| Author |
Message |
Kurt Kjelgaard (Kurtk328)
Junior Member
Username: Kurtk328
Post Number: 231
Registered: 3-2001
| | Posted on Saturday, October 04, 2003 - 3:25 am: | |
Thanks a lot, Phil
Learning something new every day |
Phil Hughes (Ferrarifixer)
Junior Member
Username: Ferrarifixer
Post Number: 210
Registered: 7-2003
| | Posted on Saturday, October 04, 2003 - 3:05 am: | |
Your first bit is correct, it's a test of cylinder sealing by comparing applied pressure to retained pressure.
It is not a test of leak over a time period.
When testing though, you can leave the test running for as long as it takes to stabilise. For example, a poorly sealing valve may seal better after a while due to the pressure assisting the valve into it's seat a bit tighter. You can use a stethoscope to determine exactly what is leaking.
A healthy engine will go straight to it's eventual reading in about 2-3 seconds, but it varies greatly with clearance volume on different engines...larger volume, slower stabilising.
Perfect sealing isn't necessarily a perfect environment. Many racing engines use less compression rings (ie only 1 or 2 instead of 2 or 3) than road cars to reduce frictional losses, so when testing a race engine slightly higher tolerances are accepted....I say about 8-10% is good, but it's a very "grey" area.
It can even be affected by the position of the ring gap around the piston....If the gap is opposite the thrust face of the bore, the leak will be higher. |
Kurt Kjelgaard (Kurtk328)
Junior Member
Username: Kurtk328
Post Number: 230
Registered: 3-2001
| | Posted on Saturday, October 04, 2003 - 2:16 am: | |
So what you are saying is, that you supply a continous pressure to the cylinder(s),
monitoring the applied pressure and measuring the resulting pressure in the cylinder.
The difference indicating pressure "disappearing" due
to worn rings, valveseats or even a leaking head gasket?
This difference is then expressed in percent?
I had an idea that you applied pressure to the cylinder and then
checked the residual pressure after a certain time - just like the way you check
hydraulic systems, including fuel supply systems. |
Phil Hughes (Ferrarifixer)
Junior Member
Username: Ferrarifixer
Post Number: 205
Registered: 7-2003
| | Posted on Friday, October 03, 2003 - 5:41 am: | |
Procedure is fairly easy.
You need to be clear we are talking leak down, not compression that is being measured.
Many types of leakdown testing gauges are available. Basically you simply remove all the spark plugs. Go through each cylinder in sequence of firing order, by setting the piston to TDC on firing stroke, select top gear and apply handbrake.
Then, using a compressed air line and the twin gauge tool, you simply measure the difference between the air pressure supplied and the air pressure retained in the cylinder, through a connection in the spark plug hole.
Most tools are graduated in a percentage scale and easy to read.
The hard part is knowing the firing order and identifying the cylinders correctly, and getting the engine on the right stroke at near perfect TDC.
The test should be carried out with a warm engine, but a comparison between warm and cold is also a good indication.
A compression test is quicker and easier, but only tells half the story, if that. |
Kurt Kjelgaard (Kurtk328)
Junior Member
Username: Kurtk328
Post Number: 228
Registered: 3-2001
| | Posted on Thursday, October 02, 2003 - 9:11 am: | |
Phil, can you enlighten me (and probably others)
on the eact procedure for a "leak down" test.
What tools are needed, how do you arrive at the percentages.
Thanks. |
wayne skiles (Bad_tt)
Junior Member
Username: Bad_tt
Post Number: 83
Registered: 8-2002
| | Posted on Thursday, October 02, 2003 - 8:45 am: | |
Phil
Appreciate the info.I hope the 355 Spider
I'm trying to buy passes the test this morning!
Wayne |
Phil Hughes (Ferrarifixer)
Junior Member
Username: Ferrarifixer
Post Number: 201
Registered: 7-2003
| | Posted on Thursday, October 02, 2003 - 6:57 am: | |
1%-5% =Perfect
5%-10%=Normal
10%-20=Worn but acceptable
20%+ =Something wrong.
Remove oil cap, air inlet pipes, cooling system cap, engine breather pipe and open the throttle wide.
Then listen using a hose or stethoscope up inside the exhaust, down the inlet and in all openings to ascertain where the most leaks are. Usually it's all in the crankcase coming past the piston rings as they have gaps. |
wayne skiles (Bad_tt)
Junior Member
Username: Bad_tt
Post Number: 82
Registered: 8-2002
| | Posted on Thursday, October 02, 2003 - 6:04 am: | |
What are the numbers I should be looking for
i.e. what are acceptable readings??
TIA
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