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MP Distributor Tuning Tutorial
I've had some time on my hands lately so I thought I'd put the tune on my engine. I'm not a "set it and forget it" type of guy as I always believe that I can get more power and/or better driveability out of my street builds. Strange as it may sound, tuning for the street is much harder than tuning for the strip as the engine must perform reliably in all temperatures, conditions, and throttle openings. There's nothing more embarassing than an engine that gasps and dies when the light turns green or rattles bad on the entrance ramp.
By running a timing loop, I found that my engine likes 16 degrees of initial advance and 32 degrees of total timing, all in by 2500 rpm and no vacuum advance. I came to this conclusion by starting at 10 degrees inital, adjusting the idle mixture to highest vacuum and going for a low rpm (under 2500 rpm) city cruise. I would take note of off-idle performance and engine temp, then add 2 degrees of timing and repeat. My MP distributor out the box delivered a total of 28 degrees of mechanical advance with 24 of it being all in at 2500 rpm on the light centrifugal advance spring and another 4 coming in progessively to about 4000 rpm on the heavy spring as witnessed by my Snap-On digital dialback timing light. This is a problem. If I set my initial timing to 16 degrees, I now have 42 degrees of total timing. Not cool.
Next, I pulled my inital back until I was at 30 degrees total and went and did some full throttle runs and also took note of engine temp and any detonation around the torque peak (3000-4000 rpm in my case) where cylinder pressure is highest. Keep in mind that I was already close on my jetting (74's all around on my Holley 650 dbl pmpr) but some tweaking will be necessary once the timing is spot on. It doesn't hurt to keep a notebook on where you are and where you've been in this process in case you get interrupted to get a gallon of milk, lol. Ideally, this could done on a dyno but I don't have one and niether do most people. A dyno won't tell you how stable your idle is in gear at a stoplight either.
"Ok man, I've got my numbers but how do I adjust my Mopar Performance distributor so it all works? I don't have a distributor machine in my garage." Glad you asked, lol. I don't have a distributor machine either but I do have a degree wheel.
I removed the distributor from the engine and mounted it in a vise with rag. Rubber or plastic jawed vises are ideal as you don't want to mar up that spendy distributor. Remove the cap, rotor and the vacuum advance unit. The unit is held in by two screws so remove the screws and remove the arm from the vacuum advance plate. This is done by gently prying up on the plate while simultaneously pushing down on the vacuum advance arm pin that rests in the plate. It's a little tricky but no big deal. Once the canister is free you need to bac off the pick up to prevent damage and remove the reluctor. This is done by using a 3/32 straight pin punch and tapping down the roll pin till it just bottoms out. At this point the reluctor can be gently pried up with two screwdrivers until it's free or by using a small two jaw puller. The reluctor has two roll pin holes, be sure you mark which one the roll pin was in. The correct hole on my reluctor was marked a directional arrow, but I can't gaurantee yours will be. Here you can see I used a drill bit to align the hole in the reluctor with the slot in the shaft during reassembly.
Now that the reluctor is off, you can remove the vacuum advance plate and base as an assembly by removing two screws. Although it seems wordy, this entire process took me about 10 minutes. Some things are harder to describe than do, lol. Now you can see the mecanical advance mechanism in plain view. Note the two mechanical advance springs that I had mentioned earlier. One is obviously thicker and stiffer than the other and has larger "eyes" to delay when they come into effect. This is what gives the second "stage" or "dogleg" to the advance curve.
Here's your two #15 Torx head screws that limit your mechanical advance. These MP distributors have a ton of adjustability and you can completely lock out the advance if you desire or even add more advance.
At this point we need to get the distributor spring clamps out of the way so the degree wheel will sit flat. I just snapped in two large sockets on each side so the wheel has a flat stable surface to rest on.
Drop the degree wheel over the shaft, pop the rotor on and tape a pointer to the rotor. I just used a ty-wrap for a pointer. I used an adjustable wrench to hold the flats on the bottom of the shaft and against the vise so it wouldn't budge, then I rotated the wheel so the pointer was at zero and while still holding the wrench so the shaft couldn't move, I twisted the rotor and read on the degree wheel the amount of advance. In my case it was 14 degrees. Remember that 14 distributor degrees is 28 crankshaft degrees. I want 16 intial and 32 total so 32 minus 16 equals 16. 16 divided by 2 equals 8. I need to limit my advance to 8 distributor degrees. I loosened the screws and adjusted until I could get only 8 degrees of advance and tightened the screws down securely.
Now we can reassemble the distributor by installing the pick-up and advance plate assembly. Don't forget to reset your reluctor to pick up gap to .008" with a brass feeler gauge.
I accomplished the entire process from pulling the distributor to dropping it back in and setting and checking the timing once installed in about 1.5 hours. This could be done with the distibutor installed, but it's a pain in the butt to do this on a hot engine, constantly setting and resetting, banging your head on the hood, resting your hand on a hot header, buggering up the Torx screws, etc. A big block would be much easier to do installed as the dizzy's in the front.
I hope this helped someone and feel free to ask any questions.
