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Re: Is Vaccum Advance Really Such a Bad Idea [Re: Diplomat360] #2889172
02/16/21 12:49 PM
02/16/21 12:49 PM
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Benton, IL.
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DaveRS23 Offline
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Most of my experience has been with our big blocks and Hemis. I have messed with a some small blocks, but have much more time with the BBs and Hemis which really respond to ignition advance so this is directed primarily at the BBs and Hemis.

I don't really understand the place that manifold vacuum has in this. Our big blocks/Hemis like a lot of lead at idle and the bigger the cam the more lead they like. They also like the advance in as soon as can be, but kept out of detonation. That is the part where for me, the manifold source fails. It drops advance at a time that my experience shows that the engine wants it.

On my street combos, I typically run 25+* of initial. As much as I can and it still not hit the starter on hot days. And then all in by 2,000. On the wedges the total is usually 32* to 34*. On my current Hemi, it likes 31* for what ever reason. As I said before, I use a hand held vacuum pump to experiment with how much it likes at cruise. The change in exhaust note is very noticeable as you change the amount of advance and is a good starting point to set the amount of advance for cruise.

I use a stock Mopar distributor and JB Weld the slots to get the amount of advance that it likes. I also use JB Weld to add weight to the advance weights along with micro springs from a hardware store to get the rate right. It takes some trial and error and multiple disassemblies of the distributor, but after a time or two, it goes pretty quick.

For me, the toughest part is getting the vacuum can stop at the right place. The opening point is easy of course, since Mopar made that adjustable. There used to be a wide selection of vacuum cans with different amounts of vacuum advance. But that is not true anymore, so modifying what we have or can get is the way to go.

So my preferred mix is modifying the weights and springs in the distributor to get the initial timing and the mechanical advance where the engine is happy and then using ported vacuum for a modified vacuum advance. That's the way Mopar did it and the way that has worked very well for me forever. twocents


KOS
Re: Is Vaccum Advance Really Such a Bad Idea [Re: DaveRS23] #2889219
02/16/21 02:09 PM
02/16/21 02:09 PM
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lewtot184 Offline
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Originally Posted by DaveRS23
Most of my experience has been with our big blocks and Hemis. I have messed with a some small blocks, but have much more time with the BBs and Hemis which really respond to ignition advance so this is directed primarily at the BBs and Hemis.

I don't really understand the place that manifold vacuum has in this. Our big blocks/Hemis like a lot of lead at idle and the bigger the cam the more lead they like. They also like the advance in as soon as can be, but kept out of detonation. That is the part where for me, the manifold source fails. It drops advance at a time that my experience shows that the engine wants it.

On my street combos, I typically run 25+* of initial. As much as I can and it still not hit the starter on hot days. And then all in by 2,000. On the wedges the total is usually 32* to 34*. On my current Hemi, it likes 31* for what ever reason. As I said before, I use a hand held vacuum pump to experiment with how much it likes at cruise. The change in exhaust note is very noticeable as you change the amount of advance and is a good starting point to set the amount of advance for cruise.

I use a stock Mopar distributor and JB Weld the slots to get the amount of advance that it likes. I also use JB Weld to add weight to the advance weights along with micro springs from a hardware store to get the rate right. It takes some trial and error and multiple disassemblies of the distributor, but after a time or two, it goes pretty quick.

For me, the toughest part is getting the vacuum can stop at the right place. The opening point is easy of course, since Mopar made that adjustable. There used to be a wide selection of vacuum cans with different amounts of vacuum advance. But that is not true anymore, so modifying what we have or can get is the way to go.

So my preferred mix is modifying the weights and springs in the distributor to get the initial timing and the mechanical advance where the engine is happy and then using ported vacuum for a modified vacuum advance. That's the way Mopar did it and the way that has worked very well for me forever. twocents
the base of the stem of the vacuum advance has a notch in it. the length of the notch is how many distributor degrees are in the can. i haven't hit this exactly but about .011"-.012" travel in that notch is one distributor degree. i will limit the travel by gluing a shim on the canister to limit stem travel at the notch. i use permatex ultra gray silicone so i change the shims if i don't like them. i try to limit travel to 6-7 distributor degrees. mech advance has .0154" inches of travel for every one distributor degree.

