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Message |
rob guess (Beast)
New member
Username: Beast
Post Number: 18
Registered: 5-2003
| | Posted on Wednesday, June 18, 2003 - 9:19 am: | |
Compression test are just a relative test to determine the engines ability to compress the intake charge for the combustion event. there are several factors that can cause the results to be inacurate.
1. not having the throttle plates opend up ( results in lower #'s)
2. leakage of the tester hoses, connectors, gauge, or schrader valve.
3. cold motor or lack of lubrication on cyl walls
4. Air density, elevation, and air temp
This last issue has had even the most die hard engineers arguing until i proved it to them in person. In NM with some of my racing jet ski's i could perform a compression test in the morning when the temp are low and the air is dense then perform the same test on the same unit during the hottest part of the day and he readings averaged 15-20% lower!!
compression test should be used a first test to determine if further testing is required for example 10% or more difference between highest and lowest or a cylinder showing a very low reading then it is time to break out the leakdown tester. Also the compression readings should always be taken with a grain of salt to begin with i have seen a motor crank 180 PSI and have 60% leak down on the same cyl. |
James J. McGee (Dr_ferrari)
Junior Member
Username: Dr_ferrari
Post Number: 110
Registered: 7-2002
| | Posted on Tuesday, June 17, 2003 - 10:54 pm: | |
Agreed, no one test is the standard for engine condition and I would NEVER base condition of an engine on just that one or any one test.
but as I have stated, There is an indication only,
If i was to perform a cylinder leakdown test, I would most probably note a difference in the two cylinders.
Best regards, Jim |
JRV (Jrvall)
Intermediate Member
Username: Jrvall
Post Number: 1713
Registered: 11-2002
| | Posted on Tuesday, June 17, 2003 - 10:44 pm: | |
Jim,
No test spans the gamit. That's why we have to use multiples of tests to root out whether problems exist and what they may be.
A compression test is to determine whether leaks in the top end are present and to what extent if present. A healthy non-leaking cylinder can compress air until it reaches "an absolute", ie: it's ability to compress new air against pre-compressed air and the gauge spring runs out...at that point endless cranking produces no more compression. We move to a leaking cylinder...we crank...depending on the size of leak, at some point the cylinder cannot compress air beyond some lower number because the air trying to be compressed escapes thru the leak with each stroke...resulting in a lower reading than the healthy non-leaking cylinder was able to compress.
The test is to determine whether or not leaks exist...and that is done by measureing "Ability to Compress Air". |
Jeffrey Wolfe (86mondial32)
Junior Member
Username: 86mondial32
Post Number: 93
Registered: 5-2003
| | Posted on Tuesday, June 17, 2003 - 10:34 pm: | |
JRV is totally accurate. I was taught that you rotate the engine until the meter stops climbing. And as to the temp of the air in the cylinder.... you always should test on a warm engine anyway so why worry about the temp of the compressed air? You need the engine to be warm to assure the ring seal. YOu are looking for difference overall |
James J. McGee (Dr_ferrari)
Junior Member
Username: Dr_ferrari
Post Number: 109
Registered: 7-2002
| | Posted on Tuesday, June 17, 2003 - 10:30 pm: | |
JRV,
I understand the meaning of compression and that we are performing a compression test,But we are mostly trying to determine the condition of the engine. The indication that cylinder#1 reaches 190 psi much faster than cylinder #2 leads me to believe there is more cylinder leakage on #2.
I know we are not doing a leakdown test, but that is an indication of the condition of the engine that is shown during the compression test.
Are you saying that according to my example, Cylinder #1 and Cylinder #2 are is the same condition?
Best Regards, Jim |
JRV (Jrvall)
Intermediate Member
Username: Jrvall
Post Number: 1712
Registered: 11-2002
| | Posted on Tuesday, June 17, 2003 - 10:18 pm: | |
Sorry guys back to the books ;-).
It's a "compression" test used to measure the amount of air that can be compressed by a cylinder without escaping thru a possible leak.
Look up compression in Websters...use google to help your i-net search.
