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Old 07-26-2004, 03:43 AM
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Ignition timing advance question.

I'm just curious.

How does advancing the ignition effect horsepower, and fuel efficiency?

It seems to me that a lot of the people here keep recommending over and over to connect the vacuum advance to manifold vacuum. But that doesn't seem right to me.

I was told by someone else that "the vacuum advance is supposed to advance the spark during non-load situations for better fuel economy.

If this is correct, then it means that the engine puts out more power when the spark is not advanced, and produces better fuel economy when it is advanced.

From the page about tuning Holley carbs at:
http://www.bob2000.com/carb.htm

"The other type of vacuum connection on a Holley is ported vacuum, which is weak at idle, strong at cruise, and weak at WOT."

This means that there is no advance at idle, lots at cruise, and none at WOT.

idle - no advance - no load
cruise - lots of advance - low load - good gas mileage
WOT - no advance - high load - lots of power

So, if no advance means more power, then why would so many people keep saying that on a high performance engine with lots of horse power one should connect to manifold vacuum?

Basically I don't understand this crap at all.

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Old 07-26-2004, 06:25 AM
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Sometimes a motor with a lot of cam will benefit from full manifold vacuum to the advance can. My motor has no vacuum advance hooked up. It really depends on your combo, try it to the ported source, full vacuum and without and see which works best.
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Old 07-26-2004, 12:29 PM
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I guess messing around with hooking the vacuum advance hose to random ports will help me find the best port to connect to, but it would be nice to actually understand what's going on.
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Old 07-26-2004, 04:56 PM
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The only difference between full and ported is at idle. Both will have no vacuum at WOT.
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Old 07-26-2004, 06:16 PM
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Re: Ignition timing advance question.

Quote:
I was told by someone else that "the vacuum advance is supposed to advance the spark during non-load situations for better fuel economy.
This has nothing to do with performance/power. When discussing advancing timing for best performance most people are referring to mechanical advance, not vacuum advance. Increasing your distributors mechanical advance will usually increase performance as long as you don't go too far and cause preignition/detonation. At WOT under hard acceleration your manifold vacuum is very low and therefore your ignition is relying pretty much on your distributors mechanical advance adjustment. Cruising down the highway under light load most engines can handle extra spark advance without causing preignition/detonation. The vacuum advance provides this extra advance to help fuel economy and improve drive-ability. As soon as you step on it the vacuum drops which reduces total spark advance back to whatever mechanical advance your distributor is set to provide for maximum performance at WOT. That is why you always set up your mechanical advance first, calculating base timing, amount of mechanical advance provided by the distributor, and total timing at WOT. After that is done then you add your vacuum advance to help improve fuel economy and drive-ability/response under low speed acceleration. Hooking up vacuum advance to either ported or manifold vacuum is a personal preference. Hooking up to manifold vacuum will simply provide extra advance at idle, this can help reduce engine temps in a car that has difficulty maintaining temperature. Hooking up to ported vacuum provides no additional advance at idle. Base timing at idle usually reduces emissions and can tend to make a mild camshaft sound a bit healthier 'cause the lower the amount of timing at idle the lumpier the cam usually sounds. If you still don't understand something just ask so we can explain.

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Old 07-26-2004, 07:02 PM
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yup....

Dmorris 1200

You need to copy your reply... I started to write this down a few times and it never came out sounding that good. The only thing i would change is add the word detonation with preignition.

Good job. I hope all the people read this that have been having the vacuum advance problem.

