Acceptable compression/ leakdown #'s

too tight = not adequately closed = lower pressure or leaking pressure

 

BTW, this is serious valve burning condition as the exhaust valve is usually the one running tight.

 

A tutorial/clarification:
Tight means that the valve-cam clearance is too little because the valve/valve seat wear has let the valve back out towards the cam.

Unless periodicly adjusted, the clearance can go to zero, meaning the valve can't completly close.

 

Thanks, gentlemen.

Is there any way besides using new shims to adjust this clearance on 348's?

 

Rick, could you please walk me through what is happening to cause such a "valve burning condition" when the valve-cam clearance is too tight...meaning that the valve isn't fully closing...meaning that gasses are blowing through when they shouldn't.

I mean, when the valve is open it is fine for gasses to be blowing by. When the valve is closed/seated then the valve itself sees the hottest gasses but none pass by...so why in between is there such a serious condition?

 

Quick and easy answer: Most of the exhaust valve's intense heat is transmitted (conduction) to the valve seat when the valve closes. When the valve can't close tightly, the valve overheats to the point where the metal burns.

 

If the valve doesn't close, there is no heat transfer back to the block, and the valve over heats. At least that's the layman's explanation...

 

Thanks (Rick and Bo)! That makes sense.

 

Agreed. I can generally tell on the test drive within 300 yards whether I need to "dig deeper" with a compression test, then further with a leakdown test, only if the compression test reveals a problem.

I'll do it if someone asks me to as part of a PPI, but if the power is adequate, throttle response is good and the idle is smooth, that'll tell you more than highly variable test results. Unless there is a substantial compression test value differential in one or more cylinders from the "average," it's good to go. These things don't just go to pot...

Besides, in the last 25 years of doing major services on V-6, V-8, V-12 and flat-12's, I have yet to see a valve clearance close to zero during regular maintenance checks. While I'm sure there are burnt valves out there, it is not the norm, or even very common.

Now, mid '80's Alfa exhaust valves? That's what bought me my first Ferrari! <grin>

-Peter

 

I hate to keep bringing aircraft into these discussions, but a leak down test is FAA required on all aircraft at least anually. There is no compression test. No matter what engine or who made it, 10% leakage is getting close to time to ground the airplane. Cars are the same, and it makes no difference if its a Honda or a Ferrari, they all have rings and pistons and valves and they are all supposed to seal tightly.

If I were looking at airplanes, $50K+ Ferraris, or any high value car for that matter, outside of looking over the rest of the car myself, I would have a leakage test done by someone who knows what they are doing if I were far from home, etc, otherwise I would do it myself.

While its true a compression test could be all over the place, a leakage test doesnt generally lie. Not only do you see the actual percentage of leakage, you can hear where its leaking as well. Anything showing much over 5% loss I would probably walk away from, unless the seller were willing to make drastic consessions. Valve timing or throttle position will not effect a leakage test either, so it pretty much tells you whats up.

 

I agree a leakdown test is a good diagnostic for an engine that's having trouble. Cars without tight sealing valves will invariably smoke and the test will let you know from whence it came. I assume a leaky intake valve is not cause for immediate panic while a leaky exhaust valve is not long for this world.

I know a lot of mechanics say they only do a leakdown as you know the compression test will be bad if you have a leak, so it's a waste of time to do both.

Ken

 

You're right and on reflection, I probably shouldn't have brought it up. Sorry for the detour.

 

Relying on only 1 or 2 pieces of data to determine whether or not to make a significant expenditure is foolish. As a prospective purchaser, or as an owner trying to make the decision as to what to do with a car, I would think one would want as much information as possible. Compression test, leak down test, road test, spark plug readings, throttle response, idle condition, opinions of experts, and anything else that is related. Only then can an intelligent decision can be make.

As to Ben's original question(s), based on the numbers that have been given posted, and if you (Ben) trust those numbers, I would concur with the mechanic's assessment. If you really want the car and plan on using it for several years to come, I would suggest you put aside an additional $10K - $15 for contingencies.

 

You have received a lot of advice and all of it points in the same direction. If you are paranoid, do all the tests. If you are trusting, do none. The best answer, like in most things in life, is neither end member. Do enough testing ro reassure yourself you are not making a stupid mistake and rat-hole away money to fix the things you didn't catch.

The big dilema here is that Ferrari ownership is a really emotional thing and car-buying should be anything but emotional. Thus, the dilema. What we have to strive to do is throttle back our "have to have it" emotion but without inducing "paralysis by analysis" paranoia. I mean, as much as we want to idolize our cars, they are just 'used cars'. When one buys a used car they are buying it from someone who wants to get rid of it! Why, we should ask.

Another poster commented that he spent 10 months selecting the right car. There is NOTHING wrong with that plan is the search is part of the hobby. We must remember that life is a journey, not a destination. If the hunt is fun, go for it. If you have to have the car NOW, buy one from Ferrari of Somewhere and get a warranty. Then again, there's thet end-member thing again.

