Re: 440 Vacuum advance, hook up ported or constant vacuum?
[Re: Robbins]
#718028
06/08/10 06:07 PM
06/08/10 06:07 PM
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Joined: Jan 2003
Posts: 931
dulcich
super stock
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super stock
Joined: Jan 2003
Posts: 931
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My preference is ported. This keeps all the idle advance purely in the distributor setting, and constant. If the idle vacuum drops (like putting the car in gear) or as a result of a change in idle rpm, or even altitude and air fuel ratio, the timing is the same static amount it was set at.
With the manifold vacuum advance at idle, if the vacuum level varies, so will the timing. This can cause idle instability, and adds another unnecessary variable. Let's say the vacuum can adds 10 degrees at idle, and you have a change in conditions like any of the things listed above, now the change in idle quality will have an exaggerated effect since a drop in vacuum will pull timing out at idle, which will lower the rpm and idle vacuum even more.
Say a plug fouls and vacuum and rpm drops at idle. With manifold vacuum the idle will also lose a bunch of timing, and if the engine is tuned to require that timing to idle, it may not want to idle at all. Or if you drop 3-4"-hg dropping in gear, the idle timing will drop too and the rpm drop in gear will be more severe.
I really don't see any advantage to manifold vacuum and vacuum advance at idle. I would rather set the static timing to optimum and then dial in the right amount of mechanical advance to get the total timing where it needs to be. Then you can add the ported vacuum advance to provide additional timing during low-load cruise if the engine will tollerate it for improved economy.
If someone cares to explain specifically how adding vacuum advance during idle is a benefit, I would like to hear it.
I'd say that most of the time when manifold vacuum is used the engine is over advanced at idle, and that is why it can work without showing the symtoms of instability. -dulcich -
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Re: 440 Vacuum advance, hook up ported or constant vacuum?
[Re: SomeCarGuy]
#718030
06/08/10 06:56 PM
06/08/10 06:56 PM
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Joined: Jan 2003
Posts: 5,788 Hamilton, Ontario Canada
Magnum
master
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master
Joined: Jan 2003
Posts: 5,788
Hamilton, Ontario Canada
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Quote:
I would like to know why some think it hurts to have vaccum advance hooked up at all?
... because everyone knows max power is obtained by finding a certain total timing. So for most members that is ALL they are willing to pursue. If idle is not that important to you and mileage is not that important to you. Set the total slam the hood shut.
For the rest of the members. Idle and mileage are 2 more things you can tune into your distributor.
Being called an idiot when debating a topic. Grow up and learn your facts before you start with the name calling. Explain why or STFU. This is not YB.
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Re: 440 Vacuum advance, hook up ported or constant vacuum?
[Re: Magnum]
#718031
06/08/10 08:16 PM
06/08/10 08:16 PM
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Joined: Feb 2003
Posts: 10,178 Someplace you aren't
SomeCarGuy
I Live Here
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I Live Here
Joined: Feb 2003
Posts: 10,178
Someplace you aren't
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So you are saying people actively do not want to use the least amount of gas for a given combo? Less money for mods and cruising? I still haven't seen a reason as to why it hurts to run run a tuned vac advance. Most all of the comments against running it seem to indicate that it is a bad thing to use. I don't understand the line at the bottom of your post. If you want to act like that, then maybe you need to find another board to hang out on.
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Re: 440 Vacuum advance, hook up ported or constant vacuum?
[Re: Magnum]
#718032
06/08/10 08:33 PM
06/08/10 08:33 PM
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Joined: May 2003
Posts: 15,487 Florida
scratchnfotraction
I Live Here
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I Live Here
Joined: May 2003
Posts: 15,487
Florida
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I get the STFU...but whats YB??? and cant we all talk/discuss stuff with out insulting each other I am trying to learn here and it does not help any one ..(i have a learnig disability) I have tried it with ported...full vac...no vac never got it right and went back to ported..but i dont under stand any of it now I got a duel point mech advance only mallory dist to try...hows that gonna work with full or port vac just set the inital to were it is at with the points dist and ported vac is set at?????
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Re: 440 Vacuum advance, hook up ported or constant vacuum?
[Re: scratchnfotraction]
#718038
06/08/10 11:59 PM
06/08/10 11:59 PM
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Joined: Sep 2005
Posts: 12,481 Chino Valley
RodStRace
I Live Here
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I Live Here
Joined: Sep 2005
Posts: 12,481
Chino Valley
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I prefer ported vacuum. Okay, now that everyone has gone to one side or the other, here's my reasons. Initial advance. The fire must be lit and the biggest boom happen at the right time. The lighting is usually BTDC so that the big boom happens at about 10- 15 degrees ATDC. better and small combustion chambers don't need as much. Go out and pedal you bike around the driveway. If you push harder earlier, it doesn't do much. If you push harder later, you have lost some momentum and the pressure will still be there when the pedal reaches the bottom. So initial timing is for the design and airflow at idle. Fuel can affect this too. A slower buring fuel (some race fuels, NOT Octane) will lower the idle because it's like retarding the timing. Now the mechanics of the burn (the time between the fire and the boom) don't change, but with increased RPM, the fire must start earlier to make the boom happen at the same point. This is mechanical advance. Pedal that same bike fast, and you will probably have a point when you mis-time a push 'cause you are pedalling so fast you are anticipating that next push and shove down on a straight up pedal. That's max advance under some load (depends on if you were going uphill or down). it hurts, and it can break parts. Vacuum advance is for load. More load means the engine has to work harder and is breathing deeper. This means less vacuum, less timing. Now, the vacuum does match fairly well above ~1600 RPM, depending on how radical the cam is. At idle, there is one less variable with ported vacuum, and with fairly mild engine, this means it holds steady better. It also does decrease NOx. However, if you have an engine that is not happy idling, often due to a big cam, it makes sense to get as much help as possible to keep the throttle blades from the transfer slots and the idle smoother. Here is where manifold vacuum can help, expecially on a stick car. Today's pump fuels tend to have fewer light hydrocarbons, and more heavy ones, so it burns slower. This would call for more advance, but the stuff they sell doesn't have the anti-knock additives it used to (lead), and often has alcohol added. It can ping or knock. An older car can sometimes benefit from a bit more advance, tied to load just off idle. In each case, the initial must be tailored to which vacuum source you use. A 512 street engine should have a torque cam, not something with huge overlap. Even a fairly big cam in a 500+ cu. in. engine will idle pretty well. I'd go with ported, especially with an auto. However, if you struggle to keep the engine from dying at anything under 1200 RPM and you have made sure it's not a mechanical issue, try mechanical vacuum.
