spark plugs

[captaincaveman]

just a quick question, why does the AT have four spark plug?

[Whealie]

Try it with just two and see if it works.

[robelst]

just a quick question, why does the AT have four spark plug?

That's to raise the level of entertainment when replacing them

The idea of dual ignition is to speed up the burning of the mixture, enabling later timing. This improves the combustion-efficiency without risking engine pinking or the need of running hi-octane fuels.

[doglegdon]

just a quick question, why does the AT have four spark plug?

That's to raise the level of entertainment when replacing them

The idea of dual ignition is to speed up the burning of the mixture, enabling later timing. This improves the combustion-efficiency without risking engine pinking or the need of running hi-octane fuels.

Taught me something for sure, but to be honest I think I preferred Whealie's reply.......

[YEN_POWELL]

just a quick question, why does the AT have four spark plug?

Apparently, someone called Mr Enjeekay rang Honda and said that their new V-twin should use 4 sparkplugs and they fell for it.

[Les666]

Its because of the design of the bike for the Paris Dakar. Most bikes broke down or lost power because a cheap little spark plug failed, when your paying a fair wedge for a team this must have been a real bummer...so 100% redundancy was set up by putting 2 plugs in each pot, this was carried over into the road bike.
Probably a load of tosh but thats the story I heard, and it sounds convincing.

[captaincaveman]

just wondered cause most v-twins only have the two and the AT hasn't got massive pistons or high output and seemed a bit much

[djadams]

OK - usual justification is due to a large bore so twin plugs will allow you to start the burn in two locations so speeding up the rate at which the flame front moves across the mixture. AT does not have unusually large bore.

However, the other reason is if you cannot get the plug central due to having two valves as opposed to four per cylinder, or as in the AT's case, you've got a single overhead camshaft in the way. So - you slot one in either side and get good balanced flame behaviour.

On early models, they also provided twin coils per cylinder to improve reliability in the middle of the desert on the redundancy principle - the engine would still run OK on a single plug.

To my knowledge (and I'm not a gasoline engine expert) there's no timing/anti-knock benefit to multiple plugs. I could be wrong though, I might have an ask around in the gasoline half of my department...

[captaincaveman]

OK - usual justification is due to a large bore so twin plugs will allow you to start the burn in two locations so speeding up the rate at which the flame front moves across the mixture. AT does not have unusually large bore.

However, the other reason is if you cannot get the plug central due to having two valves as opposed to four per cylinder, or as in the AT's case, you've got a single overhead camshaft in the way. So - you slot one in either side and get good balanced flame behaviour.

On early models, they also provided twin coils per cylinder to improve reliability in the middle of the desert on the redundancy principle - the engine would still run OK on a single plug.

To my knowledge (and I'm not a gasoline engine expert) there's no timing/anti-knock benefit to multiple plugs. I could be wrong though, I might have an ask around in the gasoline half of my department...

gotcha the getting it central think make sense, my buell didn't have the ohc problem cause of its pushrod design

i did wonder as its my first twin spark bike ive ever had

[Jenna]

not 100% sure but i think you will find the first dakar (pre '89) bikes in all builds used the hawk engine which i get a feeling was actually a single spark, the NXR engines all had custom heads that were one offs so the spark arrangement on those is of no relevance to the production bikes other than in PR/marketing terms... so that leaves the marathon bikes which actually came post 'AT' launch.

not sure if that either makes the 'because of the Dakar' idea any clearer or muddy's the water more

on a similar note isn't the XT single plug and i think its fair to say they had no issues with plug failures on the Dakar with any XT which has a larger bore and would be prone to advance/retard issues.

From personal experience the normal reason for dual plugs is to aid combustion in engine with adverse induction/exhaust flow, wouldn't swear to it (15 years sense i did engine tech at uni!) but there is no advantage with dual plugs in an application like this to combat detonation.

Ducati played with dual plugs (on the 900ss i think) and the results they published showed that the dual plugs gave a minor increase in power and really only helped the engine when the timing for whatever reason was at 2 deg less advance than stock, this also really only helped with torque at 4000 through to 4600, which is pritty well pointless with a motor that revs freely upto 8000rpm anyway.

I guess one route to check is if the plugs are single or dual fire as i would guess that a twin plug system would produce a smoother low rpm engine which is probably what Honda though a typical AT owner would want as apposted to the more simplistic and 'rural' design of the CB build engine.