That wouldn't change the parts of the flow that we're concerned with.

Naybe some day, but for now, what I've done shows my point.

So the advance port is just to the left of the CENTER of the plate, but I think that's what you meant. I'm not sure that I understand the rest of your question.

 

Im running no mech advance and vac advance only and it seems to be working ok

 

*If that is the case then could you please show that with your flow modeling software? At this point I'm not sure what exactly we're concerned with! Are you standing by your statement that the throttle plate and its position relevant to the advance port has little to no effect on the vacuum advance?


**I thought we were trying to come to a conclusion here and get to the bottom of this, not prove anyone's point right or wrong.


***Yes, I meant left of the center of the pivot for the plate and/or left of the plate in its naturally closed state. As for the rest of the question, I'm not sure what's lacking but I could try to rephrase it if that would help. However, if you are saying that the throttle plate has no influence on the vacuum at the advance port whatsoever then I'm not sure that would be helpful.

 

What I've shown is that the position of the advance port allows it and the plenum to have nearly the same pressure. There's no need to run more simulations to prove that point.

At no time did I say that the throttle plate has no effect, in fact, I've said the contrary a couple of times. There is a difference between "no" effect and relatively little effect.

 

Alright guys, I haven't been posting much info lately because I have been doing some "hands on" research... Here is what I found.

With the vac advance line feeding off the 2nd nipple from the front: Tried to verify timing, but found that it was advanced at idle. Disconnected the vac line and the timing dropped back to line up with the mark, but idled badly due to the vacuum leak I had created by removing the line from the nipple. So...

I removed my cap from the 1st nipple from the front, and placed that cap on the 2nd nipple. This sealed the vacuum leak, which leads me to believe that the 1st nipple is ported vacuum and the 2nd nipple is manifold vacuum. Then I connected the vac advance line to the 1st nipple from the front. The results....

After verifying that the timing was correct, and was not being advanced at idle due to the vacuum advance system, I took it for a drive (actually, I've been driving it like this for three days now). The car feels very different now. Specific changes are:

1. No more stumbling when coming off idle (first depressing the gas pedal). I used to have to "blip" the throttle when starting out to avoid this stumble/stall symptom.
2. No more "hunting" at idle. Idle is now smooth and not jumping around like before.
3. Car feels like it has much more torque at lower rpms (around 3k). The car just feels much more "peppy" or "zippy". Just more lively all around. Acceleration from 55 in 5th gear is noticeably improved.
4. Roughly a 2mpg increase in fuel economy.

Overall, the car runs better and feels better. I would welcome anyone with their vac feed attached to the 2nd nipple to try this and report their results....

 

Great info, and real-world results. So I guess that means the rats nest tutorial was wrong then?

This thread is really making me wonder about why they recommend just leaving vac advance open to the air with an aftermarket carb install like my dcoe. Actually that's what made me start following it, lol.

 

After reading this, I am definately going to be hooking my vac advance back up. I too have to blip the throttle in order to get passed the slight stumble right off idle.

Kentetsu = THE MAN

 

im about to go outside to walk to my neighbors shop and work on my SA. ill be sure to stop by FB and swap the lines. ill post what happens. does timing need to be reset?

 

Kentetsu - It sounds like you found the vacuum advance port alright. Is that the same port that's shown in the shop manual or under the hood? Did you try any of the other ports besides just the two?

How did you measure the fuel economy change? Without adjustments to the carb, I don't see how you'd get any kind of improvement. You'll get better performance because your pressure pulse is at the right time, but it won't change the amount of fuel being pulled through the carb.

 

Im kinda confused. Pics always help me

 

You know, I completely forgot about the sticker under the hood. lol. I've been looking for my Haynes manual, but I think the wife borrowed it to prop up a broken table leg or something....

As far as improved economy, all I can tell you is that there is a big difference in how far I need to depress the gas pedal in order to cruise down the road. Even on the freeway doing 80, I barely had to give any throttle to maintain my speed.

In addition to this, there is such an increase in available torque that I find myself not needing to open up the secondaries or downshift at times when I would normally have to. Can you imagine driving at 55 in 5th gear, and passing somebody without downshifting or opening the secondaries?

Seriously, my car performed great at wide open throttle before I did this. But anything less than that is where I was missing out (although I did not know it). Now I don't need to use WOT all the time because just a little pressure on the pedal results in a rush forward. So my theory is that the improved timing is making better use of the available fuel. Give it less fuel, but a better ignition point to get better performance. I think the gain in economy is a combination of that plus the fact that I'm not hitting the secondaries all the time now.

Of course, I do have other mods as well so I can't gaurantee that everyone's car will behave the same way. All I can do is post my results and thoughts on the matter. But, now who's going to try this and report back? A little secondary confirmation would be helpful here....

 

So are you comparing to being plugged into the plenum port or not plugged in at all?

