| Author |
Message |
rob guess (Beast)
New member
Username: Beast
Post Number: 22
Registered: 5-2003
| | Posted on Saturday, June 21, 2003 - 10:08 am: | |
JRV;
I am not trying to get into a shouting match on this issue. I respect your views from your perspective as a professional technician.
If it was not due to a wrist injury 3 years ago i myself would be twisting the wrenches myself. Now i have severe tendonitis and after about 1 1/2 hous of wrenching i lose my grip and my hand starts to lock up. So now i spend my time teaching the future professional technicians. If i had my choice i would rather be building motors and making them run much better and last longer.
I tend to look a lot deeper into issues due to my engineering background. You can call myself "Anal Retentive" in my view and i would take it as a compliment. The main reason i prefer dynamic tests over static is just the way i have learned to belive in the results. |
JRV (Jrvall)
Intermediate Member
Username: Jrvall
Post Number: 1752
Registered: 11-2002
| | Posted on Friday, June 20, 2003 - 10:16 am: | |
>>Not always some new motors have bad leakdown numbers and low compression until the rings seat in and the crosshatch peaks are worn off of the cylinder walls<<
That's completely irrelivant to the known numbers a good new engine produces. I saw a bad spark plug out of the box once, doesn't mean all new plugs are bad out of the box. ;-)...So, conclusion is relivant comparisons automaticaly discount irrelivant data.
Rings are designed to expand towards and seal agaist both cylinder wall and groove land surface when pressure is applied..if they don't there is pressure leakage and this condition among others is viewed as a problem.
Tools can be invented to replace tools that already exist, I probably won't be standing in the line to buy newer magic or slicker snake oil. ;-) |
rob guess (Beast)
New member
Username: Beast
Post Number: 20
Registered: 5-2003
| | Posted on Friday, June 20, 2003 - 7:59 am: | |
JRV;
Not always some new motors have bad leakdown numbers and low compression until the rings seat in and the crosshatch peaks are worn off of the cylinder walls.
a few things to remember here is as the piston is going up on the compression stroke the rings are flexing on the cylinder wall. also dont forget how high the peak pressure is in the combustion chamber at top dead center compression durring the combustion event makes the cranking compression number look like a walk in the park.
The reason i say that getting the sealing ability of the engine in a 1/50th of a second gives you a more realistic picture of what is happening durring combustion. Take a look at the needle on a leak down test when you hook up the air line, it will drop then rise back up to where you take a reading. what is happening is the rings are flexing from the shock of the pressure spike then flexing back to seal the chamber.
I am not saying that a leakdown test as currently done is flawed, but when i personaly am building a motor for maximum performance i am looking at every small detail. In Racecar Engineering Magazine v8#10 they had a story where engine tunners were using pressure transducers and cameras to film and measure the combustion event. I am hoping that some day there will be a digital leakdown tester that will be able to pulse a blast of air or perhaps 400 PSI nitrogen in to a motor and the transducer will be able to read the pressure spike the drop and the rise of pressure to see what the engine does in a more dynamic state. |
JRV (Jrvall)
Intermediate Member
Username: Jrvall
Post Number: 1747
Registered: 11-2002
| | Posted on Thursday, June 19, 2003 - 8:01 pm: | |
>>if there was only a way to determine cylinder leakage over a time period of say 1/50th of a second we could get a more representative indication of engine sealing ability<<
The reason that's not neccesary is because we know that new motors will hold at least 98% of the pressure pumped in during a static test, so, "the how used up is it" equation is based on what we know about new condition. |
Andrew A. Illes (Andyilles)
Junior Member
Username: Andyilles
Post Number: 107
Registered: 10-2002
| | Posted on Thursday, June 19, 2003 - 7:33 pm: | |
Rob.. Yup.. gotta agree with ya! When I was a kid on our farm, we had an old John Deere Model A you could hold your finger over the compression release spigot, but sure 'nuff... just like you say, it'd run. Dunno about a Ferrari though.
