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vacuum advance on 110hp...best practice?
Posted by: potvinguy ()
Date: September 24, 2022 04:12PM

As built by GM the vacuum advance hose goes to "ported" vacuum on the right side carb. Now this was a common deal in the 1960s to meet new govt emission stds. But it made the idle rough and hot because combustion was still in progress when the exhaust valve opened. As soon as car guys caught on they moved the hose to full manifold vacuum, giving full vacuum advance at idle and curing the problem. That's what I've done with the 1966 Monza I recently acquired, and the idle jumped up and smoothed out.

So...any experiences or thoughts? I'm always willing to learn. Maybe Corvairs are different from other cars of that era.

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Re: vacuum advance on 110hp...best practice?
Posted by: RobertC ()
Date: September 24, 2022 04:28PM

Vacuum advance hose goes on the vertical port (tube) on the (pass. side) carb.

On driver's side carb, this port is capped with a plastic cap.

If you have idle problems with this setup, you have other problems.

Is your vacuum advance good? Does your vacuum advance "hold" vacuum?

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Re: vacuum advance on 110hp...best practice?
Posted by: The Stig ()
Date: September 24, 2022 04:33PM

BS

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Re: vacuum advance on 110hp...best practice?
Posted by: potvinguy ()
Date: September 24, 2022 04:35PM

You've described the as-built setup. Why is it better than full manifold vacuum?

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Re: vacuum advance on 110hp...best practice?
Posted by: MattNall ()
Date: September 24, 2022 04:52PM

Stig is correct! If you look at our distributor "CURVE" charts you'll see no mechanical advance at lower RPM

MODERATOR
Sea Mountain, between Charleston Harbor and Coos Bay! SW Oregon Coast
Click HERE for My Website...Click HERE for My TechPages!
...............110-PG.................................................Webered-Turbo

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Re: vacuum advance on 110hp...best practice?
Posted by: 63turbo ()
Date: September 24, 2022 05:17PM

The Stig Wrote:
-------------------------------------------------------
> BS


Not BS if its causing the engine to run right at idle. Some 60's era engines really did have the vacuum advance engaged at idle, others didnt. My turbo has vacuum advance, and it is fully engaged at idle, and produces way more vacuum (17-18") than stock. It is just not a big deal to run a lot of timing at idle, especially on low compression engines. Mine idles at 1000 rpm and 28 degrees of timing, all in. Real nice throttle response (among other things) compared to stock.

------------------------------------

Kevin Nash
Friday Harbor Washington
63 Spyder, Daily driver, EFI read about my project here: [corvaircenter.com]
first test start on EFI here:[www.youtube.com]
first official EFI boost test here:[www.youtube.com]
My new fan! [corvaircenter.com]
engine less 62 Spyder
Canadian 64 Monza Parts car
Test Start#2 [www.youtube.com]



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Re: vacuum advance on 110hp...best practice?
Posted by: RobertC ()
Date: September 24, 2022 05:25PM

potvinguy Wrote:
-------------------------------------------------------
> You've described the as-built setup. Why is it better than full manifold vacuum?

As in: Why did the engineers who developed induction systems use ported vacuum?

You think they were wrong?

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Re: vacuum advance on 110hp...best practice?
Posted by: RobertC ()
Date: September 24, 2022 06:03PM

Basically, you fixed your symptom; not your problem.

Based on your description, the engine timing changed - meaning the vacuum advance is "activated".

The harmonic balancer can "slip" - changing the position of the timing mark. There is a reference mark on it.

The pivot on the stock point plate can come loose.

Vacuum leaks.

Carbs not balanced correctly.

Mismatched primary carbs.

Worn linkage / carbs.

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Re: vacuum advance on 110hp...best practice?
Posted by: potvinguy ()
Date: September 24, 2022 06:51PM

RobertC Wrote:
-------------------------------------------------------
> potvinguy Wrote:
> -------------------------------------------------------
> > You've described the as-built setup. Why is it better than full manifold vacuum?
>
> As in: Why did the engineers who developed induction systems use ported vacuum?
>
> You think they were wrong?

