Posted By: 1KoolBee
Vacuum advance or not??? - 07/31/13 05:44 AM
First off thanks to all who have helped me with my engine build so far. Its coming together slowly, but I'm almost there. Trying to decide if there is a benefit to running a vacuum advance on my street/strip motor. Its 500 cubes, 11.2 static compression, MP .557 cam, 1.6 rockers, Eddie Heads, sixpack induction, 4-speed trans. should I stick to straight mechanical advance, or is there a good reason to keep vacuum advance?
Posted By: sshemi
Re: Vacuum advance or not??? - 07/31/13 08:54 AM
Only reason for vac advance is a little better milage.
But then the carb have to be calibrated for vac advance.
Posted By: 67_Satellite
Re: Vacuum advance or not??? - 07/31/13 11:37 AM
What fuel? At that compression probably better than standard pump swill. If it doesn't ping, run it to manifold vacuum.It will give you the extra timing for a better idle/mileage and automatically goes away under heavy/full throttle.
Posted By: Dragula
Re: Vacuum advance or not??? - 07/31/13 12:50 PM
Absolutely not...Too inconsistant...And as for mileage. I did a test on one of my street/strip vehicles, and I think that is a wives tale. If you have a cam more geared for racing than stock, it will never happen.
On mine I did the mileage check back to back one week with a distributer without vacuum advance, and the next week a distributer with vacuum advance. If I saw .4mpg difference it was a lot....Since it had a small 11g tank, and my drive to work is almost 30 miles I typically put fuel in it every day and checked the mileage...11mpg without, 11.4 with it...and it didn't always get the extra mileage...yippy....less then 1/2mpg on one of the five days, saving the world now. It even ran better without it.
Posted By: Mr.Yuck
Re: Vacuum advance or not??? - 07/31/13 02:01 PM
FWIW if you are building a 440 w/ milage in mind you should have your head checked..... I can tell you w/ the 6-pack you'll do fine. I avg almost 15mpg to and from the track. Back roads, constant 45-50mph 727, 3.91's Now once you stand on it and the ouboards open that changes drastically... but in a good way.
Posted By: sshemi
Re: Vacuum advance or not??? - 07/31/13 06:55 PM
Dist and carb should be calibrated for it.
Lean mixtures burn slower so the benfit is to lean the carb at highway speed (too lean without vac) and with the increased timing ot should run with better milage.
Posted By: Thumperdart
Re: Vacuum advance or not??? - 07/31/13 07:20 PM
I don`t like variable timing on anything not stock and my dist. is locked out and starts fine even at 12.1.1 comp. Call me I`m close and can help you out..........760-900-3895........
Posted By: LAR_414
Re: Vacuum advance or not??? - 08/01/13 01:47 AM
Do it.
15.5 mpg here. 11 sec 34 degr mech and 50 with vacuum.
Posted By: 1KoolBee
Re: Vacuum advance or not??? - 08/01/13 05:15 AM
LOL...Yeah you're probably right Dom! Keep it simple is a good creed to stick to.
Posted By: MR_P_BODY
Re: Vacuum advance or not??? - 08/01/13 06:30 AM
I have trouble focusing sometimes so this stuff drives me
plus vac. adv. is for wussy cars NOT muscle cars..............
Maybe they are for wussies but now days I'll take that
extra 1/2 to 1 mile a gal any day... it might take
a few hours to fine tune it.. well worth the effort
to me.. once I'm up and running good I hope to have time
to dial in a vac advance before DW
I hooked up the vacuum advance on my 470 street motor. (
242@.050 duration) It adds 14 more degrees at part throttle. I noticed the engine temp drop about 20 degrees while cruising. But the part throttle response doesn't seem as snappy as without the advance. Which is opposite of what I thought the extra timing would add. I still have it hooked up for now.
Posted By: DaveRS23
Re: Vacuum advance or not??? - 08/01/13 03:24 PM
Doesn't the fuel economy improve because the engine is burning it's fuel more completely and more efficiently?
My engine sounds a bit smoother and quieter at cruise with the vac advance. That's why I have kept it, not for the additional economy.
Posted By: Thumperdart
Re: Vacuum advance or not??? - 08/01/13 07:17 PM
It was a joke but those #`s mean nothing if you can`t get it down the track.............
Posted By: Thumperdart
Re: Vacuum advance or not??? - 08/01/13 07:31 PM
It`s a beoch trying to make as much power as we can only to have troubles hooking or whatever but if it was that easy everyone would have 8 and 9 second street cars............
Posted By: Thumperdart
Re: Vacuum advance or not??? - 08/01/13 07:55 PM
I`ll be on slicks from now on................yes we did.........
Posted By: 383man
Re: Vacuum advance or not??? - 08/01/13 09:05 PM
As you can see I dont have a vacum advance on my dist. I got a good deal on this Mallory race dist and it works good. I run 37 total all in by 1800 rpm and it runs and drives very good. That said if it had a vacum advance I would have no trouble trying it to help my cruise even more. My old 440 got good milage with this dist as it was about 14 miles to a gallon. But this 493 wanted alot more fuel and gets alot less milage then the old 440 did. But I was only figuring I would get about 8 miles to a gallon with the 493 and 4.30 gears. If I was really worried about gas milage then I am in the wrong hobby.
But no problem using or trying the vacum advance on a street car. Ron
Posted By: dodgeboy11
Re: Vacuum advance or not??? - 08/02/13 03:39 AM
I got tired of reading all the responses so this may have been covered, but just in case it hasn't, here goes: vacuum advance was brought into existence because mileage and lean fuel mixtures became important. In order to burn a lean mixture you need to light it off sooner, hence, the vacuum advance. Light load, lean mixture, not typically a recipe for detonation, though there is a fine line from light load to tip in on the throttle where it can and will detonate if the vacuum advance doesn't drop off fast enough or you have too much vacuum advance. There may be a recipe for finding out what these numbers are, but I did mine by trial and error. Leaned the truck out until it lean surged, went back up two jet sizes, 50 cc pump to get rid of the lean bog off idle, and vacuum advance to make it run when cruising. Richened up the secondary side a tad to make up for the primary (though in hindsight should have just opened up the power valve fuel passages), and I was knocking down 14 mpg around town regularly in a '77 W100 with a 2" lift and a 4spd. Maybe that doesn't seem too spectacular, but it beats the 11 it was getting. Now it's got a 440 and 11 is about all I can do. Torker II doesn't help there.
Posted By: Thumperdart
Re: Vacuum advance or not??? - 08/02/13 09:28 PM
Now I`m REALLY confused............
Posted By: polyspheric
Re: Vacuum advance or not??? - 08/02/13 09:58 PM
#1 reason why not vacuum advance: don't understand how it works
#2 reason: understand it, too much trouble for the benefits
Also: it's another way the point plate can wobble, possible spark scatter.
Warning: if you have a Holley 4150 variant and already have your mixture dialed in, remember that cruising fuel mixture is from the primary mains, which get reduced. To maintain the same WOT mixture you have to increase the PVCR size - buy an aftermarket adjustable block or get out the drills.
There is a little more work to get it right with a vacuum advance system, but there is a pay-off if you street drive. You will find the stock vacuum advance unit will usually have about 14 degrees in it. That's going to be too much ( when you ad in the mechanical and the totals we run )with most performance build applications. You are going to have to cut it back to around 10 degrees by ether using an adjustable can ( as said) or by limiting the rod pull. I don't have any pics of the latter, but I have made them in the past. Not hard to do. 
