Eliminating vacuum advance

[Ghoste]

I know it's function and setting it up and the fact that most performance cars do not run one at all, but, what I am wondering was just how much difference does it really make when you eliminate it?
I am talking about highway cruise mileage of course, since that seems to be it's primary function.

[RallyeMike]

In a race application, the load is pretty much constant and the rpm's are all in right off the bat, so vacuum advance is not needed. For a street driven car, vac advance is important because it adjusts advance to varying engine load, improving driveability.

[Ghoste]

But... a LOT of street performance cars are running without it.  Given that, how much are they really giving up?

[dodge freak]

Most likely a lot of headaches. If the light springs are in the dist. so the advance comes in by 2000-2500 and the car has 3.91's or higher - which lots of street performance cars have- I would guess not much. I have to guess cause if I were to hook mine up , it would not run as smooth. Seems like as soon as the motor gets more than 38 degrees or so I can see the motor start to shake at 3500-4000 rpm's. The only reason I can think to use it if the motor pings with both of the light springs in the dist. so the advance is not in till 3000-3500 rpm, then a little vacuum advance would help, just enough to get the advance back to around 38 degrees. I would never run 45-50 degrees of advance even if it is only lite throttle , motors like 38 or so. The best way to increase gas milage is to slow the motor down 3.23 gears  or lower yet and or increase the compression, which if we could run 12 to 13-1 would help out a lot.

[firefighter3931]

The problem with vacuum advance is that it's designed for stock motors which make in excess of 15in vacuum. Once you hop up the engine with a cam, manifold, headers etc...tuning with it enabled causes issues (surging/pinging etc...). If your distributor is curved properly with total advance all in by 2500-2800 then you'll never miss it, inmo. The other issue is detonation...why risk the engine by pulling in extra timing for what is a minimal milage increase...if that.  

Ron

[Ghoste]

That is kind of why I was wondering.  I disconnected mine last year or the year before and I haven't seen the great drop in mileage that I was told I would get on the highway.  My car is pretty screwed up anyway but it still made me wonder.

[Chryco Psycho]

I have seen milage improve with the vacuum disconnected

[Ghoste]

Really?  Any idea why that would be Neil?

[Chryco Psycho]

at higher RPM the dyno I use has proven the timing can or needs to be retarted so adding more vacuum at a stable higher RPM such as highway cruzing may be the wrong direction & by eliminatiing the vacuum advance the timing cannot increse at higher RPM , the last car I dynoed had a retard system & showed loss of power as the retard at higher RPM was reduced 

[RallyeMike]

My problem is that I'm still lost at about 1979 and think people still drive their Chargers every day...      

Firefighter is probably closer to target. This probably is not a stock regular driver.  I still drive a 1971 Mopar every day, with an occasional fling in a 1972 car, and the factory advance set-up is pretty optimal for a daily 6AM ride to work.

[Ghoste]

Well, as Ron and Neil already know, my car doesn't make a whole lot of vacuum.  It is an adjustable vacuum advance which I would think could compensate for it.  CP's point makes a lot of sense though and both those guys know I don't do a lot of highway driving other than the 5 hour trip to Columbus each summer.  The rest of the time is more local and spirited driving.
Since it sounds like it doesn't make a tremendous difference to start with and likely less with my setup, I guess there is no sense trying to reset the thing prior to next weekend.

[chrisII]

in my small block circle tracker i wanted more advance than the stock setup alowed but did not want the vacum adv, if you take different dists apart you find that they have different amounts of mechanical advance, but i could not find enough so i elongated the slots in the mech advance till the weights almost hit the housing. then i used 2 light springs, toped it with the pickup from a lean burn dist (its fixed). I did this beacuase i was runing a limited compreshon 2bbl carb class and i was actually making the best power at about 44-48* @3000 rpm. by playing with the springs i was able to get the 48* and still have it back down around 25-30 so i could start it. nobody even neads to tell me that 48* is WAAAYY too much advance,it did work well for what i was doing tho the engine took it and made gooobs of power. the amount i was able to get would work well on most modified street cars with litle vacume, but it is rough on the springs to stretch them that much.  I have also seen guys remove the vac diaphram and install a choke cable into the drivers area so they can manyally fine tune the advance as they are driving (less for start and idle, more at hwy speed)

[dodge freak]

You can also wire a toggle switch for the ignition. This way you can flip the switch off so there be no spark and you can then crank the motor over, once it is cranking flip the switch on for the spark to start. Now of days most of the MSD boxes can get a start retard wire in, it retards the spark 20 degrees or so until the motor starts.

