incorrect spark plug gaps from the factory and fuel economy

Squily

Well-Known Member
Feb 23, 2016
189
122
133
Esperance Western Australia
So I just serviced my AT for 48kkm. Replaced the plugs and noticed that the old plugs that came out did not have the specified plug gap, but about 0.1 to 0.2mm smaller than specified.

Since the service (I'm on my third tank now), I noticed quite a sudden improvement in fuel economy. The bike used to average 5.6-6.1l /100km in commuting. Over the last three tanks, I'm consistently getting 5.1l/100km average (so at least a 10% improvement).

I will keep an eye on it, but FWIW, if you have an AT that may be less frugal than you think it should be, maybe consider checking the plug gaps. Checking all of them is a major PITA, but you'd be able to check at least the one plug on the left side of the motor easily, which should give an indication whether the rest might be correct/incorrect.
 

Shadowjack

Active Member
Mar 15, 2018
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16
38
Great Plains
The US car manufacturers had to upgrade their ignition systems 40 years ago because the old coil and points wouldn't fire the wider plug gaps required to light the mandated leaner fuel mixtures. The theory is that if you have a wide gap, you get more fuel molecules between the electrodes, so something might catch.
If the ECU was compensating for difficult lean mixtures by increasing the fuel, you might see what you described. Or maybe not.
 

Graves

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Aug 14, 2016
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Back in the day when I had my smog license and was testing/repairing vehicles here in California I often re-gaped plugs wider to help get rid of HC that was a tad high when all else was fine. If a plug gap is too small the available current in the secondary side of the ignition is not used to it's fullest, open the gap and you get a much hotter spark, igniting more HC's just like Shadowjack states. The only down side is that the hotter spark tends to be hard on plugs and will find leaks in any marginal insulating components.

The US car manufacturers had to upgrade their ignition systems 40 years ago because the old coil and points wouldn't fire the wider plug gaps required to light the mandated leaner fuel mixtures. The theory is that if you have a wide gap, you get more fuel molecules between the electrodes, so something might catch.
If the ECU was compensating for difficult lean mixtures by increasing the fuel, you might see what you described. Or maybe not.
 

Graves

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Aug 14, 2016
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To expound on this a bit further: The increase of secondary ignition current back in the eighties was to reliably light leaner fuel mixtures that contained inert gasses in higher compression engines. Higher compression forces more molecules between the sparkplug electrodes creating more resistance, molecules that didn’t always contain enough HC and o2 to ensure a planned flame travel, a hotter spark was required. The requirements for hotter and hotter sparks went so far as to require more than one coil per engine to increase primary saturation time, to the point today where each plug has its own coil. So it’s not necessarily just about lighting hydrocarbons between the electrodes of the plug, but to spontaneously light surrounding hydrocarbons due to the heat generated by a much hotter and longer lasting spark (think a spark from static electricity compared to lightning). In general, all things being equal, if you open a plug gap the spark will be hotter, but shorter in duration, so it depends on the design of the combustion chamber and gasses available as to whether or not a hotter spark of shorter duration is more beneficial than a cooler spark of longer duration. We’re fighting a lot of forces here, we want to generate heat, heat is power, but not so much that we lose control of combustion and create unwanted gasses and degrade materials. One must remember that engine designs are not perfect, there’s a lot going on in there to make them work where they’re not supposed to. Multiple plugs per cylinder, butterflies that alter one intake valve’s flow, variable ignition timing, variable cam timing, injector timing and duration… all there to make the engine run outside of the cam’s window.

Back in the day when I had my smog license and was testing/repairing vehicles here in California I often re-gaped plugs wider to help get rid of HC that was a tad high when all else was fine. If a plug gap is too small the available current in the secondary side of the ignition is not used to it's fullest, open the gap and you get a much hotter spark, igniting more HC's just like Shadowjack states. The only down side is that the hotter spark tends to be hard on plugs and will find leaks in any marginal insulating components.
 
