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Old 11-03-2005, 02:56 PM
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Compression testing question.

I got to talking to a buddy at work today about what the correct way to check compression is. I've always pulled all the plugs and then checked one cylinder at a time. He say's you can check one at a time and the readings will be the same as if you pull all the plugs. So, who's wrong? I realize that for a ring test you don't want to pull them all again, and it seems to me that the more you have to turn over (say 7 other plugged cylinders), the lower the psi would be. I could be wrong, just curious though.


Thanks in advance, Chet.
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Old 11-03-2005, 04:24 PM
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Pulling all the plugs just makes it easier on the starter and battery.

Larry
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Old 11-03-2005, 04:43 PM
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Wheather all the plugs are out or not won't make much difference other than being easier on the starter and battery, as coldknock said. But the result will be more accurate if the throttle blades are held wide open, whether FI or carb. therefore letting the combustion chamber load with a full charge from the intake. It will eventually fully load if you turn the motor over enough times, but with a wide open throttle, it should give an accurate reading within 3 revs of the crank.
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Old 11-03-2005, 04:46 PM
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can i know the compression ratio from that kind of tester ?



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Old 11-03-2005, 04:51 PM
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Oh. Sorry. To answer your question, with a healthy motor, you will get the same result either way, regardless of cranking RPM affected by whether the other 7 cyl.'s have plugs or not. There's a physical formula to get the CR with a healthy motor.
With a motor leaking compression, it may need to turn over an extra time or 2 to reach it's max with the plugs in those other 7.
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Old 11-03-2005, 04:54 PM
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Quote:
Originally Posted by camaro 77
can i know the compression ratio from that kind of tester ?
Yep. Make sure the pressure relief valve is closed.
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Old 11-03-2005, 04:54 PM
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Doc here,

Compression, yes...

Ratio requires more math...

Compression , from your question, I think is what you want to know..like 160 per cylinder..ratio is 10:1 to 1 etc...

Doc
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Old 11-03-2005, 05:01 PM
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Quote:
Originally Posted by mike 96 ws6
Yep. Make sure the pressure relief valve is closed.
ok , how do i convert the tester reading to Ratio (CR) ? is the any mathematic formula ?
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Old 11-03-2005, 05:04 PM
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Yea I meant just the compression, not the ratio. I should have been more clear. Long day. Sorry guys. Thanks.


In a while, Chet.
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Old 11-03-2005, 05:04 PM
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Quote:
Originally Posted by docvette
Doc here,

Compression, yes...

Ratio requires more math...

Compression , from your question, I think is what you want to know..like 160 per cylinder..ratio is 10:1 to 1 etc...

Doc
Ratio will not change whether 7 more plugs are in the motor or not. Or whether the compression rings are trashed or not as far as the math equation, correct?
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Old 11-03-2005, 05:13 PM
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Quote:
Originally Posted by camaro 77
ok , how do i convert the tester reading to Ratio (CR) ? is the any mathematic formula ?
I'm no math head, but you may be able to do this backwards / divided by # of cylinders, but that's a quess.
My math =


Enter Bore/Stroke Designation Type
1 = Inches 2 = Millimeters
Enter Cylinder Bore Size
Enter Piston Stroke Length
Enter Head Gasket Bore Diameter
Enter Compressed Head Gasket Thickness
Enter Combustion Chamber Volume In CCs
Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-')
Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') :


Calculated Engine Compression Ratio xxx
Total Displacement Volume xxx cc
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Old 11-03-2005, 05:28 PM
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Einstein might have had a formula that would work for the conversion but it's not something that I want to think about with a good buzz.

I will tell you that a 12-1 compression engine can have the same cranking cylinder pressure as a 9-1 compression engine with the right camshaft in it.

That said, assuming you're not Einstein's nephew or something, no you can't determine the exact ratio from the cylinder pressure. You could get lucky and come close but close don't count.

Larry
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Old 11-03-2005, 05:46 PM
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thanks ,, Mike 96 ws6

Larry you got it right !! i missed that point : more flow = more volume --> more compression !!

a blown engine with 9:1 could equals 12:1 at the end of the equation compared to the same engine without the supercharger
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Old 11-03-2005, 06:09 PM
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That's part of it.

The largest part of the change in pressure is the valve timing. Specifically when the intake valve closes, compression begins. When the valve closes later, with a long duration cam, the piston is farther up the bore. Less volume being compressed = less pressure read at the gauge. When it closes earlier, with a short duration cam, there's a larger volume being compressed so you have more pressure read at the gauge.

That's why you see minimum compression ratio specs listed with big cams. Not enough compression and it'll be a dog with a big cam because of low cylinder pressure. Too much compression and a small cam will be a detonation monster because of too much cylinder pressure.

There's a small relationship to engine size as well. Larger engines have a larger volume of air so a longer duration cam will work with less compression, to a small degree.

It's all about choosing parts that work together.


Larry
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Old 11-03-2005, 06:09 PM
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I tell my students to take all of the plugs out so it is easier to count the compression pulses. With all of the plugs still in, the engine cranks over but you must watch the gauge like a hawk to count the number of compression strokes each cylinder does. With all the plugs out every time the engine comes to the compression stroke of that cylinder the starter loads-up and you can coult the pulses without even looking.

Scholman
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