With more than a few years at it, I only recall one vehicle with compression over 200psi and it wouldn’t run because the exhaust was restricted. I can’t imagine how the engine can operate without heavy detonation with that much compression.
I agree with Rod Knox of course. The highest compression of any car I’ve ever run across was around 190 PSI and even after working for VW quite a few years I never saw a VW gas engine anywhere near 200 PSI. With about 20:1 compression ratio 500 PSI, and higher, is normal on the VW diesels. Halving a compresson ratio to 10:1 does not equate to roughly half of the diesel compression.
The camshaft profile, or the cam being out of time, has a lot to do with it.
I think some of these inflated figures are from a manual and many times those figures in the manuals are dredged up from who knows what. I’ve seen a manual on a Dodge once that specified 120 PSI as “acceptable”. Acceptable if one likes low power, wheezing uphill, potential oil burning/fouled plugs, and getting by while staying close to home I guess.
One can also have perfect compression and still burn oil like mad.
Good point and one I’m surprised no-one has already asked.
The engine is derived from a VW diesel, is therefore built for high compression and has direct port injection, this allows the injection system to squirt the fuel into the cylinder at a much later time in the compression cycle than would otherwise be possible.
Note the VR6 also runs at 200psi+.
Agree with all your points, I remember the blurb being published on that engine in 2000, I thought it was a typo until I double checked. And you’re correct on the math, half the compression doesn’t equal half the psi, many other factors are involved including valve timing.
Here’s an example of a vanilla VR6 engine under test, the reading is a little probably due to piston crown and combustion chamber carbonization but the VR6 runs at lower pressures than the 2.0 liter anyway : http://forums.vwvortex.com/zerothread?id=1510186
I think you’ve found a mechanic that is full of hooey and did a piss poor job of rebuilding your engine. Rings are flexible, they are supposed to be, their purpose is to fill the gap between the piston and cylinder and keep the piston from wollowing around in the hole.
I don’t know what the compression is or is supposed to be in that engine, but given the BS he’s given you so far and the evidently poor rebuild job, I think you are in for paying to redo his work at another shop. IMO, he owes you a refund for his improperly performed work. You’ll never get it likely, but it sounds as if he owes it.
Skip
I’ve got to agree with Skipper that something is not right on this ring job. There’s a dozen things that can wrong with a set of improperly installed piston rings and I’m curious how this guy knows the rings are becoming weak unless the engine has been torn down yet again.
If those rings only lasted 1500 miles then the mechanic did something he should not have done or overlooked some flaws.
Wonder if he even replaced the rod bearings at the time?
I can sort of tolerate the 200 PSI reading because I’ve seen a few close to that neck of the woods but 225 is bit of a stretch.
If this engine (with obviously a pretty lousy ring job) is coughing up 225 PSI of compression then my humble opinion is that this could be caused by early cam timing and the timing marks should be rechecked.