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84 Spider lived(?) through an electrical fire. After much advice here (THANKS!), pulled engine and trans which is now sitting on the floor. (Victory!)
Now I have (2) engines sitting on the floor (car came with a spare) and need to decide which one should go in the car. A Corvette Guy suggested I might be able to put 12-volts to the starter to crank the engines over enough to do a compression test. He also suggested reading the plugs for clues but of course the plugs in the engine I just pulled are brand new. So not much to learn there. Anything else I can look at to help decide which engine to put in the car? Any reason to NOT do a floor-compression test? Any thoughts and opinions would be appreciated!
 

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Just secure the engine so it doesn't fall over from any torque reaction. If the intake is still attached block the throttle wide open.

Or you can do a leak down test.

Both tests are usually done a warm engine (after it has run for a while) but there's no reason you can't do it on a cold engine.
 

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A differential (leak down) test would probably be the best option and would give you more valuable information from the results. Problem is you most likely don't have the equipment needed. Next best option is to just crank it over with an impact gun or such. Speed of rotation is not important, just keep it going until the needle stops.
 

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If this was me ...I would take the heads off and do a visual. Way easier when they are out of car. Prolly need a head gasket anyway.

Compression test don't show bad valve guides. Looking at P&L's can tell a lot.
 

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As suggested, strap the motor down and crank away. Remove all the plugs to make it easy on the starter and run it WOT. Good motors should have no more than 10 percent variation between the highest and lowest readings. Record the readings and then repeat after squirting a bit of oil in each cylinder. If the compression increases significantly with oil then you got ring problems.

A leak down test will be a better indication of the condition of the rings, seated valve sealing, and the head gasket. I think some of the auto parts places will lend you the equipment.

Before I tore into the motor I'd build a test stand and run it off. You don't need anything fancy and can build up an acceptable stand with some 2x4s. You'll need to mount a fuel pump, radiator and some plumbing but with a couple of motors it would be worth the effort.
 

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How does a leak down test indication of the condition of the guides?
 

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Doing a compression test cold will yield really low numbers, so you still won't be able to tell just how good the compression is. If you liked how the engine ran before the fire, why not keep it?
 

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The fire occurred at least 2 owners ago. I have never heard either engine run. All I know is what I have been told: The engine in the car was meticulously maintained by an owner who babied the car. The engine has about 70k miles on it and went through an electrical fire that got hot enough to warp the hood and peel some paint. The spare engine has around the same mileage and came out of a car that might have been beat and or raced. Apparently the car it came out of was rough at best. I am leaning toward the engine that came out of the car but I would like to learn what I can before going through the trouble of installing, wiring, rebuilding the car around it and getting it started. I appreciate the advice!
 

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Doing a compression test cold will yield really low numbers, so you still won't be able to tell just how good the compression is. If you liked how the engine ran before the fire, why not keep it?
Hot is best but I think a cold compression test is still valid. Obviously if it won't run, you can't get it hot.

You should be looking for consistency between cylinders of no more than 10%. Usually, if you've serious trouble you'll see one or more cylinders barely able to make any pressure.
 

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On one occassion I started up and ran a small 4 cylinder engine mounted on a tractor trailor truck tire, both sitting on the floor of my auto repair shop. Jury rigged the electrics and added a limited quanty of fuel supply. Engine was held upright by a chain from the ceiling and prevented from counter rotating by a couple of guy ropes going out at right angles. I did not have cooling or exhaust and only ran the engine for 30 seconds or so to check if it was a functional used engine before going to all the trouble of installing it into a car.

It was a loud but functional bench check of an unknown used engine. My primary concern was how easily it started up and what the operating oil pressure was.

Cold compression test of a used engine would be easier. Biggest problem was getting hold of a used tractor trailer tire to use as a test bed.
 
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