compression question

[slickster]

when i start then at lower idel i got some wobble lobey feeling kind of seems like low comp on a cylinder. how mamy lbs should i look for when doing a comp test

[droppointalpha]

IIRC, the 4.6L in the marauder should be 10.1:1...

But i've only ever done compression tests on large generators at work...not sure what you should be looking for.

[fastblackmerc]

It's not 10:1...... :shake:

I'll check the shop manual when I get home for the actual poundage.

What your looking for is a consistent reading across all cylinders. I believe the readings should be within 10% of each other.

[justbob]

It's not 10:1...... :shake:

I'll check the shop manual when I get home for the actual poundage.

What your looking for is a consistent reading across all cylinders. I believe the readings should be within 10% of each other.

Correct. 10% I do a test yearly as part of preventive maintainance and log it. I pop all plugs, pull the FP fuse, open the TB, and have my sexy asssisstant (Melanie) crank it over till the guage stops climbing (about 4-5 seconds) I get 218 - 225 readings.

You might want to do a leak down test as well.



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[BlkMamba24]

If it isn't 10:1 what is it?

[fastblackmerc]

If it isn't 10:1 what is it?

10:1 is the compression RATIO.

Different than how many pounds of pressure.

"The compression ratio of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity. It is a fundamental specification for many common combustion engines.

In a piston engine it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke. Picture a cylinder and its combustion chamber with the piston at the bottom of its stroke containing 1000 cc of air (900 cc in the cylinder plus 100 cc in the combustion chamber). When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be proportionally described as 1000:100, or with fractional reduction, a 10:1 compression ratio.

A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency.[citation needed] High ratios place the available oxygen and fuel molecules into a reduced space along with the adiabatic heat of compression–causing better mixing and evaporation of the fuel droplets.[citation needed] Thus they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree.[citation needed]

Higher compression ratios will however make gasoline engines subject to engine knocking if lower octane-rated fuel is used, also known as detonation. This can reduce efficiency or damage the engine if knock sensors are not present to retard the timing. However, knock sensors have been a requirement of the OBD-II specification used in 1996 model year vehicles and newer."

He's trying to find out if his rings, pistons or valves / valve seats are in good condition by seeing how much pressure each cylinder holds.

[fastblackmerc]

when i start then at lower idel i got some wobble lobey feeling kind of seems like low comp on a cylinder. how mamy lbs should i look for when doing a comp test

Have you cleaned your IAC?

[BlkMamba24]

10:1 is the compression RATIO.

Different than how many pounds of pressure.

"The compression ratio of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity. It is a fundamental specification for many common combustion engines.

In a piston engine it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke. Picture a cylinder and its combustion chamber with the piston at the bottom of its stroke containing 1000 cc of air (900 cc in the cylinder plus 100 cc in the combustion chamber). When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be proportionally described as 1000:100, or with fractional reduction, a 10:1 compression ratio.

A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency.[citation needed] High ratios place the available oxygen and fuel molecules into a reduced space along with the adiabatic heat of compression–causing better mixing and evaporation of the fuel droplets.[citation needed] Thus they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree.[citation needed]

Higher compression ratios will however make gasoline engines subject to engine knocking if lower octane-rated fuel is used, also known as detonation. This can reduce efficiency or damage the engine if knock sensors are not present to retard the timing. However, knock sensors have been a requirement of the OBD-II specification used in 1996 model year vehicles and newer."

He's trying to find out if his rings, pistons or valves / valve seats are in good condition by seeing how much pressure each cylinder holds.

Good Stuff!!!! :up:

[RF Overlord]

Mod motors are not known for compression problems. Although it's *possible*, I'd be willing to bet you have a weak/bad COP or a sticky injector first, or a vacuum hose leak.

[slickster]

Have you cleaned your IAC?

Not yet but this sunday I will start with cleaning iac maf etc then look at cops and plugs hopefully it will stop I don't want to find out I have a bad ring etc:( oh well at least I would of caught it early

[TooManyFords]

"The compression ratio of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of its combustion chamber from its largest capacity to its smallest capacity. It is a fundamental specification for many common combustion engines."
Not sure where this was quoted from, but it is wrong. The definition described calculates the total cubic inches of the motor, not the compression ratio. What the quote fails to take into account is when the intake valve closes. However far up the cylinder is when this event happens controls what is called the "dynamic" compression ratio.

[TooManyFords]

And what the H3LL is an "external combustion engine"?

[99SVT]

And what the H3LL is an "external combustion engine"?

A jet turbine?

The quote is right for static compression ratio, the wording doesn't make it easier to understand.

An easier one would be:
Compression ratio is the ratio of the swept volume compared to the compressed volume of a cylinder. ie: a 100 cubic inch cylinder at BDC compressed into a 10 cubic inch space at TDC would have a 10:1 ratio.

[na svt]

Static compressio ratio: the ratio of the combined volume of the cylinder and combustion chamber with the piston at bottm dead center and the combined volume with the piston at TDC.

Dynamic compression ratio: the ratio of the combined volume of the cylinder and combustion chamber with the piston at the point where the intake valve closes and the combined volume with the piston at TDC.

You don't have to pull any fuses when doing a compression check. To ensure no fuel is delivered to the cylinders all you have to do put the gas pedal to the floor (full throttle), doing so shuts off the signal to the injectors.

So, remove the plugs, install the compression gauge, put the pedal to the floor and spin the engine over 3-4 times. Check all cylinders and compare the readings.


A jet turbine?
nope

[boatmangc]

And what the H3LL is an "external combustion engine"?
Anytime I have experienced external combustion engines it has usually been in their last few seconds of life :lol:

[RF Overlord]

And what the H3LL is an "external combustion engine"?An engine where the combustion take place outside the cylinder that delivers the power. The steam engine is a good example. The Stirling engine is another.

[droppointalpha]

It's not 10:1...... :shake:

I'll check the shop manual when I get home for the actual poundage.

What your looking for is a consistent reading across all cylinders. I believe the readings should be within 10% of each other.


IIRC, the 4.6L in the marauder should be 10.1:1...

But i've only ever done compression tests on large generators at work...not sure what you should be looking for.

Hmmm... Someone didn't read my post very well...


I'll check the shop manual when I get home for the actual poundage.

not sure what you should be looking for.

[zman]

The indicated compression pressures are considered within specification if the lowest reading cylinder is at least 75 percent of the highest reading.

If one or more cylinders reads low, squirt approximately one tablespoon of engine oil on top of the pistons in the low-reading cylinders. Repeat the compression pressure check on these cylinders.


If compression improves considerably, piston rings are faulty.
If compression does not improve, valves are sticking or seating incorrectly.
If two adjacent cylinders indicate low compression pressures and squirting oil on each piston does not increase compression, the head gasket may be leaking between cylinders. Engine oil or coolant in cylinders could result from this condition.