Believe what you want to believe.

This was common knowledge.

https://en.wikipedia.org/wiki/Ignition_timing#Mechanical_ignition_systems

 

apparently the sudden movement/speed of the piston creates that vacuum which will reduce gradually as we open the throttle. means more % volumetric efficiency at WOT.

That is do to with resonance, and is only effective at a certain RPM range, not over the whole WOT time. The piston it's self creates no extra vacuum at WOT, At WOT there is less vacuum in the intake manifold, but less restriction.
Quote, "This process is called inertial supercharging and uses the resonance of the intake manifold and the mass of the air to achieve pressures greater than atmospheric at the intake valve" from - https://en.wikipedia.org/wiki/Volumetric_efficiency

 

This was common knowledge.

https://en.wikipedia.org/wiki/Ignition_timing#Mechanical_ignition_systems

That's fine for the centrifugal advance and I don't dispute it and in an earlier post said as much. But that is not how vacuum advance is used.

Quote, "Vacuum timing advance
The second method used to advance (or retard) the ignition timing is called vacuum timing advance. This method is almost always used in addition to mechanical timing advance. It generally increases fuel economy and driveability, particularly at lean mixtures. It also increases engine life through more complete combustion, leaving less unburned fuel to wash away the cylinder wall lubrication (piston ring wear), and less lubricating oil dilution (bearings, camshaft life, etc.). Vacuum advance works by using a manifold vacuum source to advance the timing at low to mid engine load conditions by rotating the position sensor (contact points, hall effect or optical sensor, reluctor stator, etc.) mounting plate in the distributor with respect to the distributor shaft. Vacuum advance is diminished at wide open throttle (WOT), causing the timing advance to return to the base advance in addition to the mechanical advance. " From the same Wiki you posted. Read the first full sentence.

 

Yes indeed! From wiki:

"Vacuum advance is diminished at wide open throttle (WOT), causing the timing advance to return to the base advance in addition to the mechanical advance."

Just like I stated.

 

Stop and think about it, please. Don't know your age or mechanical ability but know mine. I grew up working on these old cars. Have you ever did a "tune up" in an old points ignition car? And set the timing? what is the first thing you do? You disconnect the vacuum advance. Then you set the "base timing" with a timing light and the timing mark depending on the engine that is either on the damper on the cranks front or on older cars the flywheel. When you hook the vacuum advance back to the line it retards the timing, because the vacuum line goes on the far side of the vacuum canister and it pulls the points plate to retard it by a certain amount. As the throttle opens, there is less vacuum and the points plate then starts to advance back toward the "base timing" you set earlier. As you open the throttle farther the vacuum gets less and the timing is at the base setting, but can still use more advance, that is when the centrifugal advance takes over to it's full amount.

 

When you hook the vacuum advance back to the line it retards the timing, because the vacuum line goes on the far side of the vacuum canister and it pulls the points plate to retard it by a certain amount.

Sorry, but this is not correct. Read the single sentence I quoted from wiki. The reason it's called "vacuum advance" is because it adds advance. Why would you want to retard the timing at high vacuum? The lower pressure intake charge at idle burns slower, and requires more advance, not less. I am not that young. The 1991 Mustang I played with back in the day had computer controlled ignition, but my friend's 1974 IH Scout did not. From that same wiki page:

"Vacuum advance works by using a manifold vacuum source to advance the timing at low to mid engine load conditions by rotating the position sensor (contact points, hall effect or optical sensor, reluctor stator, etc.) mounting plate in the distributor with respect to the distributor shaft."

 

These guys outta know:

https://help.summitracing.com/app/answers/detail/a_id/5127/~/what-is-a-vacuum-advance?

See point #4.

 

Or it doesn't advance at all. Many small engines run at a set ignition timing point, no advance or retarding at all. What is your bike's model?

thank you
bike is a Honda CBR250RR 1993. it's an inline 4 engine. ignition module is a TCI. has a wasted spark system with two ignition coils.
this is the inside of our TCI ( same bike model ) i found from the internet. sorry i have zero knowledge about these type of complex things. all i see is a big IC ( OKI and INTEL printed on it ) and three big transistors. according to Honda diagrams and explanations they use two transistors to open the grounded path of the ignition coil primary winding with the help of signals given by the pick up coil.
sorry if this is a silly question. i recently got to know that people repair these TCIs. is it possible ? i mean do they just replace the common things like resistors capacitors ? are those transistors and big IC replaceable ? the people who are in to these things say that these TCIs are pretty much robust and not prone to failures. that's why i asked. thx again

That is do to with resonance, and is only effective at a certain RPM range, not over the whole WOT time. The piston it's self creates no extra vacuum at WOT, At WOT there is less vacuum in the intake manifold, but less restriction. Quote, "This process is called inertial supercharging and uses the resonance of the intake manifold and the mass of the air to achieve pressures greater than atmospheric at the intake valve" from - https://en.wikipedia.org/wiki/Volumetric_efficiency

yea true. as we open the throttle the pressure inside venturi gets closer to atmospheric. i think if we consider volumetric efficiency, at WOT mixture rushes to the cylinder with less restriction. so has a greater actual volume. when it comes to a fraction over the theoretical volume it has a higher value than when it's in idle or low rpm. hope i'm correct.
by the way i didn't get that resonance thing in wiki. have to google it with some more examples and get back here again. thank you

 

These guys outta know:

I'm done beating my head against the wall.

