spark plug gap vs lean mixtures

[peejay]

On the other hand, lean mixtures burn slower and need more advance for optimum fuel economy.

That is true, however, we're not looking to run at "lean" mixtures, so much as we're looking at "not rich".

We're also not looking to run for best economy, we're trying to make it light off cleanly at stoich.

Any time I've had an engine that did not want to run stoich, it was because it was choking on its own exhaust gases thanks to some hefty cam timing. A rich mixture is easier to light off in those situations. Retarding ignition timing also reduces intake vacuum, which reduces the amount of exhaust gases in the chamber.

Pulling timing does hurt economy, as well as power, (makes it feel right soggy, actually!) but if it makes enough of a difference that the engine can run acceptably at stoich, then it will meet the regulatory body's requirement.

[Firedome8]

I have read that the instantaneous impedance at the gap determines the ignition delay and misfire at part throttle and that tailoring the spark profile to each individual combustion event will stabilize delay time and minimize misfire. The book was by C Jacobs.

[Circlotron]

As we close the mixture gap the required firing voltage diminishes...up to a point.
When the gap becomes less than the quenching distance, the spark will still jump the
gap and ignite the flame kernel. But the kernel size will be too small to sustain the
heat loss to the disc electrodes and the flame kernel will extinguish itself.

No amount of additional electrical energy will produce a sustainable flame.

Would that still hold true if the spark energy and in particular, duration, was sufficient to offset the heat being absorbed by the electrodes (real world diameter, not just test discs) and therefore allow the kernel to grow large enough to outgrow the gap and become self-sustaining?

[Tuner]

My cam has something to do with it, but I see it misfiring if I try to go to 14.7/1 which I'm required now to pass tech inspection/emissions. (recently changed rules here ) Due to the CNP I think I have enough to go wider, but not sure if it would help with that. The car is not exactly one that allows for easy plug replacement without consulting a chiropractician afterwards...(especially left bank).

I think that right now they are gapped out of the factory like .035 or so.

What are the HC and CO numbers you have to meet?

Is this a simple stationary two gas test for idle and 2500 RPM or on a dyno?

To reduce HC and CO increase the idle speed and retard the timing as much as necessary, even a bit ATDC if it needs that to get the numbers you need.

When the spark fires closer to TDC there is more compression pressure so combusstion is more easily initiated, though will require more voltage. TDC is no problem for old school points systems so you should have no trouble.

Adjustment like this can only be temporary to pass the test because such retarded timing will obviously increase exhaust and engine temperature excessively, but as a means to pass the test the heat improves combustion of the exhaust diluted idle mixture.

Back in the '70s when two gas testing was just looking for HC and CO it was common to make highly modified engines pass a sniff test, though sometimes the timing needed to be so retarded the exhaust will be glowing red (in the dark).

Another thing: With your EFI can you change the timing of the injector pulse? The idle emissions are sensitive to this and it is almost always best to fire the injectors when the valves are closed.

You need access to a 4 or 5 gas tester for tuning so you can see the results of changes as you make them.

What are the emissions standards you have to meet?

[David Redszus]

On the other hand, lean mixtures burn slower and need more advance for optimum fuel economy.

That is true, however, we're not looking to run at "lean" mixtures, so much as we're looking at "not rich".

We're also not looking to run for best economy, we're trying to make it light off cleanly at stoich.

Any time I've had an engine that did not want to run stoich, it was because it was choking on its own exhaust gases thanks to some hefty cam timing. A rich mixture is easier to light off in those situations. Retarding ignition timing also reduces intake vacuum, which reduces the amount of exhaust gases in the chamber.

Pulling timing does hurt economy, as well as power, (makes it feel right soggy, actually!) but if it makes enough of a difference that the engine can run acceptably at stoich, then it will meet the regulatory body's requirement.

Mixture strength (A/F ratio) do have an effect on laminar flame speed. But laminar flame speeds are very slow compared to
turbulent flame speed generated by temperature and charge motion. Most often, the lean burn retardation was due to increased
ignition delay and not flame speed. Squish velocity and chamber/piston design play a large part as well.

Would that still hold true if the spark energy and in particular, duration, was sufficient to offset the heat being absorbed by the electrodes (real world diameter, not just test discs) and therefore allow the kernel to grow large enough to outgrow the gap and become self-sustaining?

