Compression PSI

[Mad Mac]

An update:
I went ahead and did a compression test without the special adapter. The motor was not hot and I also forgot to open the throttle. The test showed all cylinders the same at 50psi. Its a completely stock engine with 25,000 miles on the clock and runs well, but has 21 inch wheels and a 3.54 diff.

I am pleased that all cylinders are the same, but should I be happy with only 50psi? She is a bit sluggish on long gradual hills, which is what triggered off the comp' test.

[PC/SR]

With those wheels and rear end ratio it will be a slow on the hills. The compression will be better hot and oiled and with open throttle per Mikes explanation above.

[BILL WILLIAMSON]

Chief would probably say, "What reading you get would depend on how HOT your battery is & how much you paid for your tester. Them CHEAP ones read high, to make you FEEL GOOD"! Bill W.

[Tom Wesenberg]

Originally Posted by Mad Mac

An update:
I went ahead and did a compression test without the special adapter. The motor was not hot and I also forgot to open the throttle. The test showed all cylinders the same at 50psi. Its a completely stock engine with 25,000 miles on the clock and runs well, but has 21 inch wheels and a 3.54 diff.

I am pleased that all cylinders are the same, but should I be happy with only 50psi? She is a bit sluggish on long gradual hills, which is what triggered off the comp' test.

My 28 Phaeton totally stock motor also has 50 lbs. on each cylinder and I'm happy with the power it has.

[BILL WILLIAMSON]

No one mentioned if they were cranking with 6 volt or 12 volt or if running a 6 volt starter on 12 volts or have a starter that's been converted to a true 12 volt. Bill W.

[Mitch//pa]

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i never found much of a difference throttle open or closed

[BILL WILLIAMSON]

I, personally, on a warm engine, always run at least 10 compression strokes, or until gauge no longer rises. If one is low, go back & retest it several times as sometimes carbon is dislodged when taking out plugs, and a chunk might be stuck under a valve & giving a low, false reading.
Bill W.

[Tom Wesenberg]

Originally Posted by Mitch//pa

i never found much of a difference throttle open or closed

I also have never seen a difference.

[ctlikon0712]

It makes a noticeable difference the larger displacement the engine is.
It made about a 20-25 PSI difference on my 71 Buick GS 455, and my airboat with a Continental 0-470.
It’s just a good rule of thumb. JMHO

[QGolden]

Originally Posted by PC/SR

My engine guru says the test of rings is if you get 75% of the final number on the first stroke. Less than that and the rings are starting to wear. Final number should come with 3 strokes. More than that and the rings are starting to wear.
So long as the numbers are within 5 psi, drive it 'til it smokes.

Dry test/Wet test to check rings. Open Throttle, crank engine to determine Compression for all 4 cylinders. Dump a teaspoon of oil down each cylinder, redo the compression test, compare the two numbers. Leaking rings will seal up with the oil raising the compression tested.

[Jim/GA]

Originally Posted by roccaas

Ran Hot compression test with throttle wide open.

65 lbs. across all four cylinders.

I found the gauge on ePay some time ago, some gentleman was making custom A Model compression gauges.

This is what I would have expected. No issues here. Just go drive it. Have fun while the weather is nice.

[woodypecker]

If you have a 4.5 to one compression ratio and volumetric efficiendy of 100% It would be hard to get more than 4.5 times 14.7 or 66 psi.

[BILL WILLIAMSON]

HAY! Roccaas, Chief "here" "If it's runnin' SO good, whatcha' messin' around with it for? Iffin' you fiddle around long enough you'll mess up sumpthin' else"! "Ain't you got work that needs to be done"? "CHIEF"

[roccaas]

Yeah, like painting the house, trimming hedges. I lock myself in the garage where "she who must be appeased" can't find me.

I had this neato tester and the horror story of the Samba guy, so I figured compression testing would indicate ring or carbon issues. Lucked out on both.

On to Parking brake adjustment as a way to stay out of the house (Hallmark Channel on all weekend), or the dreaded trip to Le'Target.

Not looking for problems, looking for ESCAPE, particularly since I'm down to one old car, I have to get creative.

[Terry, NJ]

Take the compression ratio (4.2) and multiply it by atmospheric pressure (approximately 14 lbs p.s.i.) 4X14=56 + approximately 3 lbs = 59/60 lbs. The compression is right where it should be. Nobody's leaking, But did release the pressure in the gauge after each test? when they come out that good for me, I generally forgot to release the pressure in the gauge.
Terry

[roccaas]

Yes, quick release button just below the gauge.

[MikeK]

Originally Posted by woodypecker

If you have a 4.5 to one compression ratio and volumetric efficiendy of 100% It would be hard to get more than 4.5 times 14.7 or 66 psi.

Originally Posted by Terry, NJ

Take the compression ratio (4.2) and multiply it by atmospheric pressure (approximately 14 lbs p.s.i.) 4X14=56 + approximately 3 lbs = 59/60 lbs. The compression is right where it should be. Nobody's leaking, But did release the pressure in the gauge after each test? when they come out that good for me, I generally forgot to release the pressure in the gauge.
Terry

That simple arithmetic gives you an isothermal answer that does not apply to an internal combustion engine!!!

