Compression test results what do they mean

A lower number is better for leakdown. 3% is great/13% marginal. Did he give you compression numbers for each cylinder also?

 

Is this a car you are considering purchasing or one you already have? Compression numbers are important to determine true engine condition. My '85 308 with 26K miles was less than 4% leak down on all 8 cylinders and compression was 180-190 on all 8. From what I have read, anything less than 20% leakdown is acceptable. Actually, I should retract my statement that 13% is marginal, it's probably OK. Get the compression numbers though!

See here:

http://ca.autos.yahoo.com/maintain/answer.php?cylinder_head_answer5.html

http://www.2carpros.com/topics/compress.htm

 

with no. 1 being what it is as compared to the others I wonder if the valve seat had a small amount of crud/carbon on it to cause it to leak a little. The rest look good and no. 1 isn't that bad. As was said above the compression nos. are a must to look at along with the leak down nos. Regards, Vern

 

Lots of guys say compression tests are a waste of money. The leakdown is much more important. If there's "no" leakage than the compression will be fine. If it's leaking then you know there's an issue. Your numbers are probably fine as far as it goes. Lots of cars "pass" a compression test yet have major issues anyway.

What prompted you to test the car in the first place?

Ken

 

13% is NOT high for a high performance street car, even a Ferrari. As a matter of fact, your numbers look darn good for an eleven year-old car.

You're in good shape. Driver it and enjoy it.

-Peter (not concerned until 20% or more, then it depends on where it comes from...)

 

I love reading threads on leakdowns. I could swear that about a year ago folks on this board were advising others that a leakdown of >8% meant your engine was near death...

Today, 13% is good .

I searched this extensively a couple of years ago...

Seems that in the porsche community 2-3% leakdowns are considered good.

In the Ferrari community, 5-8% or less is good.

In the Volvo community...12% or less...

Talk to a oldschool american iron mechanic...20% or less is good.

Talk to a ferrari dealer, and, if you are buying, less than 13% is good. If you are selling, over 6% requires head work .

Pick your resource .

 

13% is high. Its most likely top end related and cosidering 355s have valve guide issues I would think that it probably has loose guides which over time effects valve seats and how they seal, not to mention oil consumption of some sort. A compression test is a waste of time becasue there are so many factors that effect it and the most important observation of a compression test is how much of the final reading was achieved in the first compression stroke. The majority of the final reading should be in the first pump to indicate a good cylinder rather than the end reading being 150psi for example. I would repeat the leak down test again in a few weeks to see if the results are the same and then keep it in the back of my mind that when its time for an engine out service I should yank the heads and do the top end while im there. If thirteen percent is acceptable then you have an engine that is just acceptable rather than great.

 

Read what bpu699 is saying there are a million opinions on this subject your numbers are not bad. Go out and run the car hard for awhile bring it back to the garage and do a leakdown and compression test yourself. There is nothing you can do to make 13% into 3% short of pulling the motor pulling the heads and doing a valve job. Only one cyclinder is that high and that isn't that bad. Drive it, have fun, keep an eye on it, watch your oil level. At this point whatever you do don't lose sleep over it. When they are at 20% in all cylinders then worry about it if you keep an eye on it you will know long before it gets to be a serious problem. Regards, Vern

 

I would drive the car and deal with it in the future when some other major repair like the engine out service is taking place. Performance loss wont be noticed unless measured on a dyno before and after the repair. Oil consumption might be the only symptom you see, that is if you drive it regularly and change the oil twice a year. Im afraid to do a leak down test on my boxer because the results will eat at me if they arent what I want to see and then Ill get myself up to my eyeballs in valves and cams when I could just drive it and be happy. Drive it, enjoy it and if it puffs blue smoke on start up you know why.

 

Just for general reference...you have to be careful when comparing leakdown results done with different leakdown testers. The leakdown gages have a flow restrictor orifice in them, and different brand gages may have different size restrictor orifices in them causing different readings between two different brand gages on the same cylinder. The gages work as follows: the source air pressure is adjusted using the gage regulator to apply 100 psi to the restrictor orifice. Air flows through the restrictor orifice and pressurizes to 100 psi the chamber that is connected to the cylinder pressure reading gage, and through a quick coupler and hose, to the cylinder to be tested. Once connected to the cylinder to be tested, the leakdown gage shows the pressure that results from the 100 psi flowing to it through the restrictor orifice, and whatever pressure is leaking away from it due to leakage sources at the cylinder. A larger restrictor orifice will result in a lower leakage reading on a given cylinder when compared to the reading resulting from the use of a smaller restrictor orifice.

 

The problem with all of these opinions, is that they are like butt holes and we all seem to have one. There is only one source of a true opinion, IMHO, and that is regarding aircraft, aircraft engine manufactures, and the FAA, as well as other aviation related government regulations worldwide. There are no laws or regulations regarding any other internal combustion engine in any other product anywhere in the world that I am aware of. Simply put, aircraft that don't run well are deadly and dangerous, hence annual and hourly inspections that include a "leakdown" test. They do NOT do compression tests on airplanes, they do leakage tests.

Aircraft engines are totally comparable to any other engine. They have piston rings and valve seats. There is no differentiation between aircraft engine manuafctures, or size. Some have huge cylinders, some small, etc., but the specifications are identical. Read advertisements for aircraft for sale, and ALWAYS cylinder leakdown numbers are included.

Generally speaking, 10% would be time for overhaul. 13% could get you grounded by the right mechanic. 2 or 3% isnt any problem, neither is 5%. But as leakage increases, the volume of hot flame going past the area leaking, creates a cascade effect. I personally would want another test done on a hot motor after a hard run, and not only confirm the numbers, but know "where" its leaking. If its mainly the rings, I would watch it. If its a valve, its going to burn much hotter and could eventually melt under the right conditions.