Hello, recently had my 350 sbc rebuilt for my 67 malibu. After about 1000 miles the engine is using around 1 qt. of oil to 200-250 miles. Ran a compression -leakdown test on it today. Here are the results- cyl. #1 150lbs.with 8% leakdown, cyl. #2 155lbs. with 7%, cyl. #3 145lbs. with 9%, cyl.#4 150lbs with 9%, cyl. #5 150lbs. with 11%, cyl. #6 145lbs. with 9%, cyl. #7 145lbs. with 6%, and cyl. #8 145lbs. with 14%. #5 and #8 cylinders had the higher leakdown %. Are these in satisfactory percents?? Any response would be great. Thanks, Tim. :confused:

I've never done a leak down test before but I had a friend do a leak down on a 402 motor in my garage last summer, he said it should be 8% or less. My motor was 80%! I guess this motor is toast! Your motor I'm not sure. Dave

Anything less than 10% leakdown is great, at the dealership I work at and Mopar as a company don't recognize a problem until you see numbers around 15-20%. You probably are having valve stem seal problems. Good luck.

Anything less than 10% leakdown is great

10% of what?

10% is a percentage of whatever air pressure is being run into the cylinder. example if 100psi is placed into cylinder and the cylinder will hold 90psi, then that cylinder leakdown is 10%. You can determine where the leakage is occuring by listening( I use stethoscope) at tailpipe, carb, and pcv or breather, to determine ex valve, in valve or rings.Simple enough? :D

Tim,your results are fine..if that was my 350,I wouldn't be upset with those numbers.As far as the oil consumption issue,it could be valve seals or the intake gasket leaking and sucking a little oil into the ports.

Originally posted by dennis68:
10% is a percentage of whatever air pressure is being run into the cylinder. example if 100psi is placed into cylinder and the cylinder will hold 90psi, then that cylinder leakdown is 10%. You can determine where the leakage is occuring by listening( I use stethoscope) at tailpipe, carb, and pcv or breather, to determine ex valve, in valve or rings.Simple enough? :D It's not really that simple. How do you arrive at 10%? I have a 5hp compressor that will keep 100psi of air in a cylinder without any problems at all. Does that mean I have 0% leakdown?

Thanks to all for the replys. Yea, my mechanic and engine builder both are alittle puzzled. We put new dart heads on the engine during the rebuild. The machanist tore apart the heads and looked them over. He lapped them in also. They had the small cc runners with 2.02 valves. They also didn't have the bronze valve guides. We installed moly sealed-power rings. Could it be the rings haven't seated in yet?? I wondered about an internal intake manifold leak, so I replaced the gaskets with the fel- pro #1205 set. I've read up where the pcv valve sometimes will let oil thru causing oil consumption like what I'm experiencing. I have noticed the hose that plugs into the carb port is alittle wet. I'm going to run without the pcv for awhile and see if this helps.

I was going to ask if Moly Rings were used, I have built engines with moly rings that have used oil for 2k-4k miles and then just stop and they have been fine since. IMO Its nothing to be concerned about at this time.
Duane

You need a leakdown tester with dual gauges and a regulator. Sorrry, I assumed we had already established that. Dart II cylinder heads, I have the same heads on my 350, has been using oil since the day I built it. I've been through twice and still have same concern, the only constant has been the heads. I won't run them again.

So now we have two gauges, one with line pressure in it and one with some lesser reading. How is the lesser reading calculated?
If I put 100# on the system and the second gauge reads 90# what does this mean? If I put 50# on the system and the second gauge reads 40# does this mean the same thing?

oneovakind
There is an orifice in between the two guages. My Tavia leakdown tester uses a .040 orifice. The percentage is the difference in air pressure across the orifice. If air is leaking out of the engine side (there will always be some), that side of the orifice will have a lower pressure. Do a percentage calculation on the difference in pressure compared to the inlet pressure, and there you have it. Some leakdown testers like mine, have the second guage already reading in %.

another explanation (http://www.4x4wire.com/lists/isuzu/200108/msg00070.html)

In your examples, 100 high and 90 low, the leakdown would be 10%. (100-90)/100 = 10%. In the 50 high 40 low, it would be (50-40)/50 = 20%

Chris

Now I'm really confused. If I want to measure flow I can measure the pressure drop across an orifice. If I have 100# on one side and 90# on the other, I have 'x' amount of flow. If I have 50# on one side and 40# on the other, I have the same pressure drop but you're telling me it's a different flow rate? How does that work?
Does the flow from the cylinder reach critical flow? The calculations for critical and sub-critical flow are quite different.
If I have a leakdown tester with a .050" orifice will my engines have more or less leakdown than your Tavia tester shows?

nova, I so badly want to drive up to fairfield(to choke you), no really. It would be much easier to show you how it works than to explain it. There is a manifold set, air is pumped into one side and flows through a gauge that measures the amount air going in, then through another gauge into the engine spark plug hole. If there is leakage in the cylinder the second gauge will read different than the first gauge. That difference is the amount of cylinder leakdown. Since it is based on the amount of air going in, it is a percentage. If you only allow 50psi into the engine the amount of leakdown is different than if allow 100psi into the engine. I hope to be educating and not confusing. If it is still unclear, let me know. I think I have junkyard run coming up and it may take me north-maybe. :D

onavakind, you're close....you do have the same pressure drop 100-90 or 50-40, but that 10psi is is a different percentage of the total going in. Dennis had got it right on smile.gif Think about it this way...if you did the test at 10psi, and the second guage shows 0, then you have 100% leakage, while still having a 10psi drop. It's easier to see that the side with 0psi is holding nothing, so it is 100% leaking down.

