Re: Is Vaccum Advance Really Such a Bad Idea
[Re: fatman]
#2887826
02/13/21 08:58 PM
02/13/21 08:58 PM
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Joined: Dec 2004
Posts: 10,553 Rittman Ohio
fourgearsavoy
I Live Here
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I Live Here
Joined: Dec 2004
Posts: 10,553
Rittman Ohio
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I went a few rounds with Eberg on this topic at Carlisle years ago and we kind of agreed that for a pure stock engine sure it's a good idea. But for most engines with modifications like a cam and moderate compression you need to just customize a simple distributor with mechanical advance will do the job. Gus
64 Plymouth Savoy 493 Indy EZ's by Nick at Compu-Flow 5-Speed Richmond faceplate Liberty box Dana 60
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Re: Is Vaccum Advance Really Such a Bad Idea
[Re: Stanton]
#2887858
02/13/21 10:03 PM
02/13/21 10:03 PM
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Joined: May 2009
Posts: 511 Temperance, MI
68 HEMI GTS
mopar
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mopar
Joined: May 2009
Posts: 511
Temperance, MI
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I run vacuum advance on my 10sec street car. Works well, gets great mileage, and the plugs always look great. Just requires the distributor to be set up correctly and you need a adjustable vacuum advance to limit the total.
68 Dart GTS "HEMI" 10.30 @ 131 pump gas street car 3780# 69 Roadrunner 511 six pack 10.92 drive to track street car
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Re: Is Vaccum Advance Really Such a Bad Idea
[Re: calrobb2000]
#2887884
02/13/21 10:53 PM
02/13/21 10:53 PM
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Joined: Jan 2003
Posts: 7,489 Minnesota
Hemi_Joel
master
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master
Joined: Jan 2003
Posts: 7,489
Minnesota
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Every street engine will benefit from properly set up vacuum advance using non-ported manifold vacuum. Smoother, cleaner idle, better mileage, cooler running, longer life. You can plug it for racing. Drag race only engines should not have it. I'm not saying nobody gets their car to run good on the street without vacuum advance, but you can make it run better and more efficiently at part throttle with it. Most of the time on the street is part throttle.
[img]http://i.imgur.com/boeexFms.jpg[/img]31 Plymouth Coupe, 392 Hemi, T56 magnum RS23J71 RS27J77 RP23J71 RO23J71 WM21J8A I don't regret the things I've done. I only regret the things I didn't do. "Wise men talk because they have something to say; fools because they have to say something. ~ Plato"
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Re: Is Vaccum Advance Really Such a Bad Idea
[Re: calrobb2000]
#2887887
02/13/21 10:58 PM
02/13/21 10:58 PM
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Joined: Jan 2003
Posts: 1,759 Windsor, ON, Canada
Diplomat360
top fuel
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top fuel
Joined: Jan 2003
Posts: 1,759
Windsor, ON, Canada
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hi why would you not use vac adv on a steet car ?
you are just wastin gas on any cruse time . Have to completely agree with that statement...no reason NOT to use a vac advance, it has a number of benefits and literally is one of those freebies that we can take advantage of.
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Re: Is Vaccum Advance Really Such a Bad Idea
[Re: Diplomat360]
#2887905
02/13/21 11:40 PM
02/13/21 11:40 PM
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Joined: Jan 2003
Posts: 7,489 Minnesota
Hemi_Joel
master
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master
Joined: Jan 2003
Posts: 7,489
Minnesota
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At part throttle, the air/fuel mixture in the combustion chamber is very sparse, so it burns much slower than the piston travels away from it. For efficient use of the fuel to make power, the maximum flame spread and cylinder pressure needs to occur near the beginning of the downward movement of the piston on the power stroke when it has the most mechanical advantage and before the exhaust valve starts to open. With the slow burning part throttle charge, it needs a bigger head start to be maxed out in the sweet spot. Vacuum advance gives it the head start it needs, so the piston is not past 90 degrees atdc when the flame front is at its peak. Ever try to pedal hard on a bike when the pedal is past 90 degrees? And the exhaust valve on a performance mill will start opening +/- 70 degrees before bottom dead center. Lack of advance will send part of that still expanding charge out the exhaust.
[img]http://i.imgur.com/boeexFms.jpg[/img]31 Plymouth Coupe, 392 Hemi, T56 magnum RS23J71 RS27J77 RP23J71 RO23J71 WM21J8A I don't regret the things I've done. I only regret the things I didn't do. "Wise men talk because they have something to say; fools because they have to say something. ~ Plato"
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Re: Is Vaccum Advance Really Such a Bad Idea
[Re: Hemi_Joel]
#2887921
02/14/21 12:37 AM
02/14/21 12:37 AM
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Joined: Feb 2008
Posts: 714 Central TEXAS!!!!
sr4440
super stock
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super stock
Joined: Feb 2008
Posts: 714
Central TEXAS!!!!
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At part throttle, the air/fuel mixture in the combustion chamber is very sparse, so it burns much slower than the piston travels away from it. For efficient use of the fuel to make power, the maximum flame spread and cylinder pressure needs to occur near the beginning of the downward movement of the piston on the power stroke when it has the most mechanical advantage and before the exhaust valve starts to open. With the slow burning part throttle charge, it needs a bigger head start to be maxed out in the sweet spot. Vacuum advance gives it the head start it needs, so the piston is not past 90 degrees atdc when the flame front is at its peak. Ever try to pedal hard on a bike when the pedal is past 90 degrees? And the exhaust valve on a performance mill will start opening +/- 70 degrees before bottom dead center. Lack of advance will send part of that still expanding charge out the exhaust. hemi Joe has this correct, when setting up a street/strip engine always run some form of advance. On the dyno, with a part/ light throttle engine load, as soon as you start adding advance you will see the BSFC number start dropping like a rock. (BSFC measures how efficient the engine is. A lower number is more efficient) How much to add, that is going to be trial and error. I wouldn't worry about "washing" out your rings or cylinder walls, you are under light load and the fuel is still being burned, just going out the exhaust. Joe
Without Data, you’re just another guy with an opinion.
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Re: Is Vaccum Advance Really Such a Bad Idea
[Re: Hemi_Joel]
#2887953
02/14/21 07:34 AM
02/14/21 07:34 AM
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Joined: Jan 2003
Posts: 6,391 Abilene, Texas
fastmark
master
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master
Joined: Jan 2003
Posts: 6,391
Abilene, Texas
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Every street engine will benefit from properly set up vacuum advance using non-ported manifold vacuum. Smoother, cleaner idle, better mileage, cooler running, longer life. You can plug it for racing. Drag race only engines should not have it. I'm not saying nobody gets their car to run good on the street without vacuum advance, but you can make it run better and more efficiently at part throttle with it. Most of the time on the street is part throttle. I am working on a 440 six pack with with the Promax metering block. It does not have the fitting in the metering block for the normal vacuum line for the distributor. Now that port on a stock metering block is ported, correct? Meaning it does not get vacuum at idle but at throttle, correct? So, I should run the vacuum line straight to manifold vacuum, right. I read on one of the Corvette forums and they were saying the first Tri powers had the vacuum advance straight to the manifold vacuum.
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Re: Is Vaccum Advance Really Such a Bad Idea
[Re: lewtot184]
#2888016
02/14/21 11:02 AM
02/14/21 11:02 AM
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Joined: Sep 2014
Posts: 4,457 Washington
madscientist
master
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master
Joined: Sep 2014
Posts: 4,457
Washington
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Absolutely, positively for damn sure you NEED vacuum advance.
You also need to run a curve. Not sure why you only have 30 total, because that seems really low. Plus if you have ANY sort of ignition box, the timing WILL retard with RPM, so you actually will have LESS than 30 total with the distributor locked out and RPM retard.
So get a curve in it and add vacuum advance.
When you run the timing that far retarded you get a TON of heat way too late in the power stroke. On blow down, when the exhaust valve opens you still have combustion happening and all that heat goes into the exhaust valves, the exhaust port and out the exhaust.
That’s not only a power killer it’s a parts killer. So yeah, you are wasting fuel and power and I’d bet everything I have, everything you have and a bunch of stuff neither of us have that your exhaust gas temperatures are through the roof.
I used to see it all the time when I was tuning at the track and guys were logging EGT’s. The EGT would be where they thought it should be, or some other hero told them the EGT was too high, so they start adding fuel (usually wrong) and pulling timing (usually wrong) and the EGT would keep on climbing.
Then when you tell them they are doing it backwards, all the guys who’ve never looked at a data log ever tell them why they need to go to a colder plug ([censored]??????) and get the timing down some more.
Get some timing in it. And get a curve. And vacuum advance.
Just because you think it won't make it true. Horsepower is KING. To dispute this is stupid. C. Alston
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Re: Is Vaccum Advance Really Such a Bad Idea
[Re: 68 HEMI GTS]
#2888017
02/14/21 11:04 AM
02/14/21 11:04 AM
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Joined: Sep 2014
Posts: 4,457 Washington
madscientist
master
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master
Joined: Sep 2014
Posts: 4,457
Washington
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I run vacuum advance on my 10sec street car. Works well, gets great mileage, and the plugs always look great. Just requires the distributor to be set up correctly and you need a adjustable vacuum advance to limit the total.
