454 Compression Test vs Leak Down Test?

[savatreatabvr]

I known that a leak down test is good determining if the valves or rings are leaking and a compression test is good for determining how mush PSI the cylinder has. To perform a leak down test everyone says to bring that particular cylinder up to TDC and use the proper set of gauges to fill the cylinder with air. That's fine and dandy but isn't it more accurate to remove the valve cover and loosen the two rocker arms for that cylinder so the valves are 100% closed then take a air fitting and screw it in to the spark plug hole and supply air to the cylinder straight from the compressor and listen for leaks? A leak is a leak, who really cares how much it's leaking because any leak is a major issue right? If the valves leaks the head must be removed anyways so determining the actual percentage of pressure leaking is kind of irrelevant. So spending the money on a leak down tester is kind of pointless. Just my opinion.

[MountainGoat1966]

Removing the valve cover will allow you to hear air escaping past the rings.
But then again, removing the oil fill cap serves the same purpose.
You don't need to back off the rocker nuts as the piston will be at TDC on compression stroke and both valves will be as closed as they can be.
When I read the other person's post about the cylinder leakdown test, I noticed they didn't mention the cylinder being at TDC compression, I should have spoke up sooner.
And no, it's not a pointless test as you can easily determine what is wrong and which cylinder has the problem.

[savatreatabvr]

Originally Posted by MountainGoat1966

Removing the valve cover will allow you to hear air escaping past the rings.
But then again, removing the oil fill cap serves the same purpose.
You don't need to back off the rocker nuts as the piston will be at TDC on compression stroke and both valves will be as closed as they can be.
When I read the other person's post about the cylinder leakdown test, I noticed they didn't mention the cylinder being at TDC compression, I should have spoke up sooner.
And no, it's not a pointless test as you can easily determine what is wrong and which cylinder has the problem.

Just my point......(((And no, it's not a pointless test as you can easily determine what is wrong and which cylinder has the problem.)))....... The only thing using air straight from the compressor does not do is tell you what the percentage of air is leaking but if you've got semi normal hearing you'll be able to hear air leaking and getting TDC is not as easy as it seems, yes I know the procedure but to be 100% confident both valves are 100% closed loosening the nuts is the only way!

[MountainGoat1966]

I don't mean to sound condescending, so please take this in the spirit in which I am typing it.
The leakage tester has a gauge that reads percentage, so you do get an accurate reading of the loss as well as your audible notice when you hear a leak.
And what I do is bring #1 to TDC compression, use the timing mark as long as the harmonic balance hasn't slipped, use left hand valve cover removed to watch valves if necessary.
Run the test on #1 cylinder.
Put a breaker bar with socket on the bolt head on the end of the crank in the harmonic balancer, turn the crank 90° and test #8
Do the same for the rest of the firing order.

[MountainGoat1966]

As for loosening the rocker nuts, I don't see where it would make any difference.
Given the pressure the valve springs exert against the retainer, the valves are fully closed when the cylinder is at TDC compression.
When the rocker nuts are adjusted tight to specs, the hydraulic plunger in the lifter will travel down slightly, you won't be applying pressure to even partially open the valve.

[Desert1957]

Originally Posted by MountainGoat1966

Removing the valve cover will allow you to hear air escaping past the rings.
But then again, removing the oil fill cap serves the same purpose.
You don't need to back off the rocker nuts as the piston will be at TDC on compression stroke and both valves will be as closed as they can be.
When I read the other person's post about the cylinder leakdown test, I noticed they didn't mention the cylinder being at TDC compression, I should have spoke up sooner.
And no, it's not a pointless test as you can easily determine what is wrong and which cylinder has the problem.

I'm not sure if you actually posted your compression results on another thread. based on the compression results a lot can be determined.

Just say ALL your compression numbers are at 140 psi, but with the engine running there is a noticeable miss. Could be a lot of different issues, vacuum leak, ignition problem, etc...

My point is there is no need to perform a leak down test if you have 8 equal cylinders. Leak down testing on a dead cylinder is used to pin point where the bad cylinder has compression loss.

I have race engines with 5% leak down , but I have other street engines with 15% leak down that run fine.

I am assuming you have performed a compression test the correct way?

1. Remove ALL spark plugs
2. Disable the ignition so it will not start
3. Hold throttle wide open during ALL testing
4. Crank engine for at least 10 seconds per cylinder
5. Record ALL numbers for future reference

Desert

[savatreatabvr]

Originally Posted by Desert1957

I'm not sure if you actually posted your compression results on another thread. based on the compression results a lot can be determined.

Just say ALL your compression numbers are at 140 psi, but with the engine running there is a noticeable miss. Could be a lot of different issues, vacuum leak, ignition problem, etc...

My point is there is no need to perform a leak down test if you have 8 equal cylinders. Leak down testing on a dead cylinder is used to pin point where the bad cylinder has compression loss.