The ICE engineers knew better, but the car makers were in a hurry to pass emission standards. They did other tricks in the 60s (smog pumps, retard solenoids on the distributor) too. Here's a good article on vacuum advance.

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Re: vacuum advance on 110hp...best practice?
Posted by: wittsend ()
Date: September 24, 2022 07:16PM

Most every engine will run smoother with timing advanced at idle. You made the generic statement "Now this was a common deal in the 1960s to meet new govt emission stds. But it made the idle rough and hot because combustion was still in progress when the exhaust valve opened.." The wording implies ALL cars of the era. A few posts later there was a reply stating, "If you have idle problems with this setup (as the factory designed it), you have other problems."This implies that there is a problem with your specific Corvair. What has not been determined is if you Corvair idles appropriately AS DESIGNED and you just prefer the soother idle with the higher advance the activated vacuum advance provides..., or if you really have a problem. That needs to be settled first.

Because you are initially advancing at idle with the vacuum advance the advance will drop upon acceleration. The question would be does it drop enough upon acceleration or climbing a hill to avoid pinging/knock?

Pinging/knocking are things that can occur with an engine that has too much advance for a given load on the engine. If the vacuum is sufficient and advance is activating during starting (cranking) it could create a condition where again there is too much advance and the load on the starter is a potential problem.

You are likely getting away with the advance as you set it because the compression and the demand load on the engine is low. However, there might be situations where that circumstances are different and I would keep and ear open for them. I tend to agree that the factory did what they did for a reason. Long before there were smog concerns the ported vacuum advance was common. But it is your car, your choice.

FWIW the Ford inline 6 of the 60's had only a vacuum advance on many of their cars. There was no mechanical advance! I assume t was a cost cutting measure.

------------------------------------------------------------------------------

***'61 Lakewood, a "Freebie" in hibernation for 27 years - In the process of applying CPR.***

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Re: vacuum advance on 110hp...best practice?
Posted by: red monza ()
Date: September 24, 2022 09:04PM

My old super mechanic before he passed away listened to my "new" 283 I had built that was not idling good. He then put the vac advance to a constant source and the engine smoothed out really super nice. No shaking, etc and it was a screamer. That was 8 years and 9,000 miles ago. It was bored 60 over, Z28 601 super high compression heads, 327 cam, quadrajet carb, elderbrock intake, points distributor from a 68 Camaro, plugs designed for an HEI distributor but set to a points distributor and a 700R transmission etc. He said my engine was not the 1st one that ran better with full vacuum. See if you can find how much vac my engine needs to idle correctly and run like it does. I can show you how much.

And, I could not tell you the vacuum advance that is on my 283 but I can bet you it is NOT the 60 year old one that the engineer's said it needs to properly idle.

Engineers provide parts that work on a brand new engine as it leaves the factory and the new factory settings that should be used...not one that has 80,000 miles on it with lots of piston/head carbon and wear and has parts no longer manufactured to engineered specs and some made overseas, like China. I think in one of my courses the instructor said auto manufactures at that time had to provide parts for 10 years to that new car. It may have been one of my required law courses. Any way, that engine is no longer brand new with all genuine Chevy made parts and all tolerances are not as a new engine would have. In my book, the settings that make that engine run the best...are the settings and connections that are the correct ones.

And as far as having the correct distributor in your Corvair, I purchased a 339 distributor from a Ca vendor at a premium price years ago. My 140 ran terrible with that distributor. I purchased a new 319 from a local car parts place and had to change the vac advance and points within a couple of weeks that quit working. 3-4 years later, I get a CORSA Tech Guide and look up my 339 distributor by the parts it should have, the only thing that was 339 was the outer housing and the worn main shaft, the rest was an early model 298 distributor. I do not blame the vendor but it would have gone back had I known it was a 298 distributor with a worn shaft.

I would love to know how many on here have the correct vac advance (not the number, the exact identical spec one) and the engine timing as listed in their Chevy owners manual. Not one of my cars has the engineers recommended timing except for my Nissan pick up. All others are advanced somewhat. You know, like when you go to a constant vac source for the vacuum advance. Some engines like it and some like my L-79 327-350hp Corvette just likes 4 degrees above the special recommended Factory timing tag in the engine compartment stated but the vac advance is hooked to the quadrajet like it came from the factory. I am pretty sure it still has the factory advance.

