Slant Six Forum
https://slantsix.org/forum/

ported or manifold vac
https://slantsix.org/forum/viewtopic.php?t=33733		 Page 1 of 1
Author:  75duster [ Wed Feb 11, 2009 4:54 pm ]
Post subject:  ported or manifold vac
ok im sure this will cause a ruckus but ill ask anyway. on both my vehicles (75 duster 225) (76 dodge truck 318) i have the vac advance pod hooked up to manifold vac. i have noticed a few posts referenceing vac advance being hooked up to ported vac. so is there any rhyme or reason or tips about which one to use? i was informed that by hooking it up to manifold cruise mileage can be enhanced. i have never had any issues aside from the hose falling off the vac advance can a couple times on the slant
Author:  Joshie225 [ Wed Feb 11, 2009 5:41 pm ]
Post subject: 
Ported or full-time distributor vacuum really depends upon the engine's state of tune (stock, mild or wild) and the way the distributor is set up. I recurved my distributor so I could run more initial advance without excess advance at higher engine speeds. I have all the timing I need at idle without resorting to full manifold vacuum to the distributor. I also have a stock cam in this engine.

Most of the early smog era engines specify timing near TDC and have distributors with a lot of mechanical advance to compensate. If one does not recurve the distributor and runs more initial advance for a better idle and more low speed punch then pinging at higher engine speeds becomes an issue as does a loss of power from the increased negative torque. Connecting full manifold vacuum to the distributor helps the idle, but you loose out on the low end punch because the vacuum advance falls away as the throttle is opened.

Once you get into the realm of long duration cams and high compression things change a bit. The engine will tolerate more ignition advance at idle and needs more advance to idle well. Having enough initial advance to idle well can some times make cranking difficult. In this case running full vacuum to the distributor will help the idle and permit easier starting. You won't get as much vacuum advance as with a mild cam though simply because the manifold vacuum with a long duration cam is much lower.
Author:  75duster [ Wed Feb 11, 2009 6:52 pm ]
Post subject: 
i am running a stock cam/lower end on the slant with a bbd/super six and a homemade exhaust similar to dutra duels. when i first had the car it was stock 1bbl, but iirc when we did the tune up we basically got rid of all the emissions stuff, and hooked it straight to manifold vac. my dad has been a chrysler man for years and has had it this way on all of our cars as far as i can remember and they were all stock or extreamly mild. the only real pinging ive had is since when i got my truck, i swapped the carbs between the two for a bit cause the truck was running crappy. i eventually found the cause of that issue (plugged emulsion tubes) and i swapped em back. now at high rpm i get a little ping on 87 but could that also be because of my compression increase?
i had a head off a newer hyd. motor that i think was a reman cause it was in 74 dart milled .090
Author:  SlantSixDan [ Wed Feb 11, 2009 7:08 pm ]
Post subject: 
If your setup is stock or anywhere near stock-ish (as it sounds like yours is) then the correct answer is "ported vacuum".
Author:  75duster [ Wed Feb 11, 2009 11:48 pm ]
Post subject: 
any draw back to running it manifold? most of my driving is cruise at a high vac (i run religiously off a aircraft vac guage my dad got me, talk about a sensitive guage)
Author:  Aggressive Ted [ Thu Feb 12, 2009 11:08 am ]
Post subject: 
Yes.......you get too much advance at the wrong times and pinging.

Use ported vacuum.

Do a search on the recurving a distributor and copy Doc's guide. It tells you the steps and how to dial in your vacuum advance.
Author:  75duster [ Fri Feb 20, 2009 2:35 pm ]
Post subject: 
Quote:
Yes.......you get too much advance at the wrong times and pinging.

Use ported vacuum.
iv looked at many different posts online and the only difference i have found is when th advance is activated. its my understanding that manifold vacuum advance was used to help idle charastics. poted (according to what i read) will be actived off idle as the throttle plates move from below the port to above it. from what i read the only major difference that i understood from it was that ported doesnt affect idle. i know that personally on my car the manifold vacuum does help the idle charastics and that it only pings at high rpm/high load (when im pulling a hill) which i take to mean my initial/mechanical is over advanced. in this condition i have no manifold vacuum as evidenced by an aircraft vacuum guage tapped in directly below the carb. i know that the charcoal canister is a ported vacuum so that it pulls the fumes out off idle.

if ported and manifold are basically the same with the difference only at idle i dont see what it would hurt to have the manifold.
Quote:
Connecting full manifold vacuum to the distributor helps the idle, but you loose out on the low end punch because the vacuum advance falls away as the throttle is opened.
from what i read and understand that you dont need the advance when you punch it because the additional fuel


http://www.hotrodders.com/forum/distrib ... 59033.html

of course everyones setup is different
Author:  Joshie225 [ Fri Feb 20, 2009 3:31 pm ]
Post subject: 
The sole reason your distributor has too much mechanical advance is the late timing and lean carburetor setting specified by the factory for emissions control. If you reduce the amount of mechanical advance and dial in more initial timing (rather than crutching the initial advance with full manifold vacuum to the advance can) the engine will idle just as well as it does now, have more torque off idle and in the mid-range and not ping on the top end. You should also see better fuel economy.

The correct way to implement vacuum advance is to first completely set up the mechanical advance for best wide open throttle torque and then add vacuum advance on top of that to improve light load economy and responsiveness.
Author:  75duster [ Fri Feb 20, 2009 4:54 pm ]
Post subject: 
ok :D so what is the mechanical advance curve on a stocker anyways? i imagine its quite abit, since it sounds like people on here are pulling it back and adding more initial
Author:  Joshie225 [ Fri Feb 20, 2009 10:05 pm ]
Post subject: 
The advance curve changed over the years. You pretty much have to disassemble the distributor to know what parts are in there. Chances are good you have 30° mechanical advance if the vehicle originally called for timing at TDC.
Author:  75duster [ Sat Feb 21, 2009 5:27 pm ]
Post subject: 
thanks so since the springs control when the advance occurs and the plate sets how much is there a preferred advance plate from the factory is it it the old weld and grind and file method (since i dont have a dist machine)
Author:  Aggressive Ted [ Sun Feb 22, 2009 8:17 am ]
Post subject: 
It depends if you are racing or cruising. For racing just get a different governor like a 7R and weld it if you need to.

If you have a 11R and you wanting to get good mileage, use a stiff secondary spring to limit the advance. That is what Doc suggested for my build. It works very well and stops the advance at 2500 rpm. From 2500 to 3000 rpm I only get 3 more degrees. So it is at its limit and stops the advance at 3000 rpm.

You really don't need a distributor machine, just a good timing light and tach.
Author:  75duster [ Sun Feb 22, 2009 1:04 pm ]
Post subject: 
ok :D thanks all i know things like ignition timing are one of those voodoo sciences and that what works on one may not work on another and i greatly appreciate everyones feedback
Author:  75duster [ Sun Feb 22, 2009 5:28 pm ]
Post subject: 
on a related note where is the thread or post about the dist recurve? i searched for it but didnt see it anywhere.
Page 1 of 1 All times are UTC-08:00
Powered by phpBB® Forum Software © phpBB Limited
https://www.phpbb.com/