Fuel economy and spark plug gaps
Posted: Sat Jan 24, 2015 7:59 pm
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.
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.