Hi everyone

I did a leak down test on my 1977 L48 to see if the piston rings are still in good condition.
So I am sure some new heads to bump the compression (and loose some weight) are worth it.

I did it when the engine was cold (valve covers, timing cover + W. pump, oil pan, radiator, hood and fan are off).
It was easier too find the right cylinder and TDC per cylinder. I also used a piston stop tool and a large braker bar too hold the crank in place when applying pressure.

To be sure I did test each cylinder 3 times.

Is this 'too good'? Because the engine was cold, I thought I would get worser results because of the little extra space between pistons, rings and cylinder walls.

I got the following results (%):

 

You're either reading the results backwards, or you have horrible cylinder leakage problems. Cylinder leakdown rates for a great engine are in the 5% - 10% range. Below 20% indicates some wear, but an engine still in serviceable condition. If you're over 20%, you have a problem...
Lars

 

Just to clarify. The picture of the gauge was not from when I was doing the test. It was just to let you guys know what tool I was using.

 

What do you mean by backwards? How is that possible?

 

A leak down tester compares the input air pressure to the pressure the cylinder will hold. Example, if input air pressure is 100 psi, and the cylinder gauge reads 95 psi, u have 5% leakage. 5 psi/100psi X 100 = 5%
ur gauge appears to be calibrated in psi so u have to do the math to determine % leakage.
with air in a cylinder, listen to the exhaust, carb and breather to see where air is leaking. Valves vs rings.
An aircraft cylinder leak down tester will regulate the cylinder input pressure to 80 psi. I cannot see the type of tester u are using but review the directions to insure accurate readings.
5 to 10 % would be very low leakage for a cold engine with some miles on it. 20% would be more in line. Warm the engine and do ur leak down again. That would provide more accurate numbers.

 

I did exactly what you stated with psi and % example.

I set the first gauge to 100 psi and read the second gauge. The percentages in my picture are the ones I read on the second gauge.

 

I didn’t know a cold engine gives ‘better’ values, than a warm engine when doing a leak down test.

When screwing the hose connecter in the spark plug holes it was impossible not to touch the header pipes with the hose and my fingers… There has to be another way no?

 

I see 92% to 95% Leak Down Readings. (Actually means 5 to 8% leakage)
Those are excellent readings for a street engine.
All the cylinders are in a narrow range, no "outliers" or low cylinders.

Some race engines can do slightly better. But rare on street engines.
Once you get one low outlier, or a couple down in the mid to low 80s, that is when you know you have a cylinder sealing problem.

The compression rings are sealing well, and the valves.
It says nothing at all about the oil rings.

That engine should be good for many miles, providing it is not using great amounts of oil.

My Factory original '08 GM 3.6L LLT had extremely similar leak-down readings, even after 220k miles. But the oil consumption had been climbing for years. The oil rings were completely shot at the end. It would use 1 QT of oil every 300 miles.

I changed all the rings and put it back together, and got virtually identical leak-down readings after the new rings were broken-in. But the oil consumption dropped immensely to an almost undetectable 4 ounces in 5000 miles (1/16 Qt) Vs the 5 Quarts per 5000 miles it was doing before. My oil rings had completely worn out, but at the same time, my original compression rings were still fine. Some early LS engines had similar oil consumption issues. Years later GM increased their oil ring tension specs. So did I.

A warm engine "may" give you slightly better readings, but not enough to worry about, considering the trouble involved. You're fine.

 

Novusuhu,

So what your saying is leakage rates are this....

1- 5% 2- 5%
3- 8% 4- 8%
5- 5% 6- 6%
7- 8% 8- 6%

If you've repeated those tests 3 times, your engine is in good shape.
The only question I have is is there a restrictor between the two gauges? The only experience I have is with aviation leak down gauges and they have a .040" or a .060" restrictor (depending on cylinder bore size) from the supply side to the measured value side. This restrictor is to limit the airflow volume but not the pressure, allowing leakdown to occur.
If you didn't have a restrictor, the full pressure and volume would overrun any kind of leak and give false readings.

 

Deleted

 

Yes, exactly what you state. The percentages that you say are the percentages of leakage I mean. I thought after doing the test 3 times on a cold engine those number are kind of high. Certainly on an ‘original’ block almost 50 years old…

But of course I don’t know what the previous owners did to it (internally).

 

I own the car for almost 9 years (as I remember correctly), and I don’t have any memory of it being an oil guzzler. I also never noticed any smoke or oil mist etc.

I’m going to do the test one more time. Just to be 200% sure.

But if the readings stay the same or almost the same, do you think I’m good to go removing the heads to check further (the block cylinder contact surface etc)?

 

For what reason do you feel you need to / want to remove the heads?

 

If you're so determined to repeat leakdown again; why not dope each cylinder with a spoonful of motor oil just prior to pressurizing it ?
Should reduce percent leakage.

 

To instal new better flowing heads, loose some weight and bump compression.

 

Win-Win-Win!

 

Your leak numbers are very good and consistent. I think you're in a really good place to do the head swap to make for better performance.