Compression Testers and Schrader Valve Location
6

Question

Compression Testers and Schrader Valve Location

tc7 (Mechanical)

(OP)

6 Jan 22 22:55

This question regards engine compression testers and the effect Schrader valve location has on accuracy of compression readings.

Background: typical automotive shop style compression testers consist of a pressure gauge attached to a flexible tube (typical ~16-inches long) with an adapter at its opposite end which threads into the spark plug hole. Higher quality gauge brands such as Snap-on or MAC have a Schrader valve located in the adapter end of the tube closest to the spark plug hole while other lower cost models have the valve located at the opposite end of the tube, adjacent to the gauge.
Does this difference of valve position make any difference in the accuracy of compression readings?

Some professional mechanics would argue that if the Schrader valve is located at the gauge, the entire volume of the 16-inch long adapter tube adds to the combustion chamber volume and would reduce the resulting compression readings (for the sake of this question let's say that simple compression ratio of an engine is based on the relationship, CR = (Vd + Vc)/Vc where Vd = displacement volume, the volume inside the cylinder displaced by the piston from the beginning of the compression stroke to the end of the stroke and Vc = clearance volume, the volume of the space in the cylinder left at the end of the compression stroke.) So any volume added by the flexible tube connected at the spark pug hole would accrue to the Vc term and somewhat diminish the overall CR and reduce gauge readings accordingly; this would be much more pronounced on small 2-stroke engines.

Consequently they (i.e., mechanics and techs) declare that when the Schrader valve is located at the spark plug end of the tube, no additional volume is added to the clearance volume and compression readings will be most accurate. This makes no sense to me. My contention is that when the engine is cranked over and pressure builds sufficiently to lift the Schrader valve, (at ~3-4psi) then the pressure pathway is open to the flexible tube and the tube volume then necessarily becomes part of the clearance volume just the same as mentioned above, so there should be NO difference in gauge readings regardless of where the Schrader valve is located.

Hoping for truth-tellers to offer expert opinions, technical dissertations and/or criticisms. All input gladly invited and thank you very much.

Compositepro (Chemical) · Answer 1 · 2022-01-06 23:21:40

I would say that any compression tester that uses a Schroeder valve as a check valve will not be very accurate since the cracking pressure is not low or accurate. And the location will affect the reading. However, as a diagnostic test, the uniformity of the readings is more important than the actual reading.

3DDave (Aerospace) · Answer 2 · 2022-01-07 01:05:34

Compression testers use multiple cycles of the engine to pump air through a check valve to the pressure gauge. The small volume only changes the amount of time by a small amount.

I am a bit dubious that a Shrader valve is used. They are heavily spring energized which is why every tire filling fitting I've ever used includes a plunger to force the valve open to let air in.

GregLocock (Automotive) · Answer 3 · 2022-01-07 01:13:02

The first few posts in https://piclist.mit.narkive.com/JPp7NA9M/tech-how-... is relevant, they certainly aren't precision valves. The force has to be quite high because otherwise your tires go flat on rough roads, which is bit of a downer. I have had a Schrader rep claim that the dust cap is the actual primary seal, a claim that is convenient for him but rather ignores actual customer usage. The machine that assembles them runs at machine gun type speeds, quite amazing.

Cheers

Greg Locock

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3DDave (Aerospace) · Answer 4 · 2022-01-07 01:35:24

Refrigerant valve caps are the primary seal - they are metal and have an o-ring. Not so much for tires.

tc7 (Mechanical) · Answer 5 · 2022-01-07 01:37:45

Hello Dave & Greg-
The Schrader valves used on compression tester have a much lower lift pressure than a tire valve, something on the order of ~4psi or less.

Thanks for your reply CompositePro -
I realize the gauge pressure reading will (ideally) be less than the actual pressure by an amount equal to the cracking pressure of the valve and yes these type of gauges are most useful for comparative basis'.
But you said, "... And the location will affect the reading... " and this is the very point of my inquiry here. I tried to make the case that location shouldn't matter. Can you explain why location of the Schrader valve will affect the reading ?

