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I'm trying to grasp some information on load sensing verses rpm (mechanical advance) type distributors. I know mech distributors work on weights and rpm to add advance, And the MJ uses a map sensor to determine load and put on a scale from 20 to 90....how does it compare...Hope I'm explaining it right....I'm lost

I think you might be muddling the two different types of advance and their function a little (easy to do)

First of all you have to remember that a given fuel/air mixture will pretty much burn at the same speed, no matter what RPM your engine is doing. If you imagine how quickly a stroke occurs at 5000rpm, compared to 750rpm you can see that you need to "light" the fuel/air mix earlier.

Hence the reason that you add advance as revs increase- so that the mixture has time to burn completely during the stroke, but also so that the peak pressure caused in the cylinder by the mixture burning coincides with the cylinder being almost at TDC. I think there is an exact figure but i'm not sure of it.

This is what the mech advance does - bob-weights spinning around add in extra advance as the revs increase and they fly outward.


Then you have the second part of ignition advance. Vacuum advance.

During cruise, or light throttle openings most car makers tend to fuel their engines run lean to improve fuel efficiency. Lean mixtures take the longest to burn, and so again you have to ignite things earlier.

Similarly, you normally setup a car so that you get a richer mixture under harder throttle openings- these burn quicker and so need igniting later (less advance)


I think that there are a couple more things to it, but thats the basics.

So just think of left to right on your MJ table as mech advance, and the up-down as vacuum advance.

I started out doing this and plotting the standard values for my dizzy in this manner. I think for my rover v8 the standard figures were around 6BTDC static timing, with 22 degrees added by the mech advance as revs rose. Then you factor in the vacuum advance (mine was around 8 deg) and you can work out a rough table in a logical manner.

One thing that may cause confusion when people are talking about vacuum advance. Some people talk of advance being added, but because of the way that the MJ table is light throttle at the top and WOT at the bottom i prefer to think of it as advance being taken away as you open the throttle.



Hope that helps

Diggity diggity diggity, that is a very good explanation of how it all works.

Top marks on the lesson.

Quagmire wrote:
So just think of left to right on your MJ table as mech advance, and the up-down as vacuum advance.

That makes it easy....but what about cranking advance, any idea's where to set that

Mine is set at 5 degrees, but really anything between 0 and 10 will work. Most cars were probably around 7.5 I think at idle, so that should work for crank on most any VW.