Manifold Vacuum Experiment
- Thread starter RustyRatRod
- Start date
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Now that's funny right there. Didn't have a chance to drive the old girl today. We had tons to do around the house, but we HAVE to go into town tomorrow so I will report back then.
Aight den. I have an update. I can tell beyond a shadow of a doubt the engine idles cleaner. By a good bit. It also has more balls at part throttle pulling up inclines. A lot more, actually. No spark knock whatsoever with the can turned all the way clockwise, so I don't think any adjustment is necessary there. All in all I think this was a beneficial mod and I may revisit it on Vixen. I didn't put the vacuum gauge on it but I will in the coming days.
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I tried running MVA on the Demon as the 660s don't have ported. I found a part throttle stumble was due to MVA and a very fast timing curve. For me, I had to pick one. I eliminated the vacuum advance all together and it runs so much better.
Have you got any idea as to how your centrifugal curve is laid out? Interested in the interaction between the 2.
Yeah. Before I changed to manifold vacuum, I had it on 20 initial, 34 total with it all in by 2500. It's still basically the same since when removing and blocking the vacuum hose, it drops the initial timing back down to 20 initial.
If I have this correct.
After replacing the vac adv. idle timing was at 30 deg. connected to manifold vacuum. 20 deg. at idle w/ out vac.
After turning the adjustment screw full clockwise Idle timing increased to 40 deg. and idle time w/out vac remained at 20 deg.
Your observations were that the engine idles cleaner by a good bit and the engine has more balls at part throttle pulling up inclines.
Separating the two observed results. First the cleaner idle. I think this can be attributed to using manifold vac.
The second result, "more balls at part throttle", can be attributed to the full clockwise adjustment and a working diaphragm in the new vacuum adv. This second result would have still been realized even if the vac adv. was connected to ported vac. only.
Looks like your combination benefited from the changes you made so far.
Good job testing and tuning. Thank you for sharing.
After replacing the vac adv. idle timing was at 30 deg. connected to manifold vacuum. 20 deg. at idle w/ out vac.
After turning the adjustment screw full clockwise Idle timing increased to 40 deg. and idle time w/out vac remained at 20 deg.
Your observations were that the engine idles cleaner by a good bit and the engine has more balls at part throttle pulling up inclines.
Separating the two observed results. First the cleaner idle. I think this can be attributed to using manifold vac.
The second result, "more balls at part throttle", can be attributed to the full clockwise adjustment and a working diaphragm in the new vacuum adv. This second result would have still been realized even if the vac adv. was connected to ported vac. only.
Looks like your combination benefited from the changes you made so far.
Good job testing and tuning. Thank you for sharing.
No, not quite. The change from 30 to 40 was me turning the distributor. lol I never took data with the adjusted "as it came", which was "about" half way between CW and CCW. The only data I took was after I turned the vacuum can all the way CW. This expands and "softens" the spring tension so that less vacuum is required to move the diapgragm.
I have to do one more thing to it today and that should get it all sorted out. In my efforts to try and lean it out at idle, (remember how I said it was rich at idle?) I put in some smaller idle fuel restrictors. I'm going to put the stock ones back in, because now I have a stumble immediately off idle and I know that's what's doing it.
I'll try to get it right this time.
After turning the adjustment screw full clockwise Idle timing increased to 40 deg. and idle time w/out vac remained at 20 deg. 20 deg. initial is where you had it set prior to replacing
the vac. adv. So the assumption is the mechanical advance curve hasn't changed.
Your observations were that the engine idles cleaner by a good bit and the engine has more balls at part throttle pulling up inclines.
Separating the two observed results. First the cleaner idle. I think this can be attributed to using manifold vac.
The second result, "more balls at part throttle", can be attributed to either the full clockwise adjustment (previous diaphram was set to mid travel) or a working diaphragm in the new vacuum adv. or a combination of the two. This second result would have still been realized even if the vac adv. was connected to ported vac. only.
One last caveat, the Idle fuel restrictor was also changed. If this was changed and test diven before the vac advance change then the back to back results should be valid. If not we'll wait to hear back after the restrictor is changed back to original to see if it changes results one or two.
Looks like your combination benefited from the changes you made so far.
Good job testing and tuning. Thank you for sharing.
