Converting Vacuum Advance Distributor (Proform 440-426) to Mechanical Advance

[killerplymouth]

Been a while since I've posted, life in general, put my mopar project on hold, getting back into it now happily. Really appreciate any help I can get here and hope I can help others.

I recently got an AED 650 Carburetor which I replaced my 1990s era 670 holley street avenger with. The AED has no ported vacuum source, just a constant vacuum source. I talked to AED today and they told me 'don't hook the distributor vacuum advance hose up to the "Dual vacuum ports"', because it's a constant vacuum source. What I should do is run mechanical advance and have timing all in by about 2k RPM.
The motor is a '72 340 with an Edelbrock LD340 Intake. Not sure if I can pull manifold vacuum from the intake, but based on what AED said I shouldn't be chasing that anyway.
The ignition system I have on my Mopar is the Proform 440-426 electronic conversion kit.

Can/Should I plug the vacuum port with something like one of those vacuum advance blocking plates?
Can I tune the mechanical advance on the Proform 440-426 Distributor? Does anyone happen to know which springs I would need to do that or if they make a kit for that distributor?
Should I look for a mechanical advance only distributor and replace my distributor entirely?

I asked ChatGPT (which has been pretty helpful with car problems in my experience) and it told me to just leave the vacuum can disconnected, cap it, and run Mr. Gasket 925D springs and that's it, I could install a blocking plate if i wanted, but not necessary. Does that sound like a good plan to others here? I hope it's that easy!

Thanks in advance Mopar maniacs

 

[RustyRatRod]

Plenty of people run full manifold vacuum to the distributor. I do on both mine. Lots of factory vehicles did too. Nothing wrong with it. Just adjust the timing according and go with it.

 

[Bewy]

^^^^^^^^^^

 

[Mattax]

Plenty of people run full manifold vacuum to the distributor. I do on both mine. Lots of factory vehicles did too. Nothing wrong with it. Just adjust the timing according and go with it.

True. But be's been told

'don't hook the distributor vacuum advance hose... because it's a constant vacuum source. ... run mechanical advance and have timing all in by about 2k RPM.

Translation: AED told him to not use vacuum advance, and set up the advance like a points drag race set up (ie. fully advanced by 2000 rpm). It will work fine for an evening cruise to the local ice cream shop and car show.
For racing it will be OK assuming its not heads up (ie dial in)

 

[Bewy]

And then there is the detonation...... [ AED= And Expect Detonation ]

 

[Dave999]

simple route
find out what it likes at idle
it will be something pretty big 15-30 btdc for a nice clean tail pipe at No load on the engine
find out what your max mechanical advance should be 32 34 something like that
set the timing to achieve that with vacuum ports plugged
Then
plug the vacuum advance into 1 below throttle plate carb port and block off any others

see how it goes, the vacuum will pull in a load more advance over the base setting at idle where its all fine... it will disappear and you will be back on the mechanical curve pretty quick as the throttle opens because the high vacuum state under the blades drops away quickly

it won't be right but it won't be wrong either
if it pings just off idle as you pull off slowly or urgently
adjust the vacuum can to increase spring pressure. make it harder to pull in the advance
see how you go

you then have a base to work from and hours of messing about will follow
but you will get it right eventually

all in by 2500 is fine if you always drive at 5000 rpm, and the guys who like that do tend to, and are vocal about the virtues.... Trouble is they don't take into account that you don't or that you have a standard-ish converter, a 2.92:1 - 3.5 rear end or that you have a 4 speed.

(just ask a legion of VW people who went with the drag racers mantra of all centrifugal distributor is best, only to discover years later that their motors went better with the orginal distributor with the vacuum connected... the power of magazine articles was strong with the VW crowd and a lot of it centred on race cars..some things just seem better because, you spent $$$ on it, and what you replaced was totally worn out, as i discovered to my cost)

for the rest of us who drive (quickly) to the track/pub/shops, it depends totally on how efficient the burn is
it will want what it wants
near enough, with clean plugs and tailpipe is where you want to be

you are basically trying to make it better than what a room full of automotive engineers, sitting around a dyno. did... its hard work

vacuum advance --> load based advance --> like advance based on a throttle position sensor
mechanical advance --> rpm based advance

having both is a good thing, if you drive your car... drag racing, is racing, its a different thing, every part of the drive train is set up for that, spinning it at 5k all of time... l advance curve becomes a bit of a moot point. in a race car your throttle is an on off switch for maximum effort...not that useful halfway up a ramp in a multi story car park

I've based this on the idea that you will drive this more than you will race it

Dave

 

[AJ/FormS]

