What do compression #'s tell me?

[jess5engine]

As you can tell from subject of this thread that I'm a complete novice. I got my very first Porsche about a month ago and I'm spending way too much time (at least according to my wife)learning as much as I can. I got Wayne's book(s) and plan on tackling some of the easier stuff and working my way up to the more difficult tasks.

But back to the original question. When I got the car the PO gave me all the records and on one of the invoices were the compression numbers tested when the car had about 123K miles. It has close to 124K miles now. The numbers ranged from 180-170. What do these mean? I've read in the Tech forum about the 3.2 having probs with premature worn valve guides. Do these numbers indicate anything related to that? Thanks in advance and someday I hope I can answer this question for some guy that's just starting out.

Jesse

[Rot 911]

170-180 across the cylinders is very good. To find out about worn valve guides you would really need to do a leakdown test. If you do a search under my name and "compression" and "leakdown" you will find a posting of my problems with worn valve guides and a burnt exhaust valve on my 3.2 with 133,000 miles.

[Wayne 962]

Read Chapter One of the Engine Rebuild Book - it's a pretty good, basics look at what these numbers tell you...

-Wayne

[rick-l]

Originally posted by Kurt V
you will find a posting of my problems with worn valve guides and a burnt exhaust valve on my 3.2 with 133,000 miles.

Someone told me to do a leakdown test to see if I had worn valve guides. Is this BS?

[Wayne 962]

Chapter One tells you all about compression and leakdown tests, as does this technical article:

http://www.pelicanparts.com/techarticles/mult_Engine_Rebuild/mult_engine_rebuild-1.htm

-Wayne

[911pcars]

"Someone told me to do a leakdown test to see if I had worn valve guides. Is this BS?"

No, but maybe uninformed.

Assuming both valves are closed during a leakdown test (otherwise the open valve is a big leakdown source), there's no way the valve guides will be subject to air pressure pumped into the combustion chamber. Anyone have a nice illustration of a combustion chamber to show us?

I think the usually way to tell is to remove the valve cover, then push sideways on the valve stem to see if it wobbles.

I doctored this pic from a Bill Verberg post to illustrate. The area in red is the combustion chamber that is under pressure in a leakdown test. The valve guides are outside of this area.


Sherwood
http://members.rennlist.org/911pcars

[Wayne 962]

Sherwood,

I kindof agree with you - in principle. As the guides wear, the valve has a tendency to no longer sit straight in the seat (sometimes), causing a loss of compression (sometimes). Excessively worn guides can sometimes show up in this manner on a leakdown test.

But, in general, you're right (as per usual!) - you can't measure how much oil is getting past the guides if the valves are closed...

-Wayne

[911pcars]

"Someone told me to do a leakdown test to see if I had worn valve guides. Is this BS?"

No, but maybe uninformed.

Assuming both valves are closed during a leakdown test (otherwise the open valve is a big leakdown source), there's no way the valve guides will be subject to air pressure pumped into the combustion chamber. Anyone have a nice illustration of a combustion chamber to show us?

I think the usually way to tell is to remove the valve cover, then push sideways on the valve stem to see if it wobbles.

I doctored this pic from a Bill Verberg post to illustrate. The area in red is the combustion chamber that is under pressure in a leakdown test. The valve guides are outside of this area.


Sherwood
http://members.rennlist.org/911pcars

[rick-l]

Originally posted by Wayne at Pelican Parts
Chapter One tells you all about compression and leakdown tests, as does this technical article:

http://www.pelicanparts.com/techarticles/mult_Engine_Rebuild/mult_engine_rebuild-1.htm

-Wayne

I got the rebuild book on Friday. I haven't gotten very far reading it late at night in bed.