Author Topic: Timing and vacuum advance - theory and practice?  (Read 14836 times)

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Yellow Truck

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Timing and vacuum advance - theory and practice?
« on: October 22, 2016, 07:31:31 PM »
I admit that I've been reading up on the subject and a little knowledge can be a dangerous thing, but I'm seriously confused about timed and manifold vacuum for the timing advance.

David Vizard in his book makes the case that you need a leaner mixture at idle and at cruise, and that a leaner mixture needs more advance to complete the burn and create maximum combustion pressure at the appropriate time ATDC. We can leave cruise out of it because at cruise there will be some throttle engagement and timed vs manifold will be less significant.

Running the vacuum advance off a timed port means that the advance at idle is just the initial timing. Since one of the goals is to get a reasonably lean mixture at idle, it stands to reason that to get as smooth an idle as possible (and best vacuum), if the vacuum advance is run off the timed port it would require more initial timing advance to compensate for the fact that the vacuum advance won't kick in until partial throttle is applied.

It also seems that if I set the initial timing lower, and ran the vacuum advance off the manifold port, it would have the advance it needs at idle without having to wait for the plates to open to engage the advance.

I've read an argument by a retired GM engineer where he argues you should NEVER run the vacuum advance off a timed port. I'm sure you all have seen this argument before. I'm interested in what the experts do, but also why they do it.
1969 F100 4WD (It ain't yellow anymore)
445 with BBM heads, Prison Break stroker kit, hydrualic roller cam, T&D rockers, Street Dominator Intake with QFT SS 830.

Paul.

Drew Pojedinec

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Re: Timing and vacuum advance - theory and practice?
« Reply #1 on: October 22, 2016, 08:29:07 PM »
Learn how to make your distributor do what your engine needs.

ericwevans

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Re: Timing and vacuum advance - theory and practice?
« Reply #2 on: October 22, 2016, 09:13:34 PM »
To expand on what Drew said, I'd get the base timing and advance curve where the engine likes it with the vacuum advance disconnected completely.  Once that is right then add vacuum advance and the engine will let you know what it wants.  If you have an adjustable vacuum canister that will throw another variable in.  My current combination doesn't like vacuum advance at all.  At idle if you use manifold vacuum it would have random misses, about 1 every 5 seconds.  At cruise you could hear concussions in the exhaust.  So I currently run no vacuum all.
Eric Evans

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Drew Pojedinec

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Re: Timing and vacuum advance - theory and practice?
« Reply #3 on: October 23, 2016, 06:34:57 AM »
Ok, how I do it......


First,  I make an educated guess where the total and initial need to be.
For my last build I figured 18 initial and 32 total.  In my case, it's a good baseline, prior to testing.  The small amount of advance is due to my BBM heads, which have a faster burn chamber.

Here is the regular slot,


So, being I use a Duraspark, I weld up the advance slot.


I then use a grinding wheel to cut a little slot, then I clean it up with  a die grinder carbide bit and some sandrolls.


Since I was only looking for a little advance, I ground a small slot to start with.
.300 or .350 I forget


Plug vac advance at this time.
I install the springs I think I might need to get the advance in when needed.  Things to consider, vehicle weight, gearing, convertor, etc.  Think about how loaded the engine must be to get things rolling.  In my case I have a loose convertor and didn't see the need for the advance to come in too soon, so I aim for 2800rpms.  My car cruises around there so I figure it's a good starting point.  Plus with a duraspark you can bend the tab for final adjustment.

I then install the distributor and fire the engine up.
First thing I'm looking for is how the vehicle idles at the initial advance.  Hot start should also be a consideration, tho with a mini starter, I was pretty certain I could go a little either way.
idles at 18 just fine, maybe it likes 20, maybe it prefers 16.  Figure it out.

Once I have that number pegged.  I get the total to be where I want to be.  Go for a drive.  Think you need more?  add more, think you need less, add less.  Want it to come in sooner, make it so.  I don't worry about initial at this time.
Once I get a good idea of my total, I take the distributor apart and reweld or grind (it's easier to grind) to get my initial where I want and total where I want.
Sometimes with me, this takes 3-4 times of reinstalling/taking apart (I do it on the engine) to get it all perfect.

At this time, in order to figure and get a baseline for vacuum advance I run the engine.  Vac advance isn't plugged, it's attached to my handheld vacuum pump.
At max advance/cruise rpm I use my vacuum pump to bring the advance in as needed.  If I need to change what I see for a baseline I do so.
Once again, hook everything up and go for a ride.  like it?  don't like it?  make adjustments.

