Printable View
I just bought a leak down tester to test my Kent motor. The unit came with a gauge that is marked in percentages, ie. 10%...20%...30% etc.
My question is, if the incoming pressure gauge reads 50 psi and the gauge measuring the leak down reads say, 10%, does that mean the cylinder is leaking 5 psi?
And, does it really matter what the actual pressure leak is or are we interested primarily in the percentage leak?
Thanks for you help.
Ah...leak down stuff. Your tester should have two gauges; when active, the differential between the two, in percentage is your leak. If the first reads 100psi and the second reads 96psi, you have 4 % leak. If your gauge has % already on it, then I'm not familiar with the unit. At the end of the day, its the percentage that matters. I've been told you need to use at least 80psi to get any proper results (I usually use 100psi as a baseline). In a good motor, I rarely see more than about 2% leak; 10% and I can usually pinpoint the issue by sound.
could calibrate the unit if u r worrried by inserting a block and reading percent, I use 100lbs like BWubb and had 100, 100, 98, 97, then a couple years later had 98, 98, 97, 96 not too many races, don't have many moolaas.....soooo, still passing people on strait, no need to rebuild ..... fyi .... they say 10% leak warrants retouch, runoffs bound, talk to DWubb
Oh, guage is from the Spruce Goose, Aircraft Spruce, and at as good a price as Summit almost, figure the flyguys have to measure that to keep their toys airborn and passing inspection, funny how a couple F1 guys made it thru F1 and then died cause they couldn't get enough of the manual dexterity thing and insisted on flying their own planes, thinking of Graham Hill and South American guy, maybe Pace or from same era. As a fiberglass guy once told me, drivers can't do fiberglass, no patience, that goes for cockpit checks, weather situations, I imagine as well ........ ok some can, can't generalize
http://en.wikipedia.org/wiki/Graham_Hill ......
PS Remember Hill as saying racing driving is a sport of anticipation and not neccessariy hand speed (thinking of the the slap the others hands before he can move them childrens game) He also said it's easy, get on it early before a straight, and brake late after a straight, all the rest is mish mash (made that part up but u get his point)
What percentage is acceptable will vary from brand to brand.
This is because there is an orifice in the line going to the cylinder that restricts the airflow between the two gages.
Bigger hole with same motor will read lower percent leak down.
I made my own with some pipe fittings, a couple of pressure gages and a flow control valve that I scavenged from a trash can at work.
to calibrate it I drilled a hole in a pipe plug, soldered it up then drilled an 0.040" hole in the solder. (did not want to try to drill a hole that small in steel. breaks too many bits).
Then put the plug where the fitting that screws in the plug hole goes and adjusted the flow control to read zero with my baseline pressure of 80 psi applied to the inlet.
Hence the equivalent of an 0.040 hole will give me 100% leakdown. YMMV.
:DQuote:
Originally Posted by Steve Demeter
Shouldn't that be 50% ? 100% would be zero on the engine-side gauge.
The zero reading with the calibration orifice installed and the flow control adjusted is on the cylinder side gage.
80 psi in, 0 psi held with the 0.040 hole.
Whatever interpertation one wants to choose.
I just record psi on the cylinder side gage anyhow.
Anything above 50 psi means a very good Pinto motor (at least by my standards).;)
Now on other motors with better ring packages available I have frequently seen 80 psi on both gages. Zero leakdown.
Thanks for all of the input. Now, I have another question. How do I keep each cylinder at TDC when the 80-100 PSI of pressure wants to push the piston down?
I have the motor in the car and even put it in gear to keep from moving but it still moves the piston. Am I missing something?
Ralph,
If the piston is moving when you apply air pressure then the crank throw is not in line with the piston pin's path. Remember that often the piston pin is offset from the cylinder bore centerline and so your TDC crankshaft position may have to be just a bit off (of TDC) to keep the piston from moving. The likely answer is that you're not at TDC, but the only other possibility I can think would cause movement at TDC, is lots of wrist pin offset. Experiment with crank positions around TDC and you will find a spot that works.
Good luck,
Randy
There are different ways to do it, but I usually put a socket with a long ratchet handle on the crankshaft bolt and rotate the engine over by hand with the leakdown gauge pressurized until the valves are closed and the reading stabilizes. You will hear the air leaking down through the open valves until it seals up and then just hold it there.
I use 50 psi with a regulator because my compressor tank pressure goes as low as 90 psi. At that pressure, it is easier to keep it at TDC, and the readings are, IMO, just as good. All you are looking for is the ratio between the input pressure and pressure after the orifice.Quote:
Originally Posted by Ralph Z.
The key to good readings, IMO, is to have a small enough orifice that normal readings are near the middle of the pressure range (20-40 psi with 50 psi input). I use a 0.020 orifice to get in that range. If you are always at 90% and above, it's hard to tell what you really have.