Keith
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Old 07-27-2004, 02:39 AM
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Read this:

http://stevesnovasite.com/forums/showthread.php?t=5220
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Old 07-27-2004, 06:28 AM
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Quote:
Originally posted by jimfulco
Read this:
That's a good link for explanation of the system but the only problem I have with it is the fact that ported/manifold vacuum advance is a preference. These 'manifold is the only way to go' guys need to get a grip. Just because manufactures used ported vacuum to reduce emissions at idle doesn't mean it's bad. How do they know for sure that there weren't other reasons for switching to ported as well? If on ported vacuum my Hydrocarbons and CO's are reduced that must mean the engine is burning the fuel/air mixture more efficiently. Why would that be bad? Why would I want to purposely advance my timing at idle to the point that I'm not burning half my fuel? Maybe before manufactures were forced to look at emission outputs they just didn't know the fuel mixture burned better at idle at base timing. If the main difference between the two methods is advance amount at idle what's the big deal? Mechanical advance is always going to dictate WOT performance/power, that is the important one to set up correctly. Vacuum advance can be set up either way depending on application. I have been using mostly ported vacuum (a few exceptions) for years with no trouble. My current Mustang uses ported vacuum for the advance. I have no cooling system troubles (runs right at 180*), my mild cam sounds tougher without the extra advance at idle, my emissions are lower (efficient burning of fuel), and the second I step on the gas my system is no different than if she was hooked to manifold vacuum advance. So what's the big deal? This whole topic has been gone over multiple times before at this site.

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Old 07-31-2004, 07:57 AM
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dmorris1200, first I'd like to thank you for your long explanations.

jimfulco, thanks for the link. I read the information and believe that I understand it.

Here's my understanding so far of what's going on.

If the engine sparks to early, then the engine will spend more energy compressing burnt fuel. Too early is bad for performance.

If the engine sparks too late, then all the air/fuel mixture won't be burnt by the time the piston reaches the bottom. Too late is bad for performance.

Rich mixtures burn faster than lean mixtures.

A faster moving engine needs to spark earlier because there is less time for the air/fuel mixture to burn.

The mechanical advance is meant to take care of the engine speed issue and provide more advance for a faster moving engine. If we had a perfect carb that always delivered the exactly correct air/fuel mixture at all times and under all conditions, then the mechanical advance would be all that we'd need.

But, a richer mixture delivers more power than a leaner mixture. At the same time a leaner mixture gives better fuel economy and less emissions.

Therefor a perfect carb actually delivers a richer mixture under load when power is needed than when not under load. And a leaner mixture when at idle and cruising.

Race car drivers don't care a bit about idle and cruise, emissions or fuel economy, I do.

Because of this, under these leaner conditions, to achieve optimal performance I need vacuum advance.

Now, for my particular situation, I have a 1977 Chevy C-20 with the AIR system. Because of this, my vacuum advance is connected to a ported vacuum on my carb. Maybe I would get better performance if I had an engine with the vacuum advance connected to manifold vacuum, but on my particular engine my mechanical advance is designed for my vacuum advance being connected to a ported vacuum port. Therefor if I were to take my advance hose and either disconnect it, or connect it to manifold vacuum, I would screw up the performance of my engine. In order for me to switch my engine over to manifold vacuum, I need to get a different distributor that is meant to be connected to manifold vacuum, or adjust my current mechanical advance if that's possible.

If I were to switch out my distributor with one meant for manifold vacuum, then I would put out different kinds of emissions that my AIR system wouldn't be able to handle, and therefor I would be putting out a lot of emissions and would need to install a catalytic converter.

An issue discussed in the article linked to by jimfulco is performance. My engine currently does not run like crap or lack performance. But it does lack good gas mileage. So far I'm figuring that I'm getting about 9 miles per gallon. But, I have a 1984 Mercury Cougar which also has a 350ci engine and also seems to run fine. Same size engine, but about 15 miles per gallon. Could this difference be because it has a CAT instead of an AIR system?

I have not had any problems with my Cougar yet so I don't have a clue how it's emission systems work, or whether it's vacuum advance is connected to a ported or manifold vacuum. I'll be checking that out soon just for curiosity and also now that I've learned a lot more about it, I'll actually know what I'm looking at.
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Old 07-31-2004, 09:14 AM
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Just a few more thoughts...

