Here's what I built essentially:


For the snubber, I used a #60 (.040") orifice.

Here's what I have before hooking it up to the cylinder:


Here it is hooked up:


Question is... do I have 10-15% leak down? or 96%?

Valves were closed. Air could only be heard coming by the rings, and it wasn't a large leak sound. Certainly not 96psi worth...

 

If you had no leak both gauges would show 100 PSI. So yes, you have quite a bit of leak down. But the reading will depend on how much air you allow to flow into the cylinder. Higher flow would show less leak down. Since your gauge is DIY, I have no idea how it would compare to a commercial leak down tester.

As comparison, my 454 was in bad shape before the rebuild also with over 90% leak down past the rings. After the rebuild before the first start up it was 5%.

 

I don't understand why it would show that much leak though... the motor ran great, has fairly low miles and no ridge or noticeable cylinder wear. I just did a simple heads and cam swap.

And this reading is fairly consistent across the board. There's no way all of my cylinders have a 90+% leakdown. As far as the guage setup is concerned, it's built just like this:



from:
http://en.wikipedia.org/wiki/Leak-down_tester

 

If you make the orifice larger, you will get a lower leak down reading. So the question is if you have the same orifice as the commercial meters, and if your setup has enough airflow to where it is really controlled by the orifice. Your supply side gauge going down to <90 PSI makes me question if you have enough flow?

But in any case it would seem your rings are not in great shape.

 

I believe my mistake was not setting the reg back to 100psi after it was hooked up. I can re-run the test, but I don't think I will see appreciable gains on the other side of the orifice.

I have a hard time believing it, but maybe it is time for a rebuild.

 

What size orifice, and are you sure the engine is at TDC for the cylinder you're testing. If you had 90% leakage you should be able to hear the air rusjing out the oil fill, exhaust, or carb.

 

#60 orifice (.040"). It may not have been right at TDC, but I loosened the rockers so the valves were closed regardless.

 

You can also squirt a little oil into the cylinders. If the rings are bad the readings should drastically improve.

 

There is no universally accepted input pressure for automotive leakdown testing. But the FAA (Federal Aviation Administration) has established 80 psi input pressure as their standard for leakdown testing on piston aircraft engines. And they allow up to 25% leakdown in those aircraft engines.

That 80 psi input pressure works perfectly fine for car engines too, so I use that as my input pressure also.

FAA specifications state that engines up to 1,000 cu in displacement require a 0.040 inch orifice diameter, 0.250 in long, 60-degree approach angle.

I’ve tested the 3 different types of leakdown testers.

One is a single gauge tester that reads leakdown percentage directly. This one is NOT recommended because its accuracy is typically not the best.

Another one is a dual gauge “low input pressure (typically around 35 psi or less, depending on the particular unit)” type that has one psi gauge and one gauge face that shows leakdown percentage directly. These are usually fairly inexpensive, and are also NOT recommended because of their typical inaccuracy.

And the last type is a matching dual psi gauge “high input pressure (usually can go up to 100 psi)” type. This type is convenient to use, and has good accuracy, making it clearly the best of the 3 leakdown tester types.

Note: Input pressure can be referred to in two ways, static and dynamic. Static means you set the tester’s regulator to the desired input pressure, say 80 psi (more on that below) with the tester NOT connected to the engine yet.

Then once you do connect the tester to the engine, the pressure will drop somewhat, becoming dynamic input pressure. You can then readjust the regulator to bring that dynamic input pressure back up to the original 80 psi, if you want. But I’ve found no difference at all in the final leakdown percentage results between doing that, or just letting the pressure drop somewhat and leaving it there. So, the most convenient method is to simply set the tester’s static input pressure to 80 psi and simply leave it.

The way to get to the final answer for a given test is:

For example, after you connect the 80 psi static pressurized tester to the engine, the left side regulator controlled gauge may say something like 70 psi after it drops, while the right side engine leakage gauge may say something like 65 psi.

You just plug a few numbers into your calculator, in the following manner:

You ask yourself, 65 psi on the leakage gauge is what % of the 70 psi on the dynamic input pressure gauge? And you punch into the calculator 65/.70 (don’t forget that its “point” 70 here) and the answer comes up 92.8, which means that the right side leakage gauge is showing or holding 92.8% as much as the left side input gauge. And because the original 70 psi dynamic input pressure was 100% of the dynamic input pressure, you simply punch into your calculator 100 – 92.8 = 7.2% leakage in that cylinder, which is your final accurate answer for that cylinder. That’s all there is to it.

For those who don’t use much math, that may seem like too much trouble. But if you read through what was done a couple of times, and then actually do it a couple of times, you’ll see that it’s no big deal at all. And you’ll be crunching the numbers freely after the first couple of cylinders.

And the reference chart I use for COLD leakdown testing on High Performance Engines is:

0-10% = good condition

10-15% = though not ideal, still acceptable

over 15% = tear down and repair recommended for optimum performance

(for non-performance daily driver/grocery getter type vehicles, over 30% = tear down and repair recommended)

As a point of reference, my 540ci BBC Street/Strip engine shows a COLD leakdown of about 3%, using conventional Speed Pro rings, with a top ring end gap of .021” and a second ring end gap of .027”. And keep in mind that anytime you do a leakdown test, at least with conventional rings, you will hear some air leakage. Even for the small amount of leakdown that my engine shows, I can still hear some air leakage hissing out of the breathers, from the ring end gaps.

Here’s the excellent leakdown tester that I use and like real well. It’s from “Goodson Tools and Supplies for Engine Builders”


http://www.goodson.com/Dual-Gauge_Leak-Down_Tester/

540 RAT
Member SAE (Society of Automotive Engineers)

 

Dumb question:
You had the crank locked in position so the air pressure wasn't pushing the piston back down the bore during this test, right?
Just checking...