If there was a like button I’d hit that thing like a lab rat hitting the button for more meth until it killed me!!!!
Just because you think it won't make it true. Horsepower is KING. To dispute this is stupid. C. Alston
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Re: Is Vaccum Advance Really Such a Bad Idea
[Re: madscientist]
#2888021
02/14/21 11:18 AM
02/14/21 11:18 AM
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Joined: Nov 2006
Posts: 2,255 IL
furious70
top fuel
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top fuel
Joined: Nov 2006
Posts: 2,255
IL
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To NOT need or benefit from vac advance, someone would need to be able to explain what they did to the engine to no longer require it, if we all agree that the OEM application of it is correct and useful.
Possible things people might say are modern shape combustion chambers, quench pistons, computerized engine controls....essentially trying to tell Hemi_Joel that the control he explained that is needed because of inefficiencies in the engine are combated in some new way.
I'm not saying that at all, just laying out the argument someone would need to make because what he said is still true with a big cam and more compression...
70 Sport Fury 68 Charger 69 Coronet 72 RR
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Re: Is Vaccum Advance Really Such a Bad Idea
[Re: fatman]
#2888025
02/14/21 11:23 AM
02/14/21 11:23 AM
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Joined: Jan 2003
Posts: 12,076 Benton, IL.
DaveRS23
Special needs idiot
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Special needs idiot
Joined: Jan 2003
Posts: 12,076
Benton, IL.
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Adding, let alone tuning vacuum advance is just about the same as choosing and tuning a carb for most guys. It is just over their head.
For a car that cruises at light load, steady throttle why wouldn't you want vacuum advance? What could possibly be the down side? I agree that on some combos the advantages could be modest but there would still be advantages. It would seem that the biggest hurdle to widespread use of vacuum advance would be the misunderstanding of it's benefits and the lack of ability to tune it.
I use a hand held vacuum pump to test and tune my vacuum advance. You can hear the difference in the exhaust note as you add advance at cruise. I find that finding a good amount of advance is not difficult at all. The hardest part is making the necessary changes to the advance stop.
It would be nice if there were some more definitive data on this. Maybe an episode of Engine Masters?
Last edited by DaveRS23; 02/14/21 11:26 AM.
Master, again and still
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Re: Is Vaccum Advance Really Such a Bad Idea
[Re: fatman]
#2888109
02/14/21 01:18 PM
02/14/21 01:18 PM
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Joined: Oct 2007
Posts: 4,206 New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,206
New York
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For engines close to stock tune (any size, but mild cam) full vacuum to the distributor works well. As the cam approaches 240 degrees @ .050", full vacuum works at idle, but as you open the throttle vacuum quickly dies, which reduces RPM, etc. Ported fixes this, since idle is based solely on initial advance it's completely stable until a later throttle position. You can even fab a new port position by inserting a small tube right through the casting to a point (generally slightly higher to delay vacuum) than the original, which will not add vacuum until slightly higher throttle disc angle. Hint: doesn't need to be horizontal. Try both, keep the one that works best, cap the other (don't blank it), it may help the next time you change your tune-up.
Boffin Emeritus
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Re: Is Vaccum Advance Really Such a Bad Idea
[Re: DaveRS23]
#2888120
02/14/21 01:29 PM
02/14/21 01:29 PM
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Joined: Jan 2021
Posts: 983 rust belt
Moparite
super stock
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super stock
Joined: Jan 2021
Posts: 983
rust belt
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I am working on a 440 six pack with with the Promax metering block. It does not have the fitting in the metering block for the normal vacuum line for the distributor. Now that port on a stock metering block is ported, correct? Meaning it does not get vacuum at idle but at throttle, correct? So, I should run the vacuum line straight to manifold vacuum, right. Yes to the first question and a big NO to the second! The distributor vac advance needs ported NOT manifold vacuum. Manifold vac is high when the throttle blades are closed giving max advance until you open them up then it drops to nothing. This is backwards of the way it's supposed to work. Ported vac is higher during open throttle and nothing when it's closed. I can't comment on what GM did but this is the way Chrysler works. And to make it short the advantage in a street motor is it will apply vac advance before the rpm's get high enough to apply the mechanical advance. For a strip motor it doesn't make sense because as soon as the light turns it's full throttle and the rpms are up where the mechanical advance kicks in.
Last edited by Moparite; 02/14/21 01:33 PM.
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