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 Compression Testing

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 Posted: Jun 22, 2020 10:22AM
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The rule of thumb i go by is no more than 10% difference between the cylinders.

If in doubt, flat out. Colin Mc Rae MBE 1968-2007.

Give a car more power and it goes faster on the straights,
make a car lighter and it's faster everywhere. Colin Chapman.

 Posted: Jun 21, 2020 02:38PM
 Edited:  Jun 21, 2020 02:46PM
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How even the numbers are is more important than the numbers unless quite low.

 Posted: Jun 21, 2020 12:28PM
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To both echo and add to what ctr wrote:

General consensus is plugs out, throttle wide open and + feed to the coil unplugged. I would add that, with an SU carb, you would likely want the dashpot off/open to get full, unrestricted airflow into the intake ports.

However, the most important thing is to recognize that a compression tester is a bit like a household bathroom scale. You will very likely get a different reading when you step on different scales, at different homes, wearing different attire, and at different times of the day. If you pick a repeatable methodology (like the previous advice, plus like engine cold or engine hot) then you will be able to have a picture of your engine's health as you go. Best used to compare your own engine to your own engine rather than one person's engine to yours.

Lots of variables such as is engine hot or cold? is there oil stuck to the cylinder walls from a drive an hour ago, or is it dry from sitting for a few weeks? How fresh is your battery?

Just be consistent and you'll get some meaningful relative numbers.

Note that, just like jumping on ANY bathroom scale while wearing ANYTHING will still be able to tell you , "Holy cow, I've got to lose some weight!" so too can a compression test tell you that there's something not-quite-right with your motor. If one cylinder is 10% or more different from the other cylinders, it can tell you that there is something different about it like a valve isn't sealing in that cyl as much as in another cyl. Or if you've got a very large discrepancy you can discover you have a blown head gasket.

So if you just want to rule out big time badness, there's nothing wrong with unplugging the + coil (or just putting the middle coil lead from the cap onto a head stud to ground it out) leaving plugs in, throttle closed while cranking. You'll just likely suck some atomized fuel into the ports if you crank for a while but it'll still tell you if you've got a blown head gasket or burnt valve.

hope this helps.

 Posted: Jun 21, 2020 08:25AM
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US
I pull all the plugs, unhook the coil and open throttle plate all the way and kill the fuel pump if electric.
Start where you want and crank till needle don't go up any more. Write it down and move on. Some
like a wet and dry. Test first as is then check again with a squirt of oil in the plug hole. Diff can indicate
ring condition. Steve (CTR)

 Posted: Jun 21, 2020 05:48AM
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What is the proper way to do a compression test of the cylinders? With a screw in type gauge.