It finally quit raining on the weekends (3 in a row) and I got a chance to hook up a vaccum gauge and a timing light. I noticed that as I advanced the timing the RPM's went up and the vacuum went up too. There was a point where it would go back down as more advance was applied, but starting at about 10BTDC all the way to about 20BTBC the vacuum(20 in climbed to 21") went up. Is this engine trying to tell me something? It sure seems like it is.

bwhitejr

_________________
'72 Duster (Performance 360)
'83 Ramcharger (Performance 318)
'80 TrailDuster (360)
'80 D-150 Truck (See Below)
CompCams 252S, Holley 390cfm, Offy manifold
Ported, Polished and Gasket Matched
P4286813 Springs,0.040 Overbore,
0.090 Shaved Head

Based on my rather limited understanding of distributor advance curves and engine vacuum related to engine performance, the optimum base timing setting would be the one that gave you the most vacuum at your curb idle speed while still allowing your starter motor to start the engine. However, this also has to be taken into account in designing your distributor advance curve. Lets say you slant likes to have the base timing set at 20 degrees BTDC. Well, most experts agree that a slant shouldn't get more that 32 degrees total advance, so you would have to have the advance arm slots in the distributor welded up to only allow 12 degree of advance. Additionally, you could only have a vacuum advance pod that provided 12 degrees of advance.

My advice is to pull the distributor and see how much mechanical advance your distributor makes, then subtract that number rom 32 to determine where your base timing should be. Get your motor to run well at that setting. Then you can start to fiddle with the advance springs and vacuum advance, or build a completely custom distributor on your workbench and install that later.

All that initial timing might give you a great vacuum reading at idle, but I don't think it will be practical for building a truck advance curve.

You would need some very heavy springs and limited mechanical advance. You might also find that you can't keep her out of ping when climbing a hill against the wind with a load in the back.

Many trucks got curves with hardly any advance at idle and lots of mechanical and vacuum advance pretty quickly. Dodge must have known a little something when they designed them that way. Not to say you can't make some changes based on your desired use, but don't expect a race distributor to give good gas mileage in a truck. I'm still trying to get better than 13mpg out of mine. Mine is set at 12 btdc initial with 32 total with mechanical, but it seems to come in too quickly and pings on hills. I think it could work with a little stiffer light spring.

You may also want to consider that as you advance the timing, you also need to maintain a steady rpm setting to compare vacuum readings. That means adjusting the timing and the carb idle mixture and idle speed at the same time.

Please keep me posted with your results because I could use all the help I can get.

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1980 Aspen 225 super six

I have installed one Mopar spring from the recurve kit (replaced the biggest spring). I am running a 9R governor with 12 BTDC initial timing and a 11R vacuum advance can. I am getting 15+ MPG in a D-150 1980 Pickup. I was hoping to get better than that.

bwhitejr

_________________
'72 Duster (Performance 360)
'83 Ramcharger (Performance 318)
'80 TrailDuster (360)
'80 D-150 Truck (See Below)
CompCams 252S, Holley 390cfm, Offy manifold
Ported, Polished and Gasket Matched
P4286813 Springs,0.040 Overbore,
0.090 Shaved Head

Oh! I forgot to mention, we have no hills in deep southeast Texas. The featureless hydrocarbon plain.

bwhitejr

_________________
'72 Duster (Performance 360)
'83 Ramcharger (Performance 318)
'80 TrailDuster (360)
'80 D-150 Truck (See Below)
CompCams 252S, Holley 390cfm, Offy manifold
Ported, Polished and Gasket Matched
P4286813 Springs,0.040 Overbore,
0.090 Shaved Head