[SuziBlu]

lots of dizzys have mechanical only.

[SuziBlu]

Thro a harley carb on, any side draft, and use mechanical advance only, or so I thought till just now, educate me, I have never done it, just read shitloads about it, and how it is far superior for modified engines, as you can calibrate the timing curve far more accurately.

[joe5701]

I assume the centrifugal advance comes in at the higher end of the rev range and the vacuum works in the lower revs? so I spose if I was to stretch the springs a bit the centrifugal advance would come in a bit lower, but it would be at full advance before it needs to be.

In saying that the old dizzy only use's centrifugal advance, maybe differant spring tensions.
want33s said he used vac advance on a 1ltr and it worked great so i'll give it a go, I just hope I manage to get the drill through without destroying my carb

[joe5701]

dunno if they fit but I could pull the weights and springs out of the old dizzy and put them in the new one.

[royce]

Why the hell must every freakin ignition topic end up a roundabout of crap?

Vacuum advance does three fifths of fuck all, it gives ignition advance at low throttle high vacuum periods to aid fuel consumption, another 2psi in your tyres would have a greater effect

vacuum advance is also on top of centrifugal advance, all the springs and weights are still under there

[joe5701]

well fark me i'm going out to put more air in my tyres in the morn . what I was getting at is are the weights and springs out of a 1.3 dizzy the same as the 1.0 dizzy? differant engines, just thought if I used my old weights I would know they are suited to my engine (if they fit). but fuck it will work either way so i'm not fussed just curious.
Royce are you ment to take medication for that temper of yours ?

[JrZook]

royce wrote:

Why the hell must every freakin ignition topic end up a roundabout of crap?

Vacuum advance does three fifths of fuck all, it gives ignition advance at low throttle high vacuum periods to aid fuel consumption, another 2psi in your tyres would have a greater effect

vacuum advance is also on top of centrifugal advance, all the springs and weights are still under there

Really?

Well ya better go write an email to all those vehicle manufactures that they should put 2psi more in there tires instead of wasting their time mapping ignition to manifold vacuum and RPM......

Got to do all about efficiency which effects fuel economy yes, also throttle response and engine running temperatures.

The major factors influencing volumetric efficiency (VE) of the engine is engine speed (RPM) and throttle plate opening (load). Mechanical advance mechanism takes control over the ignition advance requirements for engine speed at maximum VE (at WOT).

Vacuum advance, weather ported or full alters the ignition timing at lower VE's. The greatest effect on VE is at low/mid throttle openings (greatest percentage change). Under light throttle openings the manifold vacuum is lower, thus the cylinder pressures are lower, the burn rate is slower and therefore the VE is also lower. Since the burn rate is slower, the flame remains in the CC longer and as an effect also increases the heat within the CC which will have to be dissipated by the cooling system.

Will add in here, ported vac has little to no vacuum at idle but will have manifold vac once the throttle is opened beyond, where as full manifold vacuum has full vac at idle. Possible reasoning to use ported manifold vac at idle would be to increase the combustion temperatures and thus effect the chemical composition of the exhaust gasses for emissions.

Will the slowed burn rate and thus lower VE the engine also will react slower to throttle opening transients (engine response, linked to drive ability). Obviously the full effect of these are dependant on engine type and what the engine is used for.

Suziblu: Those high performance engines most likely spend most of their time above half throttle in which the percentage change of VE is less drastic. Added to that, lumpy cams and highly fluctuating manifold vacuum at low speeds/throttle openings, the use of vac advance would not very viable as you will not have a stable source.

In a nutshell, running no vac advance (less than optimal ignition timing) at low loads can lead to economy, running temp and response issues.

My 1/2c worth.

[UBZ]

Umm

The Vac advance on a G13 Dizzy is plumbed to a port on air cleaner side of the secondary throttle plate in the carburetor..
Under heavy acccelloration , or load the secondary throttle opens.
The dizzy Vac advance is exposed to manifold Vac , Which advances the timing to balance the extra fuel.

the F8A / F10A carburetor is a single throat side draft with no provision for Vac plumbing.
If you just plumb the advance in before the throttle plate , the dizzy is going to go to full advance every time the throttle opens past idle.

a carry carburetor with the right plumbing would be Awesome !!! Any more info on this ????
It would save the hassle of trying to calibrate the correct vaccum restriction to ensure Vac advance only when the secondary fuel valve opens F Carb.

