Compression test results

I am sure I will be corrected if I am incorrect but the Pico diagnostic results are from resistance in cranking analysis. Unlike an actual compression test which measures the pressure generated by the 4 cycle process of the engine. If the Pico results are within the limits, I would say that is acceptable. However, if you are worried, maybe pay for an actual compression test with a gauge and then decide on how you feel about the results.
Why did it need head gaskets and a reseal?

 

I too would wonder why it needed head gaskets. In the real world anything operating at 75% capacity is worn out. Compression tests are done with a gauge attached to a cylinder. Anything else is a waste of time and an option provided to a "Technician" who is clueless and who very likely knows nothing if not told to him on the screen. Get a real mechanic to do a real job the correct way.

 

The major components that affect compression are piston rings and valve seats. If you have a concern about your engine’s compression, have an actual compression test performed as recommended above. The rule of thumb for compression results is that all values should be within 10% of each other. If the results are out of range, then have a leak-down test performed.

 

I'm a little curious about the 10% rule. Does this apply to both modern and vintage cars? The reason I ask is because in the Roush Daytona book under how to buy, they quote under 20% variation for the compression test as OK. I can see 10% variance for modern cars but how about vintage.

 

A 20% rule would not apply to a car built in the 30s any more than it would to a car built in the 21 century. Technology evolves.

And if I had a Daytona I would be in a very bad mood it it had a compression variation of 20%.

 

Correct

There are two ways voltage and current ..current being more accurate. You can also add an transducer in one cylinder to add calculated pressures to the scope based on actual pressure from that one cylinder. It is a quick way to identify any major issues or to pinpoint a bad cylinder. 75 percent does seem indicative of s problem but without knowing , like in a real compression test, the variables in the engine that would cause more piston resistant (allowing more air) ( different flap arrangement or timings) on the 488 I would be able to comment.

And yes nothing better than a true compression test with gauges

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I'd say it's more a matter of "degree of OK" -- if all within 10% = things are good; if 20% variation = you're probably getting near where something will have to be done soon, but if between 10% and 20%, it will probably run OKish. Regular use versus disuse gets into this equation, too -- a compression test on a regularly used car is pretty trustworthy; on a long disused car, not so much.
If you'd like to see some examples of just how resilient ICE are when in poor-ish/neglected condition, check out "Dead Dodge Garage" on Youtube.

 

Modern engines many of which come with various types of coated, hardened cylinders which have slowed wear and maintained higher compression retention and lower oil consumption show very little wear for a very long time as long as no unusual influences change that. We have seen once wear does set in and our experience on the Ferrari engines anyway, once that wear is measured as getting over 10% leakage it starts to rapidly accelerate. 10% just isnt a problem in terms of its ability to run well, generate power, pass smog, etc. It is however an indicator. In an engine like a Daytona it could maintain 10% loss for a great many miles and would be of no concern. 10% in a 355 or a 360 or similar will soon be 20% and rapidly down hill after that.

 

How many miles?

Anyway, I'm with Brian - compression test and leak down test using an actual calibrated gauge. WTH is a pico test anyway?

For me, personally, when it comes to a compression test (done when the motor is warm, not cold), I like to see all the numbers hoovering around each other by a few percent with the highest and lowest range not too much over 5% difference.

I just ran a compression check on my Toyota motor a few weeks ago (while adjusting the valves) and here are the results (see attached). In the results below cylinder #2 has a slight score on the wall above the 2nd compression ring (but roughly at the top ring), so its reading is probably a touch lower due to that.

But having one cylinder 25% away from the rest - and again, I have no clue what a Pico test means - sounds awfully bad to me. Normally with a compression test, if you see any single cylinder more than 15% out from the group, that's generally cause for alarm or at least concern and further investigation as to exactly why. A leak down test will tell the real story. Is the system capable of holding pressure or not?

While rebuilding my Toyota motor last time around, I did a compression check on the cylinder head/valves - while diamond lapping the seats - until all leak down across all 4 cylinders was 0% at 95-98 PSI (max reading on my leak down gauges). I haven't performed a leak down test now that the motor is back together, but it's on the todo list soon here.

