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Compression Numbers What do they mean?

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56freebie

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I have a AH100-4 with unknown milage and unknown condition of the motor sitting for a 10 years. Possibily rebuilt at one time due to its red color. I did a compression check and the results were cylinder
#1-95 psi
#2-87 psi
#3-95 psi
#4-95 psi
The test was done when the motor was cold. I removed 1 spark plug--screwed in compression tester and cranked the engine over until I obtained the highest reading and went to the next.
Is this the correct way to do a compression test and what do the numbers mean?
 
With an engine that has been sitting, squirt a little oil in the cylinders first?

(from Wik....)
In a piston engine it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.
Picture a cylinder with the piston at the bottom of its stroke containing 1000 cc of air. When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be proportionally described as 1000:100, or with fractional reduction, a 10:1 compression ratio.
 
One tip for compression testing is to make sure the fuel supply system is wide open, allowing for maximuim air flow. Actually compression readings can be indicators and also use the readings - 4 in this case - as a comparison. The actual value is not so critical. In your case, all four cylinders are relatively close to each other. If you had readings of 95, 95, 46, and 95, that would be a good indicator of problems in that 3rd cylinder. Just my 2 cents...
 
Good news, bad news--

First as stated above, your readings are pretty close to each other so this is good.

The 100 is a fairly low compression engine in stock form, but I think these figures indicate some wear.

In fact they are similar to what my car had when I got it.

I think the figures probably indicate some wear the the rings and bores, on the other hand, my car had similar readings and I drove it for 8 years (maybe 2,000 miles or so a year) before I had the engine rebuilt.

So especially if you do not blow much blue black smoke, indicating oil burning, and the engine runs OK, you probably don't need to give the motor any imediate attention.

I will add that after my rebuild compression went up to the 170 range for each cylinder. But that was also with higher compression pistons.

Interstingly, the diffeence in perfomance before and after the rebuild, while noticeable, was not huge, but my gas mileage went up considerably.

Have fun with your Healey, they are great cars.
 
I have a recent rebuild to stock 100/4 and have 155 -155psi across all four. Just as a reference point for you to use.

Might want to make sure the valves are operating correctly, not sticking open. Also a little oil in the pistons to help free any stuck rings might be good as well.


Michael.
55 BN1
 
And to restate, make sure your carbs, etc. are wide open. I didn't know this when I compression tested my Mini the first time. When I did it again with the carb wide open, the readings increased.
 
On a 100 Four engine with high mileage but in good condition, 130 psi could be pretty good.

Be aware that altitude will have a noticeable affect on the readings.

Very Important --------
Also different compression gages can vary the readings on a given engine by as much as 50 psi, depending on the non-return valve that is installed in the gage's nose. Different valve cores have greatly different opening pressures. Tire valve cores can lower the gage readings by 50 psi or more.

Cores made for refrigeration/air conditioning can have drops as low as 5 psi.

Cores made specifically for compression gages can drop as little as 2 psi.

Tire valve cores have a red marking band. They drop about 50 plus psi of pressure.

Air conditioning valve cores have an orange marking band. They drop about 5 psi of pressure.

Compression gage valve cores have a white or black marking band. They drop about 2 psi of pressure.

Measured spring tension:
Red = 56 oz, Orange = 2 oz, black = 1 oz.

Black valves for the Lysle compression tester #STL20100 are available from Carquest Auto Parts, maybe some others.
D

PS - There is no good way to predict compression pressure from compression ratio. The cranking measured compression pressure is largely determined by the point at which the cam closes the intake valve to start the actual compression stroke. This closing typically happens some 40 to 80 degrees after the piston has reached BDC & started back up on the compression stroke. Nominal or rated compression ratio is based on the intake valve closing at BDC which is never the case.
D
 
From your responses it looks like I did not do a compression test correctly.
I retested the engine again this time with the throttle plates wide open and removed all four spark plugs

Readings this time were
#1-125 psi
#2-115 psi
#3-120 psi
#4-120 psi

I went one additional step and added oil to each cylinder
Readings went to
#1-131 psi
#2-125 psi
#3-135 psi
#4-130 psi

I am also in the Los Angeles Ca. Close to Sea Level

Correct me if I am wrong but this motor is in good condition (not excellent) due to the readings are close to each other and adding oil to the cylinder to seal the rings to the piston walls did not raise the psi drastically. This is also assuming that I have an accurate compression gauge. It should run with minimal oil burning and in the future will probably need a rebuild.
 
