vacuum advance goes to manifold vac?

[sc2dave]

According to this article,it should go to manifold vac.3/4 of the way down, "For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don’t ask Summit or Jeg’s about it – they don’t understand it, they’re on commission, and they want to sell “race car” parts.” What the heck.Have I had mine hooked up wrong all these years?

[Troy Patterson]

Well, it's totally application dependent. That's a crazy article if it says to run it in every case. It is worth trying both venturi (ported) and manifold vacuum sources to see what the engine / application like best. It may be necessary to reduce the amount of advance provided by the vacuum advance unit when connected to a manifold vacuum source.

Too much initial timing will cause a nervous idle and negative torque which can / will reduce mileage and wreck havoc on the rod bearings and so forth.

Troy Patterson TMPCarbs.com TMP Carbs

[cjperformance]

also depending on intake vac and advance unit vac requirement when set up to straight intake vac in an auto or air cond or power steer car the load at idle will drop intake vac and this causes the advance to decrease so the idle rpm decreases so the vac decreases and so on, so it can get hard to maintain a good steady idle rpm.
In pretty much most cases, running the advance unit to ported vac is the best option.

[novadude]

also depending on intake vac and advance unit vac requirement when set up to straight intake vac in an auto or air cond or power steer car the load at idle will drop intake vac and this causes the advance to decrease so the idle rpm decreases so the vac decreases and so on, so it can get hard to maintain a good steady idle rpm.
In pretty much most cases, running the advance unit to ported vac is the best option.

Agree. This is how many of the factory 60s SBCs were set-up too. It seems that there is a lot of propaganda on the internet about running full manifold vacuum on ALL applications. It seems to me that this is just a crutch for a too-lean idle circuit. I've found I get the best drivability on my application with ported for reasons stated above, plus less rpm drop going from neutral to drive w/ a tight conveter and an automatic.

[69-CHVL]

Once the throttle is cracked, the ported and manifold vac's effectes are the same. Fella somewhere actually had it graphed - somehow he hooked-up a sensor to an engine and was able to gather data on this. Plotted the differences between the 2, and they pretty much mirrored each other EXCEPT for idle.

But I agree, try both.

[swatson454]

I know that Performance Distributors sets up 90% of their D.U.I.s to run manifold vacuum. It's like anything else, there's no such thing as "always" and "never".

[novadude]

Once the throttle is cracked, the ported and manifold vac's effectes are the same. Fella somewhere actually had it graphed - somehow he hooked-up a sensor to an engine and was able to gather data on this. Plotted the differences between the 2, and they pretty much mirrored each other EXCEPT for idle.

But I agree, try both.

I'd agree with this... both are giving the same advance at light throttle cruise. My question for the manifold vacuum proponents is this:

Why do you need to light the fire sooner at speeds below 1000 rpm and no load?

[swatson454]

Once the throttle is cracked, the ported and manifold vac's effectes are the same. Fella somewhere actually had it graphed - somehow he hooked-up a sensor to an engine and was able to gather data on this. Plotted the differences between the 2, and they pretty much mirrored each other EXCEPT for idle.

But I agree, try both.

I'd agree with this... both are giving the same advance at light throttle cruise. My question for the manifold vacuum proponents is this:

Why do you need to light the fire sooner at speeds below 1000 rpm and no load?

I've run tests between the two on several occasions and the deeper you get into the throttle, the more the signal balances out between the two sources. What I've noticed when testing ported vacuum signal is that the apertures placement in the carbs venturi seems to make quite a bit of difference in how the signal behaves at lower throttle positions.

Some models are quite linear and others more-closely resemble the manifold signal.


Shawn

[69-CHVL]

Once the throttle is cracked, the ported and manifold vac's effectes are the same. Fella somewhere actually had it graphed - somehow he hooked-up a sensor to an engine and was able to gather data on this. Plotted the differences between the 2, and they pretty much mirrored each other EXCEPT for idle.

But I agree, try both.

I'd agree with this... both are giving the same advance at light throttle cruise. My question for the manifold vacuum proponents is this:

Why do you need to light the fire sooner at speeds below 1000 rpm and no load?

If your distrib has too much mechanical advance which requires a low initial timing, manifold will give you the extra idle timing.

I personally would rather just shorten the mechanical curve up, or better yet, lock it out and run ported!!!

[6sally6]

Let me get this straight CHVL......My 5.0 with duraspark distrib. and MSD, big-ass HR cam with 108 LSA..I have full 36* advance@aprox1200RPM. It is hooked to manifold vacuum side of Edelbrock carb. Since this is ALMOST a "locked advance" set-up....PORTED vacuum may be better?

Also Troy Patterson.......could you elaborate on the part were you said it could play havoc with rod bearing? I THINK(haven't checked,yet) I have a rod bearing rattling issue...or MAYBE just a poorly adjusted lifter.
Thanks for your input,
6sally6

[69-CHVL]

Let me get this straight CHVL......My 5.0 with duraspark distrib. and MSD, big-ass HR cam with 108 LSA..I have full 36* advance@aprox1200RPM. It is hooked to manifold vacuum side of Edelbrock carb. Since this is ALMOST a "locked advance" set-up....PORTED vacuum may be better?

If you have full adv + vac adv hooked to manifold, you have like what 45-50* at idle?? Hey, if your motor likes it, it likes it! Ported will just reduce the timing added idle. I'd try ported, see if it smooths out. If not, return it to manifold. But if your curve is in by 1200, why not lock it out?

My motor idles best with 40* at idle.

[torquefan]

in my experience, manifold vacuum is needed with a low cranking compression setup. a long time ago, i had a low comp. 400 pontiac with aluminum heads and mild hyd. roller cam (230/.565"). something on the order of 9.5:1. didn't want to run without locked out timing. responded to, now bear with me, utilizing vacuum advance with a mechanically locked distributor. it ran better, but VERY MUCH SO, A CRUTCH! btw, rod bearings were TOAST! too much advance while cruising. oh well, you live and you learn right? so much for trying to make up for compression with timing LOL!

[MadBill]

Touching on various of the preceding posts:
o "Ported vac." timing is dependent on the port location and the butterfly's opening at idle.
o The more mechanical idle advance, the less likely manifold vac. will be a good choice.
o The more timing the vac. advance 'can' adds, the less likely the engine can handle full manifold vac. (some GM apps used 30°)
o The engine runs cooler and is less prone to run-on with more idle advance. (regardless of how achieved)
o The more idle advance, the less 'robust' the idle, i.e. bigger speed increase idling in neutral Vs. drive.
o The more idle advance, the higher the minimum stable idle speed.
o More idle advance can 'crutch' a carb that delivers insufficient airflow before excessive transfer slot exposure occurs.
o Some factory apps in the eighties used a port-triggered regulator to give partial vac. advance on idle.

[cjperformance]

Let me get this straight CHVL......My 5.0 with duraspark distrib. and MSD, big-ass HR cam with 108 LSA..I have full 36* advance@aprox1200RPM. It is hooked to manifold vacuum side of Edelbrock carb. Since this is ALMOST a "locked advance" set-up....PORTED vacuum may be better?

Also Troy Patterson.......could you elaborate on the part were you said it could play havoc with rod bearing? I THINK(haven't checked,yet) I have a rod bearing rattling issue...or MAYBE just a poorly adjusted lifter.
Thanks for your input,
6sally6

I know you aimed this at Troy but im just jumping in, under what condition do you have this rattle?
What intake vac when rattle is present?
What rpm?
Driving condition?

Have you unhooked the vac advance and tried it out for rattle.?
Is it a sharp 'pinging like' rattle or more of a dulled version of pinging that occurs at light throttle cruise in the 1800-2500rpm range.?

[6sally6]

Hey CJ......thanx for "coming back"!
Haven't tried it wit the vac advanced switched to ported.
Its a low dull rattle @ 1800/2200rpm range.
It ONLY started after I replaced a rocker stud on side of the road. Kinda half-way adjusted lifter by bumping starter over.
Makes me HOPE its just a poor lifter adjustment.
"EVERYBODY" else sez it sounds like a rod bearing.
Drove it a couple THOUSAND miles with it making this noise and didNOT get any worse. Physical issues have prevented me from swapping out studs and re-adjusting lifters.
I had played around with timing both initial and mech after big cam install.
According to Joe Sherman's article years ago ...these SBF, like mine, do BEST with 41* total advance with significant HP decrease with the timing dropped back just 5*.
Mine sure sounds crisper-er with the big total advance!
Sooo...sound like a rod bearing rattle to you?!
Thanx
6sally6