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Old 07-14-2013, 04:35 PM   #1
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I was chatting with an engine builder I know about vacuum gauge readings and I mentioned that mine stays at 18inhg at idle but with a small 1-2 inhg fluctuation either way. He asked me what my plug gap was and when I said I run 2 steps colder and .020in gap, he said that explains it.

He said it sounds like you have very slight misfires occurring at idle due to such a small gap and setting them back to .035-.040in should produce a rock steady vacuum reading.

Anyone else experiencing this type of vac reading at idle? I think most of us run very small gaps. I may just go experiment with it because easy.
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Old 07-14-2013, 05:26 PM   #2
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This is why people install COPS or LS coils. Allows us to run the big spark plug gaps even at high boost.

If you run the stock ignition system, then small spark gaps is a bandaid to avoid spark blowout in boost. It does, however, lead to a less than optimal idle.

I just switched to COPs and restored a nice, large spark plug gap. Idle and small-throttle operation is noticeably better.

You should also check your idle mixture. What does your WBO2 read at idle? That amount of fluctuation is excessive, even with a small gap.
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Old 07-14-2013, 05:52 PM   #3
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Here is a short vid of my 99 @ idle and snap throttle. The frame rate of the video hides the fluctuation, as it shakes quicker in person but only moves that amount total.
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Old 07-14-2013, 07:06 PM   #4
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That's completely normal. Not a 1-2"Hg fluctuation at all.

I'm at 19-20" on a very fresh motor.
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Old 07-14-2013, 10:49 PM   #5
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mine is at solid 20 and goes to 19 with a/c on.

but basically what hornet said:
its a band aid with stock ignition. get proper cops if you wanna solve the "problem"
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Old 07-15-2013, 03:02 PM   #6
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Very healthy motors should drag down 22-23in.hg.
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Old 07-15-2013, 03:05 PM   #7
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actually you're right. the 20 was with untuned idle on an MP62 setup during cold start last week
I'll double check just to make sure tho

This engine is in seriously good health comp/leakdown wise
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Old 07-15-2013, 08:34 PM   #8
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This engine is, at least according to comp and leakdown testing, in great shape. Compression is 185psi even on all four (cold), the leakdown #'s on a cold engine is 5% on all four. I've gotten the same results three times now ever since the rebuild 5k miles ago.

22-23inhg is high imo
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