I have a street machine small block '71 Duster. Currently, it has the MP P3690430 vacuum advance distributor with the vacuum canister removed. Car runs great, so no problems ignition wise. I don't think these are available any more.

On my shelf I have another long dc'd part, a never run MP P4120701 which is the aluminum small block tach drive 'race' distributor from the '80s. I want to use this unit with my Moroso mechanical tach for a vintage vibe.

As I understand it though, the advance curve on the tach drive units is 'all in' very early, like 1,200 rpm, which to me is almost like having the timing locked out since launch rpm is generally above 1,200. The thing idles @ 850 in gear.

Since this is a street car, I'm not zinging the thing to 7K at every stop light, there is plenty of part throttle driving. I'll be honest, I have not checked where the advance comes in and/or timing with the current set up so I don't know how quick it is. I thought it was a somewhat quicker rate though.

Here's my question - can I swap the advance mechanism parts from the vac. advance distributor to the tach drive unit? Just thinking the vacuum advance parts are designed to be more street friendly. Its already running well with this setup so its a known good part with no issues.

Am I way off base with swapping parts from housing to housing? Would they even interchange? The distributors seem pretty similar and I believe they were both made by Mallory. Visible differences between the two that I can see is the advance plates and reluctors. Externally is the gear for the tach cable.

Or would it be better to just alter the advance rate on the tach drive unit? Seems to me that might be defeating the purpose of the 'race' distributor. Obviously the part doesn't know if it's 'race only' or not but it is designed for a relatively narrow operating range (WOT).

I really like the tach drive distributor and mechanical tach set up but I don't want to sacrifice drive ability. What kind of issues could I run into with such a quick advance rate on the street? Economy is not a major concern, it's not much to speak of. The vacuum canister is already gone anyway.

Rest of set up - stock stroke 340, W2 heads w. hardened valve seats, MP .557/296 solid cam, Victor W2 intake, Proform 750 DP, runs a mix of pump 93/100 octane unleaded when I can get it, usually around 6-7" of vacuum. Mopar ignition ECU, Taylor wires, regular canister type coil. Planning on ditching the MP ECU for an MSD 6AL soon. Drivetrain is a low gear, forward manual 904 with Hughes 3,800 converter, 4.30:1 gear (may go to 4.10 soon), 28 inch tire.

Thanks for any input.

If desired I'd install the tach drive unit & install a vac can either the other one or one from a parts house if one will fit as it's a plus on a street driven unit. I'd set the initial/total/springs/vac adv in order & before you pull the current dist you might note the adv curve/total/cans' range etc if you have a vacuum pump/dial back light (or tape) just for reference

Used the p412701 distributor in my 73 340 duster for about 5 yrs ...never had any problems with it. As the engine had eddy heads and hughes hydraulic cam..230/236 cam.

As you said..full advance around 1000 rpm...

None sure if you can get a vacuum can on the P412701 distributor as dont think there is any place to mount it...

I got 3 of the aluminum versions and one of the old cast iron ones.

You can take the springs out of the vac unit and install them in the race unit, you might even be able to swap over the weights also, not sure what the difference will be in them , if you have a scale you could weight them. The biggest problem with either of them is they have 30 degrees or so mechanical advance .

Neither unit was made by mallory , that's only the current MP units.

The tach drive is made by Presolite and you'll need the dual point distributor cap for it.

What John RR wrote is basically correct.
The advance slots in the tach drive dizzy are for alot of degrees. The slots in the older MP vacuum dizzy are around 20 plus degree. I could look it up if you need. (send PM to get my attention)

I ran the tach drive distributor on a much milder combination, and there's absolutely no issues with the quick in. Comp cams 280 cam and then latew with an approx 230 duration at 0.050 cam, stock 340 with only 9:1 cmpression and 3.23 gears, stall speed 3000 rpm.
Looking over your combo, I see nothing that hints at any reason to start swapping parts.

The issues with the tach drive distributor are fuel milage (due to no vac advance) and slightly unstable timing at idle. If the carb's idle circuits are working well with the vac advance unit, take some notes, note the timing and the prmps at idle in gear and in neutral. And of course you need to know the full mechanical advance, 3000 rpm is good place to check.

When the tach drive unit is installed, you must set the timing at full advance. It is mostly in by 1200 but pretty much all in by 2000. Then go measure the timing at idle, both in gear and out. It will not be stabile, but it should be about where it was before.
The more important one is when in gear, particularly if it is not enough advance.
You can (or could) get an even lighter light spring from Mopar Performance. Replace ONLY the light spring. The beauty of the tach drive distributor is the heavy spring wit hthe long loop. This is what keeps the higher rpm timing just right.

example. Say it was about 18 degrees at 800 rpm in Neutral with the acuum dizzy, and that's about what your getting with the tach drive.
But with the vacuum dizzy, in gear the rpms dropped to 730 and the timing to 17 degrees.
With the tach drive dizzy, in gear drops initially to 700 rpm and the timing drops to 15 degrees and then the rpm drifts down a little more to 650 and the timing goes down with it to 14, and the engine not happy with this combo. (It then becomes a bigger issue on the street with wipers, defroster blower and headlights on, espcially a high current draw ignition system. less so with with a points system.)

Yes you could weld or braze up the advance slots so the weights are further out to begin with. Or, put in the super light spring and consider it done old school style.