After turning the adjustment screw full clockwise Idle timing increased to 40 deg. and idle time w/out vac remained at 20 deg. 20 deg. initial is where you had it set prior to replacing
the vac. adv. So the assumption is the mechanical advance curve hasn't changed.
Your observations were that the engine idles cleaner by a good bit and the engine has more balls at part throttle pulling up inclines.
Separating the two observed results. First the cleaner idle. I think this can be attributed to using manifold vac.
The second result, "more balls at part throttle", can be attributed to either the full clockwise adjustment (previous diaphram was set to mid travel) or a working diaphragm in the new vacuum adv. or a combination of the two. This second result would have still been realized even if the vac adv. was connected to ported vac. only.
One last caveat, the Idle fuel restrictor was also changed. If this was changed and test diven before the vac advance change then the back to back results should be valid. If not we'll wait to hear back after the restrictor is changed back to original to see if it changes results one or two.
Looks like your combination benefited from the changes you made so far.
Good job testing and tuning. Thank you for sharing.
I guess you can say it that way. I'm unsure adjusting the vacuum canister did it as I adjusted the canister full clockwise FIRST and THEN proceeded to pull the timing up until it stopped idling the engine up. That was at 44 degrees BTDC. Then I backed it off slowly until the engine just began to idle down and that was at 40 degrees, where it is now.
Since the vacuum signal to the vacuum canister is the same at part throttle with ported vacuum as it is with manifold vacuum I was looking at the variables (canister adjustment and diaphram condition) as the potential cause for the change.
That's the part I'm unsure about. Since I've not measured it, I'm not sure it is the same. It could be, some theories say it should be, but it might not be. I think it's all dependent on "how far" into "part throttle" we're discussing and since I've no measure on that, I cannot say for sure.
jos51700
Green Bearing thread connoisseur
This whole thread doesn't make any sense.
You're running manifold vacuum to vacuum advance. In other words, you're forcing the timing to advance at idle and low end, but it's also advanced all the time at the low end. So then you're readjusting the distributor timing at idle?
Vacuum advance is an economy measure, nothing more. At WOT, the vacuum drops, the timing retards. At cruise, vacuum is high, so vacuum advance kicks in advance to insure full burn of a relative low volume of air/fuel.
Hooking up vacuum advance to manifold vacuum is adding in advance, but it's doing it at a point in load/RPM where you should be altering springs on the advance weights, or dialing in the distributor twist.
Adjusting the screw in the vacuum cannister only adjusts the rate of vacuum advance, not the total amount (the analog when using manifold vacuum goes right back to when you should be altering the springs). So when you improperly use tuning vacuum advance settings to improve low end performance (via manifold vacuum), you're going to lose economy and increase ring wear due to fuel wash at cruise, because you're using the vacuum advance in place of properly tuning the distributor and not when it's supposed to be used.
NONE of this talks about the actual vacuum stop limits in the advance canister. If you don't know this number from your canister, then someone elses' experience with tuning using manifold vacuum is potentially very different from yours.
Ping Pong covers the base of this, but the magazine articles were much more in depth.
TLDR: If you're seeing improvements in low end performance by going to manifold vacuum, that means you need tune your timing in the distributor, not use the vacuum advance as a crutch.
You're running manifold vacuum to vacuum advance. In other words, you're forcing the timing to advance at idle and low end, but it's also advanced all the time at the low end. So then you're readjusting the distributor timing at idle?
Vacuum advance is an economy measure, nothing more. At WOT, the vacuum drops, the timing retards. At cruise, vacuum is high, so vacuum advance kicks in advance to insure full burn of a relative low volume of air/fuel.
Hooking up vacuum advance to manifold vacuum is adding in advance, but it's doing it at a point in load/RPM where you should be altering springs on the advance weights, or dialing in the distributor twist.
Adjusting the screw in the vacuum cannister only adjusts the rate of vacuum advance, not the total amount (the analog when using manifold vacuum goes right back to when you should be altering the springs). So when you improperly use tuning vacuum advance settings to improve low end performance (via manifold vacuum), you're going to lose economy and increase ring wear due to fuel wash at cruise, because you're using the vacuum advance in place of properly tuning the distributor and not when it's supposed to be used.
NONE of this talks about the actual vacuum stop limits in the advance canister. If you don't know this number from your canister, then someone elses' experience with tuning using manifold vacuum is potentially very different from yours.
