Ian_D

Note: I edited this to be clearer after posts below. Where posts appear strange that is my fault not the posters.

I see lots of compression test results and discussion of values. However, I can find nothing on whether the pressures are absolute or above atmospheric (ie the increase in pressure seen).

Putting it another way, does the compression tester use the actual pressure for a chamber or the increase in pressure during the test?

eg does a result of 6.9 imply that the actual maximum pressure was 7.9 kgf/cm2 (absolute) or 6.9 kgf/cm2 (above atmospheric, which is the same as the increase in pressure)?

I assume that a tester uses the pressure above atmospheric (ie the increase in pressure). However, I want to be able to say "Mazda (or a reliable source) says here . . ." rather than "I think. . . ." where someone has used a pressure sensor to record chamber pressures over time.

Anyone have anything definitive?

ps. This is nothing to do with normalization (adjusting the values taken on test for RPM and elevation/height)

Update AM Sun 24 May: Lots of useful comments but contradictory thoughts on gauge/absolute. Please pitch in if you can help.

nycgps

iTrader: (1)

huh?

normalization ? I assume u mean the 250 rpm thing?

blckninja

iTrader: (1)

I think what he's referring to is something to do with altitude. Lower altitude will give you higher pressure, higher compression, whereas higher altitiude will give you lower pressure, lower compression.

IF i'm assuming right, scary thing I know, then compression tests are SUPPOSED, notice the emphasis, to normalize for sea level.

Hope this helps, if not please explain so us dumb americans can understand.

Loki

iTrader: (1)

I think he's asking if it's reading "pressure in combustion chamber" or "pressure in combustion chamber over the ambient atmospheric pressure". Absolute pressure vs. gauge reading with ambient as reference.

Educated guess more than fact, but it would have to be gauge pressure (so not absolute), but so are the compression targets that are used to evaluate the readings. So if the reader says 6.9, you're looking for 6.9 on whatever scale you're referencing. So it almost doesn't matter if its gauge or absolute for the purposes of comparing results to targets.

Ian_D

Guys, thanks.

I use normalization for the process of using the RPM & height to convert the pressures taken to equivalents at 250 rpm and sea level.

I'm referring to what the measured pressures are, not the height correction under normalization.

As another example, if there was no compression whatsoever would the compression test reading be 1.0 kgf/cm2 (absolute) or 0.0 kgf/cm2 (above atmospheric)?

Ian_D

Loki, thanks. Our posts crossed.

That's my educated guess too. However, I want to chant chapter and verse to counter someone presenting chamber pressures that are absolute as the pressures to use (and hence turning a poor compression reading into a reasonable one).

Loki

iTrader: (1)

If this someone is using the standard Mazda compression tester, then he should compare it to the standard Mazda compression targets. They're on the same scale.

yurcivicsux

i would imagine that if there was no compression, a value of zero, it would show zero on the test. just like it you were holding the tester into the air. the test would measure anything about or below a set 0. the rotary testers are digital and i would hope that it corrects for this before starting (setting the zero)

thats what i think.

poacherinthezoo

Since we have to correct for elevation (and RPM) - along with the fact that making separate compression testers for every possible altitude wouldn't be economically plausible - my best guess is that the Mazda gauges are calibrated for sea level compression, and therefore do not give the absolute value, but the relative compression.

nycgps

iTrader: (1)

oh, now I know what u mean

the tester doesn't know if u're at sea level or 3000 ft above. but it's not that hard to run the calculator on your computer to get u the actual value.

Ian_D

Guys, thanks.

However, my question is not about the height adjustment in normalization. It is about how a pressure figure is specified. I'll edit my original post to be clearer.

nycgps

iTrader: (1)

it just shows the pressure inside the chamber

Loki

iTrader: (1)

This is the crux of the question. In engineering you have concepts of gauge pressure and absolute pressure. If you have some pressure vessel here on earth, and you take a reading of its pressure, both the vessel and the instrument measuring the pressure are subject to ambient atmospheric pressure.

So if the gauge by itself, without the pressure vessel, reads 0, it's actually reading 14.7 psi ambient. So when you plug it into the pressure vessel and it reads, say, 100 (this is your reported gauge pressure), the actual total pressure inside the vessel is 100 + 14.7 (absolute pressure).

In thermodynamics you need to take stuff like that into account, and obviously if you're in the business of building pressure vessels. But the end user really shouldn't need to worry about it.

blckninja

iTrader: (1)

Ah I think I understand now what is happening. I would assume then the Mazda dealerships are using a digital tester with absolute pressure. In the article below it's a DIY link for compression testing. He compares his results to the ones he received at the dealership, and noticed that his results were lower, by about 10-14 psi. I'll post his results in case you don't wanna read it, first number is his testing, second number was from a dealership:

Housing #2 Leading:
Face #1 – 68psi vs. 82psi = 17% Low
Face #2 – 64psi vs. 79psi = 19% Low
Face #3 – 64psi vs. 78psi = 18% Low

Housing #2 Trailing:
Face #1 – 73psi vs. 84psi = 13% Low
Face #2 – 70psi vs. 82psi = 15% Low
Face #3 – 70psi vs. 81psi = 14% Low

https://www.rx8club.com/series-i-tec...9/#post4445340

dannobre

iTrader: (3)

Much more likely that result is due to inaccurate gauges

blckninja

iTrader: (1)

Another hypothesis, either is likely.

TeamRX8

iTrader: (25)

Yes, barometric air pressure varies regardless of altitude. For accuracy this is the variable factor that must be used rather than altitude.

