Okay, so no vacuum present at 3k, but doesn't the vacuum that was in the lower rpm count in the total to the 3k. I thought that you calculate the total timing by adding (intial+vacuum+mechanical).... Everything that I have read states that the total should not exceed 35/36 btdc (stock mopars)... I am curious where you got that 50-55 total # from...
No....
total timing of 35/36 degs refers to Intial timing + the centrifugal mechanical advance. This is a fairly common target for most engines give or take a couple of degrees.
When we say 35/36 degs at 3000 rpm, that is the maximum timing we wish to see at wide open throttle...whether that is wide open throttle takes us to 5000 rpm, or 7000 rpm...we want that much timing applied and we want it to be applied by the time the engine reaches 3000 rpm....or some might say 2500 rpm....that part is a bit subjective depending upon the total combination.
Now, remember, this is for
wide open throttle acceleration. Hopefully that will allow us to make maximum power without detonation if everything is working properly. Whatever number one decides upon, we always set this first with the vacuum hose disconnected from the distributor and plugged going back to the carb so it cannot interfere with our adjustments.
That 35/36 degs does not necessarily give optimum gas mileage at cruise where the load on the engine is low...it may only take 35-45 hp to maintain a cruising speed. What is more likely to give us maximum efficiency in conditions like this is a total timing of say 55 degs. So how do we do that? We use the vacuum advance to rotate the plate in the distributor so that we can get more timing out of it than the centrifugal advance allows....but this vacuum advance only works under high vacuum conditions which are only present when the throttle blades are nearly closed...like is the case when cruising.
So...perhaps we have 16 degs initial advance + 20 degs centrifugal advance which deals with our wide open throttle situation, and we have another potential of say 19 degs which should only be present at cruise when the vacuum is high due to the nearly closed throttle blades. That gives us our cake for performance and the ability to eat it for gas mileage.
On race engines we usually lose the vacuum part and lock the plate down so we have absolute timing control and no jitter in the curve from the plate wobbling around. If you don't drive the car very much and don't care about gas mileage, then there is no real need for the vacuum advance. If you drive the car a lot, then it helps considerably much of the time.
There are lotsa good articles online with much better explanations, I am sure.