Ping Pong covers the base of this, but the magazine articles were much more in depth.
TLDR: If you're seeing improvements in low end performance by going to manifold vacuum, that means you need tune your timing in the distributor, not use the vacuum advance as a crutch.
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I felt the same way you do until I hooked up two vacuum gauges (of similar calibration), one to manifold vac and one to ported vac. I ran hoses through the window and taped the gauges to the dash where I could see them both then went for a drive. I suggest you not take my word for it and see it with your own eyes and come to your own conclusions.
If you have a carb laying around trace the ported and manifold vacuum circuits with some compressed air or brake clean. Note the position of the ported vacuum circuit source relative to the throttle plate. Please report your findings for others to see.
If you have a carb laying around trace the ported and manifold vacuum circuits with some compressed air or brake clean. Note the position of the ported vacuum circuit source relative to the throttle plate. Please report your findings for others to see.
Can we agree on this. It's a bit of an oversimplification.
1. Engines require a specific timing based on rpm to operate at peak efficiency.
2. Engines require a specific timing based on load to operate at peak efficiency.
3. Engines require a specific timing based on the combination of load and rpm to operate at peak efficiency.
4. The more efficient the engine is running at a given throttle position the more power it makes when at 1/4, 1/2 or full throttle.
5. A mechanical advance is a means to adjust the timing based on rpm.
6. A vacuum advance is a means to adjust the timing based on engine load.
7. If an engine operates at only a single specific rpm and a single specific load the distributor would not need a mechanical or vacuum advance to operate at peak efficiency.
8. So it comes down to application and what is acceptable for engine efficiency.
Sorry about using the number thing.
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I see this a little differently. To me, vacuum advance is not just an economy measure it's an efficiency measure. Two byproducts of more efficiency are better economy and more power. If a motor wants 30 degrees of timing at 3000 rpm under full load wide open throttle and you only give it 26 degrees it will be less efficient and make less power. Same is true for part throttle at 3000 rpm under cruising load if the motor wants 40 degrees and it only gets 30 degrees it won't be as efficient.
Not saying this is the only way or even the right way but this is how I would approach it.
First I would find the timing curve that the motor wants for full load wide open throttle. lets say at 2500 it wants 25 degrees at 3000 it wants 30 degrees and 3500 to 5000 it wants 34 degrees.
Next would be all the part throttle and cruise stuff.
Last would be idle. Lets say for example the motor wants 40 degrees at idle. If I am using mechanical and or static advance to set the idle to 40 degrees then the rest of my curve for full load wide open will be way too high. So to get the 40 degrees that the motor wants at idle I can use manifold vacuum on the vacuum can. I will have to adjust the vacuum advance for rate and the maximum stop accordingly. I also will have to review my timing table at part throttle to make sure the timing added by the vacuum can does not exceed what the motor will tolerate any where in the table. If there is a place in the table where this occurs I may have to compromise my idle timing and or elsewhere to a lower number. This is the limitation of mechanical/ vacuum advance type distributor.
There are distributors/ spark boxes/ fuel injection computers that alow you to do this digitally allowing you to set up curves with out these limitations.
Just for the record different combinations require different curves. Not all motors are happy with manifold vacuum. Some will want ported vacuum ( another name for manifold vacuum that only occurs after the throttle plates have been cracked open a bit). Some engines will be fine with no vacuum advance at all for example old tractors and some race engines. Some are fine with no mechanical advance for example small lawn mower engines and again race engines (sometimes fewer parts that can fail and simplicity takes precedent).
It all comes down to application and what is acceptable for engine efficiency.
Yikes, way too long!
This is a 750 brawler. The picture shows the relationship between the ported vacuum signal hole and the throttle plate. The view is from the top of the carb looking down. The port for the source of lies directly above the throttle plate. Think of the throttle plate as a door blocking manifold vacuum from reaching the port to the ported vacuum circuit. As soon as the door is cracked open it exposes the port to manifold vacuum. From that point as the throttle plates are opened ported vacuum is manifold vacuum. They are the same thing once the door is cracked open. I think the name "ported vacuum" throws people off. It should be call "manifold vacuum that switches off at idle".
Disclaimer. Don't take my word for anything. Go to Harbor freight get a couple vacuum gauges and go for a ride. You can sell the extra gauge to a buddy when your done. Please share your results.
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