Compression \ leakdown #'s

I think the compression numbers look like about what I would like to see. Pretty much even across the board with the one exception of the #2 front bank (and even it is not completely out of range).

The leak numbers are on the high side, but again, the wear checks out even. On a 911 turbo I tell people that 16% leak is max before the engine is due for a re-ring.

It may just be slightly worn, but its an honest engine.

 

i would run a bottle of marvel mystery oil with the next oil change and just drive the car. most times with use and clean oiil, the rings will "heal" to a point and seal better. some here dont like additives, but i have used that stuff in everything i have owned, and its great.

i would NOT open the engine with those numbers, i would drive the car!

 

I wouldnt worry about that leakdown, thats a reasonable number for rings on a car this old, BTW was the car warm or cold when you did the test? if hot, how hot?

On my pro built small bore race engines, we would get 16% b4 fully broken in & 4 to 8 after a few races, with the best rings available & coated skirts. & these were on 120 hp engines where every hp counts.

Do a search on car craft magazine for how to do a leak down , they did a very complete test of what the actual leakdown meant VS Dyno numbers, they found that intake leakdown over 25% was a power killer , eveything else was not really a big performance decrease, so unless you are killing bugs with the oil burn or have so much blow by that you are blowing seals , I wouldnt even think about it. Driving it more may help seal it up if it has few miles

 

IMO, so long as your engine is not burning too much oil then you are in great shape. There seems to be different schools of thought on leakdown and compression testing. The actual compression stroke of an engine is done and over with in the blink of an eye, so many say a leakdown test measuring the amount of air escaping over an elapsed period of time is irrelevant as the engine does not operate in this manner. Leakdown numbers are great to me because they tell what part of the engine is beginning to wear, and by how much. Both are both very good tests, but with your compression numbers I believe you are in excellent condition. It's good to know where the wear is taking place with the leakdown, but in an engine rebuild won't you be replacing everything anyway? I know I would and I am by no means a pro.

 

+1 MM oil. Great stuff. I use it too.

 

We do a fairly high number of leak down tests in our Subaru service centers. There is some similarity between motors, aluminum blocks , aluminum heads, and steel liners. We use a 10% rule that being if the difference between cylinders is 10% or less then we find that within factory parameters. We do both a dry run and wet run at normal operating temps to make sure we have an accurate reading. That way you can asertain a more accurate compression and leakdown percentage reading. A dry run is, pull out all spark plugs, run the compression test, then leak down on all cylinders. A wet run is when we add a small amount of motor oil to the cylinder we are retesting. First the compression test, then the leak down. That is how Subaru reccomends testing.

With the numbers you have listed, I would not be too worried about the condition of your rings or cylinder bores. We would readily accept these numbers are "good to go" for a Subaru customer.

I really don't like the idea of a "ball hone" device. Keep in mind a hone is a follower. It only follows the hole that is already there. It is not true enough to accurately hone your cylinder. If you have a skued or oval bore the hone will not correct that condition. Just a thought on honing.

The question I would ask you what is going on with your Ferrari that makes you think you have a need to hone/rering your motor. Is is an oil burner, does it have a major loss of power, does it start quickly, what has brought you to your concerns. Is there more to the story that we see from your concerns here? If you were a customer of ours and came to us with these numbers and your concerns we would have you sit you down with a service advisor and get a detailed history on what is happening with your car. Then we would road test your vehicle with you both driving and as a passenger keeping notes on performance and runability. Then go throught our records of similar cars and similar situations . We maintain all service records in our computers and can access them by entering various search words, compression, leak down, ect. We can split screen your data with those of other customers with similar cars. We can show you very detailed comparisons. Of course privacy is a concern so owners names and detailed data is blanked out on the screen.

Howard Musolf
1981 308gtsi
1982 400i Cabriolet
Maserati Spider
2 many brass era cars

If it don't fit force it, if it breaks, it needed to be replaced anyway.

 

If you tear-down your motor and find all the cylinder walls are in good shape (308 lower ends are very robust) and are within spec, a ball hone aka grape-ball hone is great for deglazing and lightly roughing the walls for the new rings to seat to. I have done this on a Maserati, Lotus, and several Porsche engines and have never had an issue and the rings always seat well.

If your cylinders are not in spec, then they would need to be bored with a proper hone and with a proper torque plate attached to the head.

 

Was the engine cold or warm when you ran these tests? If it was cold then I'd say they were pretty good. If warm then a bit low. If you did them cold then try doing it warm & see what you get.

 

No they can burn more than that and be fine. The early 2v cars were quoted in the manual to burn 1qt per 600 miles. The 4v cars burn less, but I would not at all be concerned with a quart every couple thousand miles.

 

QVs were nikasil coated. The 2v cars were not. The oil burning issue I am guessing had a lot to do with the nitrile rubber valve stem seals which were used on almost all cars in that day. Nitrile is just crappy and cannot take the heat. Viton, teflon, and other superior sealing materials did not come into auto use for another few years.

 

No way would I be ripping into that motor with those compression and leakdown numbers.

I am however in the process of rebuilding a QV motor because it had two low cylinders due to rings. One note, the QV liners are Nikasil coated which is quite durable and hard. I would bet most ring and blow-by issues with these stem from either the car being run very hot at some point causing ring fatigue or more likely being run with the CIS incorrectly tuned and running way to rich at low rpm which washes the cylinders with raw fuel and causes cylinder glazing. Either way, if the liners are within factory spec, deglazing the cylinders and re-ringing will fix it providing the Nikasil lining is intact. If you use a berry hone to de-glaze, make sure it is an ALUMINUM OXIDE home of 240-320 grit. I'm told muratic acid can be used to deglaze instead of honing, but I've never tried it.

