PPI results on a 360 - advice?

looks like you go from about 200 to 160
I think you should have a leakdown done.

Those perameters are a little stretched out IMHO

 

Typically a good 360 motor will have over 200lbs in every cylinder. Factory specs only say how bad it can get before it requires a tear down for overhaul. The standard for a purchase should be much higher. At this point a leak down test is academic. You really do not care why the compression is bad, you know it is. It is the owners problem and spending another dime is good money after bad.

Find another car.

 

If I may go off topic here, but wouldn't it seem unusual to have a car that is only 5-6 years old, 10-15k miles and have ALL cylinders low like that. What about operator error? No real interest here, not questioning you, just curious. I realize we don't know the history and maybe it has been horribly abused or neglected but, and I may be way off base, it just seems strange that all cylinders would be similar and low.

 

The fact that one cylinder is down to approx. 160 PSI while another is up at 200 PSI is the primary indicator of a problem with the motor and not the tech testing.

A variance of more than 30 PSI is considered to indicate a problem. Personally I don't take the time to do compression without leak down testing also. The comp test alone doesn't provide enough specific information

It's not entirely unusual for a low mileage engine to have low compression, the piston rings hold oil which affects compression. If the engine wasn't run much (or at all) before testing and the cylinders weren't oiled either the rings may be "dry" and causing low numbers.

The only way to know is through a leakage test or oiling the cylinders then compression test again. Personally I prefer not to oil the cylinders before testing, it seems like a cover up.

 

I don't disagree, but come to the same conclusion more on the basis of the relatively large variation of compression among cylinders. I had heard/read (and correct me if I'm wrong) that because of different techniques and equipment, a motor could read >200 lbs. with one tech and another guy could read it at 180, but if pressures are consistent across cylinders, you should be good (but if consistently way low, say 160, then you should give up and move on).

 

At what psi do 360's start to misfire at idle?

For common everyday cars anything above 150psi will run fine and 130psi or less will have a small misfire at idle.

With the 360 having High compression I wonder if it would not be fully firing at 160psi and cause a rough idle.

Are you supposed to keep the throttle body plates open when testing on the 360?

I'm gonna test mine now and see what it looks like.

 

At 150 psi a cylinder will fire fine, if it is designed to have 150psi. The best you can say about 150psi in a 360 is that it is 30% down on power. No big deal......on a lawn mower.

 

To put oil in a cylinder before a compression test is to insure a meaningless result.

The simple fact is with the Mahle piston/ring/cylinder package it will, if good, make great compression and leakage results hot, cold or if it has not run in a week. We even test them brand new without even having been run and they show lower than 3 or 4 percent even then.


We are not testing 1966 Chevrolet straight six's people. This is a modern motor with all very modern manufacturing technology and the rules of thumb we learned 30 years ago just do not apply.

 

Their explanations do not explain away the compression test. Those problems would not cause those results. A sloppy, poorly done test by a half assed mechanic can always cause poor results but they don't want to admit to their people being that bad. You can test and test and test and always get the results you want eventually. This all highlights why getting a PPI done by anyone other than a very good mechanic is such folly.


Get it done once, get it done right and skip the outcome based testing.

 

A Jag 4.0 (obviously not nearly as well engineered as a Ferrari) built within the last 10 years actually has terrible oil retention and the damn thing won't even start if it's left to sit long enough because the rings dry significantly. Just because it's modern doesn't mean it's perfect.

 

The place with the big yellow sign needs a bunch of bodies to cover the overhead. It in no way assures quality. I know, I used to be shop foreman of the biggest. I am now a one man operation because it is the only way quality happens.

 

Rings have to seal dry. If they don't it is called severe oil burning.

 

Must be pretty recent, the one man show thing. I was there with a bud of mine and there was more then just you there. And they were working.

 

A compression test is only as good as the person doing it in a lot of cases.

The way I have always done a compression test is like this:

Warm up the car to operating temp. Then remove all the plugs (so the motor spins over freely). Next disconnect the battery and turn off the electronics, etc. Then connect a starter switch and 12V battery directly to the starter, so that you can push a button and spin the motor over, while bypassing everything else on the car.

Next, manually open the throttle bodies/butterfly valves, etc. or anything that blocks air into into the motor (i.e. to simulate having the throttle pedal down). Then attach a good quality compression gauge (the type that screws into the spark plug hole, not the type that you press up against the hole) and crank the motor to the count of 5 seconds and/or until the needle on the gauge won't bump any higher. Discharge the pressure from the gauge and repeat the test on the same hole - then (if the results are not the same for that cylinder, take the average of the two results). Normally they will be pretty spot on.

Repeat this for all 4, 6, 8, or 12 cylinders, etc.

