Compression Test Pressures
 

From time to time there are posts as to what expected compression tests pressures should be. (All the leak-down bigots can stop reading here.) For the rest of us a compression test is simple and actually measures what an engine does.

Here are the two websites that have the equations:

http://www.type2.com/library/engineg/comrat.htm

http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html

I've put together a small table with the expected pressures for different CR's and elevations. Remember too that the spread of values for a given engine is more important.

http://forums.pelicanparts.com/uploa...1220467332.jpg

Nice and handy, thanks !
Perfect timing, as I am going to check my compressions tonight.
I assume that external baromatric pressure should affect results as much as elevation.

Aurel

If you want to do it that accurately you can plug in the exact pressure in the formula in the first reference. I don't think that the formula is that accurate though.

I believe that cam timing, overlap/duration will change the results. Also if test is done warm or cold or wet/dry (wet=oil sprayed in cylinder)

Interesting article here: http://en.wikipedia.org/wiki/Compression_ratio

I believe the race scrutineers have a device to check compression ratio, but it is done by volume.

But enuf said, I apologize for the reply because I am admittedly a Leak-Down Bigot. But I'm trying, in a group that meets once a week, and the meds help.

I'm an elevation bigot. Your nice chart leaves out my part of the US.
For that matter a rather large part of the western US over 4,000 ft.
However we are somewhat used to being ignored. Thank you.
If we are ignored, we don't have to deal with folks moving here and changing our way of life.
Thank you for your efforts.
We highlanders can extrapolate the necessary numbers for our part of the world.
We'll also add in the effects of "the heat of compression" to get to a more accurate number.
It's the thin dry air, we just aren't understood by lowlanders.

interesting

- my 2.7 with 8.5 pistons and solex cams pumps 165 on 6 strokes at 2800" altitude

Yes it is interesting, either;

a) You've broken the laws of physics (Don't forget to apply for your Nobel Prize.)
b) Your gauge reads high
c) You have a higher CR than 8.5. If your engine has carbs there could be carbon buildup in the heads. This gives a higher CR but sadly not better performance.

Bruce Anderson writes in the 911 performance handbook-2 that a good 911 engine should show 130-170 psi - p.46

Wayne Dempsey's book gives a complete procedure for checking compression begining on p.14 and uses 150 psi and an example pressure