Re: Is Vaccum Advance Really Such a Bad Idea [Re: lewtot184] #2889250
02/16/21 03:30 PM
02/16/21 03:30 PM
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NW Chicago suburban area
Mopar Mitch Offline
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There was some previous talk in this thread about the PROMAX metering block not having the vacuum port.. and that is what I'm using... about 8 years ago, they (Promax) told me that I didn't need the port for the vacuum advance. They knew my driving -- some street/highway and road course HPDE/HSAX autocross racing... so, yes, it has lots of part-throttle, only occasional flat-out WOT. The car runs great -- and also now with a new SB stroker if still runs great (SB six-pack recently professionally rebuilt).

When I was ordering the Promax setup (base plates, etc, etc), they wanted to know all my specs, etc. They said they would make the metering block to better match my overall setup.... I'm an easy sell.

But... I'm curious if I should put the original OE/Holley metering block WITH the vacuum port back on and see how it would run.

Interesting thread.

(Maybe the Quick-Fuel metering block WITH the vacuum port would have been the better choice?)

Last edited by Mopar Mitch; 02/16/21 03:31 PM.

Mopar Mitch "Road racers and autocrossers go in deeper and come out harder!"... and rain never stops us from having fun with our cars... in fact, it makes us better drivers! Check out MOPAR ACTION MAGAZINE, August 2006 issue for feature article and specs on my autocross T/A!
Re: Is Vaccum Advance Really Such a Bad Idea [Re: lewtot184] #2889260
02/16/21 03:46 PM
02/16/21 03:46 PM
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DaveRS23 Offline
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[/quote]the base of the stem of the vacuum advance has a notch in it. the length of the notch is how many distributor degrees are in the can. i haven't hit this exactly but about .011"-.012" travel in that notch is one distributor degree. i will limit the travel by gluing a shim on the canister to limit stem travel at the notch. i use permatex ultra gray silicone so i change the shims if i don't like them. i try to limit travel to 6-7 distributor degrees. mech advance has .0154" inches of travel for every one distributor degree. [/quote]

Good info, thanks.


KOS
Re: Is Vaccum Advance Really Such a Bad Idea [Re: Mopar Mitch] #2889262
02/16/21 03:51 PM
02/16/21 03:51 PM
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Benton, IL.
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Is ProMax till around? Send it back to them or find someone else to add the port. Adding a vacuum port to the metering block shouldn't be a big deal. That may be better than putting an entirely different metering block on.


KOS
Re: Is Vaccum Advance Really Such a Bad Idea [Re: Mopar Mitch] #2889263
02/16/21 03:54 PM
02/16/21 03:54 PM
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lewtot184 Offline
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Originally Posted by Mopar Mitch
There was some previous talk in this thread about the PROMAX metering block not having the vacuum port.. and that is what I'm using... about 8 years ago, they (Promax) told me that I didn't need the port for the vacuum advance. They knew my driving -- some street/highway and road course HPDE/HSAX autocross racing... so, yes, it has lots of part-throttle, only occasional flat-out WOT. The car runs great -- and also now with a new SB stroker if still runs great (SB six-pack recently professionally rebuilt).

When I was ordering the Promax setup (base plates, etc, etc), they wanted to know all my specs, etc. They said they would make the metering block to better match my overall setup.... I'm an easy sell.

But... I'm curious if I should put the original OE/Holley metering block WITH the vacuum port back on and see how it would run.

Interesting thread.

(Maybe the Quick-Fuel metering block WITH the vacuum port would have been the better choice?)
my guess is that promax wants all the info to set-up idle jets and emulsion bleeds. the problem with the factory metering block is the idle jet inside the main metering well. it's too small in relation to the idle air bleed. the idle jet can be replaced or opened up but an easier solution you may want to try for a vacuum advance may be the quickfuel qft-34-8qft metering block. i'm pretty certain they have the ported vacuum provision and replaceable idle jet. if i ever get back into 6paks this is something i'll try. i never did the promax stuff because i never seen the need worth the expense. i just have a do it yourself mindset minus the gadgetry.

Re: Is Vaccum Advance Really Such a Bad Idea [Re: lewtot184] #2889796
02/17/21 09:24 PM
02/17/21 09:24 PM
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middle Tennessee
mopower440 Offline
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I was undecided on manifold or port vacuum and i was about to buy a distributor from one of 2 people that set them up on a machine to your combo. I ended up with the one set up for port vacuum but i wish i would have went with the manifold vacuum because the idle stinks like hell with the 284/484 cam and the manifold vacuum may have cleaned that up a bit..

