Compression Test 1608

[seabeelt]

New car to us. Noticed a ticking sound on the exhaust side. Did a quick “cold” compression test.

Found
#1. 125 psi
#2. 105 psi
#3. 125 psi
#4. 125 psi

I’m guessing number two cylinder sticking exhaust valve as that is the noisy side

Intake side is quiet like a sewing machine

Side note. Number 4 plug was missing it’s screw on top cap. Go figure

[SteinOnkel]

Hmmm, I would re-test at operating temperature.

Also, side note, I'm sorry you have to go through the endearing misery that is Audi Alljunk ownership My partner sold hers six weeks ago, I've never been happier to see a car leave.

[Nut124]

Do you know the history of the engine? How does it run otherwise?

Those numbers are pretty low. Did you have throttle wide open and all plugs out during the test?

Ticking valvetrain noise in these engines is usually from excessive valve clearance. Could be a leaky ex valve.

[18Fiatsandcounting]

For comparison, I did a cylinder head job on my '71 spider (1608) a few months ago, and the afterwards compression test yielded 125 psi on all four cylinders. The rest of the engine was not rebuilt (and never has been). I believe the stock compression ratio on a 1608 is 8.5, and 14.7 psi (atmospheric pressure at sea level) times 8.5 is exactly 125 psi. There will be some heating of the gas upon compression in the cylinders, but that probably doesn't change the pressure that much, so 125 psi is about all you'd expect to see. Unless you've got a blower strapped to that bad boy. https://www.youtube.com/watch?v=fV7rzFBeUlQ



And yes, Nut124's advice to test the compression with all spark plugs removed and the throttle wide open is good.

-Bryan

[Nut124]

Bryan, I do not believe one can predict cranking compression numbers from static CR and atmospheric pressure. There are many other factors in play, mainly cam timing. If you got numbers that match your observations, then, I think it was more of a lucky coincidence. Further, when compressed, air heats up and pressure increases. The formula for pressure vs volume that
you may be referring to assumes constant temp, which is not what happens in a dynamic engine case.

There are a number of calculators for this online. They use static CR, intake valve timing and a few other parameters. Intake valve timing affects the cranking compression a lot.

If the 1608 had a really hot cam, long duration, late intake closing, then low cranking compression would be the result. I still think 125PSI is on the low side even for a hot cam.

http://www.wallaceracing.com/calc-crank-press.php

[18Fiatsandcounting]

Bryan, I do not believe one can predict cranking compression numbers from static CR and atmospheric pressure.

I agree. But, I do believe it is a rough guide. Assuming you're at sea level (14.7 psi) and with a given compression ratio, if the volumetric efficiency on the intake is 1.0 (the cylinders pull in as much air as their volume allows), then all else being equal, the pressure upon squishing that intake charge would be 14.7 psi times the compression ratio. But, as you mention, all else is not equal, and gases upon compression rise in temperature, often quite dramatically. On the opposite extreme, gases cool when they expand (pressure goes down), which is the whole principle behind AC units, using Freon as the compressed gas.

You can play with cam timing all you want, but without a force stuffing more intake gas/fuel into that combustion chamber, your ultimate compression is dictated by physics. That force could be a turbo, a blower, or a carefully tuned induction system (and cams) to use the inertia of the air/fuel rushing to get more than "1.0" fillings per intake charge.

Bottom Line: When I read that Fiat owners get 210 psi static compression with their 9.8 CR pistons, I do not believe them. Tilt that gauge appropriately as you look at it, and I think you can get any number you want.

That being said, this is static compression with a cold engine and a gauge, and what your combustion chambers actually experience at 6000 rpm and 1000 oF is up for debate. Under those conditions, a well tuned exhaust system may be helping to pull in a new intake charge with larger than expected quantities of air/fuel. The volume is the same (dictated by the swept volume of the cylinder), but you may be able to stuff more than 14.7 psi into that volume given the right engine setup.

-Bryan

[SteinOnkel]

The ol' cr x 14.7 is a rule of thumb. I think you are both right.

