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Compression Test Setup Question

68vertible

Member
Joined
Jun 17, 2005
Messages
8
Location
Lavonia, Ga
Corvette
1968 Roadster, '93 40th coupe, '07 coupe
I have a 68 327 and I'm about to begin evaluating the engine. I have never fired (or even turned over) this motor. I want to run a compression test, but know I need to ground the ignition system during the test. I have some sort of converted electronic pickup in the distributor and an ignition module on the firewall with Hays written on it. I just bought a new HEI tach drive distributor to replace all this, and I'm going to be reworking the Edelbrock intake (polishing the outside and cleaning up flow on the inside).

My question is, can I pull the distributor and manifold, then run the compression test with no ignition/intake or should I leave everything in place? If I do leave it in place, how should I ground the ignition so nothing gets fried? My first inclination would be to pop the plugs into the wires and rest them on the intake manifold so they ground but I want to keep things safe so I need to be sure. Another option I've considered is to remove the hot wire from the ignition to the coil, but not sure if that is correct for an electronic conversion such as this.

I really don't have any concerns about re-indexing the distributor after turning the engine over with nothing in the hole, I've done enough teardown/rebuilds that getting the dist in right is not an issue. I've just never gotten into ignition systems before (I've always had points systems in my prior cars) so the proper techniques elude me. Thanks for the help.
 

WayneLBurnham

Well-known member
Joined
Dec 30, 2002
Messages
304
Location
Dallas County, Texas
Corvette
'75 Modified Red Ragtop: "LEFTLN"
That's one approach....or....

get one of those $10 remote starter switches which make it much easier to get to TDC anyhow, leave the key turned off, and crank it with that.

On the vette it's super easy to reach down to the starter to attach the clips to the main battery stud and the solenoid stud (usually the only other one with any wires on it.)
 

wishuwerehere82

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Joined
Sep 6, 2003
Messages
2,314
Location
Rochester, NY
Corvette
Red '82 Coupe,Sebring Silver '98 Coupe
I would leave the motor intact and pull the battery(red) wire off the coil. That way the ignition will not fire while you are doing the test. Pull all of the plugs to do the compression test and leave the wires dangling.
 

68vertible

Member
Joined
Jun 17, 2005
Messages
8
Location
Lavonia, Ga
Corvette
1968 Roadster, '93 40th coupe, '07 coupe
wishuwerehere82 said:
I would leave the motor intact and pull the battery(red) wire off the coil. That way the ignition will not fire while you are doing the test. Pull all of the plugs to do the compression test and leave the wires dangling.

Thanks, this is pretty much what I was thinking. I figured if I left the intake intact it would prevent a mess if the oil pump got enough momentum to start pumping up the lifters. Hopefully the compression check will indicate no problems that will require a tear down beyond the intake....for NOW!!! I'll worry about fire breathing horsepower dragons later.
 

bradfordsvettes

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Joined
Mar 28, 2001
Messages
549
Location
Boston, MA area
Corvette
jewel blue 61.cranberry 69 t-top.black rose 92
your readings may be low if this car has not been run up to temp and the oil to seal the ring is probably long gone take this into consideration when done also do a wet and dry test Steve
 

68vertible

Member
Joined
Jun 17, 2005
Messages
8
Location
Lavonia, Ga
Corvette
1968 Roadster, '93 40th coupe, '07 coupe
bradfordsvettes said:
your readings may be low if this car has not been run up to temp and the oil to seal the ring is probably long gone take this into consideration when done also do a wet and dry test Steve

So, would it be a good idea to spray some fogging oil into the plug holes before I start? But if I do that, wouldn't it artificially raise the compression by also sealing the compression rings? Man, nothing is ever easy.....but you do make a great point. Also, since it's been at least a year since the engine was run, should I be concerned about running the rings in a dry bore, even if it is just long enough to do a compression check? I may fog it, turn it over a time or two, then let it sit for a few days to let the oil get past the compression rings. What do you think?
 

WayneLBurnham

Well-known member
Joined
Dec 30, 2002
Messages
304
Location
Dallas County, Texas
Corvette
'75 Modified Red Ragtop: "LEFTLN"
Dry Compression Test vs. Wet

On an engine that sat a long time it's not a bad idea to pull the plugs, squirt a sec or two of WD40 in each cylinder and spin it for 30 seconds or so, but as for a ocmp test:

Comp tests are preferrably done with a warm engine, although this is often problematical - typically when done so only one plug is pulled at a time. (Edit - BTW, I'm too lazy for this, so I pull them all and do them at once, later, once I won't burn anything... :D The values are 5% or so lower then typically.)

You run the test dry first and make a point to go back somewhere near the middle of the series and retest the first couple in case warming the rings up with friction made any significant changes. Once that is done, if you find any that are typically more than 10% lower than the others, or if all are below, say, 160psi, you run a wet test.

The wet test will help determine what is leaking - valves or pistons. You use somewhere around an ounce of oil according to the texts - a tablespoon or so, or roughly 2 squirts from a standard oil can in a cylinder, one at a time. I would think WD40 for about 5 secs would be equivalent. Be cautious though - you want enough to fill up the ring area to find leaking rings and temporarily seal them but not enough to end up trying to compress it with the piston. (Oil compresses no better than water.)

If the values with the wet test are much over 15% higher than the dry test, you know the rings on that cylinder are leaking excessively.

The wet test is a subsequent test to the standard dry test to narrow down a problem.
 
V

vmrod

Guest
I pulled the plugs all at once, removed the distributor cap and wires, then spun it over. I didn't ground anything out. So far, so good.
 

68vertible

Member
Joined
Jun 17, 2005
Messages
8
Location
Lavonia, Ga
Corvette
1968 Roadster, '93 40th coupe, '07 coupe
WayneLBurnham said:
On an engine that sat a long time it's not a bad idea to pull the plugs, squirt a sec or two of WD40 in each cylinder and spin it for 30 seconds or so, but as for a ocmp test:

Comp tests are preferrably done with a warm engine, although this is often problematical - typically when done so only one plug is pulled at a time. (Edit - BTW, I'm too lazy for this, so I pull them all and do them at once, later, once I won't burn anything... :D The values are 5% or so lower then typically.)

You run the test dry first and make a point to go back somewhere near the middle of the series and retest the first couple in case warming the rings up with friction made any significant changes. Once that is done, if you find any that are typically more than 10% lower than the others, or if all are below, say, 160psi, you run a wet test.

The wet test will help determine what is leaking - valves or pistons. You use somewhere around an ounce of oil according to the texts - a tablespoon or so, or roughly 2 squirts from a standard oil can in a cylinder, one at a time. I would think WD40 for about 5 secs would be equivalent. Be cautious though - you want enough to fill up the ring area to find leaking rings and temporarily seal them but not enough to end up trying to compress it with the piston. (Oil compresses no better than water.)

If the values with the wet test are much over 15% higher than the dry test, you know the rings on that cylinder are leaking excessively.

The wet test is a subsequent test to the standard dry test to narrow down a problem.

Hoping to fog it Friday and test it Sunday. Thanks for all the help.
 

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