Cylinder leakage test

[340Cuda]

Originally Posted by Kirk Morgan

Will a gas ported piston and ring package leak the same as a conventional piston and ring package?

That has been my experience over they years even though the engines could be very different.

I could have very good or not so good readings on any piston configurations.

[Terry Drinkwater]

I was fortunate to travel with Johnny Pilcher and his family to a lot of Pro Mod races, including ADRL and many local events. There was a lot of teardown in between rounds at the larger events due to excessive cylinder leakage. In all of our mid-race repairs, the problem seemed to be detonation and/or excessive cylinder temps, which would lock up the top ring.

[Adger Smith]

As with most instrumentation the end user is usually looking for a specific number rather than using the instrument as a diagnostic tool.
I have always used a leakdown to diagnose conditions. As said earlier Valve leakage, ring leakage and even head gasket condition.
All engines are not the same due to various clearance diffenences in cylinder wall and ring lands and end gaps for the specific aplication.
I remember when EGT gauges became the vogue.
I had customers telling me that EGT "Had to Be" XXXX number because the instructions sais so.
Put the engine on our dyno and tune it and the number didn't match what they said. Do a re tune to try to achieve their numbers and the power was way off...
I always leaked a fresh engine as I built it.. then re leak after a short run on the Dyno...
For the question about gas ports. I have never done a back to back, but it would seem they might leak a little more. However with ring seal being aided by the air pressure pressing the ring out against the cylinder wall and down against the ring land it might help static ring seal.

[Dan Bennett]

Probably stating the obvious, and may not apply to some racers but the years I worked with teams running Jenkins engines he had two things that he concentrated on. Yes, he was known for reading spark plugs - a lot of teams would send somebody over to our trailer so he could give an opinion on their plugs but first and foremost he had two things he insisted on.

First, the amount of leakage. He insisted on very low numbers. Second, on variation between cylinders. If he didn't like what he saw, he always asked for the engine back for a refresh. I remember other teams thinking he was being excessive (or maybe being too cheap or just didn't have the money) ignoring the request. And of course, they'd cry real tears when they started having problems qualifying.

This was not troubleshooting if the car had lost performance, It was checked regularly. We used to talk about a lot of things, but he would never discuss the hone finish he had come up with.


Came back to edit to include another example of how much he thought of keeping things where he wanted them. He told me that shortly after starting to use the ZL1 aluminum block, he found that heat cycles would quickly take the bores out of round. He then set up a system so he could keep a block heater running 24/7, even when in the trailer (or maybe ramp truck at the time, don't remember}.

[Adger Smith]

Dan,
The Grump could have fit right into the aircraft industry.
He would have been spot on how it is done.
Leakage for aircraft is low % per cylinder and close on the variation cylinder to cylinder

[rebelrouser]

Yes, a NA engine with tight end gaps should test better than 10% on a warm engine a cold engine will read slightly more leak down. A large gap nitrous or blower engine will leak more cold. What I do is leak my engines on the stand when fresh right after taking them off the dyno cold, save those numbers. Then you have a baseline, when leakage starts to increase, I know it is time to save my pennies for a rebuild. I don't race class just bracket and index, but I see very little loss in ET until leakage gets about 30%

[Cglrcng]

Originally Posted by Kirk Morgan

Will a gas ported piston and ring package leak the same as a conventional piston and ring package?

Am no expert here (call Ed Ercis at Ross, or anyone at Total Seal and they will explain it in simple terms)...But logic says the gas ported piston ring package is a historically recent modification beyond the normal piston/ring usage (though now widely used), and using a tiny bit of cylinder pressure on the way up, and the same in exhaust gasses on the way down (to expand the top or top 2 rings outward against the cylinder walls for better compression ring to cylinder wall sealing, has to without a
doubt in my mind also allow a certain amount of those gasses/pressure to bypass the compression rings and escape to the pan due to the gaps below, unless maybe in the use of a gap less ringset (don't know, and my logical mind says that is also a further modification that may change the equation).

Now add in that a leak test is usually done static (without piston and ringset movement in the up/down directions so it (the air pressure, that you are introducing during the leakdown testing process would still be pushing the compression ringsets against the cylinder walls, but some leakage is bound to be present, just not in the
same exact manner as actual running of the engine uses it.

Who was it long ago that was actually knurling the piston skirts? It was beyond a novel concept and gave birth I think I read many years later to the gas ported piston/ringset idea.

My bet is the Guys in Lake Havasu City at EFI University having a Spintron machine, probably have tested the exact question you posed in a non-static (and an actual engine spinning but not actuallt running condition). The Spintron is an expensive yet amazing test machine. They have done testing there as a learning institution that nobody has ever attempted (like running an engine on the dyno full throttle open plenum intake without a throttle body or carb, just to see what happens)...it ran...wide open w/zero restrictions. A super crazy test for sure. No throttle blades to close off airflow. Ever seen a runaway diesel on a dyno explosion? Think ok it runs perfectly...Now how do we shut it off without intentionally going full lean by simply shutting off the fuel flow? Will the clipboard in his hand be big enough to choke off that airflow, and who would be crazy enough to enter a dyno room and approach that engine at full throttle....all on camera and uploaded to YouTube for our viewing pleasure.

Pose the question to them, my bet is they know the answer.