Re: Is Vaccum Advance Really Such a Bad Idea [Re: DaveRS23] #2889807
02/17/21 09:37 PM
02/17/21 09:37 PM
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Originally Posted by DaveRS23
Is ProMax till around? Send it back to them or find someone else to add the port. Adding a vacuum port to the metering block shouldn't be a big deal. That may be better than putting an entirely different metering block on.

i haven't looked at the promax metering block in a while but i'm pretty sure the channel for the vacuum port nipple isn't machined in their blocks. without that channel a vacuum advance port nipple can't be installed. their metering block is more orientated toward the race crowd.

Re: Is Vaccum Advance Really Such a Bad Idea [Re: mopower440] #2889812
02/17/21 09:43 PM
02/17/21 09:43 PM
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Originally Posted by mopower440
I was undecided on manifold or port vacuum and i was about to buy a distributor from one of 2 people that set them up on a machine to your combo. I ended up with the one set up for port vacuum but i wish i would have went with the manifold vacuum because the idle stinks like hell with the 284/484 cam and the manifold vacuum may have cleaned that up a bit..
there's no reason why you can't try manifold vacuum if you want. i'm a port vacuum guy, but if engine vacuum is fairly low manifold vacuum use may be the best choice. i have to wonder if your distributor is set-up for proper initial and low rpm centrifugal timing. the 284/484 is fairly aggressive with the 108lsa and a hydraulic tappet doesn't help much.

Re: Is Vaccum Advance Really Such a Bad Idea [Re: lewtot184] #2889852
02/17/21 10:36 PM
02/17/21 10:36 PM
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Originally Posted by lewtot184
Originally Posted by mopower440
I was undecided on manifold or port vacuum and i was about to buy a distributor from one of 2 people that set them up on a machine to your combo. I ended up with the one set up for port vacuum but i wish i would have went with the manifold vacuum because the idle stinks like hell with the 284/484 cam and the manifold vacuum may have cleaned that up a bit..
there's no reason why you can't try manifold vacuum if you want. i'm a port vacuum guy, but if engine vacuum is fairly low manifold vacuum use may be the best choice. i have to wonder if your distributor is set-up for proper initial and low rpm centrifugal timing. the 284/484 is fairly aggressive with the 108lsa and a hydraulic tappet doesn't help much.


Its supposed to be. I havent had a chance to try it yet. I have my current one at 18 initial and 34 total with no vacuum advance and it screams. I cant advance the total any more or it pings.Its all in by 2800 which could stand to be slowed down some. I wanted one custom curved to my combo with the vacuum advance so i had a guy on another mopar site build me one, which i have not tried yet due to icy roads. The current one i did runs great but like i was saying, i wanted the vacuum working and dialed in.

Last edited by mopower440; 02/17/21 11:03 PM.
Re: Is Vaccum Advance Really Such a Bad Idea [Re: mopower440] #2889974
02/18/21 11:00 AM
02/18/21 11:00 AM
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Benton, IL.
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18* initial sure isn't much with any cam at all.

If it were mine, I would want more initial than that. And it seems to me that the best route would be to shorten the advance slots. Can someone explain why manifold vacuum would be a better solution in this case than more initial in the distributor?


KOS
Re: Is Vaccum Advance Really Such a Bad Idea [Re: DaveRS23] #2890012
02/18/21 12:08 PM
02/18/21 12:08 PM
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lewtot184 Offline
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manifold vacuum isn't necessarily better, but with a given set of parts and low vacuum to begin with somebody may not have a choice. i always use ported but who am i to say manifold vacuum is off the table. i have one set-up on one of my cars that uses a 9 degree plate with 36 degrees total. i use a light primary spring that begins the centrifugal fairly quick and idle is about 23 degrees even though i know the initial is 18 degrees. my other car won't tolerate this kind of timing and this is why i believe that curves have to be individually set-up per engine/car/driver. i think it wouldn't be easy to buy something from a vendor, even if they have all the info, that will be perfectly tailored for an individuals combo.

Re: Is Vaccum Advance Really Such a Bad Idea [Re: DaveRS23] #2890046
02/18/21 12:58 PM
02/18/21 12:58 PM
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Originally Posted by DaveRS23
18* initial sure isn't much with any cam at all.

