Vacuum advance mandatory for a street car?

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desmo900

desmo900

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Rustyratrod brought this up in another thread and suggested someone start a thread on it as to not get the other thread too off track.

I am not trying to stir anyone's pot I just don't have knowledge or experience one way of the other and I am very curious. Is vacuum advance necessary on a car that is primarily street driven?
 
It helps mileage. Might help cool off engine at light throttle.
 
Mandatory? No. Desirable? Yes, helps with fuel economy, plug life, temp, emissions, and doesn't use any horsepower. Sort of like a PCV valve, you can run without one, but why would you want to?
 
If fuel millage doesn't matter as much as performance, leave it disconnected.
If its a daily driver and you don't need to blast off when the light goes green, leave it connected and get better fuel millage.
 
It is if you do alot of miles in a given time or if you are looking for the most fuel economy. A daily driven slant six,yes.Our 472 street Hemi,, No..Not that you can't but.....
 
I am guessing it's a zero sum game. It will cost you a bit of acceleration out of the hole?
 
It should NOT cost you anything if the distributor works right. When you give it enough throttle, the vacuum drops clear to nothing
 
Vacuum advance will improve engine performance at light throttle. It adjusts the timing per the engine load and associated mixture. At light throttle, light load, fuel mixture is leaner. Leaner mixtures burn slower, so more advance is required to get the burn started earlier. The advance related to the load component helps keep engine speed stable for varying load condition. An example is nearly stable vehicle speed while negotiating hilly terrain. An engine with insufficient advance will labor, and require more throttle to maintain speed. The retarded timing results in late fuel burn on the way out, so it heats heads, exhaust and engine compartment.

An engine with correct vacuum advance will have better throttle response, and better economy at low to medium throttle. Even cruise at highway speeds is light to medium throttle. Vacuum advance typically provides a maximum of 10 to 20 degrees adder, to the mechanical advance.

Here are some of the problems. When different carbs are swapped, the ported vacuum characteristics may change. If the idle speed is adjusted too high, it is possible the ported source will leak at idle. Vacuum advance diaphragms develop leaks, and fail to operate. Vacuum advance settings are a compromise, special fabrication may be required for improvement in advance rates. Much trial and error.

Electronic advance in modern cars use many controls to get the correct advance. There is much to gain with proper advance control, in performance and economy, when using a 3D RPM/MAP timing control.
 
Agreed with Kit's post. At part throttle, the advance is kicked in enough to give the engine a little extra. As mentioned, it will improve economy for a street car......and usually by a pretty noticeable margin. It will work regardless of vacuum signal at idle, so you can dispel the myth that engines with big cams will not work a vacuum advance, because they will.

You can certainly run without one, but why would you when running one WILL give you nothing but benefits? On a street car, there is absolutely zero down side to running a vacuum advance. People ditching them is simply one of those "race only" things that tried to migrate to the street. Some things can successfully be used from race to street, but eliminating the vacuum advance is not one of them. A street car will always be a little more efficient with one. Always.
 
I assume it doesn't change the total overall advance just modulates when that advance kicks in? (That is a question in the form of a statement)
 
It will change the total from NOT having it attached to having it attached. If that makes sense.
 
round town,light throttle: more advance ,delivers drivability,and gas mileage.Total mechanical timing.,has nothing to do with this. Until you add modifications. Big cams,are the culprit here. Then you go pure mechanical advance.If you are willing to learn to tune & learn. It takes time,wanna learn?
 
Here's experience from someone who dont have a whole lot of experience. I wish my vacuum advance worked on my truck that I daily drive every day. it sucks. run the advance.
 
desmo900 said:
I assume it doesn't change the total overall advance just modulates when that advance kicks in? (That is a question in the form of a statement)
Click to expand...

Your statement is wrong, or at least the way I understand what you said

All advance is ADDITIVE

You start with initial, "whut you set" with a timing light at idle.

You ADD mechanical which is purely a function of RPM, the springs, (for rate) and the length of the advance slots (for amount, or degrees).

So at WOT foot on the floor, RPM high enough that mechanical is "all in" you have "whatever is in the mechanical" added to the initial.

LAST

vacuum is ADDED to the top two AT WHATEVER POINT the mechanical "is."

This means that if you are at a fairly low RPM (stock engine) and the mechanical is not "all in" but that if throttle conditions are "right," you might have ALL the vacuum can ADDED to ONLY PART of mechanical.

On the other hand if you are at a higher RPM, and the mechanical is "all in" and IF throttle conditions are "right" you then might have ALL the mechanical added to ALL the vacuum.



