Leakdown and Compression

Once the spark plugs are out, a compression test takes only a few minutes.

 

What model/year of car is it?

 

A compression test is really a way to compare relative the health of your cylinders. I suspect you know that it's simply a pressure gauge with a rubber fitting which you press into the spark plug hole after you remove the plugs. Then you crank the engine over with the starter and record the readings. I doesn't take a genius to realize that the reading depends on the condition of the battery and the starter. So the actual reading isn't as important as how one cylinder compares with the others. If 7 of them read 130 psi and one shows 55 psi, then you know where to start looking. In truth, this test may reveal a bad cylinder, but doesn't really say much about the overall health of the engine. For example, an engine with a weak battery and/or starter showing 115 psi on all cylinders may be better than a well worn engine with a really strong battery showing 125psi.

Since it's done with the engine turning, it may also reveal things like cam timing problems, but usually only in the hands of a good mechanic.

A leak down test, on the other hand, tells you directly how much air is escaping past the valves and rings with that cylinder locked at TDC. You pressurize the cylinder using air from a compressor passing through a simple device (a flow meter actually) fitted with a couple of gauges and it tells you how much leakage you have. Usually a figure less than 20% is OK, but that number is not cast in stone.

The FAA has standards for aircraft engines, but as far as I know there is no similar automotive standard. Nevertheless, an experienced mechanic can tell a lot from using both tests, especially if he's tested many engines of the same type. This knowledgeable individual may be able to spot a tired engine. A smart owner with one of these devices running the test for the first time............ well it's hard to say.

By the way, the reason leak down tests are not as common is that you have to find TCD for each cylinder, lock it there, and then supply air from a good compressor. Plus the device is more expensive than the simple compression tester. A simple compression tester can be bought for a few bucks at any auto supply house and the test can be done without any equipment other than a spark plug wrench.

 

if thgis is representative of all cylinders-these are whaqt we call:" rock star numbers"

 

It is regarded in racing circles that a leak down test is superior to a compression test.


ago

 

Different tests. One is not superior to the other.

Depends what you are looking for. On a race motor all the variables are gone and you want to know one thing, ability of the cylinder to seal, sometimes referred to as wear and that is what a leak down tester tells you.

 

Leak Down Number :: Status

0%-1% :: too tight, 1% might be acceptable for a nearly brand new engine.
2%-3% :: engine is in the prime of life
4%-5% :: engine is over the hill but running well
6%-8% :: engine is nearing the end of its service life

We have history that indicates a F355 at 7% is going down hill rapidly and is unlikely to make it to another major service interval.

 

What is the test pressure?

 

the numbers listed above are way off everything I have ever heard. always been told less then 10% is good and at 5% engine is great.

 

I have had this discussion many times with customers. Here is my 2 cents. I always recommend a compression test. It is relatively easy and quick, and will give you the information you need to proceed. Even acceptable numbers across all cylinders usually means you can stop looking. It is rare that an engine will suffer excessive wear or damage equally on all cylinders. If you find one bank is higher but still even that is a good sign of bad cam timing. Easily cured during a major service. I have seen this one a few times.
Some times tight valves (non hydraulic) can cause low compression numbers but will have a good leak down because they actually seat well, just very late. Again, cured during a major service.

If I find numbers that are low on some cylinders than a leak down test will tell me where the leak is. With Ferrari it is almost always valves and guides. 355 have been known to kill a liner or two with bad valves as well, but not a high percentage.

In recap.
Compression test will indicate the health of an engine.
Leak Down test will indicate where the problem is.

If your yearly check up (compression test) at the doctors office finds nothing that a CT scan (leak down test) is not needed.

If your tech starts with a leak down and it is good, a compression test can still find poor cam timing or tight valves that don't allow the piston to make good pressure.
If you start with a compression test and have no issue, a leak down should find nothing.

Just my experience.

 

Wade,

Would you please take a look at this thread;

http://www.ferrarichat.com/forum/348-355-sponsored-bradan/456521-now-my-idea-ppi-pics.html

And give me your opinion on the compression and leakdown numbers.

Car is a gated 97 F355

 

I agree with the assessment of cylinder #6 and the others likely to follow. You can drive it for some time and enjoy the car. It will need to be repaired at some time. I use aftermarket guides and valves for head repair on older model F cars and have for the past 20 years. I have used the aftermarket on 355s for the last 5 or 6 jobs and had no issues with them.
The rack boot and wire damage appears to be rodent related, this could explain the airbag and CEL. Not a huge repair when you finally track it down and find it. This could take some time. They are odd creatures and can eat the strangest things. I can't imagine why they go some far into a car to eat wires and rubber boots that can be found very easily. I have seen them eat all of the rubber boot off of wire connectors as well.
I am not in the money side of the job. I think if you are happy with the money side and go forward understanding you have issues to address, then I would not be afraid of this.

 

That was a pretty good rule of thumb in 1960.

Things have changed.

 

Thank you.