How does that work?
9:1 CR would be 9X15 PSI so ~135.
Then assuming that the intake valve closes after BDC you would have an even lower pressure.
What am I doing wrong with the math?
I just did a compression and leakdown test today (to see if my engine is in good enough shape to add a supercharger). I read this thread and saw the quoted question.
The issue with the math is that you have to take into account that the air is heating as it compresses, which raises the pressure more than just the ratio of volumes. Assuming that there isn't enough time to transfer significant heat from the heated vapor to the walls of the cylinder, then this is called adiabatic heating in physics lingo. You can read all about it in terms of engine compression ratios on Wikipedia:
The formula is: P = P0 * CR^gamma
where P0 is the pressure at the bottom of the stroke, P is the pressure at the top of the stroke, CR is the compression ratio, and gamma is the specific heat ratio (a.k.a., adiabatic index, heat capacity ratio) for the working fluid. For air, gamma is about 1.4
If valve timing, actual pressure at the bottom of the stroke (air can't flow in fast enough to reach 1 atmosphere by the bottom of the stroke in a normally aspirated engine), leakage, and heat transfer weren't an issue, and the pressure in the cylinder at the bottom of the stroke was 1 atmosphere (at sea level =14.7psi), and for the 11.5:1 compression ratio of a 2ZZ-GE engine, then at the top of the stroke, the pressure would be 449 psi. For 9:1, it would be 319 psi theoretical maximum.
But, all those other factors do come into play, as addertooth pointed out for valve timing, which is why we end up with less pressure than theoretically possible just based on 1 atmosphere and simple compression ratio.
For dynamic compression estimates, see the “Dynamic Compression Ratio” section of the above Wikipedia webpage. With dynamic compression ratio about 25% lower, and an effective specific heat ratio of 1.3 due to lost heat during compression, you get
P = 14.7 * ( (0.75*11.5)^1.3 ) = 242 psi
which is about what we see in the 11.5:1 2ZZ-GE engine.
For a 9:1 compression ratio, this computes to 176 psi.
(Keep in mind that the 25% and 1.3 derating values are just rough rule-of-thumb values, and it varies by engine type and modifications.)
So, my results for compression today were
Trial 1 (about 170 deg F engine temperature) 244 240 235 220 psi
Trial 2 (a little cooler) 242 235 229 220 psi
It was pretty close to these values three years ago using the same compression gauge. (By-the-way, I used a high accuracy gauge and an air compressor to calibrate my compression gauge and corrected the readings to the above values. It was just a straight 8 psi offset for calibration, regardless of pressure to the gauge.) I think these compression readings are fine (the Toyota manual simply says it should be >203 psi), but I will continue to keep watch on cylinder 4. I think these are pretty good for an engine with 125,000 miles on it.
I also did a leakdown test, and the values at 90 psi regulator pressure were:
5.9% 4.7% 5.9% 7.1% (and less percent leakage at lower regulator pressures), although the engine had cooled off to 100 deg F by then, so these numbers are higher than they would be if I had reheated the engine (I was too lazy to put the spark plugs back in and reheat the engine). But, I think they are still acceptable.
By-the-way, I use a leakdown tester with two pressure gauges like the OTC 5609
I strongly recommend against the type where the second gauge tries to produce a leakdown value. This second type is very error prone, and often only works properly, if at all, at low pressures. I threw away the one I had. You want to work at higher pressures - close to 100 psi if you can. The good kind does require you to compute a ratio of two numbers and subtract 1 to get the answer, but I think everyone here can do that.