As you give it more and more throttle, the vacuum drops out more and more.

If you start looking at specs "in the book" keep in mind that these are DISTRIBUTOR degrees which are 1/2 "crank"

Likewise, some mechanical "top hat" assemblies are stamped with a number, like "15" This is THIRTY crank degrees.
 
Let me ask this a different way: If you have 14 degrees of initial advance at idle and at lets say, 5000 rpm WOT you have a total of 38 degrees full advance, is there a part throttle scenario where your vacuum advance + initial advance can exceed that 38 degrees?
 
67Dart273 said:
Your statement is wrong, or at least the way I understand what you said

All advance is ADDITIVE

You start with initial, "whut you set" with a timing light at idle.

You ADD mechanical which is purely a function of RPM, the springs, (for rate) and the length of the advance slots (for amount, or degrees).

So at WOT foot on the floor, RPM high enough that mechanical is "all in" you have "whatever is in the mechanical" added to the initial.

LAST

vacuum is ADDED to the top two AT WHATEVER POINT the mechanical "is."

This means that if you are at a fairly low RPM (stock engine) and the mechanical is not "all in" but that if throttle conditions are "right," you might have ALL the vacuum can ADDED to ONLY PART of mechanical.

On the other hand if you are at a higher RPM, and the mechanical is "all in" and IF throttle conditions are "right" you then might have ALL the mechanical added to ALL the vacuum.



As you give it more and more throttle, the vacuum drops out more and more.

If you start looking at specs "in the book" keep in mind that these are DISTRIBUTOR degrees which are 1/2 "crank"

Likewise, some mechanical "top hat" assemblies are stamped with a number, like "15" This is THIRTY crank degrees.
Click to expand...

I was typing my above question as you replied, if I read you correctly than yes I can exceed my total WOT advance setting at part throttle with vacuum advance.
 
Here's an example right out of a Chrysler manual for a street hemi



Notice that under "vacuum" they are showing different vacuum readings, and the max shown is about 11 degrees at 15" Hg vacuum.

This once again is distributor degrees

Notice down at the bottom, an initial of 12.5 BTC

Notice mechanical (centrifugal) is 9.5 at 1400 RPM. Again, X2 for crank, or 19 degrees

So for this engine, a factory initial of 12.5 + 19* mechanical gives you a total of 31.5 degrees

Add to this the max. vacuum of 11 (22 at the crank) and this means that above 1400 RPM, at light throttle, this engine would see timing of 12.5 + 19 + 22 degrees or 53.5*

It was mentioned that such things as changing carbs can change port characteristics. This can be a problem. "Back when" (70's--80's) when I had a warmed over 340 in my Landcruiser, which weighsabout 4K, I had several different carbs on the engine, from Q J to AFB. I had some trouble with part throttle ping, evidently because one / some carbs had "too much" vacuum at the port too early
 

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I took a 300 mile trip this summer. 318 with initial at 14 With 20 in dizzy total T 34. I ran without vacuum first leg, engine ran warm and took over a full tank. Ride back with vacuum engine ran cooler and used 1/2 tank. Just my 2 cents experience.
 
lots of benefits with vac advance
without it you are limited to initial 15? plus mechanical 20? for a total of around 35.
...for cruising that's not very efficient,should be 50+
...if your idling at a light or stop and go traffic you'll probably be around 20? and will run hotter.
 
Thanks everyone. I have been mistakenly assuming that the vacuum advance just modified how soon the mechanical advance came in. Clearly not the case.
 
desmo900 said:
Thanks everyone. I have been mistakenly assuming that the vacuum advance just modified how soon the mechanical advance came in. Clearly not the case.
Click to expand...

Flyweight return springs control mechanical advance rate. Weak springs bring in advance sooner. It is more complicated than that, since the heavy spring has an oblong loop, the first lighter spring determines initial slope, heavy spring plus the light spring control the second gradual slope. Slots in cam top hat, limit mechanical advance.
 
desmo900 said:
Let me ask this a different way: If you have 14 degrees of initial advance at idle and at lets say, 5000 rpm WOT you have a total of 38 degrees full advance, is there a part throttle scenario where your vacuum advance + initial advance can exceed that 38 degrees?
Click to expand...
yes.
 
Not uncommon for total WITH vacuum advance to be close to or a touch over 60*. Since it is only at light throttle, there is no spark knock. As the throttle is depressed, vacuum goes away and so does vacuum advance so there is no spark knock under accel.
 
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