Compression test numbers.

I ran a compression and leakdown test on the boxer. The numbers were all consistant within 10psi ( 145-155psi ). The leakdown test showed it to be nice and tight. All less than 5%.

From the the old rule of thumb of 17.5psi/compression point, I get an average value of 154psi ( 8.8 * 17.5psi = 154psi ) for the stock pistons/ratio. Which jives with the numbers I got.

But, ... this test was done at 2200ft altitude and the temperature was 109F ( Las Vegas in August ) Density altitude of about 5000' MSL that day. Which gives a relative density of 86.2%. So, the average value of 152psi corrects to 174psi at sea-level.

Which would seem high for an engine which is supposed to have an 8.8:1 compression ratio. These numbers SEEM to indicate that the pistons are in the 9.5 - 10.25 to 1 range. I am suspicious because it calculates out to almost exactly 10:1.

Or, am I just over complicating this ?

Anyone know the 'good' stock numbers ?

Thanks

 

Thanks for the info, David.

Your numbers square exactly with what I would expect for a stock motor with a, 8.8:1 ratio at 600ft MSL.


[ WARNING! more over-thinking follows. Includes math! I can't help it. ]

I find very little written about compression tests and altitude. There is a lot on dynomometer correction info though. I was digging through SAE J1349 to get the formulas. All I could find is that compression numbers are usually quoted for standard conditions, 623ft MSL, in Detroit, Michigan. The baseline in J1349.

I found this calculator online: http://wahiduddin.net/calc/calc_hp_abs.htm

I plug in the test day's numbers of 109F, 27.9inHg, 7% humididy and I get a density altitude of 5523ft MSL and a relative air density of 84.8%. Given that, my numbers should have been around 125-130psi

There does not seem to be any compensation mechanism in the compression gages. Just a regular PSIG gage and a check-valve. So, its reading SHOULD follow the relative air density. I don't remember, off the top of my head, the rules for heat-capacity of, gasses though.


I found this on the ASA site: http://www.asashop.org/autoinc/feb2003/techtotech.htm

There are some variables that affect the readings obtained from compression testing. They are cranking speed, altitude, temperature, worn camshaft lobes and high-performance, long-duration profile camshafts. The cranking speed needs to be maintained the same for each cylinder. This may mean jumping your battery to maintain the speed. There are factors to compensate for the different altitudes and the corresponding temperature differences.

These are as follows:

1,000 feet = .9711,
2,000 feet = .9428,
3,000 feet = .9151,
4,000 feet = .8881,
5,000 feet = .8617,
6,000 feet = .8359,
7,000 feet = .8106,
8,000 feet = .7860.

The equivalent compression reading for a cylinder that should be 135 psi by the data at 5,000 feet would be 135 x .8617 = 116.33.

These numbers SEEM to correspond directly with the air density ratio for a given density altitude.


My brain makes me do these things....

 

You think this is bad ?

Try electrodynamics

 

I am aware of the physics of dynamic compression, valve phasing, etc.

But, at cranking speeds, the camshaft/valves could only cause lower numbers than the ones predicted.

If the numbers were lower than expected, I would look into the valve timing, cam specs, etc.

But, my numbers are higher than expected ( for the conditions ). And at the few hundred RPM at cranking, this would appear to require a higher mechanical compression ratio than stock. The port velocity during test is too low to allow for any significant inertial packing of the cylinders.

The comparison with David's car ( when it was stock ) verified that the 365bb engine falls inline with the 'rule of thumb' numbers. It's my only known good datapoint

 

Hi, steve.

Yeah, I double-checked with a different gage, just to be sure. A Snap-on and a cheap Harbor Feight POS. They were within 2psi. I think I paid about $150 for the Snap-on and $9.95 for the Harbor frieght unit. Go figure ... I have a Fluke 718. But that's only good up to 100psi


Paul.

You seem to be missing my point about cam-timing at very low RPM. All of the things you mention can only cause a decrease in readings from the theotetical maximum. I have been timing cams for 30 years. A late closing event on thein take WILL cause a lower reading. But, without inertial forces provided by the velocity of the charge, you can not get a reading higher than isotropic at 100% volumetric efficiency.

The prediction made at isotropic ( adibatic ) compression at 100% volumetric efficiency represents the theoretical maximum for a given mechanical compression ratio and ambient conditions. All of the other parameters ( cam timing, leakage, weak battery, etc ) only cause a reduction in the test reading from the maximum.

It is true that when you have a low reading of say 80%, it it not possible to tell directly what the contributing factors are. And becuase of these factors, it is not possible to reverse the process and convert compression test numbers directly into mechanical compression numbers.

But, what you CAN do is calculate the MINUMUM mechanical compression ratio to support a given set of numbers. That is, even given a theoretically perfect engine, the readings will be exactly as predicted by isotropic compression. If you see an engine with a mechanical compresion ratio of 8:1 producing compression test numbers of 180-200psi at 3000ft density altitude, somethign is up.


For example a warm engine at 190F at near the SAE standard conditions 14psi and 60F with an 8.8:1 compression ratio, 365cc swept volume.
At low speeds and without combustion, t2 approaches the engine temperature. So...

p1 x v1 p2 x v2 14psi x 411.8cc p2 x 46.8cc
------- = ------- gives ------------------ = ----------------- ... p2 = 153.98psi
t1 t2 460F + 60F 460F + 190F


can be simplified to

14psi x 8.8 153.98psi
---------- = -------------
460F + 60F 460F + 190F

For those who dont like mixing units




Working this backwards, you get

(460+60F) x 153.98psi ~
CR = ------------------------ = 8.8
(460+190F) x 14psi

So, 8.8 as the minimum compression ratio that can produce these numbers under these conditions.




my conditions gave


(460F+109F) x 152psi
CR = ------------------------ = 9.71
(460F+190F) x 13.7psi


Assuming losses of the type you describe, that 9.71:1 number would have to go even higher to yeild the same psi readings.

In fact, is you allow the average leakdown number of 3% to represent those losses, you get almost exaclty 10:1

 

Wow. The editor really mangled Charles' law.

 

The whistle tester is likely just a whistle inline with a spring relief valve set to open at the required pressure. Probably with a bi-metal spring for temperature compensation. They can calculate the pressure from the equations above.

The elecrtonic one likely has both temperature and ambient pressure sensors so that it can directly calculate the density. Maybe even a compressed gas temperature sensor. With that, they can tell what the minimum CR needed to produce the reading is. So they can say You are 9.2:1 or over. But they can't tell you by how much you are over becuase they can not measure the pumping losses.

The calculation is made by Charles' Law which I tried to quote earlier. ( I guess the form I used is closer to Avagadro's law with the molar masses removed. ) But it was rendered unreadable.


http://en.wikipedia.org/wiki/Avogadro%27s_law

There is also the matter of heat capacity ratios. But it ends up being insignificant in the final readings.