[Catbird]

How many times does my spark plug fire for one engine revolution -- once or twice?

I have seen some posted RE pics that show "Twin Spark" stamped on part of the engine; does this mean that the spark plug fires on the intake as well as the exhaust stroke?

[Joel-in-dallas]

I am pretty certain that none of the UCE motorcycles for export are Twinspark models. I believe they do indeed have two spark plugs. But I suspect it is unnecessary. I would be interested to hear if anyone else has experience with the twinspark.

There are other subtle differences between the export models and the domestic models. The Oxygen sensor would be one of the biggest. Also the US gets slightly different exhaust.

That being said the motorcycles are far more alike than different.

[Arizoni]

I agree.
The twinspark engines are sold in India and they do have two separate spark plugs.  The spark plugs even use a different size thread.  They both fire at the same time.

I suppose if some really crappy fuel was all that was available the twin spark might have an advantage.  If good fuel is being used there is no real benefit to having two separate spark plugs though.

[Catbird]

... The spark plugs ... both fire at the same time.

Two plugs firing at the same time didn't even occur to me.

Thanks guys!

[tooseevee]

How many times does my spark plug fire for one engine revolution -- once or twice?

            Your engine is a 4-stroke with one spark plug. Your plug fires every other revolution.

         

[tooseevee]

I have seen some posted RE pics that show "Twin Spark" stamped on part of the engine; does this mean that the spark plug fires on the intake as well as the exhaust stroke?

         I believe the Twin Spark is just a dual-plugged head - two spark plugs for one cylinder - & both plugs fire at the same time.

         I've seen dozens of dual-plugged harleys over my life. There are many websites discussing the goods & bads. It's nothing new. It probably started in the '40s or '50s. Or before.

[Ducati Scotty]

Dual plug is one thing: two plugs and they both fire.  This helps in India where quality gasoline may be available and also adds some redundancy in case one ignition system fails.

Some engines use a 'wasted spark' method.  In a four stroke engine you only need a spark every other revolution, at the top of the compression stroke.  However, if you just fire every time the piston comes up one spark is 'wasted' on the exhaust stroke.  It does nothing but makes the spark plug firing simpler.  Is this a compression stroke?  Should I fire?  Who care!  Just fire away.

When spark was created with a mechanical switch, i.e. points, whatever drove the points  was gear driven off the engine and the spark was in synch with compression.  With modern electronic ignition the position is read by a passing magnet on the crank, but that sensor has no sense if this is a compression or exhaust stroke.  Rather than build a system to determine that, just fire every time.

All that said, I'm not sure if our engines are wasted spark or not, but Harley's are I believe.  The point it, in a wasted spark system the plug fires on every revolution.

Scott

[mattsz]

My '75 BMW used the "wasted spark" system - and I know that some owners converted their engines so that each cylinder had two spark plugs... the two-cylinder equivalent of RE's Indian dual-spark. 

BTW, I think the poster-sized wiring diagram I got from NFG shows two spark plugs...

[Pauly]

My '84 Honda V4 is wasted spark as well. I'd bet the UCEs are maybe Kevin cam weigh in

[barenekd]

Dual plugs have been around since about the beginning of spark lit engine. They have been a requirement in aircraft since the very early days. Early plug live on very low voltage and were prone to fouling. It ain't a good feeling flying around and have a cylinder quit because of a fouled plug. Newer engines with ECU that send many thousands of volts to the plugs have pretty much cured the fouled plug syndrome. However they are still airplanes engines in the thousands that still use magnetos. They have dual plugs to avoid the fouled plugs and to have duplicate ignition systems to keep a dead mag from grounding your flight.
Old race cars and motorcycles used them. The inconel plugs were developed to reduce the fouling mostly in Ring Dinger engines. They caught on with 4 strokes which generally don,t have much of a fouling problem, but if you do they can help. However a good tuneup or remap should take care of that problem alleviating the high dollar plugs.
I think some of the early Hinckley Triumph twins used a wasted spark too.
Those are just used to simplify the circuity. Certainly has no effect on performance, unless plug life would be a performance parameter.
Bare