Vaccum Advance Control

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Wilts289
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Vaccum Advance Control

Post by Wilts289 »

Just treated myself to the Aldon Amethyst Mappable Ignition Box for a Rover 3.9

http://www.aldonauto.co.uk/shop/product ... 8&P_ID=800

and very nice it is too.
Very easy to install, apart from re-fitting the distributor 180 degrees out and then seeing 2 foot flames coming out the the 4 barrell Weber, and childs play to "map" the advance curve. I have opted for the Vacuum version but haven't really got a clue as how to "map" this function. Most people recommend disconnecting this, particularly when using the Weber/Edelbrock, but now I have the ability to control this what do I do with it?


DaveEFI
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Post by DaveEFI »

With a mechanical dizzy, the vacuum advance needs to be fed from above the throttle, to prevent advance round about idle. With a fully mapped system, you can set the advance to whatever you like at any vacuum or revs. So it should be possible to map it for any carb. Just take the vacuum feed from somewhere the same as the brake servo - rather than a dedicated dizzy feed, so it is seeing true inlet vacuum at all times.
Dave
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Wilts289
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Post by Wilts289 »

Is there a basic guide to how much advance for a given amount of vacuum?
The controllable range on this unit is 0"Hg - 14"Hg in 2Hg steps so where would I start?
kiwicar
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Post by kiwicar »

Hi
this is going to be one of those times where your driving style and "feel" are going to dictate what you finally end up with.
the way to go about it is to set up the advance curve without any vac advance added so you get the best combination of power under full load and pickup off the throttle to full throttle without any detonation, if you get detonation at any point whilst doing this make a note of it. Once you have a best power advance curve and are a bit used to how it drives then run a couple of tank fulls of petrol through the car and get some good and accurate fuel consumption figures, do a tank on motorway driving and get a consumption figure, a tank full on brisk A road driving and a tank full of general daily driving.
if you can plumb in a vac gauge this will make things easier as you will be able to relate manifold vac with throttle position and speed.
Now with your base line driving figures put in some vac advance, say 2 degrees at 2 in Hg going up to 12 extra at 14Hg and go out and drive it on a dual cartridge way for about half an hour, how does it drive . . Does it pick up OK? does it cruise at a steady speed without hunting? Does it pink as you get to a slight hill? If all is OK add in another 2 degrees of advance around the middle range and another 1 at either end, drive it again how is it now? do you need more or less throttle at a given speed? Is there more manifold vacuum at a given speed? does it pink when you open the throttle? what you are aiming at is a curve that lets you drive on a motorway with the lightest throttle, highest vacuum for a given speed, but without pinking and where the engine will still pick up properly.
Now drive it on A roads, does it drive without hesitating? it should pickup better and be easier, but if it doesn't work tweak it by a degree or two (though for A road tuning you will need a vac gauge)
Once you are happy with how it drives go get some new economy figures under motorway and A road conditions, you may be able to fine tune the last degree of advance on the map using the economy figures.
best regards
Mike
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Wilts289
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Post by Wilts289 »

Great Mike, that's exactly the guidance I need and a good excuse to get out on the road. Fortunately the device/software has a graphical vacuum gauge incorporated and changing settings can be done on the fly - so will hook up the Notepad and book my son in a co-driver/mapper.!

Many thanks
Brian
Wilts289
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Post by Wilts289 »

Great Mike, that's exactly the guidance I need and a good excuse to get out on the road. Fortunately the device/software has a graphical vacuum gauge incorporated and changing settings can be done on the fly - so will hook up the Notepad and book my son in a co-driver/mapper.!

Many thanks
Brian
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Post by sidecar »

I guess I bang on about the Lucas vac advance system not working with the Eddy carb more than most people and I still stand by that. However once you have a mappable system the world is your oyster!

I run an MSD system with a locked out lucas dizzy and I guess you are doing the same sort of thing. I recently fitted an MSD MAP sensor to the system and after a bit of fiddling it seems to work pretty well. I hope my MPG has gone up and the engine is a little more responsive at small throttle openings. (I've not been out in the car much as it is an open top 'summer' car)

This is what I did:-

Fitted a vac gauge to the engine on the non-timed port then made a note of all of the vac reading at different throttle positions. At idle it pulls about 15",
very mild acceleration about 10" and cruise is about 15" or higher.

Unfortunately because the idle and the cruise pull the same vacuum I was not able to use the non-timed port, the MSD would not know what state the engine was in and therefore would add advance at idle which I did not want.

I ran the above tests again using the timed vac port....Idle 0" of vac, all the other readings were the same. I also noticed that when pulling out of junctions at normal acceleration rates the engine pulled about 5-7" from the timed port. (I'm sure the non-timed port would have been much higher but the timed port was only just being exposed to the manifold vacuum)

I then set the MSD MAP system up with it connected to the timed port, it starts to add advance at 10" and it is fully added at 15". It adds 14 degrees more to the timing. These settings mean that I don't get any nasty kick-back when pulling out of junctions, I do get some extra vac advance under mild acceleration when the VE must still be poor, I get 14 degrees under cruise when the VE is very poor and I get no vac advance when the engine is loaded under WOT or near WOT.


Just for your info I run a stage III 4.6 lump, my timing is 18 degrees at tickover up to 1000 RPM, it then advances to 25 degrees at 2200 RPM, the rate of advance then drops off because I reckon I'm in the RPM range where the torque and therefore the VE is highest...it advances to 28 at 3700 RPM. It then flat-lines at 28 degrees all the way to 4800. (28 being the figure that people like JE reckon you should run at). However I reckon above 4800 RPM the VE must be dropping so my system adds another 4 degrees bring the total to 32 at 5400 RPM. My vac system can add 14 degrees at any point if the conditions so in theory above 5400 RPM under high vacuum I could run 46 degrees although I doubt that this would ever happen apart from on over-run.


My NOS system knocks off 4 degrees when ever it is running (The MSD is a good system!)

My timing curve and theory is NOT backed up with any dyno runs at the moment!(Just with a load of book reading!) I have noticed that before I fitted the system my cruise vac was around 15" it is now up at 17" which must mean that the throttle plate is open less. however the timed port can throw a spanner in the works here because at some point the throttle is open so little that the times port figure could actually fall. I guess the non timed port is what I should look at when judging before and after changes even if the timed port is what I use to make those changes.

IHOH
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