Take a closer look at cylinder #2. You may have something out of adjustment there. Maybe a sticking valve or one of the rocker arms isn't adjusted right. In any case your engine should run fine for the time being. If I am off, I am sure someone will chime in....
 
I would recomend a 'leak down' test. Compression testing is valid information but a leak-down test pressurises the cylinder and you can learn a lot more about the condition of the valves, rings, and gasket.

Leak down testers can be had from several sources and are simple to use. Bring a piston to TDC on the combustion stroke, pressurise the cylinder with the tester, and see how much leakage there is. The real bonus is that with the cylinder pressurised you can listen for air leaks. If you hear a hiss in the carb, than the intake valve is leaking - if you hear a hiss at the exhaust pipe, the exhaust valve is leaking. If you have leakeage and it isn't audible in the carb or exhaust, then the rings and/or the gasket is leaking.

From my experience a fresh motor in good condition may have as little as 5-10% leakage, and a well used but still servicable motor may show as much as 30-40% leakage.

Like the compression test, you are interested in uniformity as well as the actual numbers from any given cylinder.

my .02 worth

Dave Phillips
 
The compression numbers that you gave would indicate that there is nothing seriously wrong.

I would check valve lash, head bolt torque, & then run it for a while. The compression could very well improve as it is run. It often does on engines that have been sitting for a long time.

As DP said, the definitive test is a leak down test but what do you do if the numbers are not to your liking?

Run it & see how it works, nothing to lose.
D
 
I agree with Dave, the number look good, if you have good oil pressure and are not burning (much) oil you may have many many happy healeying miles ahead before anything serious is needed.
 
Dave Russell said:
... Very Important --------
Also different compression gages can vary the readings on a given engine by as much as 50 psi, depending on the non-return valve that is installed in the gage's nose. Different valve cores have greatly different opening pressures. Tire valve cores can lower the gage readings by 50 psi or more.

Cores made for refrigeration/air conditioning can have drops as low as 5 psi.

Cores made specifically for compression gages can drop as little as 2 psi.

Tire valve cores have a red marking band. They drop about 50 plus psi of pressure.

Air conditioning valve cores have an orange marking band. They drop about 5 psi of pressure.

Compression gage valve cores have a white or black marking band. They drop about 2 psi of pressure.

Measured spring tension:
Red = 56 oz, Orange = 2 oz, black = 1 oz.

Black valves for the Lysle compression tester #STL20100 are available from Carquest Auto Parts, maybe some others.
D
Click to expand...
I did not know that.

Fortunately, my compression tester still has its original valve core, but certainly good information to keep the intended core should replacement be required.

A trick that has proven itself effective for me, is to use a high quality penetrating oil (Kroil comes immediately to mind) and let a couple onces sit in each cylinder for about a week when the engine is undisturbed. Blow the oil out (cranking it over without the plugs installed) before starting, and change the engine oil before ANY road use.

Both the compression and leak-down tests are more informative on a "stinking hot" engine, so your numbers, while NOT terrible, could still improve.
 
This is all good info. I am working on a 4 cylinder now that has sat for a long time. So I will:

1- Put some penetrating oil in the cylinders let it sit till the weekend
2- change the oil, filters and plugs
3 - do a compression test
4- try starting it :smile:
 
Just a follow up--I adjusted the valves and Yes the number 2 was not adjusted correctly A little tight with no clearance I adjusted all and the readings this time were

#1-125 psi now 128 psi
#2-115 psi now 121 psi
#3-120 psi now 129 psi
#4-120 psi now 121 psi

After a new gas tank, Clean out fuel lines, New points, plugs, wires, cap, coil, rebuild carbs, flush radiator and cooling system replace hoses, change oil and filter in engine, trans, rear-end, rebuild brake system New battery and a squirt of starting fluid the car came to life. A little more adjustment and drove it around the block. A little scary on 25 year old tires. Still need to adjust the carburators and timing but getting close......
Thanks for the help
 
Switch those tires as soon as you can... not only a big change in the ride, but you won't watch your tread in the rear view bounce down the road at speed!
 
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