Manifold Vacuum Experiment

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RustyRatRod

RustyRatRod

I was born on a Monday. Not last Monday.
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I've been noticing a slight stumble off idle and reduced fuel economy on my 75 F250. 400 engine, T18 4 speed transmission and Dana 60 with 3.73 gears. So I started checking it out and quickly found a bad vacuum advance canister. Got a new one and got it installed this morning. I decided to try out our friend from down under @Bewy's advice and attach the vacuum canister to manifold vacuum.

Starting the engine up and got some heat in it, then attached the canister to manifold vacuum. I also had my digital timing light on it which also has a digital tachometer. So I started pulling timing in until the engine stopped idling up in RPM. I moved the timing mark back with the light and discovered it was at 44 degrees initial. That's just too much. So I backed it down to right at 30 initial. Although it did idle down some, it was only a little. Remember, when it gets under a load, the vacuum can will fall out of the equation and the timing will fall off.

I have to go pick up a work table from a friend either later this afternoon or tomorrow across the county, so we'll see how it works. I have tried manifold vacuum on Vixen, my Valiant and it did not respond as well as it does with ported vacuum so I put it back on ported vacuum. Since the Valiant has pretty high compression (over 10:1) I believe that may be why it did not respond as well with manifold vacuum. The truck however is only around 9:1 at the absolute most and more likely around 8.5-8.7, so it may respond better, especially with the somewhat lumpy Crane Fireball cam with only about 7hg of vacuum at idle. Although I have not rechecked the vacuum with the advance canister on full manifold vacuum, it might have more now. I wanted to tag @Bewy in this, so he could put in his 2 cents, since he is a fan of manifold vacuum.
 
RRR, have you put a vacuum gauge on the different ports to see what the Hg reading is from idle up to cruising RPM?
 
Demonracer said:
RRR, have you put a vacuum gauge on the different ports to see what the Hg reading is from idle up to cruising RPM?
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Yup. It has about 14hg at cruise. It's not a vacuum miser, because of the camshaft. It's an old NOS Crane Fireball. .505/.515 228 and 232 @ .050 on a 108, so it's got some cam. Amazingly though, it operates power brakes surprisingly well.
 
Demonracer said:
It was just a thought, good luck.
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It doesn't really have a "problem" so to speak. I am just experimenting with it to see which way it runs better, or if I can even tell a difference. lol
 
RustyRatRod said:
I've been noticing a slight stumble off idle and reduced fuel economy on my 75 F250. 400 engine, T18 4 speed transmission and Dana 60 with 3.73 gears. So I started checking it out and quickly found a bad vacuum advance canister. Got a new one and got it installed this morning. I decided to try out our friend from down under @Bewy's advice and attach the vacuum canister to manifold vacuum.

Starting the engine up and got some heat in it, then attached the canister to manifold vacuum. I also had my digital timing light on it which also has a digital tachometer. So I started pulling timing in until the engine stopped idling up in RPM. I moved the timing mark back with the light and discovered it was at 44 degrees initial. That's just too much. So I backed it down to right at 30 initial. Although it did idle down some, it was only a little. Remember, when it gets under a load, the vacuum can will fall out of the equation and the timing will fall off.

I have to go pick up a work table from a friend either later this afternoon or tomorrow across the county, so we'll see how it works. I have tried manifold vacuum on Vixen, my Valiant and it did not respond as well as it does with ported vacuum so I put it back on ported vacuum. Since the Valiant has pretty high compression (over 10:1) I believe that may be why it did not respond as well with manifold vacuum. The truck however is only around 9:1 at the absolute most and more likely around 8.5-8.7, so it may respond better, especially with the somewhat lumpy Crane Fireball cam with only about 7hg of vacuum at idle. Although I have not rechecked the vacuum with the advance canister on full manifold vacuum, it might have more now. I wanted to tag @Bewy in this, so he could put in his 2 cents, since he is a fan of manifold vacuum.
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Crane Fireball Cam. Music to my ears.
 
My barracuda ran worse when I did this. Seems like it makes it work opposite of how it was designed.
 
Following along. I am trying the same thing, although my happy place for idle is 25*. I would think that the trickiest part would be the mechanical advance curve. Getting enough in, while manifold is falling off under light acceleration......Keep us posted Rob :thumbsup:
 
Mike69cuda said:
My barracuda ran worse when I did this. Seems like it makes it work opposite of how it was designed.
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It's entirely possible. So far, it made the slant 6 in Vixen run worse, too. I've still not driven the truck yet, but we'll drive it tomorrow.
 
CFD244 said:
Following along. I am trying the same thing, although my happy place for idle is 25*. I would think that the trickiest part would be the mechanical advance curve. Getting enough in, while manifold is falling off under light acceleration......Keep us posted Rob :thumbsup:
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It's pretty strange. All of them in the past I've done like this stopped idling up at what I would call a "reasonable" amount of initial timing. This one does not. It idles up right up to 44 degrees initial.

Admittedly, it takes several degrees before it begins to idle back down when pulling timing out. I also fully admit I never measured static compression, because I knew it was going to be low. I didn't care and I still don't. I haven't measured cylinder pressure with a compression gauge, either, although now that I am doing this with the timing, I'll get the gauge out tomorrow and see.

The factory "rated" compression for these engines was 8.4:1. Although where they blueprint is anyone's guess. Again, I didn't care then and I still don't. I knew unless I started whackin off .100" or more from the heads, I'd be ok. I said whackin off. I had the heads milled .060" originally when I had machine work done.