In conclusion, not every engine runs the same, Arthur built a 327 that turned 10,000 rpm and he told me he could build 10 more identical and it would be a slim to none chance any of them would turn that much rpm. However, he sure had several folks trying to get him to build them a 327 just like that one...lol


+--

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Re: vacuum advance on 110hp...best practice?
Posted by: 63turbo ()
Date: September 24, 2022 10:06PM

thumbs upthumbs upthumbs up Part of the deal with factory timing specs is that they are supposed to be retarded from best power settings enough to produce a 2% power loss, on a new engine. At 1250 rpm, on a turbo corvair, absolute best power timing is 39 degrees(!!), and advancing the timing from there to produce a 1% power loss is 45 degrees, retarding the timing from best power to produce this same 1% power loss is 32 degrees, and a 3% power loss is down to 26 degrees. These tests were done with leaded gasoline that had a Research Octane number of 105 and the Motor Octane number was 104. The point of the above is theres a huge difference between slightly too much timing (45 degrees) and a 3% power loss timing of 26. The factory recommended timing was only 24!
Back to this manifold vacuum vs ported vacuum thing... I was checking some stuff on a friend's 327 and noticed it had manifold vacuum for the vacuum advance,
and out of curiosity I checked the timing at idle with the vacuum advance plugged in. It was 45 degrees!

------------------------------------

Kevin Nash
Friday Harbor Washington
63 Spyder, Daily driver, EFI read about my project here: [corvaircenter.com]
first test start on EFI here:[www.youtube.com]
first official EFI boost test here:[www.youtube.com]
My new fan! [corvaircenter.com]
engine less 62 Spyder
Canadian 64 Monza Parts car
Test Start#2 [www.youtube.com]



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Re: vacuum advance on 110hp...best practice?
Posted by: potvinguy ()
Date: September 25, 2022 10:52AM

63turbo Wrote:
-------------------------------------------------------

> Back to this manifold vacuum vs ported vacuum thing... I was checking some stuff on a friend's 327 and noticed it had manifold vacuum for the vacuum advance,
> and out of curiosity I checked the timing at idle with the vacuum advance plugged in. It was 45 degrees!

My crate SBC runs 20 initial, 15 vacuum and 15 rpm. It idles at 35 and cruises in the 40s, and is happy as a clam (why would a clam be happy?)

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Re: vacuum advance on 110hp...best practice?
Posted by: red monza ()
Date: September 25, 2022 12:19PM

While I had an older gentleman here yesterday, I checked the dwell on the 283 and it was right on 30 as recommended. The timing at 900rpm idling with the vac advance as I stated, hooked to the manifold vs carb, the timing was right at 10 degrees BTDC. The factory specs for the timing on my 68 L-79 is 4 degrees, there was a special notice in the engine compartment that had a RED "8" degrees setting highlighted, the rest of the notice was in black. I have it setting at 12 degrees BTDC and back in the early 70's, I added some weaker springs in the distributor. Here is a video of it after just being pulled out of a 3 year storage. Its about 16 seconds long. It has a terrible old cell phone recording but tells you a little about its timing.

[www.youtube.com]

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Re: vacuum advance on 110hp...best practice?
Posted by: Frank DuVal ()
Date: September 25, 2022 01:03PM

Quote
pot
As soon as car guys caught on they moved the hose to full manifold vacuum, giving full vacuum advance at idle and curing the problem. That's what I've done with the 1966 Monza I recently acquired, and the idle jumped up and smoothed out.

Many incorrect historical statements in your post.

But, let's jump to your problem. No, I do not have 110 HP Corvairs that idle rough. If I did, I would fix them and not re-engineer the system.

Of course when you put suction on the vacuum advance the engine speeds up. If you knew how the system works, this would not be news. Of course now, the curve is backwards, and the idle is too high. What happens to the timing curve at higher rpms? I suggest putting the distributor an a distributor machine and see if there was something wrong internally to start. Parts do get swapped over 50 years! Get a copy of the Tech Guide and see if the internals are right.