If it were mine, I would want more initial than that. And it seems to me that the best route would be to shorten the advance slots. Can someone explain why manifold vacuum would be a better solution in this case than more initial in the distributor?


I think the main concern is whether the engine can crank and start with too much initial.

If the engine likes 20+ initial timing, but can't crank with that much, pretty much the only option is to use manifold vacuum to give a lower initial, or some sort of start retard box.

If it does crank fine with 20+, then sure, limit the advance slots, and set the initial at 20+.


Also in general, the engine vacuum at idle needs to be higher than the vacuum in the advance (if using manifold). If they're similar, then the timing will change with vacuum, which changes the vacuum, which changes the idle speed, which change.... panic It will never settle down and be steady. Similar issue if the step up springs in an Edelbrock are the same as the idle vacuum.


If you ever find yourself in a fair fight, your tactics suck.
Re: Is Vaccum Advance Really Such a Bad Idea [Re: hooziewhatsit] #2890126
02/18/21 03:48 PM
02/18/21 03:48 PM
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Originally Posted by hooziewhatsit
If the engine likes 20+ initial timing, but can't crank with that much, pretty much the only option is to use manifold vacuum to give a lower initial, or some sort of start retard box.

If it does crank fine with 20+, then sure, limit the advance slots, and set the initial at 20+.


Or you can do what I did... one superlight spring, one heavier spring with loop. Initial timing is 16, but when the engine starts it immediately goes to the end of the loop which is 27 degrees idle timing. Slow advance to 36 after that (limited by rotor tower/plate).

Quote
Also in general, the engine vacuum at idle needs to be higher than the vacuum in the advance (if using manifold). If they're similar, then the timing will change with vacuum, which changes the vacuum, which changes the idle speed, which change.... panic It will never settle down and be steady. Similar issue if the step up springs in an Edelbrock are the same as the idle vacuum.


True about the unstable idle (same with centrifugal advance coming in too early). But it also works if the idle vacuum is lower than the can (again, like mine). 8" at idle and the can does not move. At cruise (15") it's all in.

Re: Is Vaccum Advance Really Such a Bad Idea [Re: DrCharles] #2890191
02/18/21 05:54 PM
02/18/21 05:54 PM
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Oh, both of those are true up


If you ever find yourself in a fair fight, your tactics suck.
Re: Is Vaccum Advance Really Such a Bad Idea [Re: hooziewhatsit] #2890231
02/18/21 07:45 PM
02/18/21 07:45 PM
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DaveRS23 Offline
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I think if it were mine, I would experiment with twisting the distributor to find the limits of initial when it starts to hit against the starter and then give it all it would take by shortening the slots.

Using either mechanical or vacuum advance to bring up the idle timing would very likely end up having a guy chasing his tail as you pointed out. Just putting it into gear or easing the clutch out would change the RPM/vacuum and thereby the idle speed/quality.


KOS
Re: Is Vaccum Advance Really Such a Bad Idea [Re: DaveRS23] #2890327
02/19/21 12:53 AM
02/19/21 12:53 AM
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I’m using a Mopar tach drive distributor. I’m going to try this with it.
https://www.cbperformance.com/default.asp


Getting old just means you were smarter than some and luckier than others.
Re: Is Vaccum Advance Really Such a Bad Idea [Re: dannysbee] #2890340
02/19/21 02:34 AM
02/19/21 02:34 AM
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Bend,OR USA
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Originally Posted by dannysbee
I’m using a Mopar tach drive distributor. I’m going to try this with it.
https://www.cbperformance.com/default.asp
Try what?


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Is Vaccum Advance Really Such a Bad Idea [Re: Cab_Burge] #2890389
02/19/21 10:05 AM
02/19/21 10:05 AM
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Originally Posted by Cab_Burge
Originally Posted by dannysbee
I’m using a Mopar tach drive distributor. I’m going to try this with it.
https://www.cbperformance.com/default.asp
Try what?


LOL...I didn’t find anything from that link except a website with a bunch of pages to sort through. I certainly want to see what he is going to try.


Just because you think it won't make it true. Horsepower is KING. To dispute this is stupid. C. Alston
Re: Is Vaccum Advance Really Such a Bad Idea [Re: dannysbee] #2890402
02/19/21 10:34 AM
02/19/21 10:34 AM
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All I saw on that site was air cooler Volkswagon stuff.


KOS
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