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rob guess (Beast)
New member
Username: Beast
Post Number: 16
Registered: 5-2003
| | Posted on Monday, June 16, 2003 - 10:12 pm: | |
Andrew;
I teach engine trouble shooting and have seen motors run with compression #'s below 100 PSI and leakdowns around 50% The main thing to remember is an engine at 1200 RPM has 10 combustion events per second per cylinder. while a leakdown test is a static test with out the engine running. if there was only a way to determine cylinder leakage over a time period of say 1/50th of a second we could get a more representative indication of engine sealing ability. |
Andrew A. Illes (Andyilles)
Junior Member
Username: Andyilles
Post Number: 102
Registered: 10-2002
| | Posted on Monday, June 16, 2003 - 1:06 pm: | |
Jeremy... As others have noted, with those numbers your engine shouldn't even be running!! Since you say it runs fine, the first suspect is bad tests... do them over. If the numbers are repeatable, Rob had your most likely answer... zero or less clearance. As valves wear, stems strech, seats recede.. the valves "ride higher" and reduce clearance. If clearance is zero (or less), the valves can't close completely and you will get your pityful compression and leak down numbers... that would be the FIRST thing, as well as the easiest to check - just pop the covers and stick a feeler guage in. And yes, you'd expect some burned exhaust valves, though the intakes would probably be ok. What troubles me most is that your numbers are fairly uniform, cylinder to cylinder.
Frankly, my first suggestion would be to have someone ELSE check both compression and leak-down and see what they come up with. As is, Yugos should be passing and out accellerating you with the numbers you quote - assuming you could get it started in the first place. Bottom line - I think you got bad tests. |
Mark Eberhardt (Me_k)
Member
Username: Me_k
Post Number: 560
Registered: 5-2002
| | Posted on Sunday, June 15, 2003 - 7:54 am: | |
Stacy,
Verify that the carb linkage is working properly and the front carbs are opening completely. If thats not it, check the cam timing, one of the front cams could be off time.
Jeremy,
Well, sorry to heard the bad news....you could repeat it and like Stacy verify the cams, but it doen't look promising since there is good correlation between the 2 tests�.high leakdown cylinder have low compression.
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stacy o'blenes (Stacy)
Junior Member
Username: Stacy
Post Number: 70
Registered: 8-2001
| | Posted on Sunday, June 15, 2003 - 5:57 am: | |
Since we are on the subject, what is everyone's opinion on these compression numbers.
78 308. Was running well until started missing the other day. I pulled the plugs only to find that there was water in the sparkplug wells of the front bank plugs (the likely cause of the miss).
I decided to check compression while I was at it. Warm engine, throttle wide open, all plugs out.
The result: exactly 150 psi on all the front bank cylinders, but exactly 125 psi on all the rear bank cylinders. This patttern struck me as a little odd. anyone have any ideas?
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P. Thomas (Ferrari_fanatic)
Member
Username: Ferrari_fanatic
Post Number: 408
Registered: 4-2003
| | Posted on Saturday, June 14, 2003 - 9:41 pm: | |
Or put a vacum gauge on it (connected to manifold vacum). It would show a flutter. |
DGS (Dgs)
Junior Member
Username: Dgs
Post Number: 53
Registered: 5-2003
| | Posted on Saturday, June 14, 2003 - 10:49 am: | |
If the cylinders are really "leaking", then it's the low end that will show it most. At low RPM, all the compression will leak out before being ignited.
If those leak-down numbers are an accurate read off a warm engine, I'd expect the mill to idle like an old lawnmower.
If the engine idles smoothly, there might be some question about the one isolated test.
You might want to get that test re-done to confirm. (e.g. see if the numbers are repeatable) Heck, maybe the compression tester had a loose connection, causing the leak to be in the test gear, rather than the engine.
At least, the current owner would probably want to have that test re-checked, even if the pending buyer walks away from it.
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rob guess (Beast)
New member
Username: Beast
Post Number: 12
Registered: 5-2003
| | Posted on Saturday, June 14, 2003 - 9:57 am: | |
P. Thomas;
The main reason the you crank the engine over with the plugs out is not to get the motor to spin faster but to put less stress on the starter motor. With all plugs out the starter motor does not have to compress the other cylinders intake charge causing an increase in amperage draw and heating up the motor. This is the main reasoning behind it. |
P. Thomas (Ferrari_fanatic)
Member
Username: Ferrari_fanatic
Post Number: 405
Registered: 4-2003
| | Posted on Saturday, June 14, 2003 - 8:24 am: | |
David, <<<chances>>>: If the valve lash is too tight (o clearance)then the valve never seats or fully closes. That will permenatly fry a valve in short order.
