# vacuum advance to manifold vacuum?



## ziddey (Apr 16, 2006)

Next month, I'll be finally dropping the ABA in my Fox. The problem is that I will not be going the knock sensing route yet (probably next summer if I do), and would like to run 87 octane if possible. I understand people say to run 0*BTDC ignition timing AND to run 91 octane. Is that really necessary?

And now to the question in the title. I know the vacuum advance is supposed to be hooked up to pre throttle body. But considering the situation I'll be in, would there be any problems with hooking up the vac advance straight to manifold vacuum? The idea would be more advance at idle and perhaps light(er) loads, so I can maintain a reasonable idle (read: fuel economy?), but also not suffer pinging from overly advanced timing otherwise.

And if anyone has any feedback in general on running a c-flow big valve head ABA with 87 octane and the old centrifugal + vac advance distributor, I'd appreciate it.

Not exactly relevant but interesting nonetheless: http://forums.turbobricks.com/showthread.php?t=248607 I might consider that route if I don't go the knockbox route if/when I go FI. Or would that work alongside the knockbox?


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## WaterWheels (Aug 14, 2005)

ziddey said:


> I'll be finally dropping the ABA in my Fox. The problem is that I will not be going the knock sensing route yet (probably next summer if I do), and would like to run 87 octane if possible. I understand people say to run 0*BTDC ignition timing AND to run 91 octane. Is that really necessary??


Whether it’s an ABA or any other engine, the octane rating of the gas really is not the issue but the timing itself. Yes there are extreme situations where you just can not operate a particular engine on low octane fuel, not run but operate, but most people do not fall into that category. The knock sensor ignition was not, in my opinion at least, used because of the engine size or static compression ratio, but rather to make the car more “user friendly” while providing the most performance possible without having to make adjustments. You could use whatever fuel was at hand without worry of the engine not running right or causing damage, it just adjusts itself to the fuel. 

0° BTDC seems to be a bit extreme in the caution direction. Maybe it was due to poor ignition distributor choice (bad match) or due to poor set-up or who knows. One major problem with people trying to use “old” mechanial/vacuum ignition distributors with “modern engines” is that they don’t have the knowledge of how things work or how to alter them to work. With the onset of everything being electric many people just never learned the way things worked before electronics came into being, just like carburetors vs. EFI systems. So the short answer is no it is not necessary. If one has to set the static advance at 0° then the ignition distributor has the wrong built in advance and needs to be either swapped out or altered internally to fix things. 




ziddey said:


> I know the vacuum advance is supposed to be hooked up to pre throttle body. But considering the situation I'll be in, would there be any problems with hooking up the vac advance straight to manifold vacuum? . . . but also not suffer pinging from overly advanced timing otherwise.


To be honest, the ignition distributor should really always be connected to manifold vacuum. The “great” idea of using venturi vacuum was due to emission standards and before the EPA started screwing things up (yeah yeah) all engines used manifold vacuum for ignition advance. Also to be honest, venturi vacuum is only different when the throttle is closed or very near closed,.any other position is the same as manifold vacuum. 

Again to be honest, I have not used knock sensor ignitions for many years and convert all my motors to mechanical/vacuum (or straight mechanical). If you want a smooth idle and better throttle response then using a venturi vacuum port is not the key, use a manifold vacuum port. This may be one reason for the 0° static setting, but that falls back to the wrong distributor or the need to alter it for correct timing. Just as setting the static advance that low is a "tuning" issue so is pinging. If engine design proves to be so that it can not be made to run at it's best without raising the octane then that is what must be done. But if top performance is not the goal then you should be able to alter the ignition timing so the engine runs fine on the fuel you wish to use. Compromise always finds it way into everything, ignitions too.


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## Seax_Smith (Jun 1, 2007)

WaterWheels said:


> Compromise always finds it way into everything, ignitions too.


All actually modding comes down to a simple equation.

What you end up with = what you have + what you know + what you can find out + what you are willing to spend in time and cash.


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## Glegor (Mar 31, 2008)

Seax_Smith said:


> All actually modding comes down to a simple equation.
> 
> What you end up with = what you have + what you know + what you can find out + what you are willing to spend in time and cash.


i run ~15* advance, and im running 87 octane no problem.

ran even more advance on my GTI, something like 22*, and it would still run on regular, just not good. i advanced the timing specifically to run best on premium..


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## ziddey (Apr 16, 2006)

Thanks Waterwheels. That makes a lot of sense and is exactly what I wanted to hear . So on a "normal" setup, that might mean better fuel economy in city driving with lots of idling. I wonder just how big a difference in fuel consumption there is with just differences in idle timing. I wonder how it'll affect the fuel curve with a cis setup. Figure lambda feedback will keep me ok as long as I tune it to be slightly rich and just have it settle around a lower dpr current at idle.

Glegor, but doesn't the 85 gti have a knock box system? Seems all the posts I've been reading about ABA c-flow 8v swaps have people running 0*BTDC static timing and 91+ octane (hall effect centrifugal + vac advance distributor). If I can settle for 87 octane and 0*BTDC with the vac advance pinched, I might be ok with it. How much advance does it provide at idle vacuum (assuming 18")?


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## WaterWheels (Aug 14, 2005)

ziddey said:


> I wonder just how big a difference in fuel consumption there is with just differences in idle timing. I wonder how it'll affect the fuel curve with a cis setup. Figure lambda feedback will keep me ok as long as I tune it to be slightly rich and just have it settle around a lower dpr current at idle.


There really should be almost zero difference if the rest of the engine is tuned with the timing set that way. When you connect the vacuum line to manifold vacuum the engine speed most times goes up due to the vacuum canister adding more timing advance. Then you turn the distributor body to set the timing back to what you had/want it. So you are not really changing anything but the source of vacuum being used and the fact that the distributor vacuum advance is part of the setting where before it really was not. So the fuel curve is not altered unless you don't adjust the idle back to normal so the air sensor plate is where it was before. The mixture is set using the DPR current just like you did before, only thing is the ignition timing is controlled only by the ignition distributor, like early CIS-e, and the vacuum source is manifold rather than venturi.


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## ziddey (Apr 16, 2006)

Time to bring this up again since the ABA swap is close to being complete. 

I suppose a more basic question would be appropriate. What conditions am I expecting to see pinging at? Heavy load, low RPM?


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## WaterWheels (Aug 14, 2005)

That my friend depends . . . It can happen under a few of different conditions depending one a few different things. The engine itself does not make any kind of pinging, it is all caused by wrong tuning for the way the engine is used. You can run a 14:1 compression engine on low octane fuel with the correct tuning and an 8:1 compression engine can ping under the wrong tuning. Once you have everything together and running then you have to start doing testing and adjusting as needed.


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## B4S (Apr 16, 2003)

ziddey said:


> Time to bring this up again since the ABA swap is close to being complete.
> 
> I suppose a more basic question would be appropriate. What conditions am I expecting to see pinging at? Heavy load, low RPM?


 Detonation is most likely to happen at the torque peak, since that's where the engine is working hardest.


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