 

Wait...so kentetsu, you changed the timing or just capped off a vacuum hose and switched another vac hose to a different one??

 

The comparison is between the two nipples. I was originally plugged into the 2nd from the front, then moved it to the 1st from the front. All I did was verify the timing after that, but didn't need to change anything.

 

From what you are saying it appears that you believe timing won't affect gas mileage? How could it not? If spark is coming at the optimum point and burning all the fuel as it is supposed to then he would feel an increase in power and also realize some mpg efficency. With incorrect timing there will be an inefficient burn, thus more fuel would be required to make the same amount of power. In the thousands of revolutions required to burn up 1 gallon of gas, I don't think an extra 2 mpg is unreasonable at all to imagine as a cumulative effect of correct timing.

 

The reason I say that is because the difference between the plenum port and the advance port is advance at idle. Everything else should be the same.

 

You make a good point though, more power will change driving behavior.

 

No, not advance at idle but advance at the start of throttle increase. As soon as you start to increase the throttle (at lower to mid rpms), the timing advances to give optimum power and torque.

With the advance hooked up to manifold vacuum, the timing starts out advanced so that when you tip in the throttle it has no more to give.

 

All the vacuum advance can do is react to changes in the vacuum signal in the location we've been talking about. That signal should actually be the same whether it's a little downstream of the throttle or just upstream as long as the throttle's not closed.

 

The key lies in your last sentence "as long as the throttle's not closed". If it was hooked to manifold vacuum (and thus behind the plate), there would be advance at idle, or at any other time when the throttle is closed. Since the port is located on the front side of the plate there is no advance with the plate closed.

 

Yes, but driveability and fuel economy shouldn't be affected. Once you're driving down the road (assuming you don't have the throttle totally closed), the advance is the same whether it's hooked up to the advance port or the plenum port.

 

I'm not sure you're understanding the differences between the effects of manifold vacuum and venturi vacuum.

Manifold vacuum decreases in relation to the throttle opening (which would cause the timing to begin retarding if its plumbed to manifold vacuum).

Venturi vacuum increases in relation to the throttle opening (which would cause the timing to begin advancing if plumbed to venturi vacuum).

You wan't the timing to advance as you open the throttle. That's what gives it the "kick in the pants" and provides more efficient burn of the fuel. If the timing is retarding with the opening of the throttle, then you'll begin to lose power as you open the throttle.

 

There is no such thing as venturi vacuum when talking about ignition vacuum advance. It's either ported (just above the throttle) or manifold vacuum. The only difference is whether there's advance at idle.

Here's yet another source for you to ignore.

http://en.wikipedia.org/wiki/Ignition_timing

Vacuum Timing Advance

The second method used to advance the ignition timing is called vacuum timing advance. This method is sometimes (depending on the engine manufacturer) used in addition to mechanical timing advance. Usually, this method is used to increase fuel economy. Vacuum advance works by using a vacuum source to advance the timing at mid engine load conditions. Vacuum advance is diminished at wide open throttle (WOT), causing the timing advance to return to the base advance in addition to the mechanical advance.

The most common vacuum source for vacuum advance is a small port located in the wall of the throttle body or carburetor adjacent to but slightly upstream of the edge of the throttle plate. In carburetors having primary and secondary throttle plates, the port is located in the primary. The effect of having a hole here is that there is little or no vacuum at idle or at wide open throttle, with the vacuum signal peaking at part throttle opening.

On some vehicles, a temperature sensing switch will apply manifold vacuum to the vacuum advance system when the engine is overheating. The purpose of this is to increase engine speed to turn the engine fan faster to try to overcome the overheating.

On some vehicles, some form of switches may be used to prevent vacuum advance under certain conditions such as when the engine is cold or depending on what gear the vehicle is in. This is usually because these are necessary to comply with emission standards.

 

Most of us understand the difference between venturi and manifold vacuum. The problem is you are working on a misconception and don't understand why the vacuum advance is designed to increase with manifold vacuum and decrease with load. You will never understand how the vacuum advance system works if you first don't understand why.

Let's get centrifugal advance out of the way first. As the engine rpm's increase there is less time to achieve a complete burn so the timing is advanced. We can all agree on that.

Vacuum advance is completely different. At low engine loads, such as idle and part throttle, combustion chamber pressures are low. As you accelerate and load increases combustion chamber pressures increase. The higher the combustion chamber pressure is the faster the flame front travels through the mixture. This means you need less timing advance to achieve a complete burn! In addition, having too much timing advance when combustion pressure increases can cause knocking. This is why you retard the timing on forced induction engines as boost (load) increases.

 

Listen to REVHED, he's exactly right. Read the articles that have been posted in this thread, they're all saying the same thing, even the one dj posted. Look at the CFD plots that I've posted. There's no venturi involved.