![]() ![]() ![]() all the molicules are pulled farther away from each other and thus your burn time takes longer. vacuum advance is necessary for becasue the air / fuel mix is at a much LOWER initial energy state when it is pulled in at a high vaccum. when you have timing increasing as vaccum increases you get the equivalent of a vacuum advance distributor. The only weird change is that I like to roll off the timing in the high rpm high vacuum areas.just as a safety measure. ![]() When i build my timing maps i keep 20 as my minimum timing all the way down to full vacuum but the 3500rpm value goes up to 40 or 42 deg. so if the time of the compression stroke is getting longer due to low RPM then you want the ignition advance to be less so that you can reach peak cylinder pressures at the appropriate time (15-20 Deg AFTER TDC). the burn time of the a/f mix is a relatively fixed time for a given intake pressure. the reason for having the timing increase as you get toward 3500 is that the actual time that the combustion has to take place gets relatively long at lower RPMs. Thats a good baseline to start with.rememeber though thats 0 boost.not full vacuum. Timing should pretty much level out over 3500 rpm, like this these are some of my personal tricks, but since there is apparently a need for the info to sort out confusion i will post it. Alright.I am hesitant to post all this in a public forum as it is alot of knowledge learned form hard work and expensive mistakes. ![]()
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