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

[Kilohertz]

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/vpgcd0rf9y...acuum.mp3?dl=0

[Tinker]

If this is an 8ba you always setup the initial timing of the dizzy without the vacuum connected.

[Kilohertz]

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

[Tinker]

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.

[69a]

You don't connect the vacuum advance to the manifold. You are having the complete opposite effect.
Disconnect it!

[tubman]

Sponsored Links (Register now to hide all advertisements)

Originally Posted by 69a

You don't connect the vacuum advance to the manifold. You are having the complete opposite effect.
Disconnect it!

He's using a GM distributor, not a Loadamatic.

[flatjack9]

What cam do you have in the engine?

[Charlie ny]

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

[Kilohertz]

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

[Bored&Stroked]

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?

[1948F-1Pickup]

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.

[Kilohertz]

Originally Posted by 1948F-1Pickup

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.

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

cheers

[1948F-1Pickup]

Originally Posted by Kilohertz

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

cheers

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.

[Hal Beatty]

Originally Posted by Kilohertz

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.

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.

[tubman]

The prevailing theory with the SBC guys is that ported vacuum is better for fuel economy while straight manifold vacuum is better for power.

[69a]

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

[1948F-1Pickup]

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-

[flatjack9]

Originally Posted by Hal Beatty

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.

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.

[flatjack9]

Originally Posted by Kilohertz

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/vpgcd0rf9y...acuum.mp3?dl=0

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.

[Charlie ny]

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

[JSeery]

Charlie, isn't carburetor base manifold vacuum?

[Kilohertz]

Originally Posted by JSeery

Charlie, isn't carburetor base manifold vacuum?

I'm not Charlie but to answer your question, if the carb base port is drilled to pull from below the butterflies, it's manifold vacuum.

Awesome discussion people, I am enjoying it immensely, I just got back in from the shop. More later.

Cheers

[Kilohertz]

Originally Posted by Hal Beatty

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.

Only if you don't modify the carb. I plugged the venturi feed and only left the ported hole open just above the butterflies to give me straight ported vacuum...then realized I wanted manifold vacuum, for all the reasons discussed.


I know these are flathead engines, but read thru this article.


Cheers

[Kilohertz]

Originally Posted by flatjack9

What cam do you have in the engine?

I don't know yet, but I may pull a head and see if I can figure it out by valve movement..but not for a while. Found a 4 bbl Offy manifold and Carter WCFB carb...woo hooo


cheers

[Charlie ny]

JSeery #21.............absolutely.....It has always been my understanding that below the plates is manifold vacuum. If a convenient port in the carb base is not available use or
provide a port in the intake as close to the carb as possible. Dunno about that, it's what
I heard.
The max adv adjuster I feature is very similar to what is shown...but not as tidy...6-32 allen as well.
Audio, photos, time for a video....Kilo
Charlie ny

[Kilohertz]

Sponsored Links (Register now to hide all advertisements)

Originally Posted by Charlie ny

Audio, photos, time for a video....Kilo
Charlie ny

This was a few days ago right after I timed it and got it setup moderately well.

cheers

https://www.dropbox.com/s/6p5142j2v6...tuned.wmv?dl=0

[Hal Beatty]

Originally Posted by flatjack9

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.

So... what about the Fords '57 and up that used ported vacuum? That wasn't done to reduce emissions... and all the other 50s' and 60's engines GM and Chrysler that used ported vacuum? In that era the only emission controls used was a PCV valve, if that.

[Hal Beatty]

Originally Posted by Kilohertz

Only if you don't modify the carb. I plugged the venturi feed and only left the ported hole open just above the butterflies to give me straight ported vacuum...then realized I wanted manifold vacuum, for all the reasons discussed.


I know these are flathead engines, but read thru this article.


Cheers

Just didn't know if you had modified the carb... you were way ahead of me on that.

[Hal Beatty]

OK, so this discussion has raised my curiosity and I went out and grabbed a couple of non-Ford carb/intakes off the shelf. First one was the stock intake from a '62 Cad with the steel vacuum line still attached; traced the circuit in the 4GC base and it comes out below the throttle blades, manifold vacuum.

