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· What the Hell is That?
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Discussion Starter · #1 ·
today i decided to do a compression check. Its a SBC 327 .30 over Full roller motor. I figured its simple enough.

So I went at it and the numbers seem way off. Not different from each cylinder just way low.

1)30
2)60
3)60
4)75
5)60
6)60
7)60
8)40

I thought they should be up in the hundreds? I would get it if one or two cylinders were 60 or 30 but all of them in that range?

I pulled all plugs on the respective side I was doing. either I am doing this wrong or this engine is done. I screwed in the tester and cranked it over with no coil. it will shoot to a number then rise to a specific number. it will slow when it gets to say 60.

is it me or the motor?
 

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Did you do the test with the throttle butterflies open or closed? Sounds to me like you have a bum gauge...Should be somewhere close to around 150 PSI or so...

GT :)hand
 

· What the Hell is That?
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1,586 Posts
Discussion Starter · #5 ·
No butterfly's open, she was not warm and all the plugs were not out.

It is a very low compression engine if that matters. 6.1 Compression it was built as a blower motor.

Im pretty sure i did it wrong. But hey if you never know how will you ever learn?
 

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Plugs out is optional. Just easier on the starter. Shouldn't affect the numbers, though.

Butterflys HAVE to be open. How else can it get air to fill the cylinder(s)????

I suggest disconnecting the fuel also. You don't want to flood the motor. Shouldn't happen, unless you have sticky floats (carbed) but it is always safer without fuel pumping in.
 

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today i decided to do a compression check. Its a SBC 327 .30 over Full roller motor. I figured its simple enough.

So I went at it and the numbers seem way off. Not different from each cylinder just way low.

1)30
2)60
3)60
4)75
5)60
6)60
7)60
8)40

I thought they should be up in the hundreds? I would get it if one or two cylinders were 60 or 30 but all of them in that range?

I pulled all plugs on the respective side I was doing. either I am doing this wrong or this engine is done. I screwed in the tester and cranked it over with no coil. it will shoot to a number then rise to a specific number. it will slow when it gets to say 60.

is it me or the motor?
the motor has to roll for 3 compression strokes to get an accurate reading on each cylinder. Doesn' t sound like you did it that way.
 

· What the Hell is That?
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1,586 Posts
Discussion Starter · #11 ·
The only thing it seems is the Butterfly's were not open. I will try again tomorrow does the motor need to be warmed up? I have heard conflicting answers.
 

· Boat Nut
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4,935 Posts
The only thing it seems is the Butterfly's were not open. I will try again tomorrow does the motor need to be warmed up? I have heard conflicting answers.
Cylinders with some heat in them will generally show higher static compression. It's a very comon practice in aviation, to do a run up, prior to compression testing.
 

· What the Hell is That?
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1,586 Posts
Discussion Starter · #14 · (Edited)
well tried it again today. Cold Motor all the plugs removed, butterfly open

1)40
3)70
5)70
7)60

I turned it over until the needle stabilized.

Either I am looking at a bum gauge or a really bum motor. Does not makes sence for it to be the motor she runs fine out on the lake.
 

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1,196 Posts
Test

Regardless of YOUR procedure, all things being constant, I understand that the difference between the high and low is to not be more than 8% on all cylinders. You show a high of 75psi, and a low was 30psi, thats a bit more than 8%. If your doing the same thing from cylinder to cylinder, the gauge is reading the same pressure correctly or incorrectly from cylinder to cylinder. Thats your base line. The readings should be more consistent. Try adding a table spoon of engine oil threw the sparkplug hole. Re-test, the compression should go up, meaning worn rings/cylinders. If it stays the same there is a valve issue. Or a try different good Quality gauge. Just my .02. schick
 

· mo balls than $cents$
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11,635 Posts
well tried it again today. Cold Motor all the plugs removed, butterfly open

1)40
3)70
5)70
7)60

I turned it over until the needle stabilized.

Either I am looking at a bum gauge or a really bum motor. Does not makes sence for it to be the motor she runs fine out on the lake.
borrow a different compression tester:)bulb you'd have to have some crazzzzy azzzzz dished pistons to get cranking compression that low and i doubt your motor has em. i'd be looking for 100-120psi across the cylinders.
 

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I just did a 1969 Pontiac Grand Prix motor. A stock 400 that came in the car and has never been rebuilt. It was sitting for 10 years in a storage unit and I picked it up at auction.

Did a quick tune-up, soaked it down with Marvel, and spun it over with the starter until it had oil pressure. The the carb open, no plugs, it was 158-166 across the board. It hadn't fired yet, so it was cold. This should be real average.

120 would still be good, but definitely on the low side.
 

· What the Hell is That?
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1,586 Posts
Discussion Starter · #18 ·
So next step is a a different gauge and ill ask some guys around my work for one so I know they good and calibrated.

First I will try the new gauge then if i get the same readings i will squirt some oil in there.
 

· mo balls than $cents$
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11,635 Posts
So next step is a a different gauge and ill ask some guys around my work for one so I know they good and calibrated.

First I will try the new gauge then if i get the same readings i will squirt some oil in there.
don't do the oil squirt, if the motor was runnin, the rings aren't stuck. all you'll be doin putting oil in the cylinder is decrease cylinder volume, and of course compression will go up:D(same way it would if water got in there) just make sure the throttle is wide open, the battery is fully charged, and all plugs are out of it.

orielly's autoparts will rent ya a good compression tester kit
 

· What the Hell is That?
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1,586 Posts
Discussion Starter · #20 ·
don't do the oil squirt, if the motor was runnin, the rings aren't stuck. all you'll be doin putting oil in the cylinder is decrease cylinder volume, and of course compression will go up:D(same way it would if water got in there) just make sure the throttle is wide open, the battery is fully charged, and all plugs are out of it.

orielly's autoparts will rent ya a good compression tester kit
I was under the impression that if you squirt oil in there it will tell you wither it is the rings or not.
 
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