It makes sense logically. However, I've got one car with 11.5cr that has a whopping 225psi of compression. And another with 9.0 that I just 15 minutes ago measured at 160 across the board.

[Nut124]

Bryan, volumetric efficiency at cranking speeds is nowhere near 1.0. It is way lower due to the fact that the intake valve is open for a long time while the piston is moving up, pushing air back out via the intake.

[18Fiatsandcounting]

I appreciate the discussion, and I kinda think we're actually more in agreement than it appears. In a static compression test, there are only two things creating the pressure rise: 1) compression of the gas by the piston, and 2) a temperature rise of the gas due to this adiabatic compression (meaning the gas retains most of this heat rather than transferring it to the metal engine).

I totally agree that the CR * 14.7 psi is just a rough estimate, and that the intake charge of the gas will be less than 1.0 due to the fact that the valves don't open and close exactly at TDC and BDC.

Now that I think about it, I could be wrong about temperature rise being a small factor, especially with high CRs. If you had a hypothetical engine with a CR of 2.0, I doubt you'd see much temperature rise. For engines with CRs over 10, the temperature rise might in fact be pretty dramatic. I'm sure there are equations for this.

However, keep in mind that Steiny's 11.5 CR engine, assuming it's 2.0 liters (500 cc per cylinder) has only about 44 ccs of volume when the piston is at TDC, or about 3 tablespoons. If he squirts a bit of oil into that cylinder to compensate for ring leakage (a fairly common practice), it's not hard to see how he could temporarily reduce the combustion chamber volume and thus raise the measured CR by 10 or 20 percent. Even the spark plug hole fitting on the pressure gauge he uses to measure the static pressure might result in the combustion chamber having less (or more) volume than if a spark plug were there. I would expect this effect to be pretty small, though.

Now that I think about it, how does the gauge compensate for all the additional volume that is being added by the internal volume of the tubing for the gauge, and of the gauge internals? My pressure gauge has a metal fitting and about 18 inches of rubber hose, which could introduce a few extra cc's of volume to the existing combustion chamber volume. How does the gauge account for this, given that the gauge can be used on engines of almost any volume? Maybe it doesn't?

Man, now I'm more confused than ever! But, I've always paid attention to differences between cylinders in terms of static pressure, rather than the absolute value. For my 1608 engine that just had the head rebuilt but still has some carbon buildup on the piston crowns but is no doubt leaking some pressure past the rings, 125 psi seemed about right given the crappy original intake manifold and carb. For my 1438 engine with its 8.9 CR, 140 psi was always what I measured.

-Bryan

[Nut124]

Just for comparison;

My previous build 1800 had a static CR of about 9.7 but only cranked 135psi due to 40/80 intake cam being retarded over 5deg. So intake valve was open almost half of the compression stroke.

My current build has a static CR of 10.5. With a semi-race intake cam it cranked 160psi. With the old 40/80 street cams the same engine now cranks 185psi.

The compression testers have a check valve in the brass head. It takes several compression cycles for the hose/gauge to reach the final pressure reading. That is how they compensate, I think.

[18Fiatsandcounting]

Thanks for that. I'm beginning to think my simplistic understanding of static compression is, well, way too simplistic. Different cams, different intake systems, different engine conditions, all can result in different readings. So I guess I'm back to using static compression as a general guide, mainly in terms of differences between cylinders or decreases over time.

Given that diesel engines are generally in the realm of a little less than 300 up to 400 psi, I can certainly see how gasoline engines with compressions well over 200 could have issues with preignition.

-Bryan

[SteinOnkel]

I appreciate the discussion, and I kinda think we're actually more in agreement than it appears. In a static compression test, there are only two things creating the pressure rise: 1) compression of the gas by the piston, and 2) a temperature rise of the gas due to this adiabatic compression (meaning the gas retains most of this heat rather than transferring it to the metal engine).

I totally agree that the CR * 14.7 psi is just a rough estimate, and that the intake charge of the gas will be less than 1.0 due to the fact that the valves don't open and close exactly at TDC and BDC.