Hope that was a little more specific than my last post.
and sure, there are better ways to do with with a distributor machine.... but I don't have one.

Good luck
Drew

Yellow Truck

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Re: Timing and vacuum advance - theory and practice?
« Reply #4 on: October 23, 2016, 11:15:21 AM »
Drew,

That is a very good lesson in how to set up the full timing profile, I am glad I didn't post a smart ass response last night! It doesn't address metered vs manifold vacuum.

I am running BBM heads as well, and I'm running a stock Autolite with Petronix, and on metered vacuum in it's short period where it was working well it seemed to like around 18 degrees of initial advance. I had mine done by a local specialist (who has the machine) and he reported that he had it set up for 13 degrees of initial advance for total advance of 34 degrees, with mechanical advance all in by 2,950.

He said that it would add about 10-12 degrees of vacuum advance when hooked up. So far I've been running mine off the metered port, so the initial advance is all it sees at idle.

I think if I decide to get into adjusting the curve etc. I'll change out to a modern electronic distributor and control box. I don't have the tools or skills to do what you are doing!
1969 F100 4WD (It ain't yellow anymore)
445 with BBM heads, Prison Break stroker kit, hydrualic roller cam, T&D rockers, Street Dominator Intake with QFT SS 830.

Paul.

Drew Pojedinec

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Re: Timing and vacuum advance - theory and practice?
« Reply #5 on: October 23, 2016, 01:10:18 PM »
I am glad I didn't post a smart ass response last night! It doesn't address metered vs manifold vacuum.
I had just worked an 18 hour day in an engine room doing stuff like pulling an 1800lb gear off a shaft with 2 rosebuds and 8000psi hydraulic pressure, etc.  I was tired and my answer was too brief.  Sorry.

Quote
I think if I decide to get into adjusting the curve etc. I'll change out to a modern electronic distributor and control box. I don't have the tools or skills to do what you are doing!
I love the Duraspark, it's super simple and reliable.
The only tools I used for this was a mig welder (you could probably use JB Weld), an angle grinder, a die grinder, and a timing light.

As far as manifold vacuum vs ported, my thoughts are this:
Manifold vacuum is a bandaid for improper initial timing.

If your initial and your mechanical advance are correct you shouldn't need vacuum advance but in one place.  Lightly loaded cruising.

I'm no expert, i just tinker with these things really.  Still finding the perfect "happy place" for my engine (for me sometimes it takes a few hundred miles of tinkering before I get something perfect).

Drew
« Last Edit: October 23, 2016, 06:11:11 PM by Drew Pojedinec »

Drew Pojedinec

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Re: Timing and vacuum advance - theory and practice?
« Reply #6 on: October 23, 2016, 01:25:13 PM »
So, thinking about it.....
easiest thing for you to do, would be to find a bushing for the advance pin.

There is a lil metal piece that sticks up, the advance arm that I have pictured above is pretty much the same as your Autolite and the size of the slot is the amount of mechanical advance you get.
Well, you can use a rubber bushing on that pin to limit mechanical advance.  This would allow you to add more initial with your total staying the same.  I have a couple of those laying around, and I'm sure your local guy does too.  Or take the distributor back to him once you establish where the initial is the best for your engine (I'm sure it's more than 13)

turbohunter

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Re: Timing and vacuum advance - theory and practice?
« Reply #7 on: October 23, 2016, 01:44:31 PM »
X3 on the no vacuum advance.
I tried to make it work on my truck but it didn't like it.
Im sure I could have gotten it to run well but didn't care about spending the time to when it already runs great mechanically.
Then there's the ported/non ported debate. Hurts my little brain. ;)
Marc
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Yellow Truck

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Re: Timing and vacuum advance - theory and practice?
« Reply #8 on: October 23, 2016, 07:52:03 PM »
Thanks for the very good replies. I am VERY glad I didn't post a smart ass reply.

1969 F100 4WD (It ain't yellow anymore)
445 with BBM heads, Prison Break stroker kit, hydrualic roller cam, T&D rockers, Street Dominator Intake with QFT SS 830.

Paul.