Get one more guy to agree with you that the earth is flat and call it a consensous ;-)
SAE on Standardization of Auto Cylinder Compression Testing (scroll the list):
http://www.sae.org/servlets/SiteSearch?charset=iso-8859-1&ht=0&qp=&col=portal&qs=&sae_qt1=&qc=&pw=100%2525&ws=0&la=en&qm=0&st=1&nh=25&lk=1&rf=0&oq=&rq=0&si=1&ql=&jsp_name=simplesearch.jsp&qt=Cylinder+Compression+Testing&x=10&y=5
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James J. McGee (Dr_ferrari)
Junior Member
Username: Dr_ferrari
Post Number: 108
Registered: 7-2002
| | Posted on Tuesday, June 17, 2003 - 10:08 pm: | |
Have to agree with tom, his point does have merit. JRV, let me know site for that info, I would like to check it out....
One of the main reasons for limit of rotation (no more than 5) is to provide the same test for each cylinder.
Case in point, If cylinder #1 reaches 190 PSI in 4 rotations and stays there but it takes 7 rotations for cylinder#2 to reach the same, are the two cylinders equal in condition after 7 rotations with them both reading 190?
From my perspective, cylinder#1 is in much better condition than #2 even though they have the same reading after 7 rotations.
Also, engine must be at operating temp when test is performed, Fuel/ignition system disabled, Fully charged battery, and throttle wide open.
Best regards, Jim |
"The Don" (The_don)
Senior Member
Username: The_don
Post Number: 5277
Registered: 8-2002
| | Posted on Tuesday, June 17, 2003 - 9:31 pm: | |
Kevin,
Your first problem was you took it to blackhorse.
Stick with the FoOC results.
M |
JRV (Jrvall)
Intermediate Member
Username: Jrvall
Post Number: 1711
Registered: 11-2002
| | Posted on Tuesday, June 17, 2003 - 9:29 pm: | |
>>This isn't an accurate reading. Engines should be rotated 4-5 compression strokes with an open throttle. The more compression that you try to build the hotter the compressed air becomes giving inacurate readings<<
It's call checking "compression" for a reason. It's the ability to "compress" air that gives the relivant data. Otherwise it would be called a stroke count volume check.
The SAE site has a nice "explanation" on it.
;-) |
Tom Jones (Ferrarioldman)
Junior Member
Username: Ferrarioldman
Post Number: 129
Registered: 6-2002
| | Posted on Tuesday, June 17, 2003 - 9:18 pm: | |
Bad compression tester? It is possible but the biggest differences that I have seen is the procedure. Some mechanics turn the engine until that guage doesn't move anymore. This isn't an accurate reading. Engines should be rotated 4-5 compression strokes with an open throttle. The more compression that you try to build the hotter the compressed air becomes giving inacurate readings. Relative readings of each cylinder is very important. There is also the issue of different brands of compression guages. I trust my Snap-On more than my other one (a Milton which reads about 20 to 25 lbs. higher) for correct readings but they both will show you the difference between cylinders. Most good 355's I have tested were around 180 lbs. Anything with more that 10% difference gets a cylinder leakage test too. Just the way I do it. |
rob guess (Beast)
New member
Username: Beast
Post Number: 17
Registered: 5-2003
| | Posted on Tuesday, June 17, 2003 - 7:53 am: | |
Kevin;
The 2 test do look compariable in there results the only thing i see wrong is that there is more that a 10% variance between the highest and lowest readings.
The only reason i can think caused the the 40-60 lower readings on the first test was the technicians equipment either had a leak or the gauge was of poor quality or damaged.
Did they do a leak down test to go along with the compression test???? |
Vincent (Vincent348)
Member
Username: Vincent348
Post Number: 617
Registered: 4-2003
| | Posted on Tuesday, June 17, 2003 - 12:22 am: | |
Kevin,
Thanks for posting. That's interesting. It looks like except for #6 that the readings compare, relatively speaking.
What is the reading supposed to be on a 355?
Vincent. |
Kevin Deal (Tube_guy)
New member
Username: Tube_guy
Post Number: 15
Registered: 5-2003
| | Posted on Monday, June 16, 2003 - 11:13 pm: | |
Sooo..
I had a private shop (Black Horse) do a comp check on my 355. Results were not good. Took it to Orange County Ferrari. Numbers were quite different.
Black Horse 140 145 150 160 135 130 150 160
O.C. Ferrari 180 180 200 210 180 180 220 230
The service manager said the car looks good. They said just drive it. There is a variance cylinder to cylinder, but jeeze...worry or what?
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