A leak down test "tells" you where it is leaking from, inatke, exhaust valves, or rings. If it is the rings, a valve adj is a totally moot point. Again, if it is a valve, see above. |
Jeremy Lawrence (F512m)
Junior Member
Username: F512m
Post Number: 135
Registered: 1-2002
| | Posted on Saturday, June 14, 2003 - 7:11 am: | |
Hey guys,
The carfax report came back with a possible odometer rollback in 1990. First reading it had 50+k miles, then three days later it had 14k or 17k miles. I don't have the report in front of me but you get the idea. Every check after that was in line. And today it has a little over 59k. The mechanic said that the numbers would probably be about right for a car with nearly 100k which it might very well have if the odometer was rolled back. Any thoughts on that? He did say that the car drove strong, but did notice a lack of power on the top end.
This was a PPI so I have no idea about the last time the valves were checked or adjusted. That could make a difference in the numbers though. Right?
Erik, I will forward the PPI info to you. |
Erik R. K. Jonsson (Gamester)
Member
Username: Gamester
Post Number: 340
Registered: 11-2000
| | Posted on Saturday, June 14, 2003 - 2:51 am: | |
Engine was warm, car was run to 150 MPH a few hours prior to testing. Three hour drive, 250 miles. Wish I had tested it to have some comparison numbers. No coolant leakage. Car runs like a top. HMMMM...?? |
DGS (Dgs)
Junior Member
Username: Dgs
Post Number: 51
Registered: 5-2003
| | Posted on Saturday, June 14, 2003 - 2:40 am: | |
Are those numbers repeatable?
It almost sounds like a bad test. Those numbers are soo bad, I wouldn't think the engine would run worth a dang.
How many miles on that mill?
Were the tests done with the engine warm or cold?
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david handa (Davehanda)
Member
Username: Davehanda
Post Number: 989
Registered: 5-2001
| | Posted on Saturday, June 14, 2003 - 12:30 am: | |
Not being a mechanic, what are the chances that a valve adjustment and retorquing of the headbolts might bring the numbers back in line? Or is this a "lost cause"? |
P. Thomas (Ferrari_fanatic)
Member
Username: Ferrari_fanatic
Post Number: 404
Registered: 4-2003
| | Posted on Friday, June 13, 2003 - 11:59 pm: | |
Rob, FYI, this seems weird, but an engine with the plugs IN actually spin faster . Sounds inverted but it is true. |
P. Thomas (Ferrari_fanatic)
Member
Username: Ferrari_fanatic
Post Number: 403
Registered: 4-2003
| | Posted on Friday, June 13, 2003 - 11:58 pm: | |
Jeremy, those numbers are like the walking dead of cars. |
rob guess (Beast)
New member
Username: Beast
Post Number: 11
Registered: 5-2003
| | Posted on Friday, June 13, 2003 - 10:14 pm: | |
Jeremy;
Your numbers are awful. when was the last time your had the valve lash checked and adjusted in you motor? normaly you dont want more than a 10% variance between your highest compression reading and your lowest. When the compression test was performed was all of the spark plugs removed and the throttle held wide open?
any leakage over 10% requires investigation to the source of the leakage if the leakage comes out the air intake it is a problem with your intake valves. if out the muffler exhaust valves are the source out the crankcase breather and you have blow by the rings and pistons out an ajacent cylinder or the radiator filler neck you have a blown head gasket or cracked head.
Get that motor checked out A.S.A.P!!!! |
Andrew A. Illes (Andyilles)
Junior Member
Username: Andyilles
Post Number: 99
Registered: 10-2002
| | Posted on Friday, June 13, 2003 - 9:35 pm: | |
Jerremy.. I hate to be the bearer of bad news, but the compression numbers are FAR too low.. they should be within 5psi of each other, and over 140, at least... and good leakdown numbers should be under 5%. Really sorry.
Andy |
Jeremy Lawrence (F512m)
Junior Member
Username: F512m
Post Number: 134
Registered: 1-2002
| | Posted on Friday, June 13, 2003 - 7:39 pm: | |
Here are my numbers:
Engine:
Compression test
Cyl.# 1-120, 2-85, 3-90, 4-130, 5-145, 6-120, 7-125, 8-120
Leakdown
Cyl.# 1-50% , 2-80% , 3-70% , 4-45% , 5-35% , 6-60% , 7-50% , 8-60%
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Jeremy Lawrence (F512m)
Junior Member
Username: F512m
Post Number: 133
Registered: 1-2002
| | Posted on Friday, June 13, 2003 - 7:29 pm: | |
Anyone have the "acceptable" compression and leak down numbers for the 328 engine? |