Normally you want to see results that vary no more than about 5% across the cylinders. As Brian points out above, when dealing with a car such as a Ferrari, you probably want even more consistent (i.e. closer) readings, as you are not dealing with an Oldsmobile or something.

If I recall correctly, when I did a compression test on my 355, the results were all within 1 or 2 percent of each other. I may still have the actual PSI readings for each cylinder recorded on my other computer.

On a Ferrari with only 12K miles - unless the car was really abused (i.e. run without oil, etc.) or tracked very hard - to see such a variation among the cylinders, I think it says more about the shop doing the work than the car. I bet you that if you redid the compression test properly, you'd get vastly different results. Something as simple as not holding open the throttle bodies can have a huge impact on the results. Keep in mind, a motor is largely just an air pump. If you are doing a compression test and cutting off the intake route for the air, then you aren't really testing much...

Ray

 

That information is not correct. The sealing rings on the pistons do not really depend on being lubricated to produce compression. The rings drag on the cylinders walls and produce compression largely due to an interference fit between the rings and the walls. In fact, you can fire a motor bone dry and run it around the track (with no oil whatsoever) and it will run just fine.

Here's a perfect example of that: many years ago my friend was getting ready to install a fresh motor in his Datsun. However, instead of pulling his current motor out and installing the new one, he decided to start a pool to see how long it would take to blow the current motor up if run without any oil. When Friday night rolled around, we all placed bets on how long the motor would last. Some people said 5 minutes, some said 10 minutes, some said 30 minutes, etc. So Friday after work, my friend, Mike, went home, drained all the oil from the motor and took off around town, revving the motor as hard as he could as he drove, with the red oil light glowing on the dash. After driving around all night with no oil, purposely revving the hell out of the motor in an attempt to blow it up, he got tired and just drove home and pulled the motor out. It never did blow. Even after taking the motor apart, it really had no visible signs of damage.

Anyway, when talking about using oil during a compression test, that's generally only done to momentarily produce a higher than normal seal around the rings - for example, if you have a chamber that is showing low results. It's typically done to show up a poor seal in the rings and/or to reveal a sealing issue relating to the valves or head gasket, etc.

Ray

 

The fact that the Datsun didn't spin a bearing and throw a rod doesn't prove that it didn't have low compression. Tell me you compression tested it before and after and what the results were and you have some evidence. I've seen engines with 11:1 compression run on 145psi of compression "just fine".

Oil influences the piston ring's ability to seal in low tension,large gap or worn rings/cylinders. I've gone over enough engines to know that for fact. The Jag (for example) is known for having very low-tension rings (and compression)and therefore is sensitive to how much oil is in the rings, it's a fact that they have poor rentention and sitting creates an issue.

I concede that the real issue in the 360 specificly would be weak rings, not dry rings.

 

I think actually he did test the compression after that episode, but I don't have any specific numbers. We also inspected the cylinder walls, etc. The motor really wasn't much worse for wear. We were all pretty amazed it ran so well.

I agree that some oil is probably helpful on the cylinder walls, while the motor is running, but I don't think during a compression test you need to add any oil to what is already present and/or supplied by the motor and oil pump turning (from the starter) in order to achieve accurate compression test results. The only time adding oil comes into play is when you are trying to isolate some issue (e.g. very low compression in a cylinder) that may be related specifically to the rings.

I've done plenty of compression tests on motors and never had to add oil. The only time I ever did add oil was when I wanted to see what sort of difference it would make during the test. BTW, something to keep in mind, if you do use oil in the cylinders during a test for some reason - specifically, that oil will end up some place once you squirt it into the chambers. Normally, the motor will pump it out and it will end up up pooling in your exhaust system (typically the muffler). If you use too much oil, you'll often end up with a pool of oil in the muffler canister and then as you drive, it will generate lots of blue smoke at signals. When you then take off from the signal, you'll leave a big white cloud like James Bond.

So be careful how much oil you go squirting into your motor's induction system

Ray


Ray

 

I'm a little surprised by these statements. I recently had a PPI done on a 360 that I have since purchased. It tested at
Compression: 182, 180, 180, 178, 181, 178, 183, 180
Biggest variation: 178-183 ~ < 3%
Leak-down: 3% on all cyl.
I was told this is great, but your comments seem to indicate otherwise...? Since I have purchased the car, it is all academic, though I'm still curious about your comments.
Jes

 

There's a multitude of things that can affect the exact number you get on the compression test. These results are exactly why you look for consistency in the compression numbers, and then go to a leakage tester to determine whether or not something is sealing improperly. If you get these numbers and +/- 5% leakage you can figure the inaccuracy is in the test process not the engine.

 

Not really true. There are things that can effect cranking compression and not leak down. Cam timing is a good example of that and is quire common. Compression and leak down tests are both important tests and provide different but overlapping information. One is not a confirmation of the other.