Quote:
Originally Posted by DanEC
I don't believe I've ever heard of setting initial timing by shooting for the highest vacuum gage reading. I use it to set the idle mixture setting on the carb - but not timing.
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X2..
Now, there is a method I am aware of, that sounds somewhat similar to what TexasDoc was doing, where you set your HIGHEST vacuum-assisted spark advance, based on your highest vacuum reading, but I've never heard of that process being done at idle level RPM's ....The procedure I'm aware of is done with the engine up in the cruising RPM range.
The procedure is to hook your distributor's vacuum advance to pure manifold vacuum source, and also have your vacuum gauge connected straight to manifold vacuum as well.
Run your engine up to your vehicle's highest (or most common) cruise RPM (say 2800rpm or so- It has to be an rpm level that is above your centrifugal advance upper limit), and set your timing to the lowest number of degrees which will produce the highest vacuum reading possible (so if your max vaccum is 16Hg, then set it at the lowest number of degrees that will produce 16) -
Again, this procedure REQUIRES your distributor's base acceleration curve, and your vacuum advance, both to be "all in"- What you are essentially doing is setting the highest amount of spark advance that your engine can possibly produce, under any driving conditions.
Then lock everything back down, reduce your rpm's back to idle, and do a final carb idle mixture set... Double check your idle timing with the vacuum advance disconnected and plugged (it should be down in the happy zone- 20* BTDC, or lower). Depending on your idle quality, you can then either put the vacuum advance back on a delayed port, or hook it back up to the manifold vacuum port (your choice). Set your idle RPM's where you want them (typically to the lowest RPM which still produces steady
oil pressure), and you're done...
Now, all of that being said- This is a mega old-school method that guys would use to dial in their best vacuum assisted spark advance for fuel economy when running down the highway in old trucks.... I've ever heard of anyone using it as part of a performance tune....
(Not saying it woudn't work on Cobra - The principles are the same, and it should maximize your highway MPG, if that matters...
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