I have race engines with 5% leak down , but I have other street engines with 15% leak down that run fine.

I am assuming you have performed a compression test the correct way?

1. Remove ALL spark plugs
2. Disable the ignition so it will not start
3. Hold throttle wide open during ALL testing
4. Crank engine for at least 10 seconds per cylinder
5. Record ALL numbers for future reference

Desert

Nooooooooo, my compression read around 110psi +/- 10psi on all cylinders! I didn't open the throttle at all and I pulled one plug at a time BUT I do understand removing them all at the same time will let the 454 crank over easier resulting in less stress on the starter. Key question here is, is 110psi +/- 10psi low for a mid 70's 454 with high mileage, I mean like 200,000 - 300,000 high miles? This may be related but probably not, I recently replaced the timing chain and when I dropped the oil pan I found something very interesting! I found a piece of a "push rod" about 1" long, whomever owned the truck before me obviously had some sort of catastrophic failure but fixed the problem because all my push rods are fine. Sooooooooo, what would cause a push rod to snap like that?

[MountainGoat1966]

Not sure on pushrod failure, but compression sounds low.
General rule of thumb, you lose 10 psi on a compression gauge for every 1,000 ft above sea level.
My mild 383 stroker still has 150 psi compression, and I am at 5300 ft. It's hard to get those readings at this elevation.
And that's after 70,000 HARD work miles.
I would think at Phoenix elevation, any engine would have 150 psi or better.

[savatreatabvr]

Originally Posted by MountainGoat1966

Not sure on pushrod failure, but compression sounds low.
General rule of thumb, you lose 10 psi on a compression gauge for every 1,000 ft above sea level.
My mild 383 stroker still has 150 psi compression, and I am at 5300 ft. It's hard to get those readings at this elevation.
And that's after 70,000 HARD work miles.
I would think at Phoenix elevation, any engine would have 150 psi or better.

I agree! I guess it's time to completely rebuild the 454! The only problem is I don't have the room to pull the 454 so it would be a "IN THE TRUCK" rebuild! Has anyone attempted this before or is it even worth trying?

[Desert1957]

Originally Posted by savatreatabvr

Nooooooooo, my compression read around 110psi +/- 10psi on all cylinders! I didn't open the throttle at all and I pulled one plug at a time BUT I do understand removing them all at the same time will let the 454 crank over easier resulting in less stress on the starter. Key question here is, is 110psi +/- 10psi low for a mid 70's 454 with high mileage, I mean like 200,000 - 300,000 high miles? This may be related but probably not, I recently replaced the timing chain and when I dropped the oil pan I found something very interesting! I found a piece of a "push rod" about 1" long, whomever owned the truck before me obviously had some sort of catastrophic failure but fixed the problem because all my push rods are fine. Sooooooooo, what would cause a push rod to snap like that?

Removing all the spark plugs during compression testing is so it can relieve the starter of stress. During testing cranking speed needs to be increased to properly perform the test so I'm not sure you readings are accurate, may even be low.

I will tell you , 110 psi is low. Your motor is tired sorry.

The push-rod thing could have been a rocker arm failure or just breakage from years ago. I wouldn't worry about it.

Without getting to technical, I will tell you that GM's 454 has the worst piston to rod angle of any engine they have ever produced. While the stroke and bore combination is great for torque and power, Its not designed for long piston life. What happens is excessive side loading of the piston during its cycle. This causes cylinder bore wear which leads to ring and piston wear. Then loss of compression.

I do not remember reading in all of you other engine postings about the engine having 200,000 - 300,000 plus miles that's a lot on a 454.

Desert

[savatreatabvr]

Originally Posted by Desert1957

Removing all the spark plugs during compression testing is so it can relieve the starter of stress. During testing cranking speed needs to be increased to properly perform the test so I'm not sure you readings are accurate, may even be low.

I will tell you , 110 psi is low. Your motor is tired sorry.

The push-rod thing could have been a rocker arm failure or just breakage from years ago. I wouldn't worry about it.

Without getting to technical, I will tell you that GM's 454 has the worst piston to rod angle of any engine they have ever produced. While the stroke and bore combination is great for torque and power, Its not designed for long piston life. What happens is excessive side loading of the piston during its cycle. This causes cylinder bore wear which leads to ring and piston wear. Then loss of compression.

I do not remember reading in all of you other engine postings about the engine having 200,000 - 300,000 plus miles that's a lot on a 454.

Desert

Never thought about posting the mileage before but if I understand you correctly you're saying that.......one side of the cylinder can be worn out (kinda egg shaped) so the piston and rings are father away from one side of the cylinder than the other side? If that's the case then honing the cylinder won't do anything and a bore job is obviously needed? Is this something that can be seen with the naked eye or will I need a micrometer?