Leak down test

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Woodie

Woodie

Take-off EH!
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Thinking about doing a leak down check on one of my small block LA engines. If anyone with knowledge could chime in and give a proper procedure it would be appreciated. I think its time to refresh this mill.
 
Haven't done one in a while but this might help:
- set piston that is being tested to TDC compression.
- regulate the CLT tester to little air flow.
- connect CLT to threaded hose in spark plug bore.
- slowly regulate air pressure until you reach 100PSI.
- record reading and listen at intake, exhaust, water neck, and crank case filler for leakage.

Hope this helps.
 
Guessing you don't have an actual tester and you are just going to use line pressure and listen for leakage ? It is sometimes difficult this way because engine will tend to rotate from the air pressure. If you pull rocker arm shafts, then no problem with rotation. Block open throttle, pull radiator cap and spark plugs.
 
Being a cheap guy, I have just applied air pressure thru the hose on my Harbor Freight compression tester. It has a standard air quick-connect for the gage, so just hook your air hose to it. But, don't apply too much pressure, and insure the piston is at exact TDC or the crank may spin (don't leave a wrench on the crank bolt). I then listen for where the air is flowing - crankcase (past rings), intake (past intake valve), or exhaust (past exhaust valve). Not very quantitative, but your compression test will give you that, plus just feel for each cylinders as you turn the engine over by hand.
 
Being at operating temperature will likely give more accurate readings on the piston and ring seal if you are able to warm up the engine.
 
You will know if it's not TDC as the piston will move when you apply air. In the case of a LA motor in an A Body, I would just loosen the Rocker Shafts and eliminate the Valve closed variable. The piston could be BDC and you may get almost the same reading as TDC. 100 psi is more than a plenty. Just take the readings and see what you got.

Good luck,
JW
 
Hold on there BubbaLouie
A leakdown test at the bottom of the bore (with the rocker gear removed)will tell you if a valve is or is not leaking. And it may tell you a little about ring-seal
But it will not tell you anything about the most important part of the cylinder, the top, where all the action takes place.
And to do this test without a proper gauge, is IMO next to a waste of time.
Compression does not start until the intake is fully closed. This may be from 50 to 80 degrees ABDC. That is nearly half way up from the bottom. The bottom of the bore is seldom worn. Half way up is seldom worn. Almost all the wear occurs in the top inch of the cylinder. That is what you want to check. And the reason is that for every .001 change in the cylinder size, the ring gap changes by 3.1416; so 3 thou of cylinder wear increases the ring gap by .0094, and in terms of ring gaps that is a truckload of gap!
An engine could have very good LD results at the bottom, where almost no wear occurs and the ring gap is minimized. Yet at the top, the engine could be worn out. And what if the cylinder had at some time in history suffered damage in that top most position? Your at-the-bottom-test would completely miss it. And of course,the at-the-top test would totally miss a gouge in the cylinder wall from a wrist pin.
I only do an LD test if compression is uneven, or if it has fallen alarmingly since the previous test.Or if I want/need to prove the engine is 100%
Using 100psi is very tricky. I installed a shut-off valve on my tester that allows me to slowly bleed the pressure in, cuz if the pressure is applied all at once, it usually just spins the crank over. Even 80psi is tricky. You cannot control the crank with a breaker bar at much over 20 psi, cuz on a 4 inch bore this is 12.57 sq inches times 20 psi= 251 footpounds of force, at 1 ft, and 201 at 15 inches. Don't even try. It will tear your arm off!
The piston has to be exactly at TDC. Unless it has an offset wristpin,lol.
But the results are worth it
An engine with less than 1% leakage is a very powerful engine, or could be.
By 4% the power is failing ; Think about it,
By 8% the engine is very very tired.
Pressure is everything in an engine. The whole entire combustion process is designed to build pressure to rotate the crank to turn the drivewheels. When the engine has lost 8% of available pressure, this is the equivalent of going from a prime engine of 165 psi to an also-ran engine of 152 psi. More typically from a 130 psi cylinder to 120psi. A lot of teeners can barely make 130 when new.
This is why a stock compression teener with a 268* cam can be a real dog;a serious lack of pressure.
 