Switching the vacuum advance from ported to manifold vacuum without any other modifications to the distributor will have minimal affect on performance. Mostly it will just alter your idle characteristics.
Example Distributor:
Base timing = 10*, mechanical advance = 24*, vacuum advance = 12*
At ported vacuum source this distributor will provide these characteristics:
Idle = 10* advance
cruising light load = up to 46* advance (10 base + 24 mech. + 12 vac.)
WOT = approx. 34* advance (10* base + 24* mech.)
At manifold vacuum source this same distributor will provide these characteristics:
Idle = 22* advance (10* base +12* vac.)
cruising light load = up to 46* advance (10 base + 24 mech. + 12 vac.)
WOT = approx. 34* advance (10* base + 24* mech.)

As you can see almost identical timing potential except for idle. No real performance gain just added timing while idling. That is why distributor curving is important. Lets just say for example you have an engine that needs 36* advance at WOT and 48* at highway cruising. With a distributor that provides 30* of mechanical advance and 12* vac. advance your base timing would need to be set at 6*BTDC. With manifold vacuum advance your idle timing would still only be approx. 18*BTDC, not too bad...
Idle = 18*BTDC (6 base + 12 vac.)
cruise = 48*BTDC (6 base + 30 mech + 12 vac)
WOT = 36*BTDC (6 base + 30 mech)
Now we switch to a distributor that provide 24* mechanical advance and 12* vacuum advance on the same engine. Still needing 36* at WOT you now will need your base timing set at 12*BTDC to achieve this. With manifold vacuum advance your timing would look like this:
Idle = 24*BTDC (12 base + 12 vac.)
cruise = 48*BTDC (12 base + 24 mech + 12 vac.)
WOT = 36*BTDC (12 base + 24 mech)
With ported vacuum advance:
Idle = 12*BTDC (base timing)
cruise = 48*BTDC (12 base + 24 mech + 12 vac.)
WOT = 36*BTDC (12 base + 24 mech)
As you can see the main difference is in idle timing and the smaller amount of mechanical advance your distributor provides the higher your base timing must be to achieve the correct WOT advance. With a high base timing you may not want any additional timing at idle, unless you need it for reasons mentioned (cooling problems, radical cam, etc.). I run a small block Ford which takes about 39* timing at WOT. My distributor provides 24* mechanical advance and 12* vacuum advance. To achieve 39* at WOT my base must be set at 15*BTDC. I do not need 27* advance at idle so I use a ported signal to my vacuum advance. These are my numbers:
Idle = 15*BTDC
cruise = 51*BTDC (15 base + 24 mech + 12 vac)
WOT = 39*BTDC (15 base + 24 mech)
At these settings my car runs terrific. I can sit in traffic for hours with no trouble, and I have been caught in some fairly long parades just creeping along at idle. If my distributor provided more mechanical advance it would force my base timing to be lowered in order to maintain 39* at WOT. If my base timing was reduced enough I would probably benefit from using manifold vacuum for my advance to increase my timing advance at idle. There are soooo many different things can can factor in to whether or not ported or manifold is better. I don't see how anyone could just generalize and say "this way is the only way". Engine build, car type, cooling system efficiency, type of driving, etc. all play a role. No matter what you do I hope you at least understand why you are doing it and not just doing it 'cause some tech article said to do it that way.

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Old 07-31-2004, 12:46 PM
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I guess I'm still confused then.

I understand that an engine that produces less idle vacuum would require a different vacuum advance mechanism to generate the same advance as another engine that produces more.

And I understand how a different air/fuel mixture could burn at a different rate.

I would guess that maybe the compression might effect the burn rate as well. I would guess that higher compression would burn faster than lower compression.

Do higher compression engines require less timing advance than a lower compression engine?

dmorris1200, from what you're saying, my engine should run better on manifold vacuum with no other changes. The reason I'm saying this is because currently my idle timing is the initial timing of 2*.

I don't have a fancy timing light with the knob, so I don't know the timing under other conditions, but I do know that it advances a lot when I rev the engine, because I can see it moving far away from the mark as I rev it.