You are right that the orifice size changes things too. That's one of the problems with leakdown numbers....different orifices give different numbers. Unfortunately there is no standard size, not that i've found anyway. It's still a good tool for comparing cylinders and getting a general idea what's going on. Nice to listen to where the leaks are too. Most guages you can buy will have similar sized orifices. If you build one and use some ridiculus orifice like .250 or something, your numbers may be way skewed from normal looking numbers.

It's getting late, so don't quote me....but I think the bigger the orifice, the lower the leakdown numbers will be. The smaller the orifice, the higher the leakdown numbers will be. if you have a tiny pinhole leak in your cyl, and a giant orifice, the amount of flow that leaks will be very small compared to how much the tester can flow. This makes the two guages read very close together, and your leakdown numbers very small. If your pinhole leak is just a bit smaller than the orifice, then the amount it flows is closer to the amount that the tester flows, and you can see a bigger difference in the 2 guages, and will have higher leakdown numbers. Finally, if you have a big hole in your piston, and a small orifice, the leakdown will be 100% because ALL the flow out of the tester will be lost because the big hole in the piston can easily flow more than the tester puts out.

Nova knows whats goin on, he just likes to give people a hard time... Dont Ya .... lol
Duane

Livermore? I used to work at the Lab in the laser isotope separation program.
My original question about leakdown is about the standard for the test. If I want to measure oil pressure, I can use a guage that has a standard calibration, traceable to the NBS. 50psig of oil pressure is 50psig, period. What does 10% leakdown mean? 10% of what? I can see that you can determine relative leakdown if you use the same tester, but I see no correlation of the results from tester to tester.
In my shop I would have 2 testers, one with a large orifice and one with a small orifice. I use the small-orifice one on the incoming engines and the large-orifice one on the outgoing engines. I can take your 20% leakdown motor and make it a 2% leakdown motor for a very competitive price.
Do you know what critical flow is as it relates to compressible gases? Do you suppose the flow from the cylinder into the crankcase is critical flow?

Do you enjoy playing devil's advocate? I see your point about gauge orifice size, however I believe the tool industry has a standard for that. I can take leakdown readings with several different manufacter's gauges and duplicate readings. The reading is a percentage of flow lost in the cylinder. Example if 100psi is the regulated air flow and the cylinder only maintains 90psi there is a 10% loss. If 50psi is the regulated flow and the cylinder only holds 25psi, there is a 50% leakdown. You were a rad labber huh? That explains alot--LOL. :D

Since you've already assumed that a 5# drop across the orifice is 5% leakdown, how much of a drop across the orifice will be double that rate of leakdown?

a. 7.5#
b. 10#
c. 12.5#
d. 15#
c. 20#

I'm sorry. I played as long as I could. Now you've lost me. I can talk mechanical and slang, but the engineering stuff is where I call it quits. Seriously if I can get up there or you happen to come down for a lab convention or something I'd love to hook you up with a practical application to play with instead of theroy.

Don't bail on me now. This isn't rocket science, we're talking about flow equations developed in the 1700's by Bernoulli. The flow through an orifice is basically proportional to the square root of the pressure drop, so it takes 4 times as much pressure drop to get twice as much flow.
We can measure cranking pressure directly with a pressure gauge, why not measure leakdown directly with a flowmeter? A simple $30 rotameter, a variable area flowmeter, would give you leakdown in standard units rather than some arbitrary percentage.
We run a chassis dyno shop here in Fairfield so I get lots of practical experience. I also design and repair marine combustion control systems, which involves a lot of flow measurement.

You are killing me, you probably own several leakdown testers don't you. Was this a test of patience, knowledge, or both. My dad owns a marine repair facuility, mostly off-shore stuff out in Discovery Bay.I'm not even close to being in the same league as you. You lost me at "c'mon it's not rocket science". LOL. I know the basics of how the device works and to use those readings to fix ailing engine's. That's about it. It was an interseting debate untl you got all educated on me.

Originally posted by onovakind67:
Since you've already assumed that a 5# drop across the orifice is 5% leakdown, how much of a drop across the orifice will be double that rate of leakdown?

a. 7.5#
b. 10#
c. 12.5#
d. 15#
c. 20# If I pick "c" do I have a 40% chance of being right :D

40 of what? graemlins/clonk.gif It should be an 'e' and it is the correct answer.

Originally posted by onovakind67:
40 of what? graemlins/clonk.gif It should be an 'e' and it is the correct answer. -
Hmmmm, let's see, 5 choices with a total score of 100%,,,,,,,,,,two are the same,,,100/5,,,no, wait, if I only get one right out of five & two are the same & one is 20% then half of that,,,,,
I'll have to get back to ya as soon as I get batteries for my calculator :D

I grade on the flow curve. If it's a 5 question test:

1 right = 44%
2 right = 63%
3 right = 77%
4 right = 89%
5 right = 100%

Originally posted by onovakind67:
I grade on the flow curve. If it's a 5 question test:

1 right = 44%
2 right = 63%
3 right = 77%
4 right = 89%
5 right = 100% 44% of what???,,,,,,oh crap, now we're back to Alpha & have gotten even further from Omega :confused: