Frankie the Fink

(Oh my God - this is twice now!)

There are multiple benefits to full manifold vacuum at idle on these early GM V-8s. Cooler running, smoother idle, etc, etc.. Its well documented and discussed by several folks on here such as JohnZ.

There is NO downside to it

ChattanoogaJSB

Tux and I are going to be taking turns riding your jetski if this afternoon gets any brighter.

I'm all clear now...at least RE: vacuum advance at idle...

Frankie the Fink

(Oh my God - this is twice now!)

There are multiple benefits to full manifold vacuum at idle on these early GM V-8s. Cooler running, smoother idle, etc, etc.. Its well documented and discussed by several folks on here such as JohnZ.

There is NO downside to it....read all about it here (scroll down to the "Ported Vacuum" paragraphs near the bottom):

http://www.lbfun.com/warehouse/tech_...101Article.pdf

ChattanoogaJSB

Thank you Frankie- the Lbfun link is the article I've been "trying" to assimilate but the version I was reading was just kind of plain text. That one is nice.

Ps- I should give a little to myself for not pulling some of what I read into practical application this morning!

SWCDuke

Due to a parts consolidation decades ago, GM replaced several VACs with the ...163, but it takes about 18" vacuum to pull to the limit, so it doesn't pass the Two-Inch Rule even for a low overlap cam base engine. When John wrote the article this fact may not have been understood. The aftermarket followed, so all the catalogs reflect the error, and it will NEVER be fixed. The B1 is equivalent to the ...163, and I always refer to it as a "boat anchor".

BTW the 15" B22 is for base engines with manual transmissions. If a PG use a 12" B26 (VC1765) because automatics pull less vacuum idling in Drive at 500 than idling in neutral At 500 with an manual.

Manifold vacuum is not necessarily eliminated any specific RPM. When cruising down the freeway at 3000 the engine is probably generating enough vacuum to keep the VAC nearly pulled to the limit. Vacuum advance is purely a function of manifold vacuum and centrifugal a function of engine speed - two totally independent variables. That's why on a XYZ plot the spark advance map looks like a surface as shown in my the "Tuning... presentation.

BTW, best top end power is more like 38-40, not 36, so run it in the 38-40 range as long as it doesn't detonate.

With the engine properly configured with the new VAC the idle mixture screws may be more responsive. The engine should stall if you turn even one to the seat. If not, there is a problem in the carb. Run them as lean as you can without getting low speed cruise surge or startup stumble. Most pre-emission engines needed to idle at about 12.5:1, which is about 20 percent richer than stoich., so you can definitely detect a fuel odor in the exhaust, and percolation with modern E10 fuel exacerbates the problem.

Don't forget to tell me the number stamped on the dead VAC. I really need that.


Duke

MikeM

I believe that's the way the old time Blue Flames and thriftmaster six bangers were set up. Your Dad is how old?

If that engine was mine and I fixed one thing first I'd find out why the carb screws are all the way in and then I'd take a look at the plugs for condition and the right heat range and that doesn't mean AC 43. That's to correct a miss.

The vac advance won't make it miss, just make it sluggish. I doubt the valve guides have any influence of the miss either.

Faslane

If he would of stayed with us he would be 70

JohnZ

1. Suggest you look in the mirror before you embarrass yourself any further by calling someone else an idiot.

2. Read this, absorb it, and don't call me in the morning.

http://www.camaros.org/pdf/timing101.pdf

ChattanoogaJSB

John- thank you for the article. I myself have been absorbing it- I probably should have read it several times more before picking up a timing light!

Sorry we had some "noise" in the thread.

It really is embarrassing that I haven't contemplated how my distributor is compensating for conditions all this time. In your article you allude to those of us growing up in a "computer controlled" generation having often received little education in how our distributor is working. Clearly I'm as initiated as anyone ever was. Said again- this is FASCINATING stuff.

THANK YOU!

ChattanoogaJSB

Duke- I will most definitely post the number for you, and I'll hunt up the B22 as well.

ChattanoogaJSB

the question of my idle mixture screws not responding is excellent. If I understand Duke correctly a working vaccine advance is going to result in more complete combustion with 16 degrees or so more timing at idle. Wondering if the engine idling on 10 degrees advance alone is burning so incompletely that there is enough mixture continually available that the carb mixture screws can shut off the idle circuit fuel flow and the engine still cycle.

Mike I agree this is a key issue, not minimizing it. Thank you for your advice.

Westlotorn

Your idle condition could be checked in 5 minutes, plug the vac advance and advance your base timing to 26 for your idle test and see if the carb responds normally to adjustment. Don't drive it like this at all but for checking your idle and carb condition it may answer some of your questions as you wait for the new vac canister. Set the timing back to 10 when your done.
I suspect you might still have a carb issue after the vacuum advance is fixed.

