vacuum advance goes to manifold vac?

[twinturbo496]

I never had an engine that liked port vac for the distributor... until the first time I tried it on a very mild camshaft engine with a Duraspark... the 300 I-6 just likes it better... go figure. Also, it didn't even like it until I limited the total vac advance (cut from over 35 down to 20), and put in lighter springs. With so many variables, I don't believe that any one answer ever covers all combinations.

[panic]

I don't believe that any one answer ever covers all combinations

X2, the "one answer is always correct" is for people who don't understand what's going on, and have no patience.
There are ways to tweak manifold vacuum to get around the idle RPM shuttling effect of vacuum changes using small solenoids, very cheap, easy to adjust, just takes some planning.

[treyrags]

I've been considering trying to come up with a vacuum advance canister that is adjustable for both the amount of advance travel as well as amount of vacuum required to activate it. Instead of trying to get any advance from vacuum at idle and low speed, I'd like to run my normal 20* initial and 34* total @ 2600 and have the vacuum advance only occur at steady cruise/low load situations ~ 10* to help it run cleaner and more efficient and maybe cooler. On my current combo that would be ~ 13" hg and above. That way when I dip into the throttle and load the engine at all, the vacuum advance would go away immediately. Anybody done this? What canister and distributor?

[MadBill]

They are available with rate and total degrees adjustment, e.g.: http://www.amazon.com/ACCEL-31034-Adjus ... B00062YHRO

Also, MSD's E-Curve model distributors have an electronic vac. curve as well as 'mechanical'.

[treyrags]

Thanks for the link Bill. I've been wanting to try the E-Curve for a good while now but they're not available for Windsor's, at least as of a few months ago.

[cjperformance]

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

I would definetly look at the advance #'s @ 1800-2200 and knock the timing back in this rpm range then reasses the noise, a dull rattle @ light throttle cruise is exactly what you hear when you have too much advance.
It would help to know the ARF @ this rpm too but you can easily run very lean here and have no issues.
It may well be fine everywhere else but just needs a tweek in this rpm range.
I will guess that the noise coinciding with the rocker stud is probably just coincidence as no doubt you were listening closely to the engine after having this roadside trouble and now are hearing this noise, but i could be wrong! I have been once or twice! lol.
I'll bet less timing will cure it!
AT least try this first to rule it out!

[FishFry]

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

You are barely filling the cylinder > slow burn, so you need to spark the mixture earlier.
That also results to a much cooler idle, which is nice in heavy traffic.
In my experience, most overheating problems (given the cooling system is in order) are actually timing problems.

Also, keep in mind, that just switching the hose from manifold to ported and back for testing doesn't do the trick.
You have to recurve your whole ignition and adjust the carb accordingly. Most folks don't do that - so they often see no benefit.
If done right, you can run a AFR of 18:1 or even more at cruise speeds, which helps MPG a lot.

Frank

[FishFry]

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!

How can too much advance kill rod bearings while cruising?

[panic]

Once the throttle is cracked, the ported and manifold vac's effectes are the same

This does not happen immediately on tip-in, but when the throttle disc's edge passes the VA port.
This is determined by how high that port is above above the transition port (or the top of the slot).
In certain carburetor bodies, ported advance can be added by:
1. locating the transition port's height on the carburetor exterior
2. add ¼" (this should be a % of disc rotation in degrees, but it will vary with the disc diameter)
3. Does there appear to be any circuit in the body at this point (or is it just casting thickness)?
4. if safe, drill horizontally in directly above the transition port until you breach the throat
5. insert a short section of steel brake line until flush with the throat wall
6. the tube's outer end is the vacuum port hose connection

If you don't like the results, add another hole+tube above and/or below this one. Remember to cap those not in use.

[rgalajda]

In certain carburetor bodies, ported advance can be added by:
1. locating the transition port's height on the carburetor exterior
2. add ¼" (this should be a % of disc rotation in degrees, but it will vary with the disc diameter)
3. Does there appear to be any circuit in the body at this point (or is it just casting thickness)?
4. if safe, drill horizontally in directly above the transition port until you breach the throat
5. insert a short section of steel brake line until flush with the throat wall
6. the tube's outer end is the vacuum port hose connection

If you don't like the results, add another hole+tube above and/or below this one. Remember to cap those not in use.

What the bleep. Thank god I don't have to get you to work on my carb.

[rgalajda]

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!

How can too much advance kill rod bearings while cruising?

Light step on throttle and rattle rattle ping ping knock knock

[rebelrouser]

It is whatever works best, no steadfast rule. Most older vehicles had advance hooked to ported vacuum. Hooking it to manifold vacuum with a big cam can raise vacuum and make them more drivable. But it can also raise the idle enough to cause engine run on when you try to shut it off. Back in the early emissions days the vacuum was hooked to manifold but did not get vacuum until a certain engine temp. What you really need to do is graph your mechanical and vacuum advance with a timing light at 500 rpm increments and then make a decision. Performance trends engine analyzer will generate a WOT advance curve. A part throttle curve with the vacuum advance will be more timing, how much? Trial and error, unless somebody on here is smart enough to do the calculations. You might try looking up timing maps for a fuel injected engine, an engine is an engine, whether a computer sets the timing or an advance pad and weights.

[FishFry]

I have an electronic ignition in my 41 Chevy (261) so I can set any curves I want with a laptop, but for a mechanical system, hook up a vacuum gauge at the manifold, and a Mityvac or something similar to your vacuum can in a way that you can control both from the cab while driving.

And yeah, have somebody else doing the driving while you fiddle with it.

Now drive at 1000/1500/2000/2500 and so on RPM (cruise control is ideal, but a steady foot will also do the trick) and at each step, adjust the vacuum advance with the Mityvac till you can read the highest vacuum on your gauge.

Take notes what the gauge on Mityvac says - this way you can learn what your engine wants/needs and you can build yourself a nice vacuum curve. Than try to get as close as possible with an adjustable vacuum can.

That's pretty much the best you can do with a pure mechanical system.

Frank

[jake197000]

doesnt matter ecept for too high of idle when hooked to manifold.

[Geoff2]

It amazes me that some people don't 'get it' with comments like the idle goes up with Man Vac Adv [ MVA ]. As if it is detrimental. That is because the engine likes/wants the extra timing at idle & is making more hp because of it!!!!!!! [ RPM increase ].

Also, never heard of 'early days' of MVA being temperature activated. That happened in the 70s, not 40s,50s,60s. If engine temp got too high, vac adv switched from the USELESS PVA to MVA which increased idle speed & cooled the engine.