Compression test and leak down test

The valves will be closed to a considerable angle before and after TDC. Remember that BTDC you are on the compression stroke and ATDC you are on the power stroke. The reason to get as close to TDC as possible it to help prevent sudden rotation of the engine. As you move off TDC the pressure on the piston exerts a greater force to rotate the crank. Leak down tests are typically done with the engine assembled but when doing the heads a leak down can be done with the cams still out in which case the test can be run at TDC or BDC. With cams in the either the exhaust or intake valves may be partly open at BDC. Of course, when you do the test at TDC you are testing the cylinder seal at the top of the stroke. When doing the test at BDC, at the bottom of the stroke. Notice no cams installed when the leak down was done after my heads were rebuilt.
Image Unavailable, Please Login

 

In that pic above....
anything lower than 40% leakage is in the green... meaning? thats ok?

i thought 20% or more and you were looking at valves....

 

You have to decide what you want to do. Someone just decided to paint it green, does not mean you have to.

I, personally, would not mess with anything less than 10%

 

it says right in the pic that 40% leakage or less is considered LOW on that gauge....
is there any engine where 40% leakdown is considered "acceptable" ??

 

Original guides were Ferrari bronze. The car is a 95.

When is the leakage too high? Very subjective. % leakage changes with what brand tester you use, test pressures, who does the test, etc. There is no automotive standard. For aviation the FAA mandates the design of the test equipment, the test pressure (80 psi) and the minimum allowed cylinder pressure (60 psi). That corresponds to 25% leakage. You would think an aviation engine would be more critical than a car engine.

Hot Rod mag did a comparison test of several leak down testers:

The important thing is consistency across the cylinders. I have purchased two LD testers and am concocting some experiments where I can control the leakage to see the effect of test pressure and the difference between the two testers.

 

I don't think most people understand this but that % leakage doesn't really mean much. It's just a number concocted on the pressures across an orifice. The other thing is that actually leakage flow rate is not linear with pressure. That is, a cylinder with 20% leakage does not necessarily have twice the leakage as one with 10%.

Frankly I would place much higher emphasis on a compression test than a leak down. We know what good compression numbers should be. And they tend to be less susceptible to who does the test and what equipment is used.

However, I would not be happy with 40% leakage, assuming the test equipment was decent.

 

Ahhh ha. Now that makes perfect sense to me... Thank You John
I was also led to believe that a compression test is a much better indicator of engine condition than a leakdown but that seems to be the opposite of what is said here on Fchat.

Thanks again for the lesson

 

Especially a car thats not driven daily.

My old Panhead takes 2 hours before it runs OK after sitting

 

drbob said the motor was running perfectly. That's what's important. We are using a diagnostic test to determine if there is an issue in the first place, which is kind of strange. You'd usually use it to diagnose an existing issue, not to determine if there is an issue or not in the first place. If the engine makes good power on a dyno, has no misfire and no puff of oil smoke on acceleration, decel or on hot start after sitting for a while, I don't know that I'd break out the compression and leakdown tester at all.

 

Mitchell is correct. Each leakdown tester company makes their own based on general principals of how a tester works. So you have to read their instructions. There are basically two kinds of testers you will see. The low pressure tester and the FAA
(or FAA-like) high pressure tester. There is no consistency among testers because of this fact. However, there is consistency among FAA certified testers because the construction of each is FAA "specific." John started to talk about that above. FAA testers require an 0.040 in (1.0 mm) orifice diameter, 0.250 in (6.4 mm) long, 60-degree approach angle. The input pressure is set for 80 psi (550 kPa), and 60 psi (410 kPa) minimum cylinder pressure is the accepted standard (phrase and picture from another site). The picture will help people understand how this tester works. So if using an FAA certified tester then you can follow those general rules of thumb you see like rebuild at 15% and I'm in heaven at 2% leak. There are many who swear that the low pressure test is what you want and maybe that works if they have consistency with their tester gauge set-up compiling data over many trials. I find low pressure units all over the map so despite the heavy hitters who love the low pressure method it is the FAA standard for me. We all have to pick our own poison. Also realize that the leakdown is not the end all be all test. You have to look at more than just the numbers to decipher true engine health. quite frankly one of the best tests is the smog test. You get real smog output numbers in real time on a dynamically running car. That tells you a lot about how and engine is running. It is all about burning fuel. OBD2 has made my life a lot easier giving me even more real-time data right on my phone!
Image Unavailable, Please Login

 

agreed , it's kind of like going to a doctor and begging him to find something wrong " i'm happy and feel good , but damn it ,please find something wrong with me "

 

Well, having an advanced degree in mechanical engineering and specialized in compressible fluid dynamics for the first 10 years of my career (before switching to electronics) I feel qualified to look at a simple LD tester. . I've been working on the theoretical analysis of these dam things. It pretty simple, given a few assumptions. I'm going to do some testing on my own, as I said, under controlled conditions. When done I may put up a document on the entire thing. Let me just say that I would not have a lot of confidence in the results of a LP test. At this time, I'm not even sure that under controlled conditions the results are as meaningful as a high pressure test. But, as they say, theory is theory, data is real.We shall see.

One thing I will add is that with any of these testers that read % leakage on the scale it is imperative that the test pressure be held constant. If when you connect the tester that pressure drops and you do not adjust it back to the correct value the test is bogus.

 

LD & compression test @ 8000 ft or sea level throw temps into it and KaPoooy

 

I look forward to your posted results. It is always good to learn something. It is possible that some of what you seek has been thought out in the FAA protocol. I have only read the protocol but don't know how they got there. Somewhere there are probably published articles in how that protocol got established and it may save you some time in research. I got more questions than answers.

 

I've seen John's preliminary document but I've been too busy to wrap my head around it. What I will say is John know's his ****!

 

I always thought that the primary purpose of the leakdown test was to help diagnose a problem that was found elsewhere. For example, if compression is found to be low in one cylinder, the leakdown test can help pinpoint why.

However, the conventional wisdom in the Ferrari community seems to have become, if leakdown is greater than X% (pick your number), then the engine needs at least some level of rebuild. Given the variability that is inherent in the test (see discussion above), this makes no sense to me. My belief is that a lot of engines are being opened up that really don't need it.

Don't forget that the shops (dealers or independent) that are recommending this work are not disinterested third parties -- they stand to gain a lot of business through engine work. I'm not trying to cast doubt on any particular shop. I just think that a buyer-beware attitude is warranted.

- Eric

 

Good luck with those statements. I've tried stating the same thing, and it was brought to my attention that i am the owner of a non Ferrari exclusive shop and can't possibly know what i'm talking about . Seems that owners trust that if their Ferrari expert shop says it then it must be true , like those expert shops could not possibly be interested in LARGE profits. lol.

As an owner and mechanic , for my car , if its not misfiring , burning oil or oil fouling plugs , there is no reason to try to stir up problems on a nice running engine , that goes for all cars not just Ferrari's.

If any other manufacturer were to build vehicles that NEED engine overhauls at 30k they would be criticized for building absolute junk , but for some reason Ferrari owners seem to revel , even brag about this . i wish i had customers in my shop begging me to find something wrong on a fine running car , begging me to please take some of this money that is weighing down their wallet

If the car is misfiring , running poorly , hey go for it , there must be a problem. My friends 355 puffs a little on the shifts , doesn't seem to bother him , doesn't seem to be hurting any thing for now , sure it will need guides someday , but its got 65k on it.

But what do i know , i'm no expert.

 

Its just the game has nothing to do with reality, you should know that

Now bike engine builders, I met and chatted with a few of them last few years. Now hey there these guys are on it big time.