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  #11  
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Originally Posted by 72 Ironhead XLH View Post
Personally unless a motors been heavily worked over dual plugs are of no real advantage.
The 883 aka 900 sporty will do over a 100 mph as is , my 72 went to 105 with more left in the throttle, it had bigger balls then me . Dont believe all the hype about it.
There is no HYPE, this is scientific fact. No one is blowing any smoke, there are no mirrors used and no small animals sacrificed to obtain huge gains. And power level has NOTHING to do with the modification. A stock motor will benefit as much as a "BUILT" motor. Top speed is not what is being talked about, it is not so much a power increase it is efficiency. And gonad size has nothing to do with it.
More torque. Smoother running. Better throttle response. Better fuel mileage. Easier starting. Less chance of detonation (a more forgiving tuneup). All in all a happier Ironhead. The factory should have done this in production (but waited until the EVO XL Sport model to try it).

The huge knob on the top of an Ironhead piston blocks flame travel. If you study the affects of spark timing and combustion chamber design you will see how dual plugging an Ironhead is so beneficial. A stock Ironhead requires an inordinate amount of spark advance, there are many motors that require only 28-32° of advance to make peak power.

The mechanics of this is that you want peak cylinder pressure to occur just past TDC so it can make torque. Too late and torque is left on the table the motor runs but is not efficient and lazy. To early and peak pressure is achieved when the piston is still rising, this results in an piston destroying explosion. Cranking up the spark advance to try and wake the motor up is a band aid for a messed up burn.

The other aspect of too much spark advance is that it puts the tuneup on a very narrow ridge on one side is lack of power the other side is a drop off (Detonation). An Ironhead chamber design creates a ridge that actually overlaps in normal operation, it wants more spark advance than it can eat at peak volumetric efficiency (this is when cylinder pressures are highest). Thus it is very easy to upset this balance, any difference in fuel quality will tip it over. Changes in barometric pressure, humidity, load, ambient temperature will also tip you over the edge. An air/oil cooled motor has to run in a very wide range of temps this makes an even more tricky place.

As to efficiency, dual plugging gets more of the fuel lit BEFORE peak cylinder pressure thus there is more fuel burning after peak pressure, more push down on the piston which in turn rotates the rear tire better. And it does it with less spark advance which also provides a wider tuning window. The magic happens when you get all of the fuel lit BEFORE TDC and peak cylinder pressure happens after TDC where it can do the most work.

With a single plug you are constantly in a battle between detonation and a lack of power. Even at 45° advance the motor is lazy and wants more at times. Then there are other times when 45° is too much, you get a bad batch of fuel, you increase the load (even a hill), or you get a patch of hot weather and the oil temp gets up there. The result is this,



or this



Dual plugging you get this



I could not find Ironheads doing wheelies Im sure Shane can provide some if you need them.
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Originally Posted by TT60 View Post
On my old SE dual plug heads that have been Sledge ported I've always used V-Rods plugs. The second plug is in the middle of the rocker covers.

Bike has always ran faster with dual plugs vs single plug after lots of tuning too.
YES! I also placed the second plug from the top in the rocker box gap.
The plug electrodes (spark) is in the same location as the OEM so the two sides are equal.

I also use a reduced size plug, modern manufacturing allows for a much smaller plug to do the same function as the huge vintage plugs.

I am also a fan of extended tip plugs not the lawnmower plugs used by the MoCo (to try and widen the tuning window). You are not trying to slow down the burn so get the tip of the plug exposed to as much mixture as possible.

An NGK DPR6EA plug fits well and is easy to find and is complementary to a BPR6ES used in the OEM position.
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Ironheads doing wheelies???.....must be photoshopped. Girls bikes don't wheelie, you'd see up their skirts!
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Dude that is a Kilt!

And, Do you really want to look?
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R.R. the aluminum 4 valve head under the piston in your photo,is that from one of the midget racers?
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My stroker was dual plugged by Flo headworks, top access and 10mm plugs. Can be a bit of a pain to change but no real drama and hardly ever need doing anyway. Can't help thinking that smaller plugs are an easier fit and less likelihood of cracked heads (if that is ever a problem with ironheads, probably not). I run about 30-32 degrees advance, bike is very happy and I get 50 miles per U.S. gallon and WAY more performance than when motor was stock. All of which points to a motor that is more efficient at turning the same amount of fuel into more power.
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Originally Posted by brucstoudt View Post
R.R. the aluminum 4 valve head under the piston in your photo,is that from one of the midget racers?
That head was off a Neon I was dirt oval racing.
That head design has one of the best plug locations possible.
I think Cosworth may have been the first.
A Hemi was only 2 valve.
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