Ray


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Ray

A pico is an oscilloscope basically... mainly used by mechanics

Using the oscilloscope to map current draw from battery while cranking engine ..you can get a nice wave form showing each cylinder.. if the wave raises less or more than the others it show relative more or less resistance..... resistance being the compression

Some mechanics will stick a probe in one cylinder and use the actual compression number from that cylinder to calculate the rest


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Got it, thanks!

I'll stick with pressure gauges over here

Ray

 

Its for people who are too lazy to do a test they do not understand anyway. The industry is now full of people whose other job choice was at the counter of a self serve gas station.

 

I'm with ya. I tell people this kind of stuff all the time, especially when they start talking about taking their car to "the dealer" for service.

I won't bore anyone with my stories this morning, but yeah, the level of ineptitude rampant in the automotive service industry today is absolutely shocking.

Ray

 

I forgot to say. Diagnostic equipment manufacturers love to convince shop managers and owners how much that kind of useless gadgetry is going to make life better and more profitable. Most of that kind of stuff has been dumbed down so the quality of so many people in the industry is not a barrier to doing some level of diagnosis.

 

I was helping a mechanic from a Ferrari dealer on an older car over the phone. An exhaust analyzer would have been a big help to him. I was a little surprised not only did they not have one but he had no idea a lot of diagnostics can be done knowing the ratios of the various gasses.

 

Yeah, I totally agree with you.

Just last year, I was calling around about having a driveshaft balanced. Just not getting warm fuzzies from the shops I was talking to. I asked what RPM they balance at and wasn't getting clear answers. To what gram do you balance to? Nobody seemed to know anything other than, "I think they just put it on the machine and push some buttons". Ugh..

Finally I ran across a guy up in San Jose who has been doing it 30 or 40 years. He was able to speak intelligently, so we got to talking. Near the end of the conversation, he said something like, "I actually like to spin the driveshaft up and feel the vibration by hand; machine is helpful, but there is no replacement for the human touch". That's when you know you found the guy to balance your driveshafts...

Ray

 

Yes, exactly. Reading plugs, reading exhaust, A/F ratios, etc, etc... it's all a big mystery to people these days. We have similar issues in the computer tech world creeping into the mix. "Hackers" who don't understand binary or hexadecimal anymore. I was just talking to someone involved in regression testing for AI driving up in silicon valley yesterday and they didn't know what "asm" meant. Like whoa.. assembly language? You never heard of that? I would hope that's what your lower level AI is partially written in - and even if you don't do the coding yourself, I would think you'd have heard about it around the office kinda deal. In fact, this kind of stuff is why I give Tesla's a wide berth on the highway. FSD my ass.

But yeah, I would think jetting the carbs on an older Ferrari without having a very firm understanding of exhaust gas analysis would be rough

Ray

 

The only point I would make is it isn't necessarily laziness. In some instances you cannot access the plugs without removing the intake manifolds. The Pico can give you a good idea if you have an issue and you can trigger it cylinder number 1 so you know which cylinder you have an issue with and justify removing intake and then taking actual readings. Its a good tool and has a place providing the electrical system is in good shape but if I had one showing a cylinder at 75% of the others it would get a mechanical check. My experience using it on the F12 and 812 has been pretty accurate when using a pressure transducer on cylinder 1, within 5 psi of a mechanical reading on all the cylinders.

 

If I only had one test available it would be pre catalyst emissions readings. You know exactly what is going on in the combustion chamber and ellliminates a lot of items if you know what you are looking at.

 

Seems to me - Pico or not - 75% relative to 100% on the other cylinders either indicates something serious (e.g. valve not sealing, ring failure, head gasket leak) - none of it sounds good.

Ray

 

Its probably not but it becomes a matter of what your standards are. Also it makes some difference on the motor in question. If it has nikasil its getting ready to puke up a cylinder. A SBC will likely be the same in 25000 more miles.

 

Could some stray oil in that particular cylinder cause a lower reading via the Pico stuff (on account of reducing fiction?) whereas that wouldn't be an issue when using a traditional compression gauge?

Nikasil means they are using aluminum liners with a coating? That sounds bad...

Ray

 

It's hard to kill a well built 350

Ray