Mechanical advance starts at approx 1200 rpm and is gradual until approx 5500 rpm.

At the end of the day you are taking a Dizzy suited to a 1300 and putting it on a 800 or 1000cc engine . Most people I've talked to have not bothered with the Vac advance and are very happy with the performance of the dizzy.

My 2c.

[royce]

JrZook wrote:

royce wrote:

Why the hell must every freakin ignition topic end up a roundabout of crap?

Vacuum advance does three fifths of fuck all, it gives ignition advance at low throttle high vacuum periods to aid fuel consumption, another 2psi in your tyres would have a greater effect

vacuum advance is also on top of centrifugal advance, all the springs and weights are still under there

Really?

Well ya better go write an email to all those vehicle manufactures that they should put 2psi more in there tires instead of wasting their time mapping ignition to manifold vacuum and RPM......

Got to do all about efficiency which effects fuel economy yes, also throttle response and engine running temperatures.

The major factors influencing volumetric efficiency (VE) of the engine is engine speed (RPM) and throttle plate opening (load). Mechanical advance mechanism takes control over the ignition advance requirements for engine speed at maximum VE (at WOT).

Vacuum advance, weather ported or full alters the ignition timing at lower VE's. The greatest effect on VE is at low/mid throttle openings (greatest percentage change). Under light throttle openings the manifold vacuum is lower, thus the cylinder pressures are lower, the burn rate is slower and therefore the VE is also lower. Since the burn rate is slower, the flame remains in the CC longer and as an effect also increases the heat within the CC which will have to be dissipated by the cooling system.

Will add in here, ported vac has little to no vacuum at idle but will have manifold vac once the throttle is opened beyond, where as full manifold vacuum has full vac at idle. Possible reasoning to use ported manifold vac at idle would be to increase the combustion temperatures and thus effect the chemical composition of the exhaust gasses for emissions.

Will the slowed burn rate and thus lower VE the engine also will react slower to throttle opening transients (engine response, linked to drive ability). Obviously the full effect of these are dependant on engine type and what the engine is used for.

Suziblu: Those high performance engines most likely spend most of their time above half throttle in which the percentage change of VE is less drastic. Added to that, lumpy cams and highly fluctuating manifold vacuum at low speeds/throttle openings, the use of vac advance would not very viable as you will not have a stable source.

In a nutshell, running no vac advance (less than optimal ignition timing) at low loads can lead to economy, running temp and response issues.

My 1/2c worth.

pity you dont have a 1.3, you could go unplug yours and see how little real world difference it makes

[royce]

UBZNZ wrote:

Umm

The Vac advance on a G13 Dizzy is plumbed to a port on air cleaner side of the secondary throttle plate in the carburetor..
Under heavy acccelloration , or load the secondary throttle opens.
The dizzy Vac advance is exposed to manifold Vac , Which advances the timing to balance the extra fuel.

its plummed to a port on the primary carby throat that is exposed to atmosphere at idle and manifold vacuum at lower throttle angles which then allows it to advance and as the throttle is opened more the vacuum levels drop again to more like manifold vacuum which drops off the advance

This is totally independent of the centrifugal advance

Joe, you could change them but the base plates are going to be different weights so it may behave differently anyway, plus how would you test the difference?

[SuziBlu]

royce wrote:

JrZook wrote:

royce wrote:

Why the hell must every freakin ignition topic end up a roundabout of crap?

Vacuum advance does three fifths of fuck all, it gives ignition advance at low throttle high vacuum periods to aid fuel consumption, another 2psi in your tyres would have a greater effect

vacuum advance is also on top of centrifugal advance, all the springs and weights are still under there

Really?

Well ya better go write an email to all those vehicle manufactures that they should put 2psi more in there tires instead of wasting their time mapping ignition to manifold vacuum and RPM......

Got to do all about efficiency which effects fuel economy yes, also throttle response and engine running temperatures.

The major factors influencing volumetric efficiency (VE) of the engine is engine speed (RPM) and throttle plate opening (load). Mechanical advance mechanism takes control over the ignition advance requirements for engine speed at maximum VE (at WOT).

Vacuum advance, weather ported or full alters the ignition timing at lower VE's. The greatest effect on VE is at low/mid throttle openings (greatest percentage change). Under light throttle openings the manifold vacuum is lower, thus the cylinder pressures are lower, the burn rate is slower and therefore the VE is also lower. Since the burn rate is slower, the flame remains in the CC longer and as an effect also increases the heat within the CC which will have to be dissipated by the cooling system.

