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Old 08-09-2014, 10:42 PM   #1
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Default Will engine still build compression without plugs in?

This is a dumb question, but I'd appreciate the sanity check: I have my "new" engine on a stand, and I was turning the crank with a wrench. No oil, no plugs. It gets tighter and tighter for about 90 degrees, and then spins a little on its own.

This seems normal if the engine is building compression and then releasing it (air comes out of the exhaust once it releases).

Should this still happen with plugs out though? Somehow I figured the compression stroke would just shoot air out the plug hole, but I clearly don't understand engines very well.
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Old 08-09-2014, 10:48 PM   #2
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Old 08-09-2014, 11:03 PM   #3
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It won't have compression without plugs installed, but it still has cams and valvetrain.
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Old 08-09-2014, 11:06 PM   #4
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OK, thanks for the clarification. So without plugs, as you turn the crank with a wrench, should it still tighten up and then release, or is that a sign of bad things?

It's not like I have to really muscle it, probably 30 or 40 lb-ft, it just gets noticeably harder to turn every 90 degrees. Is this probably just the valve springs or something?
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Old 08-09-2014, 11:36 PM   #5
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You still don't understand his clarification. He said it still has cams, which have lobes, and the rest of the engine, which will make it do what you're experiencing even without building compression
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Old 08-09-2014, 11:40 PM   #6
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Well, like I said, I don't know much about engines. I just needed a double-check that yeah, the cams and the rest of the engine will build in resistance through the stroke. I wasn't sure if he meant that or if he meant there'd be resistance in general.

But I think you just said yes, what I'm getting is normal, and that's what he was telling me. So, my noob question is answered.

Thanks guys.
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Old 08-09-2014, 11:51 PM   #7
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now hurry up and boost it
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Old 08-10-2014, 01:44 AM   #8
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Haha, yep, that's the plan... eventually. The last engine spinning bearings definitely set me back, I'll get there.
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Old 08-10-2014, 07:20 PM   #9
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Opportunity for you to learn here

As the crank turns, it turns the cam shafts. As the cam shafts turn they open and close the valves. As the valves are opened the valve springs get compressed providing resistance to you turning the crank. If you take the valve cover off you can see this in action and it is easy to see how resistance gets more as the valves reach their maximum open point, and you will also see that this doesn't happen on the compression stroke, with valve train losses you have maximum resistance on the intake and exhaust strokes when the applicable valves are being opened.

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Old 08-11-2014, 10:07 AM   #10
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Interesting. I was planning to tear down the engine that spun bearings and confirm what happened, possibly look at a rebuild in the future. I'll check out what you're saying with that one cracked open. Thanks!
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