Cylinder pressure vs leakdown...

[mhinders]

Hi all,
I'm trying to figure out what needs to be done in my 67 383 before opening up the engine. I know the cylinder pressure numbers are way down, the heads are 346 and the cam is "hot" making the assumingly low compression ratio showing very low cylinder pressure...but that will be taken care of!!

What puzzles me is that the leakdown numbers don't correspond so well with the cylinder pressure numbers.  Cylinder 2 was leaking 27%, but still showing a (relatively) good pressure? The pressure numbers were recorded at a different time than the leakdown numbers, but the engine has been running very little between the tests.

Should I assume that the heads, the valve guides, need some maintenance and that there is a dynamic seating problem with some valves  and therefore the pressure and leakdown numbers don't correspond so well?

Below you find the numbers. Comments appreciated.
Cheers,
Martin

Cylinder #          Cylinder pressure (psi)      Leakdown % 100 psi testpressure
1                   128                             10
2                   123                             27
3                   119                             12
4                   119                             18
5                   113                             20
6                   119                               7
7                   116                             13
8                   119                             10

[c00nhunterjoe]

What is done to the engine? Those compression numbers are low, even wth a decent cam. While performing the leakdown test you should be able to hear where the air is escaping. Exhaust pipe, crankcase, or intake.

[elacruze]

What was your leakdown procedure?

You have to be sure that you perform the leakdown while the piston is on the upstroke. If you reverse it even a tiny bit, you unseat the rings from the lands and will show leaks that you may not have, particularly if the lands are worn. You should also test at about 20* BTC and ATC, which requires that you have a very good hold on the crankshaft so the engine doesn't rotate.

[mhinders]

The engine was somewhat modified when I bought the car...cam, intake, carb, headers, heads are 346. Cam is an unknown type (likely Compcams), vacuum at idle is very low 6-10 in, idle is powerless and engine is prone to fouling plugs if at idle for a short while. However, lambda value is good at idle...and the exhaust rumble is good!! 

Good point Elacruze, I'll check the worst cylinders again to make sure I didn't unset the rings. The leakdown was tested at TDC, I will test also at 20 BTC to see if I can notice a difference.
Thanks guys,
Martin

[John_Kunkel]

When it comes to leakdown tests I'm more familiar with aircraft; nice thing about planes is you can grab a propellor blade before introducing air into the cylinder to hold the piston at TDC and then "massage" the prop to get about any leakdown reading you want (as long as nothing is amiss).

IMHO dynamic compression test are more about the actual world the engine operates in, leakdown is only useful for finding the source of compression loss.

[mhinders]

...IMHO dynamic compression test are more about the actual world the engine operates in, leakdown is only useful for finding the source of compression loss.

Yeah, I understand your point.
The leakdown test is attractive since it's easier to perform, and there is no time limit when testing (well, pressure in the air tank drops, but not as quickly as a battery during a compression test  ).
I will see if I can get the values to correspond a little better, I might even run compression tests again on a few selected cylinders! 

[c00nhunterjoe]

When it comes to leakdown tests I'm more familiar with aircraft; nice thing about planes is you can grab a propellor blade before introducing air into the cylinder to hold the piston at TDC and then "massage" the prop to get about any leakdown reading you want (as long as nothing is amiss).

IMHO dynamic compression test are more about the actual world the engine operates in, leakdown is only useful for finding the source of compression loss.

Holy crap. You might be the 4th person I've ever heard talk about dynamic compression testing. You never cease to amaze me.  You can learn so muc more about an engine from a dynamic test. Static is a good start, but actually running the engine can tell you more about where an issue may lay.

[BSB67]

I believe that there is a bit of an art to a leak down test, and you need to spend time and repeat testing with the same gauge set to really have confidence in what the readings mean.  They all do not work the same, and size of the orifice or valving in the block effects the sensitivity of the gauge reading.  One set of gauges might read 2 or 3% while another set of gauges on the same motor will read 10 to 12%.

[c00nhunterjoe]

I believe that there is a bit of an art to a leak down test, and you need to spend time and repeat testing with the same gauge set to really have confidence in what the readings mean.  They all do not work the same, and size of the orifice or valving in the block effects the sensitivity of the gauge reading.  One set of gauges might read 2 or 3% while another set of gauges on the same motor will read 10 to 12%.

I agree. As long as you don't have a cylinder that is more then the others

[mhinders]

I bought the leakdown tester brand new so I will always use the same equipment. I realize I need to experiment a little with the stuff to see what influence different crank positions and movements have on the readings. Looks like the leakdown stuff is more interesting than I thought...

[John_Kunkel]

If you take a long look at leakdown testing you'll realize that the tester has an orifice that's .040" in diameter, that's .000126 square inches so all you need is a combined leak that totals .000126 sq. in. and you have 100% leakdown. When compared to a typical 4.25" piston that has 14.19 sq. in. of surface area (A ratio of over 112,000 to 1) that 100% leakdown is pretty insignificant.

That's why a cylinder with 100% leakdown could conceivably produce 100% of its rated power.

In the aircraft industry the cylinder leakdown test is often referred to as "the A&P mechanic's full-time employment guarantee".    The arbitrary maximum of 25% leakdown has lead to the rejection of some fine-functioning cylinders.

[mhinders]

John, interesting perspective. Well, I think I have encouraged myself to think the engine is in good condition. Will anyway tear it apart because it needs a better matching of performance parts.
Thanks for all comments.
Martin

[oldschool]

If you take a long look at leakdown testing you'll realize that the tester has an orifice that's .040" in diameter, that's .000126 square inches so all you need is a combined leak that totals .000126 sq. in. and you have 100% leakdown. When compared to a typical 4.25" piston that has 14.19 sq. in. of surface area (A ratio of over 112,000 to 1) that 100% leakdown is pretty insignificant.

That's why a cylinder with 100% leakdown could conceivably produce 100% of its rated power.

In the aircraft industry the cylinder leakdown test is often referred to as "the A&P mechanic's full-time employment guarantee".    The arbitrary maximum of 25% leakdown has lead to the rejection of some fine-functioning cylinders.

john you are so right about that. i own and fly a bonanza, and i see a&p,s reject alot of good clynders because of the leakdown test. they act like the numbers are etched in stone..........

[flyinlow]

If you take a long look at leakdown testing you'll realize that the tester has an orifice that's .040" in diameter, that's .000126 square inches so all you need is a combined leak that totals .000126 sq. in. and you have 100% leakdown. When compared to a typical 4.25" piston that has 14.19 sq. in. of surface area (A ratio of over 112,000 to 1) that 100% leakdown is pretty insignificant.

That's why a cylinder with 100% leakdown could conceivably produce 100% of its rated power.

In the aircraft industry the cylinder leakdown test is often referred to as "the A&P mechanic's full-time employment guarantee".    The arbitrary maximum of 25% leakdown has lead to the rejection of some fine-functioning cylinders.

john you are so right about that. i own and fly a bonanza, and i see a&p,s reject alot of good clynders because of the leakdown test. they act like the numbers are etched in stone..........

Always had fresh warm oil in my Cessna's at annual time and rocked the piston past TDC with the prop to pass the test.( I used to be an A&P) Of course you don't want to stretch the rubber band to far if you only have one.

[John_Kunkel]

i own and fly a bonanza, and i see a&p,s reject alot of good clynders because of the leakdown test.

So you likely have the Continental IO-520. These engines are well known for flunking leakdown test when they're brand new; so much so that Continental took to writing advisory letters urging techs to overlook the 25% leakdown "rule".