Leak down vs compression

There is a thread going in the 348/355 section "living the nightmare" I think is the title where there's quite a bit of discussion with which you might supplement your research.

But from my limited knowledge ..........A compression test is relatively quick and easy to perform and provides a certain amount of information. I guess if one does not believe one has a problem and the compression is good and (perhaps more importantly) is close to even across all cylinders one might stop there.

Leakdown testing is a more time consuming and complicated procedure from which one can glean additional information which can assist in diagnosis of poor compression. I guess it's the logical next step to perform after compression testing reveals a poor result.

Hope this is of some assistance!

 

A compression test is where you start. If the cylinder has good compression well then it has good compression & nothing to worry about.

If it doesn't then you do a leakdown which basically involves just connecting an airline to the cylinder & listening for where the air is escaping. When you identify that then you know whether the compression problem is rings/bore wear or inlet or exhaust valve (or some combination of all three!)

 

Well said from Greg & Iain.

Use a compression test to see if you have a potential problem, THEN use a leak down test to pinpoint the possible issue.

 

compression is an excellent and cheap way to gain much insight-much like an x-ray(fast and cheap, with a narrow spectrum of info)-in a 2 VALVE engine.

for a 4 and 5 valve engines, this test will only tell you of a catastrophic failure-after a 100% loss in cylinder sealing has ALREADY OCCURRED.

Therefore, in these engine families-like it or not-only a leakdown test will provide any really meaningful insight as to cylinder sealing function, and relative states of decay BEFORE there is toatal loss of compression
for example, frequently 328 engines ill show 160-180 psi compression(gages bare the principole variant on the baseline-which is another reason why it is not an aqccurate means to evaluation), and the car will fail smog miserably every time due to a HC contamination in...say #3, has the first exhaust valve is leaking a 23% , and # 7 has an 18% leak in the second exhaist valve.
Car has NO SMOKE, no noises, uses no appreciable amount of oiul, doesn't foul the plug, and runs well in most all aspects-except the chambers are leaky-the car requires a valve job....and without this TEST-leakdown-you would never have known it, as a compression test will simply pump it up...the numbers dont lie, and this is a perfect illustration as to whay ALL 4/5 valve engines require a leakdown test donne as a PPI-regardless of what the history file tells you. I own a machine shop, and do almost exclusively these blocks and heads...
FWIW......

 

Not only is 335s correct but I would do them on 2valve motors too. It is so easy to do it is a part of a good physical if you will. I'm also a fan of oil analysis too. You can see problems happening before the big kaboom! When you have baseline records for oil , compression, leak, and vacuum you god a lot of good data.

335s, as a machine shop guy do you have an opinion on FAA standard leakdown tests vs. the ultra low pressure (15-30psi) leakdown tests? I just don't buy the value of the lower psi test. Cylinder pressures are probably 800psi while we already test at 10% of that value with the FAA test. I don't think at 80-100psi we are blowing out the rings enough to fool us into thinking be got better sealing than there really is if under "real" conditions there is 800psi in there pushing the rings out for a seal. Opinions?

 

I grew up in a 2 valve world. Could someone explain how a motor could have a good compression test but a bad leak down test? Would there really be no other symptoms until the problem got worse?
I'm not doubting the brain trust here, I just don't understand.

Cheers,
TomB

 

Define good compression, please. If a motor has compression at 120 psi on all cylinders, is it good or bad? Before we get into altitude, throttle angle, temperature and gauges variances, leak down tells you what's leaking and where. Pressure differential remains the same, no matter where we are in the world. We don't do compression tests unless beaten by our customers. We always do leak-down tests, when in doubt.

 

The voice of experience and knowledge.

Read and Heed.

 

I gladly accept the expertise here, but the question is can you have significantly poor leak down numbers and still have what might be considered good compression numbers? Would valves leak enough for one but not the other?

Thanks,
TomB

 

An old tired engine could have equal, but low compression numbers...say 120 psi. With cylinder to cylinder variation of less than 10%, one might deem this engine "OK". However, if we knew what the compression should be, say 190 psi, we might be suspect. Otherwise, based on the information we had, we'd say the engine was good and be "incorrect."

So...how would we account for the apparent compression loss, then? Worn rings (on all cylinders) would show on a leak down test. Unless you did a wet and dry compression test, you'd just never know this. Generally speaking, most manufacturers don't publish compression numbers, as there's just too many variables. Leak down is based solely on differential pressures which every engine builder knows what's acceptable, regardless of engine type.

 

I think you are a real mechanic right? Its hard to disagree with you but, I disagree. You need both tests and others to get a true non-destructive impression of engine health. Maybe with your vast pro experience you can pull off the one test diagnosis? I don't think there are very many who can do that.

 

I'm not but... I have watched a few of these being done on youtube.

 

I'd compare it to blowing up an inflatable pool toy that has a small leak. If you pump fast enough and hard enough, you'll get it inflated. But it will slowly leak out.

So you can have inflation (compression) while still having an issue (leak down)

Remember how most compression gauges work - they hold a reading. If on one engine revolution the cylinder hits 130, it reads and holds at 130, if the next revolution is higher it reflects that.

However a leakdown is actively measuring flow into - and out of - a cylinder. Essentially you're dealing with two different measurements - pressure and flow, or loss.

 

Hey, didn't we watch that video together?

 

Admit it Dave, this is you working on your high performance bug right?

[ame]http://www.youtube.com/watch?v=GvG22YMOzUI[/ame]

 

Busted.

 

Young and cocky, as a Jr in HS, I showed up at the drag strip with my triple carbed 66 GTO and was paired with a pretty powder blue Bug.

Got the lights and all I saw was the front end of the Bug 3 feet off the ground in 2 gears, flustered me so bad I forgot to shift. A short time later when I figured out that real Racing involved steering and brakes, I had the oppertinity to drive a Super Vee in anger.

Having had my backside handed to me and then having been scared half to death by another, I no longer chuckle at those damn fool things. The fact the engineers forgot to put coolant in them, but instead chose to build an oversize weed wacker, is a bit disturbing to me yet.

 

LOL. You guys are just too much

 

You should have been there when we had dinner together about a year ago. Oh, boy!!

 

Dave,

Did zee old Faterland Keffer eat your lunch ..... so-to-speak?


Coolant? Vhy vould anyvone put a coolant, such as beer, in an engine?


You Amerikans are so amuzing.

 

Young and dumb, only one half of the Power/Weight argument made any difference... who gives a hoot what it weigh's? No chance a powder blue, fried egg styling effort, cross between a chainsaw and a weedwacker stood any chance against a GTO tri-carb Convertible. Oh... dont get me started... NOT a Fartvergnugen experience when that Panzer drove over my Scwhachanegle the subject of ridicule for weeks at school!

That experience led me to owning a Vega wagon, a Gremlin with flames and then a TR3 (with a spare TR4 heater box on the pass floor, spliced in series with the TR3 heater, both together amounting to the equivalent heat of a mouse fart... the stat never opened for 4 months a year) being used as a daily driver in the Minnesota winters. Try getting a date when she, the passenger, is required to scrape the inside of the windshield any time you are moving, the side windows were 8 mil plastic sheet and there is a VERY high likelihood that the hand crank would have to be used in unison with the starter to get it running again after the movie.

Sorry for the side slip on the subject.... A rather emotional subject for me, that miserable Panzer led to a long downhill spiral in my early automotive experience's and a case of frostbite that still nags me to this day.

Back to how to compress 10 #'s of crap into a one pound bag... and test to make sure it stayed there.

 

Kris will be flattered with this assessment of your chivalry.

 

That was a great story! Don't be sorry.