Compression/leakdown test results

[Larry T.]

I ran a compression check on my engine today (L72). I have been concerned about possibly using too much oil. A quart every 5-600 miles. I posted a few weeks ago that I get a dry residue on the lower rear quarters anytime I take the car out. I was trying to figure out if it was fuel or oil related. I also did a leak down test on two of the cylinders; I just did not have time to do them all. Results follow:


Compression Leak Down
#1 185 5% differential at 100 P.S.I.
#2 200 5% differential at 100 P.S.I.
#3 185
#4 180
#5 190
#6 180
#7 190
#8 190

There was a hiss coming from under the valve cover. I assume that is normal since the pressure has to go somewhere, is that correct?

I will run the leak down test on the other cylinders, but assuming they come in anywhere close it seems to me I have a pretty tight engine. 200 seems a little high on #2, I am guessing it could be due to the excess oil I am getting in that cylinder. Presumably the engine has about 3500 miles in the last 10 years.

My next move is probably to pull the springs, check the valves to see if they are loose and put on new seals. Maybe the limited use in the past ten years has allowed the seals to dry out?

I would appreciate any thoughts and suggestions.

[Joe C.]

Re: Compression/leakdown test results

That is excellent compression! Accepted rule of thumb is 200 psig maximum for street use with pump gas. There are fairly wide variations on this, but it indicates that 180-190 is very healthy. As an example, my SBC shows 225-235 psig dry cranking compression, and runs detonation free on 93 PON fuel with slightly limited vacuum advance (12 degrees instead of 17 degrees). The addition of 1 pint Kemco TEL additive per 15 gallon fill up allows me to run the vacuum advance with the limiter stop removed.

Leak down is OK also. Hiss at the valve cover says that the slight leakage is passing the ring(s) and getting into the crankcase, which is much better than if it were passing one of the valves. Hiss heard at tailpipe or carburetor would signify leaky exhaust/intake valves, respectively.

If you had oily residue on the body, that would indicate a problem. Dry residue, especially if it is black indicates carbon, and a small amount is normal for pre emissions controlled engines.

Causes of oil consumption with good compression like yours would be valve seals, or oil scraper rings.

Compression and leak down tests are simple diagnostic tools, but do not test sealing/dynamics under full load. There are other considerations which will cause oil consumption and/or loss of compression under full load.

[Larry T.]

Re: Compression/leakdown test results

Joe,

Thanks for sharing info and confirming some things for me.

Leak down is OK also. Hiss at the valve cover says that the slight leakage is passing the ring(s) and getting into the crankcase, which is much better than if it were passing one of the valves. Hiss heard at tailpipe or carburetor would signify leaky exhaust/intake valves, respectively.

If there is a differential of any kind (5% in my case) is it fair to say there will be some sort of air escape noticible somewhere?

Causes of oil consumption with good compression like yours would be valve seals, or oil scraper rings.

What is my best choice for replacement valve seals? Part number?

[Joe C.]

Re: Compression/leakdown test results

Joe,

Thanks for sharing info and confirming some things for me.



If there is a differential of any kind (5% in my case) is it fair to say there will be some sort of air escape noticible somewhere?



What is my best choice for replacement valve seals? Part number?

Anything less than (about) 10% is acceptable. Anything other than the best prepared race engines will pass more than 5% past the rings, but anything, at all, past the valves or head gaskets are not acceptable. A small leak past the valves, especially the exhaust valve, will soon turn into a big leak.

I am no expert on BBC. If you had a SBC, I would not hesitate to recommend NOTHING BUT positive stop valve seals on both the intake and exhaust stems. You might be safe to use these with BBC as well, but BBC does not have the ample top end oiling that the SBC is blessed with.

[Timothy B.]

Re: Compression/leakdown test results

Larry,

I agree with Joe, 190-200 cranking compression pressure is ideal for max torque and I would not want more. My 63 has about 175 lbs cranking pressure with a .038 head gasket and if they ever come off I will not be afraid to install the GM steel gaskets (.022). GM manual for small block says 160 +- cranking pressure.

Are you sure there is even a problem with valve seals, that engine may need to be driven some.

Joe, What is quench measurment for your engine, can I make a guess .040 Did you change the pin height or big pistions?

[Larry T.]

Re: Compression/leakdown test results

Are you sure there is even a problem with valve seals, that engine may need to be driven some.

I am not sure of anything really. Maybe that is the answer. I would have thought 3500 miles was enough to break it in. I also wonder if after 10 years of limited use the seals could have dried up and not seal as well. I personally have put fewer than 1000 miles on the car. Maybe I will go another 1000 and see if the consumption slows down.

Thanks.

[Larry M.]