 

I'm done beating my head against the wall.

I know just how you feel...

 

@shortbus ok i think they make use of the resonance to achieve volumetric efficiency above 100% in a naturally aspirated engines. thank you

 

@shortbus ok i think they make use of the resonance to achieve volumetric efficiency above 100% in a naturally aspirated engines. thank you

They do it on both ends of the cylinder. Intakes they adjust the runner length and use "velocity stacks". Exhaust the use "headers" in 4 strokes and "expansion chambers" in 2 strokes. There are books written about the theory of doing it.

 

Stop and think about it, please. Don't know your age or mechanical ability but know mine. I grew up working on these old cars. Have you ever did a "tune up" in an old points ignition car? And set the timing? what is the first thing you do? You disconnect the vacuum advance. Then you set the "base timing" with a timing light and the timing mark depending on the engine that is either on the damper on the cranks front or on older cars the flywheel. When you hook the vacuum advance back to the line it retards the timing, because the vacuum line goes on the far side of the vacuum canister and it pulls the points plate to retard it by a certain amount. As the throttle opens, there is less vacuum and the points plate then starts to advance back toward the "base timing" you set earlier. As you open the throttle farther the vacuum gets less and the timing is at the base setting, but can still use more advance, that is when the centrifugal advance takes over to it's full amount.

I wish my garage wasn't a mess right now so I could make a video. It's too cold and windy outside. Vacuum advance means vacuum advance!! I have a Ford 360 distributor right in front of me and if I pull on the advance cam it advances the timing. The rotor turns counter clockwise when running. If I pull on the advance canister like the engine's vacuum would pull on it the pickup turns clockwise. With the rotor going counter clockwise and the pickup turning clockwise it would increase the advance with an increase in vacuum.

If you set a base timing of 10 degrees BTDC and then hook up your "vaccum retard" (which on mine I believe adds 14 degrees at high vacuum) you would end up with your spark plugs firing 4 degrees past TDC which isn't what you want.

And yes Ford did make a 360 (for those who always asks "Isn't that a Dodge engine?") It's last year was 1976.

 

This chart of a standard HEI distributor (or any other one) clearly shows an INCREASE in advance with an INCREASE in vacuum.

 

this is the inside of our TCI ( same bike model ) i found from the internet. sorry i have zero knowledge about these type of complex things. all i see is a big IC ( OKI and INTEL printed on it ) and three big transistors. according to Honda diagrams and explanations they use two transistors to open the grounded path of the ignition coil primary winding with the help of signals given by the pick up coil.
sorry if this is a silly question. i recently got to know that people repair these TCIs. is it possible ? i mean do they just replace the common things like resistors capacitors ? are those transistors and big IC replaceable ? the people who are in to these things say that these TCIs are pretty much robust and not prone to failures. that's why i asked. thx again

It is possible to repair them I'm sure. A lot of modules take basic off the shelf parts and put them in a pretty housing and charge a small chunk of gold for them. Of course they are also sealed good, been tested to extremes, and everything else, but in the end it is still just off the shelf parts anyone could get.

 

They do it on both ends of the cylinder. Intakes they adjust the runner length and use "velocity stacks". Exhaust the use "headers" in 4 strokes and "expansion chambers" in 2 strokes. There are books written about the theory of doing it.

yea true. thank you

It is possible to repair them I'm sure. A lot of modules take basic off the shelf parts and put them in a pretty housing and charge a small chunk of gold for them. Of course they are also sealed good, been tested to extremes, and everything else, but in the end it is still just off the shelf parts anyone could get.

you mean including that big IC ? sorry i have zero knowledge about these so i thought it was a distinct IC made by a collaboration ( INTEL and Oki Japan ). thank you

 

you mean including that big IC ? sorry i have zero knowledge about these so i thought it was a distinct IC made by a collaboration ( INTEL and Oki Japan ). thank you

My apologies... You may be right on the IC. Even if that is the case it looks like a fairly good sized IC and it wouldn't be hard to salvage one from another unit. Unfortunately the part number is hard to make out in the picture so I couldn't come up with anything online.

 

@Autobike take a look into megasquirt if you have some time. It is a do it yourself engine controller geared towards cars and fuel injection, but the site has lots of good information on it. The unit it is actually supposed to work on any gas engine with the right tweaking. The tuning sections describe a lot of how the various sensors and stuff work along with how a typical controller would be set up to use that information. EFI or carb... it still works on the same principles, but different parts.

 

My apologies... You may be right on the IC. Even if that is the case it looks like a fairly good sized IC and it wouldn't be hard to salvage one from another unit. Unfortunately the part number is hard to make out in the picture so I couldn't come up with anything online.

thank you. got it

@Autobike take a look into megasquirt if you have some time. It is a do it yourself engine controller geared towards cars and fuel injection, but the site has lots of good information on it. The unit it is actually supposed to work on any gas engine with the right tweaking. The tuning sections describe a lot of how the various sensors and stuff work along with how a typical controller would be set up to use that information. EFI or carb... it still works on the same principles, but different parts.

thank you very much. i didn't know it before. will definitely check it