FirstEliminator

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Re: Timing and vacuum advance - theory and practice?
« Reply #9 on: October 23, 2016, 10:25:25 PM »
   Currently, I run all of my vehicles on ported vacuum. Often, I've thought about trying to run the vacuum advance direct to manifold. Here is why I ponder it: A while back I was working on a 90's T-bird. While watching data on the scanner one of the pids would show timing. The timing fluctuated around at idle. It would go up to 25 or so. So, the computer would move it around to accommodate a smoother/cleaner idle. Have you noticed when you plug in the vacuum advance to manifold vacuum the engine idles higher? It can tolerate more advance and it seems to be  more optimal of a setting.  With manifold vacuum engaging the vacuum advance at idle means a higher rpm, you'd compensate by closing your throttle to be at the same idle rpm. Seems like you'd have better efficiency at idle with manifold vacuum. So why not just set your initial timing 10 or so degrees higher? Because with too much initial it will kick back when hot. I set my initial as high as I can so it doesn't kickback while cranking on a hot day after a 10 minute hot soak. I usually find this to be somewhere around 15 degrees---in a street set-up with an RV type cam. Why would it tolerate more timing at idle than cranking? When cranking there is more time for charge to fill the cylinders. Idle is about 300% faster than cranking, so charge density is less at idle because with the same throttle opening.   I believe ported vacuum was used to reduce certain emissions at idle. Oxides of nitrogen I think.

   One thing I can't see is how a street driven engine would run better with no vacuum advance. The optimal timing at part throttle operation is going to be different than full throttle. Race cars don't need vacuum advance because they either idle or are WOT. Race cars don't care about part throttle efficiency and emissions. Load, environment, and throttle are all variable with a street engine. The vacuum advance helps accommodate optimal tuning for this application------if the vacuum advance is tuned correctly. I find vacuum advance tuning is usually between 8 and 16 degrees. If you plop a vacuum advance on that pulls 25 degrees because it was set to run on a 1975 EGR engine with 7.7:1 compression would be like swapping around guitar strings and expecting it to sound good when playing the same old chords.

     So, why do I still run ported? I don't know, I guess just 'cause.  But, I'm talking myself into it.  Think I'm going to try it to see if there are any drawbacks and if the theoretical benefits actually exist.

   I have a different method of tuning advance range and curve speed in the Duraspark. I wrote about it in this posting: http://fepower.net/simplemachinesforum/index.php?topic=308.msg2388#msg2388   The procedure may not be as complete as I'd like. But the gist of it is there. Someday I'd like to detail my set-up a little bit better.
     

   My life is overloaded with work at work, work at home, work on my house, work raising kids, work helping friends and once in a blue moon work on my car projects. So, if you can explain to me in detail why my theory won't/can't/doesn't work, please tell me. I don't have enough time left in my life to do all I want to once. So, doing anything twice really sucks.

   thanks,
     Mark
 Berkshire Transmissions
North Adams, Massachusetts
     
Mark
Berkshire Transmissions
North Adams, Massachusetts

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chris401

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Re: Timing and vacuum advance - theory and practice?
« Reply #10 on: October 24, 2016, 06:56:39 PM »
I run ported vacuum advance for more engine braking. After you set your base and mechanical you can safely test and set your vacuum advance at idle with a mighty vac and a timing light.
« Last Edit: October 25, 2016, 02:28:18 PM by chris401 »

BattlestarGalactic

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Re: Timing and vacuum advance - theory and practice?
« Reply #11 on: October 25, 2016, 11:53:05 AM »
I've never had any luck putting any vacuum advance on anything I own.  Even my stock '89 F150/302.  I converted it to carb, put Duraspark ignition on it.  I tried both manifold and ported and it didn't like either.  Ran like crap no matter what I did.  Left it all unhooked.  All the hot rods are mechanical since they don't have enough vacuum to worry about.

Like mentioned I'm sure if you spend some quality time tinkering you can make it work.  Me?  Nope.  Does it idle?  Yes.  Does it go like stink at WOT?  Yes.  Perfect, no other work needed.  ;D
Larry

Autoholic

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Re: Timing and vacuum advance - theory and practice?
« Reply #12 on: October 27, 2016, 11:28:41 PM »
This is one of those loaded questions, where you will get 1,000 different and specific opinions, all circling the same drain. There will likely never be consensus as there aren't two identical, old school engines out there. Just changing a few things from stock in a group of 100 owners can result in tens of thousands of different variables, and that's assuming the engines were all closely built to spec at the factory. Some food for thought, some engine management systems measure manifold pressure (MAP sensors) to help control timing. Something else to think about, a lot of vac cans have advance and retard functions.