For an automatic trans equipped STREETER:
I'm with @Dave999.
If you do not have a VA, none of your timing below about 3200 will ever be correct.
However, if you have a 3200 stall convertor, who cares, right?.
Well;
when you hit the hiway, at something like 65= 2400, your hi-compression 340 might want 56 degrees of timing for best fuel economy. But if your distributor is locked at 34*, and the cost of gas goes to 6 or 8 or 10 bucks a gallon, will you be able to afford to drive it anywhere?
Again, for a hi-compression 340 STREETER with iron, open chamber heads; like @Dave999 says, detonation is an ever-present danger, which you must avoid at all costs.
The VA unit will allow you run a nice safe mechanical curve, and pump up the timing for Part-Throttle acceleration, and specifically for cruising.
I was able to modify my Mopar VA to get 20/22 degrees of VA. Another guy I talked to, @TrailBeast, here on FABO, got 24* out of his.
My alloy headed 367, is happy with a two-stage PowerCurve that gets me 28*@2800 rpm. Add 22* from the VA, she gets 50 Total when cruising . To that I can add up to 15 degrees from my dash-mounted, dial-back, timing module, in real-time right from the driver's seat. I usually run it at plus 6* for a total of 56*. ...... cuz that's about where she gets best fuel economy. But, there have been times that she wanted more.
After 2800, my Power Curve slows down, and tops out at 34*@3400, so I can run 87E10 full-time, for even more fuel savings.
********
Not in a million years would I run a locked out distributor. Nor without a VA.
I would take the time to install a timed port into that AED. all you gotta do is drill a little hole in the right place, and press in a little brass pipe.
Even if you miss the right place by a few hairs, it will still be better than none at all. and, you even have a pattern right there on the bench, in the form of the old 780. Altho, I can't speak to if it's in the right place, lol
*********
As for a manual-trans STREET car;
You do not have a TORQUE convertor to help you at part throttle, so, to run with the automatic guys, you need at least 5 to 10 percent more gearing. OR, just give the engine the timing it wants at PT which could be, as I have found out, up to 24 degrees more than that nice safe PowerCurve that I mentioned earlier.
You're gunna find out real quick that with VA, you will not be downshifting as much any more, OR, you can run a more economy-friendly street gear.
Now, before you locked-out guys jump on me and embarass yourselves;
try to remember, I said for a STREETER, and for a manual-trans, and at Part Throttle.

For
RickyRacer, or
the hi-Stalled streeter, or
the guy with a Lo-Compression SBM;
I'm not talking to you guys;
altho, the VA can't hurt you guys either.

 

[RustyRatRod]

True. But be's been told

Translation: AED told him to not use vacuum advance, and set up the advance like a points drag race set up (ie. fully advanced by 2000 rpm). It will work fine for an evening cruise to the local ice cream shop and car show.
For racing it will be OK assuming its not heads up (ie dial in)

By all means, he must do what AED says.

 

[Mattax]

By all means, he must do what AED says.

That was basically my point.
It's not the best way to go for, well, just about any goal, but it will work. But it can't be half. If it is to be run w/o vac advance then the fast advance is really needed.
OP never stated what his goals were. Reading between the lines this is for a Sunday drive, coffee and donuts car show. So if its a little retarded at 5000 rpm, no one will ever know.
If its going to be run with vac advance, then it needs a mechanical advance that looks something like any other 4 bbl 273/340/360 (or 318).

Reading the OP, 90% of this thread is over his head. Otherwise he would not have asked the first question "Can/Should I plug the vacuum port with something like one of those vacuum advance blocking plates?" No one here actually answered his following two questions, but he has plug and play answers from ChatGPT. If it works out to his satisfaction, great. If not, he can blame AI and or AED.

 

[Dave999]

yeah but i was hoping to pique interest in not doing a block it off limit it and just whack in a jack of all trades mechanical curve that just stops it pinging

some encouragement for man who just got back into his project kinda lean to my post i thought.....

no don't block it off use it
no don't limit anything until you know you need to
try running the vacuum
see what it wants and act on what you find accordingly

AED might be right But if you don't explore the options you will never know

not enough detail for anything else
and short of getting it running quite nice my knowledge of v8s with fancy holleys runs out quick

nothing happening at work today so a little bit of FABO prattling on, kept me occupied.
the end of my working (ha) day has arrived, i now need to cook dinner for the kids

laters

Dave

 

[TT5.9mag]

Best advice is to pick up a book and understand the difference between vacuum advance, mechanical advance, ported vacuum, manifold vacuum, initial timing, total timing. And then learn how to make your engine happy using any/all of those things.