Loki

iTrader: (1)

You mean "gauge" rather than "absolute" though, right?
Absolute requires no adjustment, it's with respect to absolute vacuum.

TeamRX8

iTrader: (25)

the identical compression ratio will "absolutely" generate a higher compression pressure reading with a higher atmospheric air pressure and vice versa

you are measuring pressure, not compression ratio

nycgps

iTrader: (1)

its absolute

Loki

iTrader: (1)

To the OP, without directly answering the question, maybe this will help get you to the same goal. At this link you'll find the mazda tech manual page on performing the compression test with the Mazda compression tester.

https://www.rx8club.com/attachments/...l-1%5B1%5D-jpg

According to the procedure you take the numbers the machine spits out and you compare them to targets directly, they're both on the same scale. We know the Mazda compression test results don't give an extra atm of pressure compared to other, more basic, compression testers (TR-01, for example), which we know don't measure ambient pressure, so it's pretty safe to say the readings are gauge. But it also means that your friend doesn't need to pad his results with an extra atm.

Ian_D

Guys, again, thanks.

The height of the test does make a significant difference to the values. Putting inputs of 100 psi and 250 RPM into the Foxed.ca calculator gives me normalized pressures of 102.68 at sea level and 107.99 (5.2% higher) at Phoenix (elevation 1160 ft). Pressures of the Standard Atmosphere are 14.6959 psi at sea level and 14.0903 psi (4.0% lower) at 1160 ft.

I don't want to go off topic but I think it's worth my pointing out that the Foxed.ca calculator is not wrong for converting an input of 100 psi into 102.68 psi as it needs to allow for the extra dead space when a spark plug is replaced by the sensor. These values imply a dead space adjustment factor of 102.68/100 = 1.0268. This value will vary with the sensor and so introduce variability in the output calculations.

TeamRX8

iTrader: (25)

Assuming all else being equal. If you know anything about the weather rarely are all things equal. It just depends how many RCHs you want to try and split trying to convince yourself that your engine does not need to be rebuilt, does not need to be rebuilt, does not need to be rebuilt, does not need ...

Ian_D

I can readily understand why there is an adjustment for height in the Foxed.ca calculator:

• We want to compare the compression ratio in our test in order to check an engine's compression (beware: do not compare test values with the engine's compression ratio; we are not correcting for such factors as the rise in temperature from compression)
• For our purposes, the pressures in the chambers are directly proportional to atmospheric pressure (aP) at the time of the test and other atmospheric factors can be ignored.
• A lower aP will give a lower compression value and a higher aP will give a higher compression value.
• The effect of aP is significant: an engine at Mazda's limit of 6.9 (kgf/cm2) on the Foxed.ca calculator could give 7.15 on good day at LA and 5.74 (!) on a bad day at Denver unless we normalized them for atmospheric pressure. My calculations are at the bottom.

Clearly the Mazda tester must correct for atmospheric pressure otherwise its results would be useless. I've just managed to find a copy of the user manual for Mazda's 49 H07 280 compression tester. The manual makes no reference to pressure correction; however, it does require the tester to be reset with the sensor disconnected and hence recording aP, suggesting that it then uses this aP in its later calculations.

I've therefore just bought myself a hiker's digital barometer & altimeter for $20 so that I can either adjust the recorded pressures directly or input the pressure altitude as the test height in the Foxed.ca calculator.

Calcs:

• The normal range of sea level atmospheric pressure is from 980 mb to 1050 mb (0.999 to 1.071 kgf/cm2; 14.21 to 15.23 psi).
• Height has a big effect on aP: for example Denver is over 5,000 ft amsl and has a standard atmospheric pressure of 843.3 mb, some 17% less than that at sea level.
• A lovely sunny day on the coast at LA could have an aP of 1050 mb and a bad day at Denver could have an aP of 800 mb. The standard atmosphere has these pressures at heights of -1,000 ft and 7,000 ft and a sea level pressure of 1013 mb.
• An engine at Mazda's limit of 6.9 (kgf/cm2) will show, all other things equal and assuming that Mazda's test is at sea level, values of 1050/1013 x 6.9 = 7.2 on that lovely day at LA and 800/1013 x 6.9 = 5.4 on that bad day at Denver. Using the Foxed.ca calculator without a height correction I need 95.5 psi to get 6.9 kgf/cm2 (98.1 psi), 95.5 x 1050/1013 gives me 101.64 psi (7.15 kgf/cm2) and 95.5 x 843.3/1013 gives me 81.63 psi (5.74 kgf/cm2).

Ian_D

I managed to get the attached copy of the manual for Mazda's compression tester 49 H07 280.

It looks as if it displays gauge pressure in operation, implying that test results are gauge pressures, and measures atmospheric pressure to use in normalization.

Its tolerances are +/-2% in pressure (+/- 0.15 on a typical test) and +/-2.5% in RPM, suggesting that a reading is accurate to within 0.15 (kgf/cm2) and readings for tests on the same engine could vary by over 0.3 (kgf/cm2).

I found its description of calibration somewhat confusing. It describes pressurizing the sensor to 5.0 kgf/cm2 and adjusting VR1 and VR2 to get 7.1-7.2 kgf/cm2 on the tester's display. I could only assume that VR1 and VR2 are hidden adjustment dials and that the tester's calculations use mean (average) pressures rather than actual pressures (pressure traces show a waveform close to sinusoidal where an output of 7.1 to 7.2 peak-to-peak has a mean value of 5.02 to 5.09 ).