Fine wire electrodes will run substantially hotter and therefore absorb less combustion heat from the
embryonic flame kernel. The reduced mass of the fine wire electrodes permits rapid heating of the electrodes. A closer gap can
be used and might even be necessary due to the reduced electrical conductivity of some electrode materials.

Since lean air/fuel mixtures tend to be non-uniform in composition, a larger gap is useful to obtain a self sustaining flame kernel, along with increased spark duration. I have found that spark duration that extends beyond TDC does not help very much, if at all.

But...large gaps and longer duration require increased firing voltages.

[peejay]

I think you're focusing on the mixture's effect on ignition.

I'm looking at ignition timing's effect on manifold vacuum, and the resultant effect that will have on exhaust residuals.

[Belgian1979]

My cam has something to do with it, but I see it misfiring if I try to go to 14.7/1 which I'm required now to pass tech inspection/emissions. (recently changed rules here ) Due to the CNP I think I have enough to go wider, but not sure if it would help with that. The car is not exactly one that allows for easy plug replacement without consulting a chiropractician afterwards...(especially left bank).

I think that right now they are gapped out of the factory like .035 or so.

What are the HC and CO numbers you have to meet?

Is this a simple stationary two gas test for idle and 2500 RPM or on a dyno?

To reduce HC and CO increase the idle speed and retard the timing as much as necessary, even a bit ATDC if it needs that to get the numbers you need.

When the spark fires closer to TDC there is more compression pressure so combusstion is more easily initiated, though will require more voltage. TDC is no problem for old school points systems so you should have no trouble.

Adjustment like this can only be temporary to pass the test because such retarded timing will obviously increase exhaust and engine temperature excessively, but as a means to pass the test the heat improves combustion of the exhaust diluted idle mixture.

Back in the '70s when two gas testing was just looking for HC and CO it was common to make highly modified engines pass a sniff test, though sometimes the timing needed to be so retarded the exhaust will be glowing red (in the dark).

Another thing: With your EFI can you change the timing of the injector pulse? The idle emissions are sensitive to this and it is almost always best to fire the injectors when the valves are closed.

You need access to a 4 or 5 gas tester for tuning so you can see the results of changes as you make them.

What are the emissions standards you have to meet?

Yes, I have to pass HC and CO. Not NOx.
Don't know the number for HC but believe (out of my head) that CO needs to be below 1%. Don't own a 4-gas tester.

[Tuner]

My cam has something to do with it, but I see it misfiring if I try to go to 14.7/1 which I'm required now to pass tech inspection/emissions. (recently changed rules here ) Due to the CNP I think I have enough to go wider, but not sure if it would help with that. The car is not exactly one that allows for easy plug replacement without consulting a chiropractician afterwards...(especially left bank).

I think that right now they are gapped out of the factory like .035 or so.

What are the HC and CO numbers you have to meet?

Is this a simple stationary two gas test for idle and 2500 RPM or on a dyno?

To reduce HC and CO increase the idle speed and retard the timing as much as necessary, even a bit ATDC if it needs that to get the numbers you need.

When the spark fires closer to TDC there is more compression pressure so combusstion is more easily initiated, though will require more voltage. TDC is no problem for old school points systems so you should have no trouble.

Adjustment like this can only be temporary to pass the test because such retarded timing will obviously increase exhaust and engine temperature excessively, but as a means to pass the test the heat improves combustion of the exhaust diluted idle mixture.

Back in the '70s when two gas testing was just looking for HC and CO it was common to make highly modified engines pass a sniff test, though sometimes the timing needed to be so retarded the exhaust will be glowing red (in the dark).

Another thing: With your EFI can you change the timing of the injector pulse? The idle emissions are sensitive to this and it is almost always best to fire the injectors when the valves are closed.

You need access to a 4 or 5 gas tester for tuning so you can see the results of changes as you make them.

What are the emissions standards you have to meet?

Yes, I have to pass HC and CO. Not NOx.
Don't know the number for HC but believe (out of my head) that CO needs to be below 1%. Don't own a 4-gas tester.

If you can answer these question it would help.