I'm not an expert, nor am I a veteran of seeing what works. The theory behind manifold advance, is that it will detect idle and idle circuit only cruising load reasonably well. Many if not most carbs are still on the idle circuit under 2,500 RPM's. When only the idle circuit is being used, or you are in transition, port vacuum will be zero or poor. When you suddenly let off the go fast pedal at 5,000 RPM's, port vacuum will drop to zero. Port vacuum cannot differentiate between starting, idle, near idle cruising and suddenly going off throttle. There most definitely will be a difference in manifold vacuum in relation to RPM as throttle position changes. 5,000 RPM at WOT is different from 5,000 RPM off-throttle. This is where manifold vacuum comes into play and remember that it is still used today in some EFI systems (self learning EFI often uses it). Using a lot of initial timing and zero manifold vac advance means that the engine's timing won't differentiate between WOT and off throttle at a specific RPM, and what the load is. Using ported vacuum will give you advance at WOT and no advance at off throttle, but it mostly gives your timing an indication of throttle position when you think about it, aside from starting when you really aren't drawing enough air in to register a lot of vacuum. It really depends on how you setup your timing. The point about too much initial timing when doing a hot start is also a very good point to make.

Again, I'm not an expert but I do know that there is no one way to setup your timing, regardless of engine. It depends on what your engine needs to run happy for your application. Any blanket statement out there about which is better, can easily turn into bad advice. Again, some swear by manifold vacuum, some swear by ported vacuum, some swear by using one to advance and the other to retard, and some swear by mechanical only. How many ways can you skin a politician? Do a crap ton of trial and error and see what works for you, no amount of reading will provide you that insight into your vehicle.

Something else to think about. When you are engine braking, advancing the spark will cause the combustion to help resist the engine from rotating. This can help slow down the engine. Ported vacuum won't help with advancing the spark for engine braking. Manifold vacuum will, because suddenly going to zero throttle at say 4,000 RPM will register a lot of vacuum. Food for thought, just be careful of how early the spark happens. Modern EFI systems control spark and fuel injection duty cycle involving RPM, load and throttle position, in some form or another, for various situations, including engine braking. How the engine's ECU computes load depends on the system, utilizing MAP, MAF or both (Evo X used both).


In an over-simplified nut shell...

Manifold indicates load, port indicates throttle position.
« Last Edit: October 28, 2016, 02:05:49 AM by Autoholic »
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Yellow Truck

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Re: Timing and vacuum advance - theory and practice?
« Reply #13 on: October 29, 2016, 11:34:19 AM »
I have been re-reading some of these posts and mulling. I had some issues (described in my other thread http://fepower.net/simplemachinesforum/index.php?topic=4350.0) where it suddenly began running like crap after changing the headers, plus I had noticed some strange movements in my initial timing from start to start (jumped from 16-18 to 28 over night).

Took the distributor apart and had a good look and all seemed fine but I noticed some oil on the intake, and the coil had puked. Had a new Flame Thrower, so put it on, swapped out the plugs (it seems sensitive to fouled plugs and mine were a bit dirty with only 2 hours of running time). Net result was got it restarted after putting it back together and it ran very well again.

We messed with the distributor and timing, moving from a timed port to manifold, and with about 10 inches of vacuum at idle, we had 13 degrees of initial advance, 21 degrees with the vacuum pot connected to the manifold port, and at 3,500 we had 41 degrees advance. So far so good (I think), but then we tapped on the vacuum canister to see if reacted. Advance at idle dropped to 3 to 5 degrees and the engine stumbled badly. We adjusted the connecting rod as it seemed to be bent a little and on a strange angle. Haven't run it since because I need to adjust the lifter pre-load (had a tick, and in getting rid of it I over did it and have a hanging valve again).

Any suggestions on a replacement vacuum canister?
1969 F100 4WD (It ain't yellow anymore)
445 with BBM heads, Prison Break stroker kit, hydrualic roller cam, T&D rockers, Street Dominator Intake with QFT SS 830.

Paul.

machoneman

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Re: Timing and vacuum advance - theory and practice?
« Reply #14 on: October 29, 2016, 12:00:23 PM »
I'd try a Rock Auto or NAPA supplied cannister although they may all be Chinese made at this point. Be sure it is an adjustable unit with a small Allen key through the nipple....usually clockwise for less advance, counter for more.

Summit btw has Drake's units but I'm not sure they are for FE's or adjustable.

https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#tbm=shop&q=adjustable+vacuum+advance%2C+Ford+FE&spd=1164009194894188099
 
« Last Edit: October 29, 2016, 12:02:54 PM by machoneman »
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