Also, these engines were known to be a bastion of a detonation problem, because of the negative deck clearance and long stroke. That was the reason for wanting a camshaft with some duration and a late IVCE (intake Valve Closing Event). So far, I've run as much as about 28 initial with the vacuum canister on ported vacuum and heard narry a peep of detonation all the while running on 87. ......which was my goal. A cool, lumpy idling truck engine that will pull the snot out of something on the bottom end and so far, that goal has been met the heck out of. This experiment is just to see if there's something to be gained from full manifold vacuum at the vacuum canister. We'll see. lol
 
RRR,
Going to add 3c worth, not 2 cents...

Firstly congrats on going to the trouble of trying MVA. 44* at idle does not surprise me [ My GTO has 48* ]. A 440 Charger engine that I set up a few weeks back idles with 42*. Could be a few reasons for that high amount. Poor chamber, lack of quench, low compression, low vacuum [ cam ] etc.

You really need an adj VA unit. Even if it means getting a different dist; Allen Key needs to be fully CW [ softest spring setting ].
The #1 cause of comments like post #7 is bad set up leading to erratic/varying timing at idle.

To anybody setting up MVA: before closing the tool box, always check the timing at idle, in gear if auto. If it is unsteady, you need to find out why because it will cause problems.
 
RRR,
have you been for a test drive yet? Engine run cooler at idle?
 
Bewy said:
RRR,
have you been for a test drive yet? Engine run cooler at idle?
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I have not. We had tons to do in town today and not in the truck so I ran out of time. We'll drive it tomorrow and see what's up. And as in your post regarding bad chamber, low compression, no quench and big cam. That's a big "yeah" to all four. LOL So that's very likely why it wants so much. I have it at 30 right now, but I'll keep messin with it.
 
Bewy said:
RRR,
Going to add 3c worth, not 2 cents...

Firstly congrats on going to the trouble of trying MVA. 44* at idle does not surprise me [ My GTO has 48* ]. A 440 Charger engine that I set up a few weeks back idles with 42*. Could be a few reasons for that high amount. Poor chamber, lack of quench, low compression, low vacuum [ cam ] etc.

You really need an adj VA unit. Even if it means getting a different dist; Allen Key needs to be fully CW [ softest spring setting ].
The #1 cause of comments like post #7 is bad set up leading to erratic/varying timing at idle.

To anybody setting up MVA: before closing the tool box, always check the timing at idle, in gear if auto. If it is unsteady, you need to find out why because it will cause problems.
Click to expand...
Oh and I do have an adjustable can. The old one was as well, the diaphragm was just compromised and it was no longer working.
 
Oh I need to come back and add this. While not what I would call a "problem" it has exhibited a rich condition at idle. Stinky exhaust and such, which is indicative of not enough initial timing.....even though I've had it as high as about 25-28 degrees. That just shows that it obviously wants more. The carburetor is only a 650 on a Ford 400 engine, so I hardly think it's too much carburetor. Pretty sure that's timing related. I plan to find out.
 
RustyRatRod said:
Oh I need to come back and add this. While not what I would call a "problem" it has exhibited a rich condition at idle. Stinky exhaust and such, which is indicative of not enough initial timing.....even though I've had it as high as about 25-28 degrees. That just shows that it obviously wants more. The carburetor is only a 650 on a Ford 400 engine, so I hardly think it's too much carburetor. Pretty sure that's timing related. I plan to find out.
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interesting: a little off subject , but how much vacuum is too much on a r/b hotrod ???
I `m running full vacuum , has recently gone up to about 15", (think my rings have finally seated???) has been around 4 forever, sure stops oil leaks !!
Asking from u guys who probly know ...

sorry for buttin in rusty...
 
famous bob said:
interesting: a little off subject , but how much vacuum is too much on a r/b hotrod ???
I `m running full vacuum , has recently gone up to about 15", (think my rings have finally seated???) has been around 4 forever, sure stops oil leaks !!
Asking from u guys who probly know ...

sorry for buttin in rusty...
Click to expand...
I wouldn't think there could be "too much". A stock or mild engine has 18 to like 22 or so hg. The more the better IMO because a good strong vacuum signal leads to good street manners and the ability to run power accessories.
 
RustyRatRod said:
I wouldn't think there could be "too much". A stock or mild engine has 18 to like 22 or so hg. The more the better IMO because a good strong vacuum signal leads to good street manners and the ability to run power accessories.
Click to expand...

A good friend , called ''mr chevrolet around here'' said that anymore than 10 or so would/could start sucking oil thru the valve guides >hi perf. engine ...
no power brakes ,and everything is controlled by the fast fuel inj set up ....
 
famous bob said:
A good friend , called ''mr chevrolet around here'' said that anymore than 10 or so would/could start sucking oil thru the valve guides >hi perf. engine ...
no power brakes ,and everything is controlled by the fast fuel inj set up ....
Click to expand...
Oh horse feathers!
 
So here's where I am this morning. I turned the vacuum canister adjustment all the way CW. Busted old Gladys off and let her get some heat in her. Got the timing light back out and made sure of my initial findings yesterday. Wasn't quite 44 degrees initial, but 40. About 44 is where it makes a pretty hard stop idling up, so I backed it off a few and we're at 40 initial. That of course is with the vacuum canister hooked to manifold vacuum. I unhooked and plugged the vacuum hose going to the canister and reread the timing and it's right on top of 20 degrees initial, which is where I had it set before using ported vacuum, so we're right in the ballpark. It does seem to idle cleaner with the extra 20 degrees pulled in by the vacuum advance. We'll drive it later on and I'll report back.
 
Rusty, I read in another post that you are using a Brawler carb now. What happened to the Street Demon?
 
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