Now that I think about it, I could be wrong about temperature rise being a small factor, especially with high CRs. If you had a hypothetical engine with a CR of 2.0, I doubt you'd see much temperature rise. For engines with CRs over 10, the temperature rise might in fact be pretty dramatic. I'm sure there are equations for this.

That's essentially how Diesels work. Our 7.3 beast has a cr of 17:1 I believe.

However, keep in mind that Steiny's 11.5 CR engine, assuming it's 2.0 liters (500 cc per cylinder) has only about 44 ccs of volume when the piston is at TDC, or about 3 tablespoons. If he squirts a bit of oil into that cylinder to compensate for ring leakage (a fairly common practice), it's not hard to see how he could temporarily reduce the combustion chamber volume and thus raise the measured CR by 10 or 20 percent. Even the spark plug hole fitting on the pressure gauge he uses to measure the static pressure might result in the combustion chamber having less (or more) volume than if a spark plug were there. I would expect this effect to be pretty small, though.

It has a whopping 1598cc of displacement. I did actually do exactly that with the tbsp of oil. One cylinder is currently down to 60psi and the oil trick brought it back up to 90, confirming shot rings. Still runs though. Tons of blow-by and it sounds like a Subaru.

Now that I think about it, how does the gauge compensate for all the additional volume that is being added by the internal volume of the tubing for the gauge, and of the gauge internals? My pressure gauge has a metal fitting and about 18 inches of rubber hose, which could introduce a few extra cc's of volume to the existing combustion chamber volume. How does the gauge account for this, given that the gauge can be used on engines of almost any volume? Maybe it doesn't?

I think you are turning a grain of rice into an elephant. The volume added by the gauge is minuscule. I bet you the outside diameter has nothing to do with the inside diameter and there is a plastic line on the inside with a tiny cross section.

Man, now I'm more confused than ever! But, I've always paid attention to differences between cylinders in terms of static pressure, rather than the absolute value. For my 1608 engine that just had the head rebuilt but still has some carbon buildup on the piston crowns but is no doubt leaking some pressure past the rings, 125 psi seemed about right given the crappy original intake manifold and carb. For my 1438 engine with its 8.9 CR, 140 psi was always what I measured.

Mission accomplished. I think my 1800 Fiat measured out at exactly what the book specified for a new engine. And if I'm not mistaken, it adheres to the x14.7 rule of thumb.

Cheers
Steiny

[18Fiatsandcounting]

Well, being a scientist by occupation, I always try to understand things from that standpoint (yes, even Fiats), and so I thought back to my chemistry class and recalled the equation (P1*V1)/T1 = (P2*V2)/T2. I believe this uses absolute temperature, that is, Kelvin. 25 oC (77 oF) is 298 Kelvin, so if you start with 0.5 liters at atmospheric pressure and 25 oC, and compress it to 0.05 liters at 160 psi, the final temperature is around 124 oF. Ha! 124. I knew it!!

So, I guess I was wrong. For a moderate CR of 10:1, the temperature rise due to compression is fairly significant, and that would easily account for the increased pressure you read on the gauge, above and beyond the 14.7 (x volumetric efficiency) times CR.

-Bryan

[seabeelt]

I re did the test yesterday with the engine warm
Results are
1-130
2-110
3-132
4-130

Number two with a spoon full of oil still resulted in a reading of 110

Don’t know the extensive history of the car. PO had it for about 10 years or so. I would say the car suffer from lack of use more than anything else. There are the usual PO/garage mechanic issues. Example. Changed the radiator, fan, thermostat and water pump along with the timing belt. But neglected to check the overflow bottle and line which were plugged causing the radiator hose to swell like a sausage. rtv everywhere. And so forth

[18Fiatsandcounting]

Those numbers aren't terribly bad, and so I guess the choice is up to you whether to pull the cylinder head off or just live with it for a while. If it were me, I'd just drive it for while, assess what else might be wrong in the engine / drivetrain / suspension areas, and then go from there.

And, I promise not to hijack your thread anymore with all this talk of thermodynamics and physical chemistry...!

-Bryan