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DELLA

Member
Sep 7, 2018
85
26
8
Hochstadt
You both have this spot on. . Yes it will improve economy, but....

There is a problem with doing what Squilly says , and that is you will in effect shorten the service interval .
( that the plugs need to be changed ) .

The NGK SPARK PLUG SILMAR8A9S I have seen at € 39 each ,
you may well take a deep breath.


Rugged Rd do the SILMAR8A9S for under € 30

At the price these things are being sold that may put a lot of your off from tinkering with them ,
and when actually should you change them ? If you don't know the power output or the CO2 content.

I would point out that a NGK 2202 DPR8EIX-9 Iridium Spark Plug at a more reasonable 12 pounds ,
so its a case of charging the customers with new equipment , The spark plug we use is just at higher price
for what is basically the same technology.

Then I would go further and say that a DPR8EA-9 (which fits over 100 Honda models and has proved itself in hundreds more like Kawasaki/Ducatti/Suzuki etc ) would do an equally good job , for a lot less money , and they are just € 4 each.

I have seen this sort of nonsense in Car plug specifications as well , so let me confirm what happens on a rolling road by changing from NGK Iridium to a NGK Std of the same temperature grade on a highly tuned Subaru. Without tuning anything else ,
It showed less than 3 bhp increase of peak power. ( < 1% ) and actually the torque curve under 3500 rpm was slightly worse. The real reason for using them is how long they (iridium) last. And I see that as perhaps the only valid argument to be considered together with the manufacturers service interval.

However I like , Squilly's idea a lot,
Given that the spark plug gap increases by electrode burn wear, it might be advisable to keep them set (0.05mm)
just inside the plugs recommended limit. One other small point , on doing that , you may find that you spend a little longer turning it over in winter , I notice with my SD06 (small sized lighter battery) that the first time start performance that was so impressive in Sept thru Nov is now in cold -5°C is now not so impressive. I hear you all saying that you should expect that as
the battery capacitance will fall exponentially in cold conditions , it is never the less a further warning about opening spark plug gaps. As nothing starts if you cannot produce a spark , due to a reduced supply voltage under the cranking load.

Finally , that also depends on how tight your new motor is , and if it is full of oil. So in conclusion , I would not be at all surprised if Honda don't send them out with plug gaps slightly tighter deliberately.
 
Last edited:

Squily

Well-Known Member
Feb 23, 2016
189
122
133
Esperance Western Australia
Just to be clear:

The specified AT plug SILMAR8A9S should come standard at the required specified gap. My gaps were way smaller than the specified gaps and this after 45kkm. So imo the factory slipped up when installing them. Setting plug gaps to the specified settings should not detrimentally affect the plug life.

You can also replace the specified plugs with an alternate ngk unit (sorry but don't have the number on me but can check it when I get home in two weeks). This plug is a BMW specified unit and the only difference is that the preset gap is smaller than the AT specified unit, so you need to set it before installing. It is about half the price so worth the extra 5 minutes ;-)

I can confirm that my bike is way more economic with the correct plug gap. I used to get 280-320km on a tank commuting. That is now up to 350-380km. And on the open road I've seen an increase of around 1.5km/l
 

DELLA

Member
Sep 7, 2018
85
26
8
Hochstadt
Hi Squiily ,

When the weather warms up a bit and I go back to using a road tire then I'd like to check if mine are set tight like yours were,
while my factory plugs are still fresh I' d try the gap out at the NGK recommended 0.9mm.

Please do post the Equiv NGK plug # and the gap you are using them when you can. I might do a 1:1 test on them.

To your point about spark plug settings , I would expect that if yours were indeed too tight , ( due to your report of the previously higher fuel consumption ), while the required voltage (the voltage required to discharge across the gap) rises in proportion to the distance driven. Setting them to a larger gap would tend to burn the electrode quicker, also leading to the higher consumption over the length of service period as unfortunately plug life is influenced by spark temperature.

Note , that may be irrelevant depending on how often you intend to change them as an Iridium plug can last up to 60tkm but a Nickel one may only last 20-40tkm.

