456 Compression and Leak Down Test Numbers

My engine was rebuilt and leak down is < 2% on all cylinders. Anything above 10% would concern me. 14% and 16% would cause me to wring hands. Could be carbon build up on exhaust valves though may not it be a huge deal you can do various things to help that. Seafoam though intake vacuum works well on the imports. Smokes like you would not believe though. You can do the leak down and listen for air and determine where the air is going, exhaust valves or intake valves or rings. Technon does diddly as a remedy I found.

For me at least leak down is more important than compression and compression is a momentary bump, leak down tells how well its holding. People love compression tests, mainly because they can mis-lead as to condition.

 

Plus or minus 10% is considered to be very good. If you take 185 psi as an approximate median, than all your values are within + or - 18 psi of 185 psi. Everything seems to be OK.

 

For compression tests yes. Like I said everyone focuses on the compression test. As if the leak down is incidental. It should be the primary test for me at least other disagree and that's fine. People like leaky engines apparently.

Those leak down numbs don't tend to get better with time. When there is actual violent combustion a whole bunch of power escapes and is going where its not supposed to at that scale.

Some folks say 20% leak down is fine. Ok... but at some point is was likely 2-3%. See what I mean about trajectory and time. Could just be carbon next step is to determine WHERE the air is going. Seriously.

 

The point of a leakdown test isn't just to measure it, but to diagnose what is causing excessive loss so that the problem can be fixed. The numbers alone aren't worth much, but I'm in the camp that believes anything over 10% is cause for concern, even though the engine may run fine.

 

I would buy that car if those are the only test data I have. While more than 10% leak down is a cause for concern, those few numbers are not a reason to tear down the engine and spend $25k for a rebuild. Buy at the right price, drive, drive hard, drive harder, re-test, and go from there.

 

16% on cylinder 10 deserves some investigation. Could indicate a future valve guide issue. Or a valve adjustment is badly needed.

 

456s and 456Ms were susceptible to the dreaded brass valve guide problem that also afflicted the F355. Ferrari fixed it on the F355 during production, but never bothered on the V12s until the 575M and its sintered steel valve guides. Often there would only be a few non-bronze valve guides in a set, so it was really hit and miss. Most 456s and 456Ms (and 550s) were not afflicted.

The sintered steel valve guides from Ferrari are so expensive, most techs recommended using good quality bronze guides from other sources.

 

As this is an automatic I would perform a valve adjustment, remove intake and clean intake valves as it is common to see intake valves carboned up due to low RPM use. I suspect you will find this is due for a valve job.

 

I would not be afraid of this car. It is an old car and numbers will not be perfect. Compression test is very good indicator of possible issues. 175-190 is well within 10%. Leak will help pinpoint an issue. As stated already, without further inspection with the leak down, it really doesn't tell you much. It is possible that a good ole' Italian tune up will help those numbers. I have driven a number of cars really hard and you would be shocked at how much junk gets blown out the exhaust when driven hard. No guaranty that it helps, but it is a good time.

 

I remember an old Aston-Martin maintenance schedule that called for "decoking the cylinder heads" at regular intervals. Italian tune-up does some of that for you and the Ferrari V12 is a lot cleaner burning than the old A-Ms.