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JMOR
05-20-2008 21:42:13
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Re: Educate me please.... in reply to Dell (WA), 05-20-2008 20:08:37
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Pete Cowart said: (quoted from post at 23:08:37 05/20/08) I only have a 52' 8N but a post on another topic made me curious. It was stated that point gap on a front mount dist. was .015 while a side mount was .025. Why? Both engines are 4 cylinder, each cylinder "fires" 90 degrees apart. I would think almost all inline 4 cylinder gas engines would have the same point gap specs. From way back when I remember that "point gap" affects ignition timing. My shop manual on "CD" lists .015 as the spec and I'm sure thats what I've been running. To change it to .025 would affect the timing and I'm not sure how it would run. Is the biggest difference the fact that someone said the spark on the front mount is "weak"? With the wider gap the points are open longer (dwell) and closed a shorter time. When the points are closed isn't this when the coil primary side stores it's charge? But anyway, why the wider gap? I forget, it's been a long time. Todays cars have a coil per cylinder and they are turned on and off by an electronic control module and that's what I work on everyday.
thanks for your patience,
Pete |
For maximum 'charging' of coil, you want the longest dwell (period that points are closed) possible. There are limitations in gaining that dwell time by decreasing point gap, because when the gap gets too small, the stored energy in the coil is released in the arcing of the points (to the detriment of point life) rather than into the HV to the spark plug. Other considerations are things such as the contour of the breaker point cam...if it is too radical in trying to obtain maximum dwell, then the acceleration ramp is such that control is lost & points don't simply open & close but bounce open/close/open/close, etc. To control this spring tension becomes so high that wear is a problem. So, it becomes one trade off after another to arrive at "the best we can do with what we have to work with"...enter electronic ignition.
Your other question regarding 0.015 vs 0.025. That is a geometry problem. In either the front mount or the side mount, the cam itself provides an amount of "lift", being the difference between the flat and the peak of the cam. In the front mount this translates directly to the point contacts, since the length of the point arm from pivot to rubbing block is equal to the length from the pivot to the point contacts. However, in the side mount there is a multiplier effect, because the length from the pivot to the ribbing block and the length from the pivot to the point contacts are NOT equal. In fact the point contacts open 1.6 times more than the rubbing block moves. So, this geometry makes it much easier to obtain a large point opening (gap), while retaining a reasonable cam profile, with respect to acceleration, bounce, wear, etc. The quicker you open the gap and the wider you open it (within limits) , the less stored energy you waste in burning up contacts & the more you send to the spark plugs.
This is evolution in design at work. That's my story on why 0.015 vs 0.025.
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