Compression testing

[Gabriel]

1984 200ES

The book says 156 psi

It also says the compression ratio is 7.8:1

That doesn't add up so what am I missing?

Typical atmospheric pressure is 14.7 psi which would be a 1:1.
7.8 multiplied by 14.7 equals 114.66psi
156 psi would be about 10.5:1

So what am I missing?
I just picked up another 200Es and it won't run. It has some spark issues but the compression test reads 100PSI dead cold. That would be 6.8:1 and it would climb once oil circulates and everything expands. I know I need to rebuild it and I will eventually but I'd like to get it running and square away some other issues first. It has been sitting the better part of a decade. Anyway.

What am I failing to understand here? Cause if 156psi equals 10.5:1 then why does the 'high compression' pistons from Wiseco say 10.5:1?

[350for350]

I don't know how to explain it, but the 156 PSI would probably be 7.8:1. When you did the compression test on your new 200ES, did you hold the throttle open? You need to do this in order to get an accurate reading. Just for fun, I did this a while back on a YTM225DR I had. with ho throttle, it tested at 105PSI. I got 165 PSI by holding the throttle open. Big difference.

[Gabriel]

I guess the whole point is moot, I just got it running. I let it run for a minute or two. I couldn't see my neighbor's house. Seriously. It was bad!
Top end will HAVE to be gone through.

[sledcrazyinCT]

Drain the oil and see if it is milky. I have had some condensation/water in oil from a machine that sat forever and as soon as I changed the oil smoke goes away.

[El Camexican]

Google the difference between compression and cylinder pressure (might want to take an Advil first)

There’s static compression and dynamic compression. On a four stroke your piston might might have an advertised static ratio of 12 to 1, but a big cam with lots of overlap can bleed off enough pressure that it will run on pump gas.

Another thing to keep in mind is that your compression gauge is almost as unique as a fingerprint in that you won’t always get exactly the same reading from two different ones on the same engine. A 2 to 5 psi difference between two gauges that cost under $30 bucks wouldn’t surprise me.

[wonderboy]

Here is a link:
https://en.wikipedia.org/wiki/Compression_ratio#Compression_ratio_versus_overall _pressure_ratio

Even if you don't read the words, there is a graph and table that explains it. The key is that compression ratio is a comparison of the volume in the cylinder. Volume changes do not correlate 1:1 with pressure changes.

There is an exponent that depends on the medium (air in this case).

Here is the theoretical table (sorry, formatting doesn't paste well):
Compression ratio 2:1 3:1 5:1 10:1 15:1 20:1 25:1 35:1
Pressure ratio 2.64:1 4.66:1 9.52:1 25.12:1 44.31:1 66.29:1 90.60:1 145.11:1

At 7.8:1 compression ratio, your pressure ratio (theoretical ideal) is 17.7. So your 14.7 PSI atmospheric pressure would give 260 PSI at TDC.

Some reasons you don't get this high of pressure:
-The valves are not closed at the instant the piston is at BDC. There is time when the piston is still moving up with valves open, so you aren't capturing all the air indicated by compression ratio (compares TDC to BDC)
- Rings are not perfect and leak
- Throttle and intake provide restriction preventing perfect atmospheric pressure in the cylinder at BDC
- Thermal transfer into or out of the cylinder. The pressure ratio assumes no heat is added or taken away. Air will get hot when you compress it. The ideal theory says that nothing happens to alter this natural increase in temperature.

[Gabriel]

Cameixan is right, anyone got an advil...or some vicodin.