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Vacuum advance or not?

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handsofstone View Drop Down
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    Posted: 14-April-2020 at 1:28PM
I have seen lots of pics on the facebook groups that show distributors having no vacuum advance.  What is the deal with that? Any benefit to it? Is it merely a fuel mileage issue? I picked one up for short money without an advance and it is probably going to be another paper weight.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72FordGTS Quote  Post ReplyReply Direct Link To This Post Posted: 15-April-2020 at 5:53AM
I personally would never run a distributor without a vacuum advance.  What is the advantage to removing it on a street car?

Here is what David Vizard says about them:

Originally posted by David Vizard David Vizard wrote:

Too many street rodders see that the pro racers use distributors without vacuum advance and figure if it’s good for the winning car it’s good for them. Unfortunately this is not so. Although a drag race car may not realize much in the way of an advantage, none of it performance related, it’s about the only type of race car that falls into this category. Here’s how the puzzle fits together.

When you back out of the throttle the manifold vacuum goes up. This means the compression pressure at the end of the compression stroke is way lower. When a spark timed for a normally fast burning charge (as it is at WOT) fires under these circumstances it is going to occur far too late for the pressure to be at it’s peak at 15 degrees after TDC. This means that less than optimal use of the energy content of the fuel has been made. That in turn means burning more fuel to get the level of power being demanded by the driver at that particular moment. By having vacuum advance pull in appropriately more timing you can let out of the throttle more and consequently cruise on less fuel.

Also you may have based a cam selection on the basis of minimal negative idle impact. This may have limited your choice of cam to one of say a 280 degrees (advertised). If this was to be paired with a distributor with no vacuum advanced as most competition ones are then rest assured you have probably given away about 10-15 hp. If a vacuum advance is used the ignition timing at idle can be optimized where-as without vacuum advance it cannot. Assuming a realist street engine idle speed a ‘mechanical advance only’ distributor is barley pulling in any advance. What with say 12 degrees of initial and at most 5 degrees mechanical the total timing at idle would be only 17 degrees. In practice we find that a big cammed V8 needs about 50 degrees for best idle along with the lowest idle fuel consumption. I have seen some engines require as much as 55 degrees yet these engines – with cams as big as 260 to 270 degrees at 0.050, would idle tolerably well at 1000 rpm – sometimes less. The same goes for cruise. The vacuum advance can pull in the desired advance which at about 2500 rpm and a quarter throttles can be as much as 50 degrees. Normally an engine with mechanical advance only would have no more then about 25 to 30 degrees max. It does not take a diploma in rocket science to see that cruise fuel consumption is going to be much higher with the timing as much as 25 degrees out!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote handsofstone Quote  Post ReplyReply Direct Link To This Post Posted: 15-April-2020 at 9:09AM
Nice writeup.  After doing a bit of reading, vacuum source (ported/direct) was mentioned more than a few times. Definitely not using a distributor without the vacuum advance. I might try direct manifold source for the heck of it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 15-April-2020 at 10:44AM
so what is this potential paperweight, something i might be interested in?


direct manifold vac requires another level of set-up and calibrating to limit the timing pull otherwise it can pull up to 60* or more at idle! some distributors are not easily capable of being limited with an allen key through the vac nipple because there's no adjuster inside. aftermarket replacement vac pods are/used to be available but i have no idea what the current state of availability is today?

i have a link to a how-to but be aware if you're subject to epa testing the manifold vac system may not keep your CO's & HC's where they need to be. that's what the ported / timed spark vac source does, no vac pull at idle so no increase in CO/HC until the throttle blades activate the timed spark port.


" The vacuum advance unit is adjustable. You need to limit the total advance under vacuum. You adjust the unit by inserting an allen key (I do not recall what size, try several until one fits) into the front (some have two) hose nipple. Turn it one way or the other (been a while, obviously) until total additional vacuum advance is about 10 degrees. Your engine will like this.

In my daily butt hauler I run manifold vacuum advance. The car is a 1973 Mercury Montego four door. It's a pretty big car, about 4200 lbs. The engine is a 351 Cleveland 2V, it has a PAW/SSI 11339 (194/204 @.05, .458/484), a Performer intake, a 600 Holley and duals. I use a Pertronix Igniter in a Ford dizz with a Ford Duraspark coil. The Holley has a light sec spring, so it cuts in quickly.