I've had some time on my hands lately so I thought I'd put the tune on my engine. I'm not a "set it and forget it" type of guy as I always believe that I can get more power and/or better driveability out of my street builds. Strange as it may sound, tuning for the street is much harder than tuning for the strip as the engine must perform reliably in all temperatures, conditions, and throttle openings. There's nothing more embarassing than an engine that gasps and dies when the light turns green or rattles bad on the entrance ramp.
By running a timing loop, I found that my engine likes 16 degrees of initial advance and 32 degrees of total timing, all in by 2500 rpm and no vacuum advance. I came to this conclusion by starting at 10 degrees inital, adjusting the idle mixture to highest vacuum and going for a low rpm (under 2500 rpm) city cruise. I would take note of off-idle performance and engine temp, then add 2 degrees of timing and repeat. My MP distributor out the box delivered a total of 28 degrees of mechanical advance with 24 of it being all in at 2500 rpm on the light centrifugal advance spring and another 4 coming in progessively to about 4000 rpm on the heavy spring as witnessed by my Snap-On digital dialback timing light. This is a problem. If I set my initial timing to 16 degrees, I now have 42 degrees of total timing. Not cool.
Next, I pulled my inital back until I was at 30 degrees total and went and did some full throttle runs and also took note of engine temp and any detonation around the torque peak (3000-4000 rpm in my case) where cylinder pressure is highest. Keep in mind that I was already close on my jetting (74's all around on my Holley 650 dbl pmpr) but some tweaking will be necessary once the timing is spot on. It doesn't hurt to keep a notebook on where you are and where you've been in this process in case you get interrupted to get a gallon of milk, lol. Ideally, this could done on a dyno but I don't have one and niether do most people. A dyno won't tell you how stable your idle is in gear at a stoplight either.
"Ok man, I've got my numbers but how do I adjust my Mopar Performance distributor so it all works? I don't have a distributor machine in my garage." Glad you asked, lol. I don't have a distributor machine either but I do have a degree wheel.
I removed the distributor from the engine and mounted it in a vise with rag. Rubber or plastic jawed vises are ideal as you don't want to mar up that spendy distributor. Remove the cap, rotor and the vacuum advance unit. The unit is held in by two screws so remove the screws and remove the arm from the vacuum advance plate. This is done by gently prying up on the plate while simultaneously pushing down on the vacuum advance arm pin that rests in the plate. It's a little tricky but no big deal. Once the canister is free you need to bac off the pick up to prevent damage and remove the reluctor. This is done by using a 3/32 straight pin punch and tapping down the roll pin till it just bottoms out. At this point the reluctor can be gently pried up with two screwdrivers until it's free or by using a small two jaw puller. The reluctor has two roll pin holes, be sure you mark which one the roll pin was in. The correct hole on my reluctor was marked a directional arrow, but I can't gaurantee yours will be. Here you can see I used a drill bit to align the hole in the reluctor with the slot in the shaft during reassembly.
Now that the reluctor is off, you can remove the vacuum advance plate and base as an assembly by removing two screws. Although it seems wordy, this entire process took me about 10 minutes. Some things are harder to describe than do, lol. Now you can see the mecanical advance mechanism in plain view. Note the two mechanical advance springs that I had mentioned earlier. One is obviously thicker and stiffer than the other and has larger "eyes" to delay when they come into effect. This is what gives the second "stage" or "dogleg" to the advance curve.
Here's your two #15 Torx head screws that limit your mechanical advance. These MP distributors have a ton of adjustability and you can completely lock out the advance if you desire or even add more advance.
At this point we need to get the distributor spring clamps out of the way so the degree wheel will sit flat. I just snapped in two large sockets on each side so the wheel has a flat stable surface to rest on.
Drop the degree wheel over the shaft, pop the rotor on and tape a pointer to the rotor. I just used a ty-wrap for a pointer. I used an adjustable wrench to hold the flats on the bottom of the shaft and against the vise so it wouldn't budge, then I rotated the wheel so the pointer was at zero and while still holding the wrench so the shaft couldn't move, I twisted the rotor and read on the degree wheel the amount of advance. In my case it was 14 degrees. Remember that 14 distributor degrees is 28 crankshaft degrees. I want 16 intial and 32 total so 32 minus 16 equals 16. 16 divided by 2 equals 8. I need to limit my advance to 8 distributor degrees. I loosened the screws and adjusted until I could get only 8 degrees of advance and tightened the screws down securely.
Now we can reassemble the distributor by installing the pick-up and advance plate assembly. Don't forget to reset your reluctor to pick up gap to .008" with a brass feeler gauge.
I accomplished the entire process from pulling the distributor to dropping it back in and setting and checking the timing once installed in about 1.5 hours. This could be done with the distibutor installed, but it's a pain in the butt to do this on a hot engine, constantly setting and resetting, banging your head on the hood, resting your hand on a hot header, buggering up the Torx screws, etc. A big block would be much easier to do installed as the dizzy's in the front.
I hope this helped someone and feel free to ask any questions.
The heavier of the two springs now does nothing because the movement of the weights is restricted below the point where heavier spring with the larger "eye" would take effect. With that being said, the smaller spring is the only one that does anything and this is good. Advance starts at 1400 rpm and progressively increases until 2500 rpm. It's perfect for my application but MP offers a spring kit for their dizzys if a different curve is required. 
