here goes the debate---ported or manifold vacuum to distributor or neither??? I've read that ported is better for non performance engines (that would definitely be mine LOL) for daily driving and highway speeds.
this probably don't belong here but...
Shadetree55
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I have no idle problems with ported, remember, this is a powerless stock 305. My ponderings on manifold vacuum is at idle with manifold vacuum to distributor you're all in on vacuum advance. Max vacuum at idle. Under acceleration you lose that vacuum therefore losing that advance until you get to your highway speed and vacuum come back to it's strongest and that vacuum has you advanced again. kinda like vacuum wipers stop under acceleration until you level off . It's just confusing and sounds opposite of what it should be.
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As stated, ported vacuum only lasts until you touch the pedal, then it goes to full manifold, so it has nothing to do with the advance curve above idle.
WAIT: AI finally came up with a possible pro for ported vacuum.
"At idle, ported vacuum systems don't advance the timing as much as manifold vacuum, which can help reduce emissions"
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okay, so right now with vac plugged and my base timing is @ 13° and at 2800 rpm is @ 33° should I drop my base to 10° since manifold vac at idle is gonna raise that number?? Like I said I'm not looking for high performance, just dependable, coolest running engine. AND what idle speed should I check the initial/base timing . I don't want the idle so high that I'm seeing some of the mechanical advance figured in with that base timing.
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you left out part of AI's answer "
- It prevents excessive advance at idle, which can lead to engine knocking or pinging.
While ported vacuum was the norm, some engines may have been set up differently based on specific performance needs or modifications. However, the general consensus is that ported vacuum was the preferred choice for most stock engines during that era.
Using manifold vacuum was less common and could lead to issues like poor idle quality
and how many of us just sit around and let our car idle?? Unless you have an unusually high idle setting there should be NO ported vacuum until the throttle blade opens enough to allow it. with manifold vac to the distributor you lose that advance immediately under acceleration so you're losing that and relying only on the mechanical advance on an HEI system. The quadrajet was first introduced in 1965 and HEI in 1975 and all used ported vacuum.
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I suggest everyone take the time to read the article here:
This Lars guy is known to know his stuff.
The other thing is: you might want to actually measure the vacuum at the ported vacuum at idle. If it's actually higher than the vacuum spec for the vacuum advance you're using, ported versus manifold fitting is a moot point.
Vacuum_Advance_Specs PDF - CorvetteForum - Chevrolet Corvette Forum Discussion
A technical documentation that outlines the specifications and applications of vacuum advance controls in automotive engines.
www.corvetteforum.com
This Lars guy is known to know his stuff.
The other thing is: you might want to actually measure the vacuum at the ported vacuum at idle. If it's actually higher than the vacuum spec for the vacuum advance you're using, ported versus manifold fitting is a moot point.
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back in my "young" days, many decades ago we used to just get the engine warm and move the distributor until it grunted while trying to start and back it off until it didn't LOL. If it pinged we just backed it off until it didn't. LOL
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so how do you know the vac spec for a stock HEI distributor advance?? AND at a correct idle there should be NO vacuum at idle (therfore the term ported). Mechanical advance in an HEI is ALWAYS 20° no matter what. Changing springs and curve weights is what determines how fast or slow it gets to that degree.
not true there are HEI with more and less mechanical advance. I have seen as little as 14 degrees and as much as 24 degrees, and gm made like 17 different canisters for them too. then there is plastic bushings on the vacuum advance canister rod that can limit it even more.
There's a number of factors in all this.
In my case, its manifold vacuum, but I've also selected a vacuum cannister for specified rod pull (advance range), with GM springs and weights (mech advance) for my particular engine run specs and requirements to come up with a suitable curve.
I agree about not going to argue for one way or another. My point is you've got to look at the overall picture, not just where you're pulling vacuum.
In my case, its manifold vacuum, but I've also selected a vacuum cannister for specified rod pull (advance range), with GM springs and weights (mech advance) for my particular engine run specs and requirements to come up with a suitable curve.
I agree about not going to argue for one way or another. My point is you've got to look at the overall picture, not just where you're pulling vacuum.
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I'm just reporting what I've seen.
What I've seen is some ported vacuum ports are so close to the throttle plate at idle, I don't know beyond a shadow of doubt that zero inches of ported vacuum at idle is always the case. In the case of the trifive carbs, I actually had to shoot carb cleaner in the fitting to be certain whether it was above or below the throttle plate. I'm just saying that's easy to check for vacuum, and then you know for sure.
If you have a stock HEI, is there a part number on the vacuum advance? There are also specs for HEI units in the article I mentioned.
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I have read several of Lars articles over the years and mostly on tuning quadrajets. Never on the distributors. I'm not going to the trouble of trying to get any numbers off the distributor or the vac advance as I'd have to remove the wiper motor just to get the cap off. I do know it isn't pulling any vacuum at idle because I checked it to make sure it wasn't before I put the hose on it. I could put the gauge back on it and see when it does open but that would be a moot point as it needs to open under acceleration. I also adjusted the secondary linkage to where they hardly open unless above about 70 mph or if I floor it completely. Fuel mileage seems tolerable and it still opens enough to get that sweet sounding moan LOL. What I love about a q'jet---those small primarys.
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well, I guess in my case mine is 20 since my initial is 13 and all in at 2800 is 33
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this was the info I found for the majority of GM's non performance engines like my 305 is why I said that.
The stock weights and advance plate are perfectly acceptable for all but the highest HP/RPM engines. ALL stock HEIs that were installed in V8s are designed to have a total centrifugal advance of 20 degrees, +-1 degree due to production line tolerances.
1957 Chevy Bel Air Convertible; ZZ4, 700r4, 9" rear w/3:50 posi
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I used that Lars article to tune my distributor. I used a #VC1852 vacuum canister which limits advance to 10* at the crank. I've got at least 14 Hg vacuum at idle, so I'm getting all 10* advance at idle. One heavy + one medium spring which should put centrifugal advance all in around 2800 rpm. I think I've got my initial timing set around 10-12.
Slightly different numbers but this is the article and process for my particular engine parameters.
You should always check actual start and total pull vs vacuum reading on a vacuum cannister - they will vary. I use a mityvac to determine the numbers and compare cannisters, otherwise its just a guess. Too much $$ invested to guess.
Example - rod started to move at 5", total pull at 10".
One other thing, regardless of whether you use a regular HEI or a more electronic version, the unit functions best with a dedicated 12v, direct wire ignition connection. I typically use a 12ga wire, nothing else attached to it. Factory wiring to HEI was typically set up like this, but after years of fiddling, you'll see vehicles with all sorts of wiring cobbled together.
55 HT, ZZ4 w AFR's, 700R, 3.55 Posi
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Mark...Since I have a ZZ4 as well...Are you using ported or manifold vacuum?
1957 Chevy Bel Air Convertible; ZZ4, 700r4, 9" rear w/3:50 posi
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I’m using manifold vacuum.
Change the distributor to a “Progressive ignition” distributor, connect full manifold vacuum and use your iPhone to adjust curve, best mod I’ve ever done, it’s 100% electronic no moving parts. And you can change the curve live during your drive and view on your phone the timing in real time.
Extra timing at idle usually results in a smoother idle.
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not to be a smart aleck but I have all the electronics on this thing I want or would have put an LS in it. I like it old school as possible but thanks for the suggestion
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