AJ/FormS said:
Hold on there BubbaLouie
A leakdown test at the bottom of the bore (with the rocker gear removed)will tell you if a valve is or is not leaking. And it may tell you a little about ring-seal
But it will not tell you anything about the most important part of the cylinder, the top, where all the action takes place.
And to do this test without a proper gauge, is IMO next to a waste of time.
Compression does not start until the intake is fully closed. This may be from 50 to 80 degrees ABDC. That is nearly half way up from the bottom. The bottom of the bore is seldom worn. Half way up is seldom worn. Almost all the wear occurs in the top inch of the cylinder. That is what you want to check. And the reason is that for every .001 change in the cylinder size, the ring gap changes by 3.1416; so 3 thou of cylinder wear increases the ring gap by .0094, and in terms of ring gaps that is a truckload of gap!
An engine could have very good LD results at the bottom, where almost no wear occurs and the ring gap is minimized. Yet at the top, the engine could be worn out. And what if the cylinder had at some time in history suffered damage in that top most position? Your at-the-bottom-test would completely miss it. And of course,the at-the-top test would totally miss a gouge in the cylinder wall from a wrist pin.
I only do an LD test if compression is uneven, or if it has fallen alarmingly since the previous test.Or if I want/need to prove the engine is 100%
Using 100psi is very tricky. I installed a shut-off valve on my tester that allows me to slowly bleed the pressure in, cuz if the pressure is applied all at once, it usually just spins the crank over. Even 80psi is tricky. You cannot control the crank with a breaker bar at much over 20 psi, cuz on a 4 inch bore this is 12.57 sq inches times 20 psi= 251 footpounds of force, at 1 ft, and 201 at 15 inches. Don't even try. It will tear your arm off!
The piston has to be exactly at TDC. Unless it has an offset wristpin,lol.
But the results are worth it
An engine with less than 1% leakage is a very powerful engine, or could be.
By 4% the power is failing ; Think about it,
By 8% the engine is very very tired.
Pressure is everything in an engine. The whole entire combustion process is designed to build pressure to rotate the crank to turn the drivewheels. When the engine has lost 8% of available pressure, this is the equivalent of going from a prime engine of 165 psi to an also-ran engine of 152 psi. More typically from a 130 psi cylinder to 120psi. A lot of teeners can barely make 130 when new.
This is why a stock compression teener with a 268* cam can be a real dog;a serious lack of pressure.
Click to expand...

To each his own on this one. I have used a leakdown tester on everything from A to Z and the numbers dont vary that much from what i have seen. To each his own. In fact having you piston at BDC may reveal other problems.

As for percentages, leak a true nitrous built application and report back. Give me 4-7 on a street application and I will be fine...

JW
 
AJ/FormS said:
Hold on there BubbaLouie
A leakdown test at the bottom of the bore (with the rocker gear removed)will tell you if a valve is or is not leaking. And it may tell you a little about ring-seal
But it will not tell you anything about the most important part of the cylinder, the top, where all the action takes place.
And to do this test without a proper gauge, is IMO next to a waste of time.
Compression does not start until the intake is fully closed. This may be from 50 to 80 degrees ABDC. That is nearly half way up from the bottom. The bottom of the bore is seldom worn. Half way up is seldom worn. Almost all the wear occurs in the top inch of the cylinder. That is what you want to check. And the reason is that for every .001 change in the cylinder size, the ring gap changes by 3.1416; so 3 thou of cylinder wear increases the ring gap by .0094, and in terms of ring gaps that is a truckload of gap!
An engine could have very good LD results at the bottom, where almost no wear occurs and the ring gap is minimized. Yet at the top, the engine could be worn out. And what if the cylinder had at some time in history suffered damage in that top most position? Your at-the-bottom-test would completely miss it. And of course,the at-the-top test would totally miss a gouge in the cylinder wall from a wrist pin.
I only do an LD test if compression is uneven, or if it has fallen alarmingly since the previous test.Or if I want/need to prove the engine is 100%
Using 100psi is very tricky. I installed a shut-off valve on my tester that allows me to slowly bleed the pressure in, cuz if the pressure is applied all at once, it usually just spins the crank over. Even 80psi is tricky. You cannot control the crank with a breaker bar at much over 20 psi, cuz on a 4 inch bore this is 12.57 sq inches times 20 psi= 251 footpounds of force, at 1 ft, and 201 at 15 inches. Don't even try. It will tear your arm off!
The piston has to be exactly at TDC. Unless it has an offset wristpin,lol.
But the results are worth it
An engine with less than 1% leakage is a very powerful engine, or could be.
By 4% the power is failing ; Think about it,
By 8% the engine is very very tired.
Pressure is everything in an engine. The whole entire combustion process is designed to build pressure to rotate the crank to turn the drivewheels. When the engine has lost 8% of available pressure, this is the equivalent of going from a prime engine of 165 psi to an also-ran engine of 152 psi. More typically from a 130 psi cylinder to 120psi. A lot of teeners can barely make 130 when new.
This is why a stock compression teener with a 268* cam can be a real dog;a serious lack of pressure.
Click to expand...
Beauty. Thank you. :)
 
AJ has the info that I agree with (mostly). Move the piston around TDC and you will be surprised how the leakage can change.

BTW, AJ, at 80 psi, the piston is putting around 150 ft-lbs torque on the crank with the piston half way down the bore, and that diminishes to 0 at TDC and BDC. So with a breaker bar and a 3 ft pipe, you can control it all around TDC easily, but you have to be on top of it. That gets done on small aircraft engines all the times by holding the prop, which is typically 33-40" long on one side. (I sometimes get to be the 'live ballast' on the end of the prop LOL)
 
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