Re: Compression/leakdown test results

Larry:

What is your oil pressure cold and hot?? Also, have you checked manifold vacuum?? And is it fairly steady or a bit erratic?? How about PCV operation??

Do your plugs look black/oily?? Which ones??

Besides valve seals, another possibility is an intake gasket leak or porosity that allows oil to be drawn into the intake runner(s).

Larry

[Gene M.]

Re: Compression/leakdown test results

Valve seals may be a cover up for wear on valve guides. Big blocks are more prone to guide wear more than small blocks in stock street engines. When rebuilt were the guides replaced or knurled? Also the cast iron guides are better that the bronze for street use.

Big block replaced guides need to be sealed from the water jackets. Also there is not a lot of material around them for press fit, so use a good sealer.

[Joe C.]

Re: Compression/leakdown test results

Larry,

I agree with Joe, 190-200 cranking compression pressure is ideal for max torque and I would not want more. My 63 has about 175 lbs cranking pressure with a .038 head gasket and if they ever come off I will not be afraid to install the GM steel gaskets (.022). GM manual for small block says 160 +- cranking pressure.

Are you sure there is even a problem with valve seals, that engine may need to be driven some.

Joe, What is quench measurment for your engine, can I make a guess .040
Did you change the pin height or big pistions?

Quench is quite a bit less than 0.040". Don't try this with stock GM connecting rods if you intend to run a moderately long duration, solid lifter engine past its power peak!
Pistons have a pin height is 1.675" with small domes like original.

[Joe C.]

Re: Compression/leakdown test results

Larry,

Do you have oil in the air cleaner housing?

[Larry T.]

Re: Compression/leakdown test results

Larry:

What is your oil pressure cold and hot?? Also, have you checked manifold vacuum?? And is it fairly steady or a bit erratic?? How about PCV operation??

Do your plugs look black/oily?? Which ones??

Besides valve seals, another possibility is an intake gasket leak or porosity that allows oil to be drawn into the intake runner(s).

Larry

Larry,

Oil pressure is is bout 55 upon upon start up. About 40 when hot.

Manifold vacuum as measured on the carb is about 16-17 depending upon how much I lean it out. It bounces about 1/2". What would be considered erratic?

There is plenty of "suction" through the PCV, it rattles when you shake it, I am not sure what else to look for. I can not say with certainty it is the correct one. It has the number 2051 and Made in the USA on it.

The plugs are R45xls's and burn everything that goes in the cylinder. Several plugs do have fresh oil on them when I pull them out. #2 is the worst. A couple stay cleaner than the others. When I had the carb off a few months ago I did not see anything that looked like oil residue in the intake.

Larry,

Do you have oil in the air cleaner housing?

Joe,

No oil in the air cleaner housing.

Thanks,

Larry

[Larry T.]

Re: Compression/leakdown test results

Valve seals may be a cover up for wear on valve guides. Big blocks are more prone to guide wear more than small blocks in stock street engines. When rebuilt were the guides replaced or knurled? Also the cast iron guides are better that the bronze for street use.

Big block replaced guides need to be sealed from the water jackets. Also there is not a lot of material around them for press fit, so use a good sealer.

I wish I knew more about the engine. I do not know if the valve guides were knurled. I would have to drop a valve to check it out. That will be easy enough if I do replace the seals, I guess. I do not even know what type of guides it has.

[Joe C.]

Re: Compression/leakdown test results

Larry,

Joe,

No oil in the air cleaner housing.

Thanks,

Larry

A good sign, which tells you that if you DO have some blow-by, that it's not severe enough to cause crankcase pressurization under heavy load or WOT. It also indicates that the PCV valve is working properly.

Another test would focus on the valve seals/guides........unfortunately it will also reveal a problem with leakage of the intake manifold into the lifter valley (highly unlikely and easily checked by removing the intake and examining the gasket sealing impressions). Run the engine to moderately high RPM (say, 5500) in a low gear. Close the throttle abruptly and check for bluish white smoke out the sidepipe(s). Check each side separately. It's also highly unlikely that a previous rebuild would have resulted in knurled valve guides.

An excellent option is not to replace the entire valve guide, but to install new bronze liners. They transfer heat better than cast iron (especially important on the exhaust stems), and are very durable.

[Jim M.]

Re: Compression/leakdown test results

Hi Larry: Without a doubt, the first thing I would do is check to see what type of valve stem seals you have and make sure they are in good condition. The "positive" type seals do NOT cut it on a big block. You MUST use a good quality "umbrella" valve stem seal. I learned this the hard way and could not believe the difference. Regards, Jim

[Larry T.]

Re: Compression/leakdown test results

Pardon my ignorance please; how do I tell the difference between the two?