Compression Testing

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Original submission by T June 13th 2006:

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Compression Test Gauge. Click to enlargen. Photo by Jacks

Compression Testing:

Plug Drop Test:

Much quicker and more conclusive than a Compression Test is a

Plug Drop Test.

Compression Test:

This subject comes up a lot and there is usually confusion surrounding it, that's why I've chosen to publish this page.

Safety During Compression Tests:

Compression Tests must be carried out with the Throttle wide open and the ignition completely killed so that there can be no chance of a Cylinder firing or a stray spark igniting the raw mixture that will be expelled from the Spark Plug Holes as the Engine is cranked. Place the Gear lever in Neutral (manual) or Park (Automatic), the Handbrake firmly on and the Wheels chocked. On EFI Engines, make sure the Injectors cannot discharge Fuel by removing the Tachometric Relay or disabling them by some other means.

The Throttle Must Be Wide Open:

The reason the throttle must be wide open is to ensure that each cylinder fills completely. If the throttle is partially open, a vacuum will be generated in the inlet manifold and the pressures indicated will be lower than expected.

Compression Testing Gauge. Photo by HZ Cambridge. Click to Enlargen.

Valve Overlap:

There is another reason why the throttle must be jammed open and that is Valve Overlap. As each piston reaches the end of its exhaust stroke there is a point where both the inlet and exhaust valves are slightly open. This point is called "Valve Overlap". If a compression test is done without the throttle wide open, Valve Overlap will cause some cylinders to read higher than others because they will draw air back through the exhaust valve and exhaust manifold into the inlet manifold which worsen the compression readings.

Uneven Cranking:

You may have noted that a single carburettor red/blue/black 6 cylinder Holden engine has a curious beat under partial throttle cranking, whereas an injected engine will crank dead evenly. Essentially the single carburettor engine will crank in sets of three cylinders, sounding like 1-5-3 (pause) (6-2-4). An EFI VK 6 cylinder engine has dead even cranking 1-5-3-6-2-4. The difference in cranking is caused by the fact that the single carburettor engine has manifold pipes of different lengths. Add Valve Overlap to that and under cranking some cylinders are pumping much more effectively than others and loading down the starter more, hence the uneven beat.

The Combined Effect of Valve Overlap and Different Length Manifold Pipes:

Picture piston number 1 descending and drawing in charge against a vacuum. When it reaches the 120 degree point, cylinder 5 experiences Valve Overlap. The Valve Overlap in cylinder 5 causes the inlet manifold vacuum to be reduced and air flows into cylinder number 1 from cylinder number 5. The fact that cylinder 5 is a long way away from cylinder number one means that the effect of Valve Overlap will be smaller than the effect cylinder number 3 will have on cylinder 5 when that pair experience the same circumstance. Because cylinders 3 and 5 are closer together there will be more time for the air to flow from cylinder 3 into cylinder 5 and the effect will be that cylinder 3 will produce more compression than cylinder 1 would. A wide open throttle stops a vacuum from being generated in the inlet manifold and negates the effect of Valve Overlap.

Further Compression Testing:

A submission by Jacks. Need to do before test Warm up engine, chock vehicle, compression gauge, screw in type preferable, note pad and pencil, a good car battery, remove ignition coil lead, remove all spark plugs, need to crank about 10 revolution to get a proper reading, with the throttle fully wedged open. The readings should not vary by more than 10% between cylinders, if less then all is ok, if over then further checking is required.

The Compression Pressure is Too High:

This can be caused by an Inlet Valve that has excessive Tappet Clearance. It might also be caused by a build up of carbon, so maybe a steam clean might be in order. - T adds: Hydraulic lifters have been know to cause the compression pressure to increase if worn rockers have been relocated from another cylinder or from another engine. If the Rocker Face is not smooth or does not match the end of the valve stem, the lifter will be fooled into providing excessive lash as the valve stem rotates around. This will make the inlet valve close earlier. Less charge will be blown back into the inlet manifold and the compression reading will be higher than the book states it should be.

The Compression Pressure is Too Low:

If the gauge pressure is lower than normal, (see manufacturer's recommendation) then pressure is leaking out of the combustion chamber. Low engine compression can be caused by the following conditions; Blown head gasket, broken valve, hole in piston, broken valve, cylinder head seat damaged or the valve face, by combustion, worn rings, broken valve spring, seal, or retainer, valve adjusted with insufficient clearance, this will keep the valve from fully closing properly, broken valve spring, seal, or retainer, maybe the timing, loose or worn chain or belt has jumped over teeth, upsetting the valve timing. - T adds; Gummed up piston ring grooves are a major cause of compression loss. See the [PCV] section. - T adds; Rocker wear (caused by a wear pattern matched to another cylinder) can also cause hydraulic lifters to pump up. The result will be a riding valve through which compression can be lost.

Wet Test:

This is done AFTER a dry test,by pouring a small amount of engine oil into the cylinder
If compression reading goes up with oil in the cylinder, the piston rings and cylinders could be worn, and leaking pressure, as the oil temporarily coats and seals the bad compression rings and will increase pressure, however, if the compression reading does not change and stays the same, then the valves or head gaskets may be leaking; as the oil seals the rings, but doesn’t seal a burned valve or a blown head gasket. So that’s why it’s best to do a wet test as well, go sparingly on the oil in the cylinders, wait a bit so the oil can seal around the cylinder rings, too much will give a false reading, as the compression will go up, even on good rings and cylinders, about 1-2 tablespoons of oil per cylinder will do the job But if the compression test showed a low reading, well then maybe it’s a cam lobe or the timing has jumped.

The Leak Down Test:

Mainly done after compression test, to pinpoint further problem that need investigating before you start pulling down the engine, pump high pressure air into the cylinder, make sure the piston is at TDC, we used to have a spark plug with the guts cut out of it and a hose fitting brazed on the end for the hose to slip on, it worked.
Usually only did it on the sus cylinder, you then noted how quick the air leaks by keeping an eye on the gauge, it should be very slow, especially if the rings and valves are in good condition. You will need to listen for where the air is escaping, exhaust pipe, crankcase, (need to undo filler plug) intake manifold listen at the throttle body, also check the coolant if it rises when pressure is applied in the cylinder then could be a blown head gasket, if no leakage then all is O.K.
You can buy a proper leak down gauge, they consist of two gauges alongside each other, one measures the pressure going into the cylinder, the outher measure the percentage of pressure being lost, There is a knob on the gauge that lets you control the pressure going in so you get even results going out, however this type of gauge can be a bit pricey to buy
Note: If the cylinder is Not at TDC, beware as the air pressure will rotate the crankshaft.

Compression Testing Performance Engines:

T adds; - A while back a good deal was posted on Compression Testing of engines with performance modifications. Essentially the compression test results can vary between a standard engine and a modified Engine. One reason can be the valve duration. T adds; - A long valve duration can cause compression to be blown back into the inlet manifold during cranking and result in less than a predicted compression ratio. The compression ratio may increase with RPM as the duration begins to pay a dividend at higher RPM though.  

Compression Test Gauge. Photo by HRAmbo. Click to Enlargen.
Remote Starter Switch. Connect this to the Starter Solenoid while working under the Bonnet to activate the Starter. Use this when Compression Testing and make sure the Ignition Circuit has been completely disabled first. Photo by HRAmbo. Click to Enlargen.

Gauge Prices:

$10 for the hand held one, others are $55, and still others are $90, all from Repco.


Compression Testing 06:10 "Our Throttle is in the Wide Open Position"



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