63 340HP

The distributor's combination of centrifugal and vacuum used to be considered an analog mechanical computer. Learning how it works is an exercise in history.

You will likely find more issues impacting the need for the idle mixture screws to be adjusted in tight. A high float or other concern, in addition to the timing.

Don't be discouraged. It's a hobby, one that burns rubber and the midnight oil.

MikeM

Just remember that most if not all solid lifter 283's didn't even have vacuum advance and the initial timing was set around 10*.

Some of the '66-'67 calif. ennges idled with spark retard. No advance at all.

ChattanoogaJSB

That's a great creative solution! I may fall back on it, as of tonight, I'm full with family stuff so I'm going to hope I can have a correct advance on it tomorrow

Mossy66

I'm confused by this statement about idling at 12.5:1.

John's tech paper indicates that the fuel mix is lean at idle and cruise, and that is why the additional timing from the vacuum advance is necessary. I would have thought that the idle mixture was leaner than 12.5:1 on engines where vacuum advance was used. 12.5:1 seems to be the appropriate mixture for WOT.

Is it the exhaust dilution that causes the need for advanced timing at idle?

Gerry

Frankie the Fink

I believe your throttle return spring is hooked up wrong which isn't helping things either...

SWCDuke

No, spark advance within reason has no effect on combustion completeness. If the spark is later than optimum, maximum possible expansion does not occur before the exhaust valve opens. Pressure and temperature at that point are higher than with optimum advance for the specific operating condition.

You're essentially throwing useable mechanical energy out the exhaust and this will be reflected in exhaust manifold temperature. With optimum advance at idle the exhaust manifolds should be no more than 500F. Emission controlled engines with ported vacuum advance will be several hundred degrees higher, which is necessary to promote oxidation and reduction of HC, CO, and NOx.

It actually costs additional fuel to reduce emissions.

Duke

SWCDuke

You want to set the idle as lean as possible commensurate with acceptable idle quality, and no driveability issues like hesitation upon initial throttle application or lean surge at low speed cruise. From the thirties up car owners expected a smooth idle, and this requires a richer than stoichiometric mixture. Most shops set it richer than abolutely necesssary to avoid the customer coming back complaining about "rough idle".

My dad always drove Cadillacs and Pontiacs. Their big slow revving V and straight eights idled butter smooth and quiet. He was appalled at the idle quality of my 340 HP SWC. At the time when I was 16 I vaguely understood why and said it had a "3/4 racing cam".

Back in the day better tuneup shops had CO meters. CO content is a linear function of A/F ratio richer than stoichiometric. Starting there at near zero it increases linearly with decreasing A/F ratio reaching about six percent at 12.5:1, which is about 20 percent excess fuel.

Rather than setting the idle mixture "by ear" or with a vacuum gage and tach, they would set it at 5-6 percent CO.

Rich idle mixtures are easier to ignite, so there are less misfires with the marginal ignition systems of that era, which means smoother operation.

Modern engines idle at stoich. for emissions reasons, but actually don't idle as well as some classic cars. I recall a 12 or 16 cylinder Cadillac at Pebble Beach years ago. I was standing right next to the engine compartment and it was open. The only way I knew that the engine was running was that the fan was turning. It was uncannily quiet and smooth and was probably turning no more than 400 RPM.

Low density, exhaust gas diluted mixtures also have slower combustion propagation, which is why more advance is best for these conditions.

All of this is explained in more detail in my "Tuning... for Maximum Performance and Fuel Economy" presentation, which is on the CF and easy to find. You should download and study it.

Duke

ChattanoogaJSB

Wow- there are so many good replies here, I cannot tell you how much I'm trying to absorb. May I pause and say that before I do any more ignition work or the carb, I paused to do the valve guide seals. My cylinder 1 results are not good on face value. I have a Bosch compression tester which I was to use to inflate the cylinders to hold up the valves. Using the Lars (Duke? I don't recall right off) I marked my balancer and rotated my engine with all the plugs removed until the exhaust valve was opening at the first turn past the balancer mark (the original mark). Then I went another full turn so that I was on compression and stopped at rest. Neither rocker moving through that last travel. Seemed good.

However, my compression tester is leaking down FAST- in a second or so on #1! No leaks at the gauge or the fittings and I threaded it several times tight into the plug hole.

Should I put the scrade valve back in the bottom end so I can reverse the readings and run the gauge with the Piston travel instead of compressed air?

My regulator shows 120psi, I can get no more than 90 into the cylinder and it's gone in a heartbeat. No rocker arms.

Thanks guys- I feel "better" (I say with humility) about my working on the distributor and the carb is nothing that bothers me working on, but now it seems I have a general engine health issue maybe at play.

Perhaps another good question is- am I making sense?