Quote:
Originally Posted by SSLance
Trying to compare vacuum advance with vacuum in inches to Boost Retard with instead of vacuum in inches, manifold pressure...makes one's head spin.
What I need to do is decide if it's worth the effort to lock the distributor in at say 45* crank timing and plot both run retard and boost retard graphs to pull enough timing out with either or both to get the timing in perfect spot for all scenarios? I could probably end up with total timing in the 40+* range at idle and cruise RPMS and use the MAP sensor\boost retard to pull timing out under a load and WOT.
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Yeah - kind of confusing... theoretically atmosphere is 14.7", so anything under that is a vacuum. It is just whatever your reference is. You just have to unplug to figure out what your atmospheric pressure is. The crazy part is the fact that your scale is flipped. If 14.7 is atmosphere, that is what you think of as 0 vacuum. The numbers don't really matter in reality, it is all just a reference. Sounds like your "0" is WOT at 13.75, so be it.
OK... two different camps on this. Manifold vacuum and port. Based on the problem you are describing let's try port vacuum, this may give you the effect you want. With port vacuum you should only see vacuum at partial throttle. This will eliminate the extra advance at idle and at full throttle and stabilize the rpm at idle. The "port" you want to plug into should be above the throttle blades and should read 0 vacuum at idle when measured with a vacuum gauge.
I think you are still thinking of the vacuum curve backwards.
OK, two different curves! One for RPM (aka centrifugal advance) and one for the "boost" retard (aka vacuum advance). For the RPM curve let's say you want to go from 8 to 32, all in by 2800 rpm. And, for the "boost" you want 10 degrees advance max on the ported.
Your base timing with no curves would be 8+24+10= 42 degrees (pretty close to the 45 you stated). Remember that everything on the MSD is backwards, it is all retard. So, the RPM curve would start at 24 degrees retard. If your idle is 900, then maybe you start adding timing back in above 1200. Your "curve" would be 24 degrees retard from 0 rpm to 1200, then progress to 0 degrees retard at 2800. The effect is timing would be 8 degrees at idle and at 1200, it would then increase gradually to 32 degrees at 2800 rpm. This is my example... set it for power at what works best for you! I used "8" as my idle timing just as an example.
OK, for your boost curve (connected to port vacuum), if 13.75 is WOT, then you want 10 degrees retard at WOT to give you 32 total above 2800. So, left to right you put 0 retard to about 4 then increase your retard to the full 10 by about 12, leave it at 10 total all the way to the end of the graph on the right. At idle, or with the port vacuum line unplugged even, the 10 degrees retard will be pulled out and the timing should be whatever you want it to be, it should not jump around.
So add these together... say you are running 2800 rpm with the throttle barely cracked and a little downhill, you could then be running as much as 42 degrees advance. No problem under light / no load. As you tip into the throttle and start to load the engine the vacuum will drop and your timing will move to 32. Cruising speeds around 2000 (half way between 1200 and 2800) you would be running 18 advance due to RPM, and if there is no load, as much as an additional 10 which will help mileage.
Attached charts make sense? Boost: