Compression Test Questions

[texasz]

I have borrowed a compression test gauge from my brother-in-law and have a couple questions.

1 - what should the compression on a stock '82 280 be with an F54 block and a P79 head? I looked in my books and could not find this info.

2 - how much spin on the engine is required to do this test? The engine is on a stand in my garage so just turning the key will not be possible.

[That Ozzy Guy]

Sorry to jump in on your thread like this but I wanted to buy one of these gauges and was wondering how much they are? Keep in mind I'm in Australia.

Cheers

[Bambikiller240]

Originally posted by texasz

2 - how much spin on the engine is required to do this test? The engine is on a stand in my garage so just turning the key will not be possible.

Never heard of someone trying to do this on an engine out of a car. Usually requires 3 to 4 compression strokes (cranked with a starter) to build pressure to reach the maximum it is going to get on a cylinder. Multiply by 6 cylinders.

I'm not really sure you would get an accurate reading rotating the engine by hand, but I certainly could be wrong on that.

Good Luck and let us knoww what you get for readings. Maybe we can all learn if this will work.

[Bambikiller240]

George:

Sears in the USA has two. The basic one for $25 USD, and a nice kit with many adapters etc for $50 USD

[That Ozzy Guy]

Thanks Carl. I think I'll defiantely get one. Will come in handy when car shopping in the future.

[Phred]

I have heard of some people hooking up a starter to an engine on a stand, and cranking to get a reading. But...

A more proper test for compression is done with a gauge called a leak-down tester. Tune up shops and racers all use this as a diagnostic tool to clairify an engines condition. However, this devise will pinpoint where the leakage is, and tell you how much (in psi & percentage) leakage is escaping. A compression tester only tells you that a certain cyl. is 5 or 10 lbs. low. Is it a valve? gasket? rings? a crack? You won't know unless you use a leak-down test. I use these things on a weekly basis on stands and on the dyno, and I can tell you they are the way to go. All aircaft shops use these and have standard readings that will allow an engine to pass or be repaired. They sell for anywhere from $50.00 to $150.00.

Basically they work by setting the piston at TDC on the compression stroke, and pumping and setting a regulated (80 or 100psi) amount of air into the cyl. There is a specially sized restrictor in the gauge which will allow the gauge to display the amount leakage taking place. So if the gauge is set at 100 psi, and then attatched to the cyl., and then the gauge reads 88psi,

the engine has 12 percent leakage. Then you just use your ears to follow the sizzle. If an intake valve is leaking, you will hear it at the carb, If the rings are leaking you will hear it by listening at the oil filler cap, Head gasket leakage will show bubbling in the radiator. Fun huh.

Hope this sheds some light on the subject.

Phred

[biker]

George

You can get them at any car shop but go for the more pricey ones, around $80.

You will get a more accurate reading. Seen them in AutoOne on the weekend.

Thought about buying one myself but was saving my money for a colourtune which I have been looking for for a month now. Finally have found one in Melbourne from a company that specialises in camping equipment (Ansco) of all things.

biker

[texasz]

Phred,

Do you hook one of these up to an air compressor? Do you happen to know if anyone rents them?

[Phred]

I think you'll be hard pressed to find one to rent, unless you know a tune up or A&P mechanic. These gauges can be damaged if handled improperly. There are two different types, a dual gauge, which is a bit more accurate and generally more expensive. And a single gauge type, these are usually less expensive, but do a very good job if you can supply a constant air supply to them. I've used both with excellent results. Moroso has a very good single gauge unit (part #89600). A call to your local hotrod shop should provide a price.

Phred