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Re: 440 Vacuum advance, hook up ported or constant vacuum?
[Re: Crizila]
#718039
06/09/10 12:07 AM
06/09/10 12:07 AM
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Joined: Dec 2003
Posts: 5,314 Carstairs, Alberta, Canada
dave571
master
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master
Joined: Dec 2003
Posts: 5,314
Carstairs, Alberta, Canada
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WOW.... this sure got outta hand OK.. constructively. I curve distributors on my machine with a secondary system built on, for many differnt combo's. Decent results so take this for what it's worth. Question why would it ever hurt, to run vac advance? Answers - on a more radical build, the mechanical advance curve may not respond well to more advance at cruising speed. A surge at cruise is a common result. -On a 4 gear car that is driven hard, the resulting vacuum applied on the shifts, throws an unwanted variable into the mix. Q Why would you simply not bother with vacuum advance? -vacuum advance's purpose, is to make the lean cruise mixture burn more completely, giving better economy. On cams with say 235 @ 50 duration or more, it simply doesn't help. That much cam makes the economy poor, and adding the vac advance makes no measurable difference. - Some feel that there are performance benefits to running vacuum advance. The truth is, if your car runs or performs better with the vacuum advance hooked up, there is something wrong with the set up. Vac advance is for economy. That said, I do understand that fuel is expensive, and these cars use a lot of it, so trying to get some more with vac advance makes sense for most mild street builds. Running manifold vacuum over ported -It was only done by the factorys in the late 70's. By then the things going on with way the cars ran, and what they were trying to get emmisions wise from carburetad vehicles made it necessary. Makes no sense on one of these, tuned for performance (and even economy) and not emmisions Why shouldn't you run manifold vacuum? -I know Don at FBO, and learned many things from him directly in the area of distributor curving. That all said, I have no idea why he recommends guys run manifold vacuum. By Don's own school of thinking, we curve the distributor to run the MOST initial timing possible, and keep the total mechanical around 34. It improves throttle response and idle quality DRAMATICALLY to do so. SO after doing that, I don't see how adding even more timing at idle is the right thing to do. Using ported, ads it only at light cruise, and THAT is what vacuum advance is for. To improve economy at light cruise. To give the lean cruise mixture, more time to burn. Don is VERY knowledgable, and does a great job on his distributors, BUT most of the ones he curves are for more radical builds, and he is a big supporter of not using vac advance at all. Ported will always do what vac advance was designed for, while I don't see how manifold will.
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Re: 440 Vacuum advance, hook up ported or constant vacuum?
[Re: Mr.Yuck]
#718041
06/09/10 08:37 AM
06/09/10 08:37 AM
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Joined: Jan 2003
Posts: 24,562 Brookeville, Md
Mr.Yuck
Not enough dumb comments...yet
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Not enough dumb comments...yet
Joined: Jan 2003
Posts: 24,562
Brookeville, Md
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Quote:
Quote:
Quote:
hence my quest why even hook it up? Isn't this a 512cid wedge? Set it for performance and have fun.
And why not hook it up? If tuned correctly you won't know it's there, except for the mileage increases.
doubt you'll see much. Not like it's going to easy to kep ur foot out of 512 cubic inches.. I couldn't anyway
I guess i should add I have my distrib's curved for the combo; engine, cam, stall, gear, car weight.....
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Re: 440 Vacuum advance, hook up ported or constant vacuum?
[Re: dave571]
#718042
06/09/10 09:41 AM
06/09/10 09:41 AM
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Joined: May 2003
Posts: 6,545 Downtown Roebuck Ont
Twostick
Still wishing...
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Still wishing...
Joined: May 2003
Posts: 6,545
Downtown Roebuck Ont
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There are a couple of reasons why you might use manifold over ported. One would be when the timing is advanced at idle the engine is less apt to heat up. My combo is efi'd so it doesn't have vacuum advance BUT if it did I could solve a minor problem it has. The engine LIKES 20deg initial. Idles well drives well etc. Occasionally the starter doesn't care for it tho when it's hot. If I had vacuum advance hooked to manifold I could pull the initial back to say 12 and it would never argue when hot and it would still idle well plus I might gain a LITTLE fuel mileage .
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