Of course, Band-Aids worked for many years keeping these cars on the road.winking smiley

I'm not discussing 283s as I sold my last 283 car in the early 80s. Of course, they ran fine with the factory setup also.... I was not into racing, but being I owned them during the 70s, I was into maximum MPG!thumbs up I got 21 MPG in the 66 full sized wagon.

Frank DuVal

Fredericksburg, VA

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Re: vacuum advance on 110hp...best practice?
Posted by: 63turbo ()
Date: September 25, 2022 02:01PM

>
> Of course when you put suction on the vacuum advance the engine speeds up. If you knew how the system works, this would not be news. Of course now, the curve is backwards, and the idle is too high. What happens to the timing curve at higher rpms? I suggest putting the distributor an a distributor machine and see if there was something wrong internally to start. Parts do get swapped over 50 years! Get a copy of the Tech Guide and see if the internals are right.
>

Uh no the curve is not backwards if the vacuum advance is fully engaged at idle. At any given manifold vacuum, the timing rises and falls as the engine rpms rise and fall. As the load increases at any given rpm, the vacuum falls and takes out the extra timing. If you hit it at any given rpm, the vacuum timing goes away and your only riding the mechanical advance. Most that convert from ported vacuum to manifold vacuum will re-adjust the throttle blades more closed to slow the engine down. Which is what I did. Not very hard to figure out.
As has been said many times on many different forums, the ported vacuum was a emissions thing. Idle cooling is especially nice with the extra timing at that speed.

------------------------------------

Kevin Nash
Friday Harbor Washington
63 Spyder, Daily driver, EFI read about my project here: [corvaircenter.com]
first test start on EFI here:[www.youtube.com]
first official EFI boost test here:[www.youtube.com]
My new fan! [corvaircenter.com]
engine less 62 Spyder
Canadian 64 Monza Parts car
Test Start#2 [www.youtube.com]



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Re: vacuum advance on 110hp...best practice?
Posted by: potvinguy ()
Date: September 25, 2022 03:29PM

The mind boggles. For a given engine, can one deduce the optimum initial advance and vacuum curve and rpm curve? Assume all other variables don't change (temp, fuel, whatever). And we have to define "optimum;" do we want most power or most mpg or best throttle response, best idle, or some other quality? Fortunately for most of us it isn't critical, so don't hurt your brain.

For my 110hp PG the book says 14 initial, 24 vacuum and 20 rpm. But there is no description of the curves, which I assume is factory set in the distributor per some idea of how the car should perform for the masses. Think I'll get a cold drink and watch some Parking Wars...

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Re: vacuum advance on 110hp...best practice?
Posted by: 63turbo ()
Date: September 25, 2022 06:10PM

In this thread, 14 posts down (NMcarnut) is the 110 vacuum advance curve for your PG
[corvaircenter.com]

------------------------------------

Kevin Nash
Friday Harbor Washington
63 Spyder, Daily driver, EFI read about my project here: [corvaircenter.com]
first test start on EFI here:[www.youtube.com]
first official EFI boost test here:[www.youtube.com]
My new fan! [corvaircenter.com]
engine less 62 Spyder
Canadian 64 Monza Parts car
Test Start#2 [www.youtube.com]






Edited 1 time(s). Last edit at 09/25/2022 06:16PM by 63turbo.

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Re: vacuum advance on 110hp...best practice?
Posted by: potvinguy ()
Date: September 26, 2022 05:45AM

63turbo Wrote:
-------------------------------------------------------
> In this thread, 14 posts down (NMcarnut) is the 110 vacuum advance curve for your PG
> [corvaircenter.com]

Thanks! I've thought on it and what I want is easy starting, smooth and cool idling, and good throttle response. MPG and power don't matter; this is a town cruiser. It's in the shop now for a brake job. When I get it back I'll do the sync thing, verify advances, and all 3 vacuum pots and both manifolds will be interconnected. And I'll peek at the PCV valve; it's probably original.

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Re: vacuum advance on 110hp...best practice?
Posted by: Frank DuVal ()
Date: September 26, 2022 06:49AM

110 with PCV valve? AC car? Or 110 in an earlier vehicle (pre 64)?

Frank DuVal

Fredericksburg, VA

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