Fuel economy and spark plug gaps

[immortality]

Hi guys,

Has anybody experimented with spark plug gaps to improve fuel economy?

After installing a boost upgrade on the missus L67 it had a ignition miss under load. Thought it was a timing issue so I had taken timing out without improvement. I remembered that the previous owner had replaced the spark plugs with iridium type plugs so I went back over the paper work supplied with the car and indeed the previous owner had fitted BPR6EFIX-15 (as standard replacement recommended by NGK) instead of the regular BPR6EF-13 plugs. I Re-gapped the plugs to 1.2mm (allowing for the extra boost) and the ignition miss is gone, engine revs out cleanly to the limiter. There have however been some other less desirable changes as well. It seems it has lost approx 1 litre/100km average fuel economy, engine doesn't seem to have as much low rpm power as previously and even the brake pedal seems to have more travel which suggests a little less engine vacuum. I'll experiment with the timing to see if we can regain some of this lost performance.

I had a similar experience with my old VN many moons ago. When purchased it still had very old original equipment plugs. Bought/fitted new plugs and leads and lost about 50 km's per tank of gas and the car didn't run quiet so well in general. The new plugs were 1.3mm gaps (as per NGK listing), the old plugs that came out were between 1.6mm-1.75mm. Re-gapped the new plugs to 1.5mm and found back the lost fuel economy and the engine ran better.

Obviously plugs need to be gapped in such a way to prevent a miss-fire under full load but it also seems to me that the biggest gap possible (without a miss-fire) is important for vehicles where fuel mileage is as important as been able to sink the boot occasionally. No doubt modern ignition systems with a coil per plug will aid in this venture allowing for bigger gaps without a miss-fire under full load.

[vlad01]

very interesting indeed. I have experimented with bosch super 4 ( 4 ground electrodes )
made economy worse lol and thats with the standard 1.5mm gaps. best plugs I found is the genuine GM nippon denso as it has a grooved ground electrode and as a result its a much fatter and stable spark compared to the non grooved NGK equivalent.

Actually look side by side of grooved and non grooved while sparking and you'll see what I mean.

[delcowizzid]

yes a bigger gap is better fixed many fuel issues in commis with the proper gap.its partly to do with the DFI system.the bigger gap makes the coil fire all its energy out the plug in the fueled cylinder instead of sending spark out both plugs .with boost its just hard to run a big gap and not missfire we run down at 0.9mm turboed case of having too.maybe with non resistor plugs you could get the best of both worlds but only if you dont get RF interferance issues

[HQ355]

How much boost are you running ? In some high boost motors it blows out the spark, so you could possibly change the heat range of the plug or find a way of getting a bigger spark ? I have no experience with boosted motors only heard this over the years

[Holden202T]

yeah I had to close my plugs down to .030 I think they are to stop it missing with 20psi boost .... this is methanol too so im not sure how that changes the situation...

[HQ355]

Could allways run a magneto off a top fueler

[immortality]

yeah I had to close my plugs down to .030 I think they are to stop it missing with 20psi boost .... this is methanol too so im not sure how that changes the situation...

Is that with 1 coil per cylinder?

I suppose with twice as much fuel in the cylinder Vs regular petrol it may effect ignite-ability?

[vlad01]

How much boost are you running ? In some high boost motors it blows out the spark, so you could possibly change the heat range of the plug or find a way of getting a bigger spark ? I have no experience with boosted motors only heard this over the years

yes, if the electrode is hot enough they readily emit electrons to initiate a spark.

With many many years of experiments as a high voltage enthusiast I found this to be the case. Same as welding electrodes when they are still hot, strike heaps easier than cold.

[HQ355]

Welding electrodes like to be warm because it gets rid of moisture, when u buy them they are vacume sealed, that's why, once opened keep them in a stick heater, if your just doing general welding it won't realy matter, if your having dramas and there's a chance of moisture that's why

[HQ355]

And those top fuel mags run at about 80amps from memory (and there's 2 of them ) so you prob could weld with them !