The measured compression is adiabatic. Simply put, the compression also raises the temperature of the compressed gas, which in turn causes a pressure rise in a fixed volume. Quite simple if you understand the combined effect of Charles' and Boyle's gas laws.

A 4.22 compression engine @ 14.7 atmospheric can exceed 78psi if leakage is zero and thermal energy loss (Carnot efficiency, AKA heat loss to the metal) is zero.


Another way to look at it is like this:

Where 'k' is the latent energy (heat) constant for the gas being compressed, in this case a 14.7:1 air/fuel vapor mix.

[Terry, NJ]

Mike, with all due respect I would agree. I own two diesels (Case & VW) and that is what fires off the fuel. Basically , they run on compression. And with a 19 to 1 CR, in a hot cylinder, a lot of heat is developed. Speed also factors in which is what the glow plugs do when the engine is cold and has a slow cranking speed. I don't believe the mod A, with it's low CR and low cranking speed and cold engine block, heat is much of a consideration. Let me put it another way, if mod A engines (Or any other gas engine with a low CR and a cold block) was going to produce enough heat to raise the pressures approximately 30% over the pressure that I guesstimated, then we'd hear of it all the time and we don't. Moreover, This heat would increase the internal cyl. temperature to the point where the block would noticably heat up with a long cranking cycle and I'm not aware of that happening. Just My $.02
Terry

Originally Posted by MikeK

That simple arithmetic gives you an isothermal answer that does not apply to an internal combustion engine!!!

The measured compression is adiabatic. Simply put, the compression also raises the temperature of the compressed gas, which in turn causes a pressure rise in a fixed volume. Quite simple if you understand the combined effect of Charles' and Boyle's gas laws.

A 4.22 compression engine @ 14.7 atmospheric can exceed 78psi if leakage is zero and thermal energy loss (Carnot efficiency, AKA heat loss to the metal) is zero.


Another way to look at it is like this:

Where 'k' is the latent energy (heat) constant for the gas being compressed, in this case a 14.7:1 air/fuel vapor mix.

[MikeK]

Terry, There is quite a bit of heat absorption from the air/fuel mix when cranking, but not so much that it makes a simple atm. X CR = psi calculation a repeatable benchmark or tool. You can see this yourself by hooking up a compression gauge to a 4.22:1 engine and cranking with a tired 6V battery. Now immediately jump that starter with 12V, delivering four times the cranking energy (the amps double, power = amps squared x ohms). You will easily get 5 or more indicated psi.

As for feeling the engine heat during a long cranking, the ratio of air mass x latent heat to the engine mass x its latent heat is overwhelming. Air compressors, which have at least double the CR, much less mass, and crank several times faster will not show noticeable heating of the block or head if you only run it for the duration of engine cranking. If you grab the low mass output pipe, you will feel it.

I think some of the perceived 14.7 X CR = psi validity lies in the fact that cylinder leakdown is never zero and volumetric efficiency is never 100%. The result is something 'in the ballpark', but thats a pretty big, wide field. Even if you slowly hand crank an A, adiabatic factors increase the pressure. Air is a rather poor thermal conductor and the air/fuel charge in a cylinder cannot give up almost all of it's gained temperature in less than a second.

[Terry, NJ]

Mike, If I had an engine whose compression increased by cranking it faster, I'd write it of to wear. It takes time to leak past enlarged ring gaps and old valve seats.
As far as the compressor goes, It's running at say 600-800 RPM and it wouldn't take long for what you've described to kick in, namely for it to heat up. However, we're talking about 4,5, or 6 revolutions at cranking RPM. Much less likely to generate the amount of heat to affect readings.
Terry

Originally Posted by MikeK

Terry, There is quite a bit of heat absorption from the air/fuel mix when cranking, but not so much that it makes a simple atm. X CR = psi calculation a repeatable benchmark or tool. You can see this yourself by hooking up a compression gauge to a 4.22:1 engine and cranking with a tired 6V battery. Now immediately jump that starter with 12V, delivering four times the cranking energy (the amps double, power = amps squared x ohms). You will easily get 5 or more indicated psi.

As for feeling the engine heat during a long cranking, the ratio of air mass x latent heat to the engine mass x its latent heat is overwhelming. Air compressors, which have at least double the CR, much less mass, and crank several times faster will not show noticeable heating of the block or head if you only run it for the duration of engine cranking. If you grab the low mass output pipe, you will feel it.

I think some of the perceived 14.7 X CR = psi validity lies in the fact that cylinder leakdown is never zero and volumetric efficiency is never 100%. The result is something 'in the ballpark', but thats a pretty big, wide field. Even if you slowly hand crank an A, adiabatic factors increase the pressure. Air is a rather poor thermal conductor and the air/fuel charge in a cylinder cannot give up almost all of it's gained temperature in less than a second.