A NGK SILMAR8A9S on a CRF 1000 should be set to 0.9mm

Its gap can increase 0.10-0.15 ( or even 0.20mm nickel) for each 10,000km driven.

1549015942060.png Courtesy of Denso.


What gap were yours previously actually set to ?
And how did the spark plugs look like when you took them out , were they still light brown ?

This is statement is straight out of a NGK technical page

The operating temperature of a spark plug varies between 450-870°C. At 450°C the spark plug reaches its self cleaning temperature; this means that carbon deposits which are produced during combustion are actively burnt off the insulator nose. When too many carbon deposits accumulate along the insulator nose carbon fouling occurs and engine misfire may occur. If the temperature of a spark plug exceeds 870°C overheating may occur leading to spark plug and possible engine damage.

I'm only at 6 tkm on my Sept18 SD06 , so my plugs are fresh. On the standard Bridgestones on tour Sept I was getting around 6-6,5 /100km consumption Since the weather turned colder it rose to 7-7,2/100 tkm on my commute.

I think what I'm saying here is that temperature and roll resistance and where you drive have a huge impact on fuel consumption
so what I would like to ask you is , are your findings with a reduction in fuel consumption based on same commute , same tires / and if your air filter is new clean enough ( over rich mixture otherwise ).

Following me throwing the Factor Bridgestones away at 4 tkm , I have noticed a further fairly large increase in fuel consumption
using the Silca Heidenau's now its nearer 8/100 tkm. (I don't concern myself about that cost , but I can no longer get a week of commute out of the tank , Id love 5 litres more. After a 3 mile 60-80 kmh , I'm farting about in traffic feathering the clutch for 6 miles and then have a further 8 mile mostly thru frustration at 160-170 kmh, all of which is very bad for Avg consumption.
However I digress, Your commute is obviously not like mine.

Mine does have the advantage that even our relatively "cold" rated plug does get a really good heat up , that allows any carbon deposits to be burn't off well every day. They don't start doing that till the plug reaches 450°C that does not happen at below half throttle and that does not happen unless you are doing at least steady 80 or even 100 kmh.

Look forward to hearing your feedback.

Best regs Ralph
 

Squily

Well-Known Member
Feb 23, 2016
189
122
133
Esperance Western Australia
Found a reference to the replacement plug. According to itsnumber42 over at ADV forum, he spoke to an NGK technician and they found the NGK LMAR8AI-8 is identical except for the gap (as said earlier)


IMG_20181225_131208.jpg

Photo of old plug vs new. Should answer your questions about colour etc. Plug looked really good. I've been using iridium plugs for more than 10 years now because of the increased life. Never had to adjust a gap on any of them in the 1st 60kkm. I think Honda's replacement schedule of 48kkm is very conservative and i'll run the next set longer than the specified interval.

FWIW, we tend to run cooler plugs here in Australia than OEM specified. E.g. The xr600 uses dpre8 and they rarely last 20kkm with the heat etc here. A dpre9 will give extended life and run better.

As for your other comments regarding rolling resistance etc. True but not relevant to my observations. My bike has run on knobbies almost entire life. No other relevant changes such as airfilters etc. Only change was the plugs.
 
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DELLA

Member
Sep 7, 2018
85
26
8
Hochstadt
Thanks Squilly,

Please thank you contact "itsnumber42" from the ADV forum for the usefull info.
Give him a thumbs up from me, and this comment "he obviously knows where his towel is"


Indeed the plug colour looks perfectly ok.

So after my guarantee is up, the stealer-ship won't be seeing me ,
usually do my oil, plugs, filter, service myself.

I found this one in UK https://www.sparkplugs.co.uk/ngk-spark-plug-lmar8ai-8
for a fairly reasonable 12 pounds appx

and this one https://www.motointegrator.de/artikel/1066804-zuendkerze-ngk-lmar8ai-8 In Germany for appx 8 €
 

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