My theory is that Direct Connection was either; satisfied with the answer they had put together and didn't/couldn't put more time into developing this distributor, and/or, they realy like the fact that the cars were so easy to start because the true initial timing (at 50-100 rpm) when hitting the starter was around 5 degrees.

PS. The springs in the Mallory built dizzies are slightly different design than the ones chrysler and prestolite used.

I understood that you understood the higher rpm cruise and WOT. I assume you also understand that with vacuum advance, cruising at 55 will add another 12 degrees or so to the mechanical total. That's the fuel savings.
What I was saying is that the tach unit's distributor curve is also OK for chugging through a 15 mph school zone or the pits or wherever.

As far as the Engine book, look again. If its the same as mine, after advancing 20 degrees, the line does not go horizontal. What happens at that point is their is no more slack in the heavy spring's long loop. The long loop lets the advance continue very slowly. On an engine you may see it, or it may look flatter because there is slop and stretch in the engine under load.

I've used an MSD 6T for years, but only because the MP chrome box and voltage regulators were dying. I'ld set up the dizzy first, do the MSD later. There's not much time savings I can think of by doing it at the same time. You'll need to make a jumper (or splice on) connector from the distributor to so it can be connected to the MSD's signal wires.

If you're interested and patient it should be possible to get even that engine to idle below 1000 rpm (in gear). Mostly a matter of fidling with the timing and carby. It needs enough timing at idle to make as best possible vacuum in manifold, but not so much advance that the engine prefers a leaner mix. You know you're in the ballpark if at idle the carb's primary transfer slots below the throttle blades look look pretty square (.020-.025") and the idle mix screws provide some degree of adjustment (between 1/4 -1 3/4 turns out).

The Fuel:Air ratio needs to be fairly rich at idle with such a hot cam. As it the rpm comes up off of idle, at light throttle it can lean out and still have power, but at idle it needs to be richer to offset the exhaust reversion.

Just fired up a 360 with an old cast iron version of that distributor..

set full advance at 3000 rpm...34 degrees

at 1000 rpm idle..the engine shows 34 degrees of timing..

So I went back and re-read what 'ol Shep had to say. Puts me right to sleep but I managed to get this out of it.

Here's a paragraph pulled directly from the text:
"...Mopar Performance also has an "A" engine race distributor (P4120701). It has an even faster advance curve. This distributor has 20 degrees built in and is fully advanced by 1,000-1,400 rpm. This is much too quick for a dual purpose csr. However, the idle of a race engine is typically above 1,200-1,400. This means that the race distributor gives the race engine a flat advance curve which is desired for racing..."

So this confirms my thought that the quick advance curve is too much on the street (at least according to Shepard). I guess it can be interpreted any way you want but this is where my original question came from.

Larry Sheppard is wrong and the tech bulletin reprinted in the Engine Book is correct. Don't get me started on Sheppard...

See attached, and if you don't believe it, take one apart and measure the loops in the springs, and do the math. (yes I have) You can also put in on a distributor machine.

I wanted to resurrect this thread with some new info. I typed out this whole long post and the damn thing expired on me. Grrr...

Anyway... I recently got myself a working Sun 500 distributor tester, which I'm really excited about. Well, it's not 100% working since the tach is INOP but no biggie. I have an old Sears tach/dwell meter (which only goes to 2,000 crank rpm) that can fill in for now.

The good thing though is that I finally had a reason to play with the tach drive distributor I was originally asking about here.

To answer my own original question, the tach drive is in fact totally different than the regular MP vacuum advance distributor so nothing would really interchange. The shaft is different, the weights are different, the pickup plate, reluctor etc. The tach drive also does away with nylon bushings as well which seems to help keep the timing rock solid in the upper rpms. Nice piece, however unique.

So in the interest of science, I swapped out the really light advance spring that came in the tach drive with a heavier one from a factory unit I had on the shelf. (I left the monster heavy one in with the long slot) Sure enough, the advance rate slowed by several hundred rpm.

I couldn't tell exactly where it stopped since the needle on my Sears tach/dwell meter was pegged at 2,000 but it stopped advancing very soon after it got to that point. Certainly it's a less aggressive curve than being all in by 1,200.

The goal here is to use the tach drive in an application that sees part throttle driving and avoid pinging with full advance. The combo makes the car idle around 1,000 rpm and cruising in drive is usually over 2,000 so I was basically at full advance at the same points with the vacuum distributor and never observed any detonation.

I understand the tach drive is designed for WOT and though Mattax might disagree, timing is for all intents and purposes locked after 1,200 rpm.

There's no reason however that the quick advance can't be altered for street driving with a simple spring swap. I can always put the light one back in when I take it down the track.

A few more notes. The slots on the advance plate measure .044" +/- which = 13 degrees and X2 = 26 degrees initial. Centrifugal advance was 11 degrees for 35 total. Again, no vacuum advance so that's the whole curve. Reluctor gap was slightly tricky since one of the tips was much closer than the others. I got it to .008" but it took some fiddling.

So now I have a dilemma - I have a new Firecore HEI cap and plug wires and brand new SW deluxe electric tach with a tach adapter for the MP vacuum distributor that's currently in my car. Not sure I want to ditch all this stuff before I even use it for the tach drive setup. wiring that stuff was a PITA too. Yeah, I got uptown problems.

Hope to hear from some of you who responded earlier.