DAVID GRIMES

I see several people on this forum ditch their vacuum advance. May I ask why ? And if I decide to do so, how is it done ? eg. Besides pulling the lines from the distributor, do I plug them ? Leave 'em hanging ?

In case it matters, I have a 4 port 13b ( rally ported ) in an '81. No nest. Mikuni sidedraft. Haven't done the DLDFIS mod yet, but I plan to.

If I do get rid of the vac advance, should I reset my timing ? To what ?

As usual, thanks.

vipernicus42

iTrader: (4)

Yeah, I'm wondering this too.

Can you cap the two vac advance diaphrams, set your timing with them capped and be good to go? Why did Mazda make our cars have vac advance? Was it emissions-related, or is it good for power too?

I'm considering removing it eventually... when I figure out wherethehell my timing marks are and time the car. I think if it's not detrimental to power and can run fine without vac, then it'd make the rat's nest removal even cleaner/easier.

Another thing I'm wondering has to do with the rat's nest and it's role in the vac advance. If you look at the nest, there are two electrical connectors attached to valves for the vac advance (leading and trailing respectively). This would lead me to believe that when *certain conditions* are present those switches would have an effect on advancing the timing. In Carl's tutorial the vac advance is run off a straight vacuum souce. Wouldn't that advance the timing too early?!

Jon

SilverRocket

I think vacuum advance is solely to reduce emissions, rather than having anything to do with performance. I got rid of vacuum advance a loooong time ago. Do not cap the inlets on the dizzy - leave them open!

hornbm

WRONG


Everybody Ive talked to that did an after market carb setup and did have the vacuum advance connected, then later connected it said it pulls WAY WAY harder down low with the vacuum advance connected.

DAVID GRIMES

Okay. But what do I do with the lines ? Cap at source ( to clean up ) ?

Also, I asked about the timing because as I understand it, vac is mostly for low RPM advance ( and is quick about it, but maxes out low ) while mechanical advance ( centrifical ) is for higher RPM's. Is mechanical more "linear" in response across the RPM spectrum ( from idle to 8000 ) ?

SilverRocket

WRONG eh?

No offense, but I don't think you understand how vacuum advance works. It only has any effect when *vacuum* is present; ie. at idle. Once you're WOT, there is *no* effect whatsoever.

How do you explain pulling "WAY WAY" harder; is this supposed to be at idle??

BTW, most aftermarket carb setups that I'm aware of (including the one I run) have no provision for vac advance as far as I'm aware.

SilverRocket

Vacuum advance has no impact on WOT acceleration. Mechanical advance is somewhat linear until it maxes out somewhere around 4k rpm.

Yes, you should cap the source (ie. where the lines came from) - just don't cap the inlets on the distributor.

red13brx7

Yea, don't cap the dizzy, you can let them hang there. vacuum advance just helps lower RPM driving, not just idle. It really doesn't effect higher RPM performance.

CarlRx7

the vac advance does use vacum tru. but not manifold vaccum. its usually a port that sits on the other side of the throttle plates. the vac advance is mainly for idle and cruising to give you better gas mileage., as the air goes past the port (for the advance) it creates a slight suction, advancing timing.

my 2 cents

carl.

hornbm

what you say makes perfect sence. Mabey they mena pulled harder at part throttle. BTW, these were all connected to manifold vacuum. Just drilled and taped a new vacuum port in the manifold

NickSA22

I've tried both ways. It is true that it pulls better at low RPM's with the vacuum advance but at least for my car, my power flat lined at 5 to 7 grand. Thats the sweet spot so I unhooked and plugged the vacuum advance lines, and it pulls better. Why get marginal gains at lower RPM's at the expense of your major power band?

DAVID GRIMES

ok, I'm convinced. I will do away with vac adv this Saturday. Thanks for the info, guys !

Jeff20B

iTrader: (13)

I have always treated vacuum advance like 7Up.

The reason to leave the nipples open to the atmosphere is so when you go up and down mountains and stuff, the air on both sides of the diaphrams can stay equal. If you capped them, what do you think would happen?

vipernicus42

iTrader: (4)

Another thread I was reading mentioned that without vac advance your gas milage goes to ****... is this true?

Jon

DAVID GRIMES

The pressure differential would in effect be a vacuum to the insides and advance unnecessarily. Fortunately, the biggest mountains in Southwest Georgia are the landfills so I don't have that concern. Jeff, will you tell me why you don't use vac advance ? What are the gains and losses from removal ( in depth if you have time ) ? My main concern: I am going to go to your direct fore timing scheme ( DLDFIS ), so if I ALSO remove vac advance will I still get the easy starts and smooth idling and no more backfiring ???

Thanks ahead of time !!!

Jeff20B

iTrader: (13)

You could keep vacuum advance if you wanted to. I'm actually not a very good person to ask because I've always been free of vacuum advance. I have no basis for comparison with an engine that has vacuum advance.

Vacuum advance has nothing to do with DLIDFIS. Go ahead and run it with vacuum advance and without to see which you like better.

DAVID GRIMES

Thanks, Jeff. I got my "black box" that I plan to make a totally clean DLIDFIS mod with. Don't worry, it will be cool enough ( temperature-wise and otherwise ). Just as soon as my CPRacing tower brace gets here. Hijacking my own thread, I realize, but I see some 50,000 volt vette coils for the price of Blaster 2's ??? Hmmm... whadayathink ? More is better, right ?

Back to subject: Anybody out there that can tell me the possible reason(s) / merits for removing vacuum advance on a N/A 13B ?

Tranquil

iTrader: (1)

bump an old thread

I seriously would like to know the answer to removing vacuum advance.

vipernicus42

iTrader: (4)

Amazingly enough this came up in a thread on a completely different topic.

https://www.rx7club.com/showthread.p...8&page=6&pp=15

Summary: Leave it connected. It only affects idle and cruising timing anyway (when manifold vacuum is highest).

If you remove the rat's nest, just T both of them together and plug it to the second port in the carb spacer (as per Carl's tutorial, or the thread I did about PCV valve installation)

Wow... I sure have learned alot since I originally posted in this thread.

Jon

j9fd3s

iTrader: (3)

the vacuum advance, does what it says, it advances the timing while you're cruising, you get more power, better mileage etc.

in a race car, you arent at part throttle, and you want simple, so you want to ditich it. in case you're wondering, you dont have a race car.

Rogue_Wulff

Vacuum advance pulls vacuum from above the throttle plates (AKA venturi vacuum). This means it only sees vacuum when the throttle is open. Even at WOT, there is still vacuum going to the advance pots, although, not as much at lower RPM. This is the same area that the stock vacuum secondaries get their vacuum signal from.
Leaving it hooked up improves part-throttle driveability and power, and the side effect of this is improved MPG, and better emmissions.
Racers disable the vac and mech advance, and set to timing at what they want for the total advance, since their engines seldom see lower RPM under race conditions. They also don't care about MPG or emmissions.
Running a street car with the timing advanced too far can lead to the engine running warm, possibly even overheating.

novaboy009

Keep it! With manifold vac: improved drivability, idle, gas mileage and part throttle response with no effect on WOT.

Kev

Tranquil

iTrader: (1)

The vacuum advance is the only vacuum I'm needing from my carb spacer. I'm hoping to ditch the spacer in lieu of a nitrous plate I'm having made for me with some "spare" money.

Are there any alternatives to the vacuum advance? Nector was telling me something about soldering the ***** of the dizzy. I'll just have to ask his uncle, or something.