Compression/leak down #'s
 

Hello,

I am trying to understand the numbers I obtained from the compression and leak down tests I did on my 1973.5 911t targa 2.4 cis engine. the numbers are:

The lowest compression value was 95 psi and the highest was 115.

The leak down ranged from 3% to 11%.

What should the compression and leak down numbers be?

Thanks, Gib

Your compression is pretty low. Should be closer to 140. Leak down of 3% or less is ok. The important thing is where the air is escaping. You need to know if it is getting by the rings, intake, or exhaust valves. This will tell you what the problem is.
Were the tests done on a warm or cold engine?
Ed

Did you do the compression test with throttle wide open? If not you will get low readings.

Leakdown of 3% is VERY good. But a 3 - 11% variance is bad. Like Ed said, you need to know where the air is escaping so you know how to approach the fix. It will be pretty obvious if you listen at the intake, exhaust and oil tank. That will tell you if it is intake valves, exhaust valves, or rings, respectively. Could also be other issues but they are the main suspects.

Also do a search on leakdown. Lots of info already discussed.

You are looking for leakdown of 3% or less on all cylinders? On a used car ? There must be more reasonable numbers than perfection ...

No Expert but consistency between cylinders is important on leak down and up to 8% can be considered OK, not needing rebuild.

I believe a DYSer first time should expect some variation and consider some margin of error in the numbers. If your racing or sunday driving take the use into consideration. racing..... 8% plan on rebuild.

If you are racing, then 8% leakdown leads to a rebuild, OK. If you are just driving it around town, a 10% leak down on 1 cylinder does not necessarily mean you tear down the motor to bring it back to 3%.

By the way, my good leak down gauge shows 2% leak down when IT IS NOT connected to a cylinder. So, if it shows 5%, then it really means 3% after substracting the 2% base line.

The first question that comes to mind when you were given your "leak-Down" results, is where were the leaks? The leak-down test is diagnostic tool that can tell you where your engine problems are. The person testing can listen to where the air is leaking. For example, air leaking, or "hissing" out of the oil fill tube may indicate that there is air getting past the rings, this may indicate worn rings. Air escaping out of the exhaust pipe may indicate exhaust valve ware. It will also tell you if your intake valves are leaking, or not sealing.

Much of the information that I have provided below is from Wayne's "How to Rebuild and Modify Porsche 911 Engines". It is an excellent resource. I highly recommend that you purchase it.



Compression Test;

In order for a compression test to be done correctly the engine must be set up properly. The over-all value will determine the condition of the valves or rings. The following should be considered for a compression check;

- The valves must opening and closing at he right time. If they are not, there could be variances in one cylinder to the next.
- Valves are to be in correct adjustment before the test.
- Worn cams can affect the readings.
- The throttle must be pushed all the way down.
- a weak battery can change the numbers, due to it's cranking power getting weaker from one cylinder to the next.

Different compression testers will give different numbers, as will batteries of varying strength. When a compression test is completed, the numbers are to be compared to each other.According to Wayne's book "a good rule of thumb is that each cylinder should read a minimum of 85% of the value of the highest cylinder"

An excellent motor should have a variation of less than 5-10 psi. However, if the motor is not in good order, the results can be inaccurate.


Leak-Down;

- This is a test that can eliminate many of the variables that are involved in a compression test.
- Wayne's book sates that a newly rebuilt engine is expected to have a leak-down of 3%-5%.
- A generally healthy engine should be around 10 percent or less.
- When numbers are around 20%, it is acceptable. This engine is an engine that has ware, but the numbers do not necessarily indicate a rebuild.
- Numbers around 30% indicate a problem that may necessitate a rebuild.

Hope this helps, if there are any discrepancies please let me know.

James

Now that is a lot more reasonable. Nobody should be looking for 3% from a used motor as a condition for a purchase. Nobody should be expecting that a leak down of 8% means an immediate rebuild.

Wow, thanks for all the info! Did not even think about diagnosing where the leaks were. I'll look into it and report back.

Thanks again, Gib

Don't forget "accurate" gauges are not really accurate, they all have a +or- factor that can and will skew your numbers somewhat.

How many miles on the 2.4 CIS engine?
Was the comp test done warm or cold? Plugs in or out on the comp test?
Were you definitely at wide open throttle for the comp test? What were the comp #s for each cylinder? As you can from others responses, there are several variables here...

My 964 had a 20% on 1 cylinder, I have been tracking it for a year-it is first in class regulary......runs like a top.