Those lean burn pick up plates are great, one piece and cheap. Way to go Mopar.

[Ghoste]

Didn't some of the old super stock guys have "secret" switches installed so they could retard the timing at the far end of the track?

[grouseman]

What is your current set up?  What mileage do you get presently? 

[Ghoste]

If you're talking about mine, the current state is confused and I never check the mileage.  I know it didn't see a big drop when I unhooked it because if I do any long distance driving, it is generally to the same locations and one tank takes me to the same filling stations it always did before.

[myk]

So, when you unhooked your advance how did you have to adjust the engine?  When I disconnect my advance, idle speed drops quite a bit and I'm convinced that the engine won't run smoothly...

[dodge freak]

You have to make sure the timing is set right, might be but I would check , then turn the throttle opening screw till the rpm's are where you want them. You could skip the timing check and just turn the throttle screw-idle screw some call it a 1/4 -1/2 of turn maybe a bit more. You don't need vacuum advance, once the timing is perfect it should run smoother than it is now.

You might even have to put in light spring in the dist. to get the advance to come in sooner, that might be a problem for you. MSD make a good dist. thats easy to change the springs.

[myk]

Ok, so now that my advance is disconnected, it doesn't matter where the little allen screw in the advance was set at, right? 

[dodge freak]

You got it, sounds like that carb. of yours is your main headache right now. Don't know much about eddy's just Holley. My Holley hp 750 runs great but the gas milage isn't , unless you think 11 mpg on a 318 with 3.91 gears is, but it does run fine. Think that MSD 7 box helps out too. Good Luck, don't give up. Might want to jet up 2-3 sizes and see what happens, you always can go back.

[Chryco Psycho]

So, when you unhooked your advance how did you have to adjust the engine?  When I disconnect my advance, idle speed drops quite a bit and I'm convinced that the engine won't run smoothly...

I fthe idle quality is affected you had the advance hooked to manifold Not ported vacuum , so with it disconnected you need to advance the timing a bunch , your power & milage will greatly improve with it disconnected 7 the timing adavnced

[myk]

So you're saying to advance the timing on the distributor and not the vacuum advance, right?  Well, I tried bumping up the timing from 19* initial to about 10* and other numbers in between and I got the off-idle bog/stumble/hesitation from hell.  Anything else you think I should be looking at, or should my motor have not responded this way? 

[firefighter3931]

  Well, I tried bumping up the timing from 19* initial to about 10* and other numbers in between and I got the off-idle bog/stumble/hesitation from hell.   

Myk, going from 19* to 10* is not bumping up....that is "retarding" the timing. Get a vaccum guage on it and adjust for maximum vacuum on the guage. Check the timing and record that number. Readjust your carb idle mixture and idle speed adjutments. Put the light on it and record the advance in degrees from idle to where it stops advancing. Report back with the numbers.


Ron

[myk]

Thanks for your continuing help and patience guys. 
I thought that I was bumping it up, since I'm going from 19* BTDC to 10* ? 
As for your recommended timing procedure, it goes like this:
1. adjust initial timing to get maximum vacuum, document timing number.
2. adjust 'carb and idle speed.
3. adjust initial timing again, this time taking note as to where the motor stops advancing from distributor adjustment-also document the amount of degress between the 1st and 2nd timing numbers.
4. don't run over any cats
5. report back to the forums.

Once again guys, thank you all, very much...

[Ghoste]

You ran over a cat?