Has it been tested and failed? Do you have numbers from any test taken recently?

What are the HC and CO numbers you have to meet?

How is it tested, a simple stationary sniff test or a rolling road. Is this a simple stationary two gas test for idle and 2500 RPM or is it tested on a dyno?

To reduce HC and CO increase the idle speed and retard the timing as much as necessary, even a bit ATDC if it needs that to get the numbers you need. Tunnel ram radical cam engines have been tuned this way and had less than 1.5% CO and under 300 PPM HC.

When the idle timing is retarded it requires a larger air flow, be it throttle opening or IAC, to have the same idle speed as with more timing. This is particularly so with sporty valve timing. More air flow combined with the spark firing closer to TDC results in higher compression pressure at the time of ignition, which results in a more vigorous fire and more complete combustion, so reduced UHC. If the engine is mechanically sound it should run without misfire with the timing near TDC and the A/F near 14.7/1 and in that condition it should get below 1% CO and under 300 UHC. I have made some crazy engines with carburetors pass with these numbers, I don't know why EFI couldn't do the same.

To have low HC numbers the injector should not be fired when the valve is open.

[ClassAct]

Any updates on this? I'm interested in the outcome.

[Belgian1979]

No, sorry not yet. It's not the best season of the year to drive the car around. Lots of rain, freezing etc.

[Tuner]

Just got a carbureted SBC with a sporty cam to pass the idle sniff test yesterday, under 1% and less than 220 UHC, initial timing TDC and 1000 RPM idle speed.

What is the deal with some cars being forbidden to be driven in some locales in Belgium?

What is the deadline? I thought the examination was imminent. When do you have to have the car tested?

[Belgian1979]

Europe has become especially bad with regards to cars. We didn't have to go to tech inspection except only once and that was to check that the lights worked, brakes worked etc. That has completely changed. Now they check the whole nine yards, even conformity of the car to the original specs and so on (luckily for me my car has gotten a one off attestion).
The only thing they are going to allow is electric cars in the future. And I absolutely hate those bags of shit.

I'll have to go before september 2020 officially but I'm going in faster (Jan-Feb) because if I need to make changes to the car I have the time to do it and hopefully still be able to use it in spring and summer.

[peejay]

The only thing they are going to allow is electric cars in the future. And I absolutely hate those bags of shit.
[/quote]


Ever drive one?

Maybe I'm biased the way Europeans are biased toward Diesel. In the US, Diesel means rednecks in lifred trucks blowing smoke everywhere because they are rednecks,and thanks to them there are people trying to get privately owned diesel powered vehicles banned. Also in the US, electric car means Teslas that outaccelerate anything else on the road.

I would imagine that in European countries, Diesel means BMW 3ers with hydraulically smooth acceleration, and electric means Mitsubishi shitboxes.

[Belgian1979]

No, I've not driven one and acceleration is not the only thing that's important in a car.
I drive an Audi A5 sportback as a dd. TDI version with 150 hp and that is very smooth mind you. More than enough for the speed limits we have here.

As for Tesla, € 100k for a car that would normally cost maybe € 50k ? No thanks. That's not withstanding :
- I do 350 km every day for work related miles. That's not counting my wife who does another 25k kms per year. The range of those things is not sufficient.
- Charging at home is problematic. Fully charging a Tesla takes ca. 13 hours on a regular househould electrical supply. That's for 1 car. Imagine when you have 2. So I absolutely need to park the car at 18 h in the evening and can only start working at 7 h in the morning. At best.
- charging is a problem on the road.
- I have my serious doubts that it is anything more environmental friendly than a gas/diesel, because electricity doesn't fall out of the air.
- Luckily I live in a suburb. But image you live in a flat ? What are you going to do ? Run a cable 10 stories down to the street to charge your Tesla ?

[peejay]

That's the difference, isn't it.

An A5 Diesel, if one was sold in the US, would cost a lot more than a Model 3, which has much greater performance than an RS3, while being far cheaper to operate even at the US's subsidized fuel costs. (currently, where i live, about .60 Euro/liter for gasoline or .71 Euro/liter for Diesel)

Meanwhile, Model 3s are fast becoming the weapon to have for our autocrosses, where their acceleration and torque-vectoring handling ability makes them unbeatable.