I run 12 degrees initial advance, use the stock curve and use manifold vacuum for the vacuum advance unit which is adjusted to provide only 10 degrees additional advance. The result is that at idle, the engine runs smoother with the ac on (York comp that does not cycle, unless you count when I have to turn it off when the core freezes. I could store meat in that car). The vacuum provides an increase in efficiency at cruise (18 MPG with 2.75s). Being that it is manifold vacuum, the amount of additional advance is reduced when I push into the throttle. Acceleration is easy and effortless, especially in the 2-5K range where Clevelands run great. The greatest advantage of this set-up is the reduction of advance under acceleration. The US 2V Cleveland heads have an open chamber. That chamber coupled with the dished pistons of 1973 create a great deal of surface area. Detonation is a real problem here, but I am able to run the car on 87 octane with no problems, even on hot SoCal days with the AC on. When I was trying ported vacuum, I could get it to run well but detonation on acceleration was a problem. As it is, the car runs very well."
~ Tom Custom (founder of the original 351C forum)
 
check out the new 351c.net



Edited by Rockatansky - 15-April-2020 at 10:59AM
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Post Options Post Options   Thanks (0) Thanks(0)   Quote handsofstone Quote  Post ReplyReply Direct Link To This Post Posted: 15-April-2020 at 4:10PM
I suppose so. I haven't opened the box yet but it was cheap money, probably made in China. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote handsofstone Quote  Post ReplyReply Direct Link To This Post Posted: 15-April-2020 at 4:16PM
I just opened the box. Pretty well built for the price.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 16-April-2020 at 2:44PM
the $40 Special, small diameter can be very useful in some cases

not a Ready To Run type but requires some kind of module, can be used with a Duraspark box, GM module, Mopar transistor unit, MSD6AL etc. price of the ticket less than you'd think the housing alone cost, the biggest ? in my mind is what will cap & pick-up coil available be going forward? possible they are patterned on something existing but IDK what that is if anything?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote handsofstone Quote  Post ReplyReply Direct Link To This Post Posted: 16-April-2020 at 2:48PM
I hear you. Didn't even see there was no advance until I clicked buy. Tried to cancel immediaty. Zero replies. Typical online experience. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72FordGTS Quote  Post ReplyReply Direct Link To This Post Posted: 16-April-2020 at 11:52PM
What are you running now for a distributor?  This is what I run.  It's a lot more money, easy setup and works great:



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Post Options Post Options   Thanks (0) Thanks(0)   Quote handsofstone Quote  Post ReplyReply Direct Link To This Post Posted: 17-April-2020 at 12:49AM
I have an original with the Pertronix.  The new one was shiny and I was like an ostrich. I habe been trying to return it with no success. I like your setup but don't you need other components as well?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 17-April-2020 at 1:17PM
Ready To Run is an industry term used to describe a 'stand alone' distributor like OEM,

only needs a coil. Tim's dizzy's use a GM style Pertronix module
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Post Options Post Options   Thanks (0) Thanks(0)   Quote green 72 Quote  Post ReplyReply Direct Link To This Post Posted: 17-April-2020 at 3:26PM
I have bought several different ones from advance auto/carquest that are new units. have used both point ones and dura-spark ones . they are under 100 bucks and are really good distributors. they are like oem and even come with cap and rotors

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Post Options Post Options   Thanks (0) Thanks(0)   Quote handsofstone Quote  Post ReplyReply Direct Link To This Post Posted: 17-April-2020 at 6:12PM
I never had an issue with oem. Injust wanted to dress up the engine a bit and wasn't paying attention to the advance. I will put it on ebay or a parts page on farcebook and take a little loss. Won't happen again until next time. Haha
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Post Options Post Options   Thanks (0) Thanks(0)   Quote californiajohnny Quote  Post ReplyReply Direct Link To This Post Posted: 17-April-2020 at 10:18PM
take your stock one and either sand and polish it or remove the advance and chuck it up in a lathe  and machine what you can sand the rest Wink


Edited by californiajohnny - 18-April-2020 at 9:06AM
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72FordGTS Quote  Post ReplyReply Direct Link To This Post Posted: 18-April-2020 at 1:33AM
Originally posted by handsofstone handsofstone wrote:

I have an original with the Pertronix.  The new one was shiny and I was like an ostrich. I habe been trying to return it with no success. I like your setup but don't you need other components as well?

All it needs is a 12 volts of power, ground and a coil.  I used an e-coil from a 5.0L Mustang.  I used 12 volts through a relay, triggered by the original resistor wire.  So far, it works great.  I have the prototype distributor and Tim was using DUI modules when mine was made.  Seems he has switched to Pertronix modules now.  You can do a cheaper version of the same ignition, which is outlined in this thread here on Todd's Ranchero:



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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 18-April-2020 at 5:22AM
Yes a Duraspark is just a magnetic pick-up unit, similar to MSD that requires a spark box. you can wire it up to a GM module, Pertronics etc.

a stock points dizzy can trigger a 6AL box or probably anything else too. point gap is not critical at all as long as they make & break, no condenser IIRC, it just becomes a trigger unit.


a stitched wheel from the hardware store on a drill motor and a couple compound sticks will buff up a stock housing.

do yourself a favor and invest in a metal sealant so it stays nice ;)

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