TugboatEng (Marine/Ocean) · Answer 6 · 2022-01-07 04:43:04

Why is a precise compression reading important? It's a diagnostic tool to identify a bad cylinder. It only needs to show imbalance.

gruntguru (Mechanical) · Answer 7 · 2022-01-07 05:12:30

The location of the check valve will affect the reading. With a hose between the engine and the chck valve, extra clearance volume is added to the engine, in effect reducing the compression ratio.

Accuracy of the reading will be nimportant if the test result is to be compared to the limits publised for some engines.

je suis charlie

3DDave (Aerospace) · Answer 8 · 2022-01-07 10:07:23

At least that extra volume can be accounted for - unlike the volume lost to the inserted body of the spark plug vs the air plug from the tester.

tc7 (Mechanical) · Answer 9 · 2022-01-07 10:13:08

Thanks gruntguru but what you are saying is the same current wisdom that I am questioning, i.e. that a hose between cylinder and check valve adds to the clearance volume. But my claim is that with a check valve on the engine side of the setup, as soon as the valve opens (at a very low pressure) the hose then also becomes part of the clearance volume and will also reduce the compression ratio.

3DDave (Aerospace) · Answer 10 · 2022-01-07 13:24:49

The other side of the valve integrates to the peak pressure. For example - if the gauge was a 10 gallon air tank it might take a while to pump it up but it would eventually reach the peak cylinder pressure minus the cracking pressure. If that did not happen one would always require a pump with a greater displacement than the item being pressurized.

Tmoose (Mechanical) · Answer 11 · 2022-01-07 13:31:10

Several decades ago my rubber cone tipped compression tester's valve failed, and since they look the same I replaced it with a tire schrader valve.
The test readings instantly became shockingly low.
A new genuine proper compression tester valve(package of 2 or 3 fromn NAPA) restored useful function.
I recall when tested by hand (finger) the comp tester valve spring was very weak.

tc7 (Mechanical) · Answer 12 · 2022-01-07 16:08:15

Yes 3DDave, "...other side of the valve integrates to the peak pressure...", that is a very eloquent way to describe the pressure buildup with each piston stroke and it shouldn't matter where the valve is located.

Compositepro (Chemical) · Answer 13 · 2022-01-07 16:23:04

The peak compression pressure, or compression ratio, of any piston in a cylinder depends very much on the residual volume between the piston and the exhaust valve. More volume results in a lower peak pressure reading. Multiple strokes does not change this fact. Multiple strokes does in increase the pressure after the valve, so that it will eventually reach the peak pressure before the valve.

To make this obvious, imagine if the hose had a volume of one liter. What results would you expect?

Compositepro (Chemical) · Answer 14 · 2022-01-07 16:28:47

Dave, you did not think through your example of the 10 gallon tank. What happens when the 10 gallon tank is between the cylinder and the valve? You will never reach a pressure high enough to open the valve, and the gauge will read zero no mater how many strokes there are.

3DDave (Aerospace) · Answer 15 · 2022-01-07 17:29:14

Unless that gage is between the engine and the valve, then your question is moot. Not sure why you chose to misrepresent what I wrote.

Compositepro (Chemical) · Answer 16 · 2022-01-07 17:42:04

Dave reread the OP's question. Your response did not correctly answer the question. The location of the valve on the hose does change the gauge reading.

tc7 (Mechanical) · Answer 17 · 2022-01-07 17:56:20

Hey CompositePro-
In your extreme example of a one liter hose between the cylinder and valve, I can see that the valve may struggle to crack open especially when the cylinder volume is small. So, yeah I'm beginning to accept that in actual practice, valve location just might make a difference !!

3DDave (Aerospace) · Answer 18 · 2022-01-07 18:15:20

Composite - you are just skipping the rest. The original question was modified as "But my claim is that with a check valve on the engine side of the setup, as soon as the valve opens (at a very low pressure) the hose then also becomes part of the clearance volume and will also reduce the compression ratio."

Does that context fix it for you?