Drive it...and hard.

Spang

 

There appears to be some movement nowadays toward ONLY honing if you bore a cylinder, and NOT honing just to install new rings. Also dry install of new rings (bore/hone or not) is being used more and claims of virtually instant seating in both cases - bore/hone/rings or just rings in existing, untouched bores. The concept that there had to be rough surface for rings to seat properly is no longer viewed as required, or even desired.

When I quit doing this work professionally (early noughties) we had gone to very light lubes - like MMO on the cyl walls and no pre-lube lube of the pistons/rings (except for the piston pin interface) but now many of the folks I used to work with are doing it dry.

I am not making a recommendation here, just noting that there are different views of this in the precision engine building arena than what used to be "normal."

Note that a flex hone does tend to retain any slight wall imperfections in the cylinder because of its design.

 

I am going to be the lone dissenter here by the looks of things. I have personally seen auto/truck engines with over 200K miles, as well as dozens of aircraft engines with several thousand hours TSO, show 5% or less on leakdown tests on many occasions. Having worked around aircraft, it should be noted that 10% leakage on any one cylinder will ground a plane for repairs, and anything over 7% most buyers start walking, or negotiating a top end overhaul into the sale. These arent Ford's or Chevy's, or Honda's or Subaru's, 308 engines cost over $10K to do the minimum amount of work we are talking about to bring the cylinders back up to par. Those kind of numbers would be considered poor on an engine thats a dime a dozen in the junk yard.

That said, the cars not going to fall out of the sky if it quits, and there could be other factors than wear causing the loss, such as carbon deposits in the rings, and Michael's suggestion of MMO is a very reasonable idea if that is in fact the case. I have had several engines showing similar compression results that came up after a few motor flushes and oil changes, with MMO. But if your buying a car showing those numbers, I would walk away or negotiate the cost of repairs into the sale, you just wont know how it turns out until you spend some time with it, and once compromised, engines begin dying at some accelerated rate, a rate you'll never really be able to know. You can be darn sure the seller knew what it had for compression before putting it up for sale, if that not the reason in fact that its for sale. Caveat Emptor.

 

Frankly, I wasn't going to mention anything at the risk of seeming too curmudgeonly (sp?) but since he brought it up, I totally agree with Art. Those numbers are rather poor and there's no way I would buy a car with an engine in that condition UNLESS the seller agreed that the engine needed refreshing and the price reflected that.

Those figures are a perfect example of why a compression check, on it's own, can be very misleading. You can have high compression due to carbon deposits on the piston/combustion chamber. You look at those figures and think - hey compression is great. BUT you look at those leakdown figures and see that the engine in question is not in very good shape performance-wise. Certainly this doesn't mean that the car won't run just fine for many years, just that it is down on power compared to what it should be. If you go to a performance engine shop the max acceptable leakdown on a performance engine is around 5%. A newly done performance engine would be down in the "is this gauge working, it's not moving" area.

 

I am going to have to agree to disagree on this one fellas. The compression numbers are great and the leakdown is getting there, but not there yet. Once it moves past 17-20% then a refresh is in order. From my conversations with Ferrari a certain northern California Ferrari tech (most of the time a reliable source, but not always!), 355s and 348s had 4-5% leakdown from NEW. My rebuilt 3.2 911 engine had about 3% leakdown. I am sure 308s left the factory new with leakdown numbers not quite as good as they have been doing in the past dozen or so years so until it got a little worse I would not sweat it.

 

My two cents which is worth less than it costs. Knowing nothing else about the car but these numbers and the miles on car. This should be a fairly cheap car to buy, it is showing wear and not in the best shape it could be in, however it could be a fun driver for many years and the numbers may improve with regular driving, or they may not. If it runs good and everything else checks out as a good driver and the price is right the numbers aren't a critical failure. It's not at the top of it's game, it isn't completely worn out, it's an old car.

 

I've been sitting back on this one. Lots of good information both pro and con coming out. I have a question here about the data presented. How is it all are showing "leakdown thru rings"? With that many valves (QV) you have a high chance of leakdown at valves unless this was done with the heads off. Unspecified, suspect not since compression also listed.

When I bought my car it was contingent on the PO paying for the valve job as I was seeing very similar figures and worse. With the heads off, we did a leakdown on the bottom end and saw less than 2% across the board. All my leakdown problems were in the heads. (side note: motor had 56K miles, these bottom ends are really good)

Second. Do you have a stock starter or a newer high torque starter? If high torque, I'd be unconcerned about the compression figures and go with your leakdown numbers only.

Rick

 

I do recall from high school auto shop we'd drop some oil in the cylinders on the second run to isolate the rings from valves, doh! slap to forehead, why didn't I think of that.

 

A leakdown test can detect where the leakdown is coming from though normally not in a totally scientific way. It is done simply by listening. Air from the int manifold shows int valve leakage, air from exh shows exh valve leakage, air from crankcase shows ring leakage. So, for example, let's say one cylinder has leakage from the int valves and past the rings. It is possible to say the cylinder has 15% leakdown but it is not possible to accurately say that of the leakdown in that cylinder, 60% of it is past the rings and 40% is past the int valves. But you could have no air at the int and exh and air past the rings and thus state that the leakdown is past the rings only - like the test in question.

Generally, the test is to show you overall condition and if you have excessive leakdown in any area you would likely be refreshing the entire engine anyway. Though it is possible that if you had air at the intake and/or exhaust but none at the crankcase, you might do headwork, leaving the cylinders alone.

"I do recall from high school auto shop we'd drop some oil in the cylinders on the second run to isolate the rings from valves, doh!" Yes, that's the classic way to determine rings/valves on a compression test - not necessary for a leakdown test.