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MG Midget and Sprite Technical - Compression tester types

Here's a question that is not specifically aimed at Spridgets, since it is for an Austin 7.

What are the plus and minus points of compression testing with 'peak pressure' types where you spin the starter over when compared to the 'leak-down' type where you allow a metered flow into a cylinder and compare the static pressure (ie: 80psi in, 75psi remains)?

I have used both in the past and the second one is sooooo much easier to use on a car with a starting handle and cr4p starter motor.

I'd welcome any suggestions and personal experiences.

Thanks

Rob in Llandow.
r thomas

They do two entirely different things:

The compression meter measures the pressure achieved in the cylinder at cranking speed,

The leak down tester shows how good the setup is at containing the pressure regardless of the pressure achieved and where the gas is escaping from.

So it depends on what you are trying to measure!
Chris at Octarine Services

Thanks Chris.

It does suggest then that the first type is only good for diagnosing which cylinder has a leak, and nothing else, since it can't really tell you where the leak goes. I assume you can only replicate the figures if you get exactly the same rpm from the starter?

If this is so, why do they sell so many more of them that the other type?

I've used the 'second' type on the A7, only because the 6volt starter is painfully slow and you can't hand-crank it fast enough to get much of a reading!
r thomas

A compression test can be followed by other tests to indicate the likely problem. For example. Get a low reading on number 1. You might suspect rings. If a few drops of oil in the bore improves the reading, it's likely rings. If there's little or no change in the reading, it could be a leaky valve or a head gasket.

Either way, a low reading, unless it's due to a rocker clearance being too close, needs the head of to rectify it. So a basic compression test is all you need really to decided whether or not to pull the head.

I've got a cheap one. I don't know how accurate it is, but it indicates all I need to know. But I agree. I wouldn't really want to use one, if I had to crank the handle on each cylinder numerous times to get accurate readings.
Lawrence Slater

The simple compression tester is cheap and a quick health check - starter rpm is pretty well irrelevant and 5 revs on the handle will do just as well. The absolute value is less important than a balance across the cylinders - generally I would expect an engine in good condition to have readings within 10 % of each other.

The leak down tester is a more expensive bit of kit and needs a source of compressed air at 60psi - I generally only use it to follow up a problem identified by a low reading on the first type of tester.
Chris at Octarine Services

If you really want to cook your noodle....

hook up a vacume gauge to the intake manifold and learn what the gauge readings mean

Whats great about using a leak down test, a compression gauge and a vacume gauge is the readings can justify each other and off a more detailed understanding of whats happening and pin point where a problem exist

A vacume test will tell you if you have a blown head gasket, a compression test will tell how bad and where, a leak down test will confirm and remove any doulbt...that other issues dont apply....

Is any test better then the other...yes and no, is a screw driver better then a hammer, or a saw better then a drill

Obviously a drill does better at drilling holes then a hammer can....it depends on what the objective is as to which tool does the best job
Prop and the Blackhole Midget

This thread was discussed on 26/01/2014

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