10,000 ft points.

Short Response:

Your pressures are fine.

Long Response:

You have to compensate your pressures by (Sea Level Density)/(8,300 ft
density) in order to find out what your car would do at sea level. If you
do that, your 141 psi at 8,300 ft translates to 181 psi at sea level. This
takes into account both the different pressures and the different
temperatures.

If you don't want to try to take into account different temperatures, then
you merely compensate by (Sea Level Pressure)/(8,000 ft pressure). If you do
that, your 141 psi at 8,300 translates to 192 psi at sea level.

(The first case is presuming that you are testing with air at 29.4 degrees
and would be testing at sea level with 59 degree air. The second case is
presuming that you are testing in both places at 59 degrees).

Since the range is 172 to 205 at sea level, I think you're fine.

 

Great info! I'll take it that the numbers are good then.

Mike