Will add in here, ported vac has little to no vacuum at idle but will have manifold vac once the throttle is opened beyond, where as full manifold vacuum has full vac at idle. Possible reasoning to use ported manifold vac at idle would be to increase the combustion temperatures and thus effect the chemical composition of the exhaust gasses for emissions.

Will the slowed burn rate and thus lower VE the engine also will react slower to throttle opening transients (engine response, linked to drive ability). Obviously the full effect of these are dependant on engine type and what the engine is used for.

Suziblu: Those high performance engines most likely spend most of their time above half throttle in which the percentage change of VE is less drastic. Added to that, lumpy cams and highly fluctuating manifold vacuum at low speeds/throttle openings, the use of vac advance would not very viable as you will not have a stable source.

In a nutshell, running no vac advance (less than optimal ignition timing) at low loads can lead to economy, running temp and response issues.

My 1/2c worth.

pity you dont have a 1.3, you could go unplug yours and see how little real world difference it makes

Considering you asked me how a dizzy advances without a vacuum to do it, then you come up with this, ID say you did a bit of googling and found something that sounded like what you thought. But still no understanding, bit of cut n paste maybe a few words here an there chnaged so as to make it sound like your own.

Like Royce says, if you had a vehicle with a vacuum advance dizzy, or really knew, you would know what little diff the vacuum advance makes.

Disconnect the vacuum advance on 99% of the populations dizzy, and they would have no idea, power, fuel economy or temp wise.

[joe5701]

well got it all hooked up properly this morning (no vac adv) runs great as it is, I aint gunna fuck with the dizzy anymore for such a small gain if any. But I reckon it could do with a bit more fuel, I took it for a quick drive and pulled the plugs when I got back they were white.

Other than drilling jets is there any way to give it a bit more fuel up top?

[JrZook]

royce wrote:

pity you dont have a 1.3, you could go unplug yours and see how little real world difference it makes

I did have the 1.3 in the past remember? Surprisingly enough when I got it the vac advance unit had a split diaphragm. So in my young wisdom seeing it didn't work why not experiment to find out how it performed with one to see if I really needed an extra addition to it, otherwise like you said, why even bother.

When I got another vac can that actually did work I found it made fuck all difference to the power at WOT (obvious now why not) but at lower speeds and less throttle It took less 'pedal' to get going. You did not need large movements to get the bloody thing to pick up. Ended up nicer to drive so I left on on there.

That was my limited experience playing with the 1.3 wayy back without actually understanding the engine dynamics and what really was going on inside for the improvement, but it made a difference. The vac can was deffo not optimized to the engine nor was the mech advance curve and thus after I gained more knowledge I set upon trying to make a digital ignition system for the 1.3 based on load/RPM (Damo remembers this one). Only got as far as setting it up and getting it running on the 1.3 until I got a super deal on a 1600 EFI which abandoned that project. Flipside is teh project is not completely dead, its getting a new lease on life on my EFI bike project, which didn't have a vac advance system to start with.

Another real issue I came across not running vac advance different motor, Chrysler LA318 in the bash car. Previous racers decided to remove the vac advance in favour of pure centrifugal. Worked sweet as while racing and under WOT conditions. Cruising between courses at light throttle it would get hot and start to overheat. The experts replaced everything in the cooling system and hey no change! Had them fucked! Dan's alleged stupid suggestion, lets put the vac advance back on. All looked at me like I was on drugs, young electronics guy WTF would he know? We tried it and it worked! That also increased the engine's response and we could actually skid steer the thing around very low speed tight hairpins.

[JrZook]

JrZook wrote:

SuziBlu wrote:

Or you can play with the mechanical advance weights bypassing the vacuum advance.

And.................................?

How do you get it to advance the timing under low load and cruising conditions?

Suziblu please stop changing the question when it has been clearly stated previously.

SuziBlu wrote:

Considering you asked me how a dizzy advances without a vacuum to do it, then you come up with this, ID say you did a bit of googling and found something that sounded like what you thought. But still no understanding, bit of cut n paste maybe a few words here an there chnaged so as to make it sound like your own.

I tried to explain the science/actual reactions behind what is actually happening inside. It's very well know stuff and is basically an extension of the ideal gas law in which I am implementing in my EFI controller. Trust me is there was a simpler way to be doing all this I would! Hair ripping out material!