For what its worth, I have owned my '79 308GTB TWICE! I bought it in 1997 then sold it a year later when I got a Mondial QV. I bought the same 308GTB back in 2000 and sold the Mondial 6 months later. Both cars were great BTW, but like in love, your 'first' is always special...

Rick

 

Look..You can be fooled by both..I have. I had a car with a valve seating issue. it had a constant miss at idle. We went down the list, plugs, wires, swapping out injectors, intake gaskets etc. Obviously the first thing we did was a compression check and a leak down test, when the engine was warm and cold. Everything showed almost perfect. Compression was excellent, and the leakdown was only 10%.

After pulling my hair out for almost 4 days on this car, I yanked the head anyway and sent it to the machine shop to be looked at..sure enough it had a bad valve seat. I asked the machine shop why this did not show up on my leakdown test, he said its becuase the spring pressure will hold the valve shut, but when its running and the combustion chamber is hot, the valve cannot seal 100% under these conditions causeing the miss.

A leakdown and compression test ARE NOT THE BE ALL AND END ALL OF ENGINE DIAGNOSIS!! You cannot test for bad valve guides with a leakdown test either. They will only show if they are really bad, but you will know this as soon as you start the car anyway from the blue smoke out the exhaust anyway!

It is a good test, but anybody with any kind of back ground with cars and engines will know just by the performance of the car if there is a issue with the internals of the engine. On a 12 cylinder it may be harder to detect..this is were a compression test would come in handy..leakdown would follw if a problem was found to determine what happend.

 

Agreed that all these tests should be done at operating temperature, but could you please clarify what you mean by WET and DRY tests?

If WET means with the engine well lubricated, such as after a drive to operating temperature, would a DRY mean that the engine has sat a while, the oil drained back into the pan/sump and is no longer at operating temperature?

Thank you in advance for helping me learn. I've heard that compression tests performed when hot will provide the most information but that a cold will give an additional data point.

 

But why not just do the leakdown? If it passes that, how can the compression be bad?

Ken

 

It's quite a bit quicker to do a compression test.

 

Bad timing will mean that your valves are open at the wrong time, yielding lousy compression.

Your leakdown test tells you if you are getting good sealing. Your compression test tells you if each seal is occurring at the proper time.

Consider: you won't get good compression with open valves letting air/fuel flow out rather than be compressed!

 

Tbakowsky, I agree with you. I've personally seen the blue smoke from my 348 on startup when I had bad valve guides.

However, at least one well-respected Ferrari pro is telling anyone who will listen the opposite, that bad valve guides don't smoke.

http://www.ferrarichat.com/forum/showpost.php?p=135926383&postcount=8
"No visual signs only leak down test. Have yet to see one with bad guides smoke on start Dave"

 

The 360 that we sued Ferrari over had less than 3% leakage on all cyls #1-4 but cranking compression of 140. The cyls #5-8 had compression of 175-210 and leakage of up to 35%.

1-4 had a cam that was mistimed by the dealer and 5-8 were being destroyed by the debris from the clogged, partially disintegrated precat that was being ingested by the cylinders on that bank. Dealer repeatedly denied anything was wrong with the car or the performance level.


Point is both tests are valid and provide different but overlapping information. Depending on the nature and reason for the diagnostics, neither should be ignored. Both are very open to interpretation, especially cranking compression, and cranking compression is also the easiest to perform wrong and due to equipment and methodology, the results can be inconsistant. Cranking compression is like measuring anything, if you do it the same way with the same equipment every time, the results are valid to you, but maybe not someone else.


By the way, we won. Ferrari took the car back and gave the owner a great big wad of money. The State of California found the dealer in violation of 6 counts of the auto repair act of 1971 and threatened to hold up certification of the 430 for sale in Ca until FNA settled the case.

 

Apples to oranges. I've seen 355 guides that you can fit 2 valve stems in and not smoke.

Have a look at the back of the car, the fact that one says 355 and one says 348 indicates they are different cars.

In 1980 Ferrari built a car called the 308GTSi. They were famous for not being able to go between fillups without using so much oil that they were starving. They never smoked.
Ferrari also built a car called the Testarossa. In some circumstances they smoke like a train on startup brand new.

 

Some people focus on details and not the big picture, though. I personally will never pay a shop for a PPI. Ive been around cars and engines for 20 years. Thats the difference. Took me 7 months to find mine and no one is more critical of overall condition/performance than me. Im very passionate about cars and guitars, my two hobbies.

 

That may be very true but I bet due to my years of being around them every day I bet I can find more than you. I would be willing to bet that I can find enough more on any car to cover my fee.

 

Any of us can find a list of things wrong with any pre-owned car, whether its an '06 or '66. Im positive you could get more detailed on any problems then I could. Im saying if there IS a problem, large or small, I will pick up on it and weigh the decision if its worth walking away or not. Again, you have daily hands on, i have 20 years with only 6 of them hands on. I cant justify any fee to myself. Im not the average.

Didnt post it to start an argument. Im just saying that when I appraise a car, i take the entire car into account before Ill put the number on it.