SwinnyGG (Mechanical) · Answer 19 · 2022-01-07 21:40:00

Quote (tc7)
Can you explain why location of the Schrader valve will affect the reading?

It definitely does, and the explanation is easy.

It's true that whenever the schrader valve is open, the entire tube volume is being compressed.

What's missing in the thought process above is that the cylinder volume, and anything exposed to the cylinder volume, returns back to ambient pressure during each cycle. Anything on the other side of the schrader valve never returns back to ambient pressure.

Near the end of the test with the valve on the chamber end of the hose, you are cracking the valve against the little column of air inside the tube which is already at 150 (or whatever) psi. When that valve opens, the volume behind it doesn't create an artificial drop in compression ratio because it's already pressurized.

If the valve is at the gauge end, and the whole length of the hose is part of the compression volume, that pressure is allowed to bleed during every engine exhaust cycle and your pressure reading will be lower.

Answer 20

[Post Deleted]

tc7 (Mechanical) · Answer 21 · 2022-01-07 23:45:07

Very nice explanation SwinneyGG, thank you for posting.

gruntguru (Mechanical) · Answer 22 · 2022-01-08 02:21:27

Positive displacement compressors are designed for clearance volume as close as possible to zero. Any compressed gas in the clearance space at TDC will expand on the intake stroke effectively delaying start of intake until expanded to intake pressure.

je suis charlie

3DDave (Aerospace) · Answer 23 · 2022-01-08 02:50:57

Each cylinder of a 2 liter 4 cylinder is about 500 ccs. A hose ID of ID 3 mm X 500mm is 3.5ccs; added to the original 500 ccs the resulting ratio of running compression to measured pressure is 503/500, less than a 1% error due to the tubing when the valve is at the far end of the tubing from the plug port. A one meter hose takes that to slightly more than 1%.

This only applies to conventional valve timing engines, not those with delayed intake valve closing.

GregLocock (Automotive) · Answer 24 · 2022-01-08 02:52:16

Bear in mind that the hose is not rigid.

Cheers

Greg Locock

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3DDave (Aerospace) · Answer 25 · 2022-01-08 02:54:19

Yeah, you can practically blow them up like a balloon animal. /s

SwinnyGG (Mechanical) · Answer 26 · 2022-01-08 10:00:07

Quote (3DDave)
Each cylinder of a 2 liter 4 cylinder is about 500 ccs. A hose ID of ID 3 mm X 500mm is 3.5ccs; added to the original 500 ccs the resulting ratio of running compression to measured pressure is 503/500

It's more complex than that.

Typical combustion chamber volume for a 2.0 liter 4 cylinder is 50cc or so.

Assume a perfect flat top piston to make the math easy. For a 500c displacement cylinder:

CR = (500+50)/50

CR = 11:1

Now add the hose:

CR = (500+53.5)/53.5

CR = 10.3:1

7 tenths of a point in compression ratio is a huge change.

11*14.7 = 161.7 psi

10.3*14.7 = 151.4 psi

10 psi is significant.

Compositepro (Chemical) · Answer 27 · 2022-01-08 20:17:34

The 14.7 in the above calculation is in units of absolute pressure, psia. So the answer is in psia.

The pressure gauge reads in psig so the gauge readings will actually be 147 psig and 136.7, respectively. Details are important in engineering. Like, upon which end of the hose should the valve be placed?

SwinnyGG (Mechanical) · Answer 28 · 2022-01-08 21:17:38

Details are important... like how I never specified that those calculated pressures were gauge readings. smile

Anyway I think we all get the point by now.

3DDave (Aerospace) · Answer 29 · 2022-01-08 23:18:24

Sure - do what I originally thought but then f'd it up. /s

gruntguru (Mechanical) · Answer 30 · 2022-01-11 08:12:56

Wow. All that maths.

Clearance volume = 50cc.
Hose adds 3.5cc. (7%)
Reading changes about 7%.

je suis charlie

3DDave (Aerospace) · Answer 31 · 2022-01-11 13:34:41

The nice thing about the math is that it can be applied in other cases than just the one and the full explanation shows why it makes a difference.

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