I would be willing to guess that the other two numbers are similar to your numbers because I believe that under those conditions the my mixture can't burn that much faster or slower than yours.
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Old 07-31-2004, 03:11 PM
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You really need to know exactly what your distributor is doing. Why are you at 2*BTDC base timing? Your answer should be "because I have 'x' amount of mechanical timing giving me 'y' amount ot total timing at WOT". If your distributor is only providing 24* of mechanical advance at WOT you only have 26* of advance. You can't just say 'my timing really jumps when I hit the gas'. With mechanical and vacuum advance applied it should put you in the high 40's of advance, that is a lot of movement. You need numbers and mechanical comes first. If you can't find out how much mechanical/centrifugal advance your distributor has you need to do some tests. How far do the timing marks go on your engine? You need to rev her without your vacuum advance hooked up and find out exactly how much the mechanical advance is advancing. To run my base timing at 2*BTDC I would need a distributor that advances 37* mechanically, that is allot. If that was the case I would probably use manifold vacuum for my advance can but the mechanical needs to be correctly calculated first. If you are really into learning timing curves and outcomes I would suggest investing in a decent timing light, they are not that expensive and are a great learning tool.

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Old 07-31-2004, 03:27 PM
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Some 70's Mopar 318s were designed to run a 2o BTDC initial. This is because the distributor's mechanical advance was increased to compensate. With low initial advance throttle response is greatly diminished.

I like to run as much initial as I can. The limit to how much you can run is fairly easy to watch for. If the starter kicks back on start up, ease up on the initial. If there is LOW Rpm detonation, ease up a little.

With more centrifugal, (distributor) advance you reduce the chance of the low end detonation, and ease start ups. Race engines don't need these features, which is why racers can lock-out their distributor advance curves.
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Old 07-31-2004, 04:07 PM
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The 2* initial advance is because that's what the smog sticker on the air filter cover says.

I've never fiddled with timing at all to see what the symptoms are of having the timing off, but when I got the truck and took it in to get it smogged, the smog guy said that it was at 28* initial timing and that I had to set it to 2* or else it will fail the visual.

I didn't watch close enough to see if he disconnected the vacuum hose. If he does, I don't know if the machine tracks the timing to make sure the advances are functioning within specs.

So, it's very possible that if I took it in to get it smogged with the vacuum advance connected to manifold vacuum it would fail with the machine claiming that the advance is not functioning properly.

The truck is currently very easy to start, but it was also pretty easy to start when it was at 28*. But, when it was at 28* the choke was stuck and the fast idle cam was upside down.

I used to have a really nice timing light with a knob on it, but I'm not sure right now where it is, so I've been using this really crappy one I got for free. I'm definitely going to look for the good one and start messing around to see what happens.

There is a slight delay before full power is experienced when I floor it from driving slow, but that seems to have improved a bit since I added a little tension to the springs holding the secondaries closed.

But at the same time, I'm experiencing this huge bog when I do the same thing when the engine is cold.

Since I didn't have this truck before it was screwed up, I don't know how it's actually supposed to perform. So I don't know if there's still a problem or not.
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Old 07-31-2004, 05:03 PM
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The factory advance settings are where it needed to be with all things original to operate one specified octane gas (probably regular), carb settings, emissions laws, etc. Most likely you could pull some more advance out of her but not much without other modifications to fuel delivery, etc.. Like lluciano said, you could try bumping her up little by little and see how she performs. Without a timing light and a little work on your part you still may be able to pick up a little extra power but you robbing yourself of the reasons behind the power. Start playing with a light and you can say for sure "I max out performance at 'x' total degrees of advance" and you will better understand the relationship between mechanical advance vs. vacuum advance. Also with a timing light and working knowledge of what you are doing you can always adjust your settings when you need to for an inspection sticker and then readjust for regular driving (not that I would advocate any adjustments that would by-pass state emission laws).

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