Manifold vacuum at idle... vs without...interesting observations

Kilohertz

Kilohertz

Senior Member
I was able to run the flathead engine today until the T-stats opened and everything seemed to find it's happy place. I was able to set the timing just so and played around with the vacuum advance and I made an interesting observation. I am using my modified GM distributor and vacuum advance can which currently advances 14* at idle, a little too much but for now it's okay.

When I first started the engine a few weeks ago it had that classic flathead sound out of the open headers, you know the burble....today I made some comparisons with and without vacuum advance at idle. With the vacuum can disconnected, the idle sounded rough, the exhaust was quite loud from the open headers, ran fine, just sounded a little...well, unhappy. I connected the vacuum advance (manifold) and the engine just seemed to sigh a happy sound and it just ran smoother, and was very much quieter out of the exhaust. I know it's to do with the timing of the firing and firing 14* before the exhaust opens will be quieter than opening them at 2* BTDC, I was just amazed at how much different the engine ran. I plugged and unplugged the line a few times during the recording.

I thought some of you might enjoy this analysis.

Cheers

https://www.dropbox.com/s/vpgcd0rf9yvoxb5/flathead%20vacuum.mp3?dl=0
 
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If this is an 8ba you always setup the initial timing of the dizzy without the vacuum connected.
 
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I should have given more details, it's a '51 C1BA block with EAB heads and a GM dist I have machined to fit, with vacuum canister, stock 94 carb (for now) .


cheers
 
I put a Bubba mechanical chevy in a friends car a while back with a 8ba. It couldn't do a burn out, then it did.


There is always more then one way to skin a cat with more then one carb.
 
You don't connect the vacuum advance to the manifold. You are having the complete opposite effect.
Disconnect it!
 
Kilo,
What is your total mech advance.....I'd be somewhat more concerned about
that at this point. Delco did install a limit bushing on some vac cans....usually nothing more than an appropriate diameter plastic sleeve, might be an easy way to reduce your 14 deg.

Your 'audio' was pretty heads up.
Charlie ny
 
Thanks for the input guys, appreciate it.

As mentioned I machined a GM distributor to fit the flathead so manifold vacuum is where I have connected it. This is the classic location before emissions based ported vacuum came about. I will experiment with both once the truck is driveable but for now, the engine just runs smoother like this.

Charlie, thanks for the compliment. I was having issues with my borrowed Snap-On adjustable timing light but from what I could manage, it was only 16* at ~2000RPM, and it has the original bushing on the mechanical pin, I will try a sleeve on the vacuum can pin to see how it changes. I just found it interesting how different the exhaust at idle sounded with the difference in timing.

Cheers


My distributor is 1112047
 

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You may need an adjustable vacuum advance cannister - they used to produce them, but I have not tried to buy one for awhile. Your total advance (all in) should be about 24 degrees (under load).

I'm not an expert in how much vacuum advance you should have to support partial throttle "cruise" conditions - who has some numbers here?
 
Here's the Crane adjustable canister for the points style GM distributors.....
99601-1
I altered mine to get rid of their hokey means of limiting advance, substituting my own.
The other photo is one means of putting an adjustable limiter on the mechanical advance, rather than using the factory arrangement with its different thickness bushings.
 

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1948F-1Pickup said:
Here's the Crane adjustable canister for the points style GM distributors.....
99601-1
I altered mine to get rid of their hokey means of limiting advance, substituting my own.
Click to expand...

Cool! Is that little threaded block welded on the canister arm? Did you make that block? Pretty cool, tiny little threading job.

cheers
 
Kilohertz said:
Cool! Is that little threaded block welded on the canister arm? Did you make that block? Pretty cool, tiny little threading job.

cheers
Click to expand...
Yup, it's carefully tacked on there with a MIG.
I tried to "cheap" out on it and find something off the shelf but ended up machining a tiny chunk of material. I used a fairly long 6-32 allen set screw.
 
Kilohertz said:
Thanks for the input guys, appreciate it.

As mentioned I machined a GM distributor to fit the flathead so manifold vacuum is where I have connected it. This is the classic location before emissions based ported vacuum came about.
Click to expand...

And that myth just keeps perpetuating... every engine from the late '50's that I ever worked on used ported vacuum; long before emission controls entered the picture.

You say you are using the stock 94 carburetor. If it was the one used with the Load-a-Matic distributor it has different passages for the dist. vacuum and will give you a venturi vacuum signal.
 
The prevailing theory with the SBC guys is that ported vacuum is better for fuel economy while straight manifold vacuum is better for power.
 
If you set your timing at 0deg at idle with vacuum disconnected as soon as you connect the vacuum line you have 14deg advance at idle. WTF. SBC dizzy is designed to have the vacuum line connected above the throttle body. I have had a SBC dizzy for 35 years starting with a Mallory twin point. Then HEI. Standard HEI has about 20 mechanacal advance all in at 2500rpm. Perfect for a Flathead
 
Ported vs full manifold vacuum.
It's a pretty pervasive story if it's got no substance....
Here's some of that tale on the article's page 57 & 58-
 

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Hal Beatty said:
And that myth just keeps perpetuating... every engine from the late '50's that I ever worked on used ported vacuum; long before emission controls entered the picture.

You say you are using the stock 94 carburetor. If it was the one used with the Load-a-Matic distributor it has different passages for the dist. vacuum and will give you a venturi vacuum signal.
Click to expand...

No myth involved. Ported spark was developed to reduce exhaust emissions. I worked on engine development in the auto industry in the 60's and 70's. Ported spark was a way to reduce hydrocarbons before better systems were developed. Back then, engines didn't run worth a damn because of this and other attempts to reduce emissions.
 
Kilohertz said:
I was able to run the flathead engine today until the T-stats opened and everything seemed to find it's happy place. I was able to set the timing just so and played around with the vacuum advance and I made an interesting observation. I am using my modified GM distributor and vacuum advance can which currently advances 14* at idle, a little too much but for now it's okay.

When I first started the engine a few weeks ago it had that classic flathead sound out of the open headers, you know the burble....today I made some comparisons with and without vacuum advance at idle. With the vacuum can disconnected, the idle sounded rough, the exhaust was quite loud from the open headers, ran fine, just sounded a little...well, unhappy. I connected the vacuum advance (manifold) and the engine just seemed to sigh a happy sound and it just ran smoother, and was very much quieter out of the exhaust. I know it's to do with the timing of the firing and firing 14* before the exhaust opens will be quieter than opening them at 2* BTDC, I was just amazed at how much different the engine ran. I plugged and unplugged the line a few times during the recording.

I thought some of you might enjoy this analysis.

Cheers

https://www.dropbox.com/s/vpgcd0rf9yvoxb5/flathead%20vacuum.mp3?dl=0
Click to expand...
Your observations are spot on. At idle, the engine receives a less dense mixture and also there is some exhaust dilution due to cam overlap. All this means the engine needs more spark advance to better burn this dilute mixture.
 
HMMM, any Chevys or Oldmobiles I owned from '55 up to HEI's used vac from the carb base....below the throttle plates.....all were standard trans if that means anything.

Any Chevy /8BA distributors I sell...now over 1000....with the optional vac advance
I tell my customer, if asked, hook the vac line to the carb base or close.
Charlie ny
 
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