Then grabbed the set-up off of a '54 Cad, traced the vacuum passage and it comes out above the throttle blades, ported vacuum. So some variation here...

I'm not debating the merits of ported vs. manifold vacuum advance here, every engine is different and you'll have to experiment with it. Just the idea that ported vacuum didn't appear until smog controls...

[Talkwrench]

Just as a matter of interest since we're talking vacuum. Someone may correct me if Ive got this wrong I was told our Falcon GTHO's in Aust had both ported and manifold, no doubt to do with starting a high comp engine.

Also the effect of vacuum, today here is very humid and sticky noticed I was barely getting over 20Hg on a cool crisp clear morning I will get close to 22 Hg.

[Charlie ny]

Kilo,
That exhaust is a scream......you may have started a whole new performance
arena.....installation of hi-po vacuum fans to help pull the exhaust out of the motor....
super evac vs super charging.
Charlie ny

[Kilohertz]

So my Dad came over to hear the flathead again after I got her tuned up, and I was mucking about with the timing light trying to figure out the advance curve on the mechanical springs with the vacuum can disconnected and his first comment is..."it sounds like it is retarded". One thing I have noticed is it has a hard time accelerating without stumbling and you kind of have to ease into the throttle to get it up to 2,000RPM, I figured I needed to adjust the accelerator pump. He said no, lets check the timing, but I assured him I had it set to the mark on the crank pulley and he said forget that, lets time it by ear. I loosened the clamp and we spun it around until the idle smoothed right out and when accelerating it jumped right up like it was a new engine! Ran great! So for grins I put the timing light on it and it was at about 10-12* advance at idle, and made it to 21* at 2500 RPM, this was still with the vacuum can disconnected. Well what the heck? My Dad used to hot rod these engines back in the 50's and knows them very well, way to go Dad, thanks!

Now this crank pulley looks like it has been changed, it is new, with a little bit of surface rust on it....is it possible it's marked wrong, or is the wrong pulley? Anyone else find this? I am not sure if I can accurately determine TDC without pulling a head off, and even then, there is a fair amount of crank* movement while the piston is at TDC.

For now I'm leaving it as it as it runs so well, but ultimately would like to know what is going on with the marks on the pulley and why it runs so good at this advance setting.

Cheers

[Charlie ny]

Kilo,
This happens all the time.........had '51 Merc in the shop with lag complaints,
timing was on the money mark wise, reset the dwell, changed the accel pump all for
nothing. I bumped the timing up a bit, took the Merc out it was better ,4th trip out it was real happy....same thing ,the pointer and mark were way off, but the Merc ran like it should....I remarked the pulley.
Charlie ny

[JSeery]

Originally Posted by Kilohertz

Now this crank pulley looks like it has been changed, it is new, with a little bit of surface rust on it....is it possible it's marked wrong, or is the wrong pulley? Anyone else find this? I am not sure if I can accurately determine TDC without pulling a head off, and even then, there is a fair amount of crank* movement while the piston is at TDC.

Cheers

Fairly easy to check/establish TDC with the heads on. There have been several threads on the process and I think Mart did a video. Basically you insert something through the spark plug hole that the piston can bump up against (something soft enough to not damage anything). The head of a zip-tie works. Then establish a pointer near the crank pully, turn the engine over until it stops against the object in the cylinder. Mark the position on the pully. Turn the engine in the opposite direction until it again hits the stop, mark the pully. TDC is halfway in between the two marks. Several repeats is good to verify your marks. Attempting to establish TDC at actual TDC is difficult because there can be a lot of crank movement with little noticeable piston movement.

[Kilohertz]

Thanks boys for the replies. Interesting Charlie, QC? what QC? Oh boy, thankfully things have improved over the years.

Jseery, cool idea for finding TDC. I will try it tomorrow when I get a moment. It will be interesting to see how far off the pulley mark is.

Happy the engine is running so well. Oh, and the compression seems normal again at ~100psi. Seems one of my gauges was out of calibration and the other had a faulty Schrader valve.

Cheers