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  #1  
Old 2 Weeks Ago
Profour77 Profour77 is offline
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Default Flat top vs dome pistons

Questions::
My 1000 is currently.060 overbore, it has 10yrs and countless miles on it sence. I am having the motor rebuilt and redoing the bike. It is a hard tail chop and will continue to be!
Anyway i was looking at the pistons that sit inside the jugs know and they are dome. I was thinking that with such a high dome that they can not possibly be very efficient!!
There is no possible way that the burn is complete or can the discharge of exhaust be any good! With that saod and anyone answeing this should understand what i am saying! So is thete a .070 flat top piston if i need to go that route? Or am i over thinking this??
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You need those domed pistons since the Ironhead is a hemi-head design. Any flat top piston will drop your compression so much that you'll probably not be able to start the motor! Best thing to do is go to a dual plug setup with a new electronic ignition. You'll not only get better performance but mileage will increase as well with the better combustion efficiency. Lots of places will machine the dual plugs for you.

Eric
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To understand why the pistons are the shape they are you need to look at your combustion chamber.
These engines were designed back when the road to performance was with bigger valves. Obviously (they thought) bigger valves would flow more air than smaller valves, biggest is best.
To get the very large valves in they had to "cant" the valves a long way from the vertical. In fact, our Ironheads have valves at 45 degrees to the vertical. What we call a 90 degree included valve angle.
This angle then dictated a very deep combustion chamber and in fact our engines have what's termed a hemispherical or "hemi head" being half a complete sphere.

But if you just ran flat top pistons inside this combustion chamber you wold have very little compression. Flat top pistons would probably give you something in the range of 4-5 to 1 compression. Not really anywhere near enough.

Viola ! The domed piston to increase the compression ratio to something usable. 8 or 9 to 1 at least.

Is it a poor design ?

In a nutshell, yes. We now know a whole lot more about hat constitutes a good combustion chamber and how to make valves flow sufficient air to feed such an engine.
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Default

Thanks for responding!! And in a way that makes perfect technical sence! I have a good ignition now. Maybe dual plugging the heads and upgrading the ignition for a dual plug set up is the way to go with that. And have the heads cleaned up around the guides so they flow a little better. Im not looking for unusable h/p. But definitely more streetable torque upgrade is always good!!
Thanks again!!!
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I use 10 to 1 pistons and mess with the base gaskets so the piston comes all the way up in the cylinder.
I don't think you will see any gains from a flame slot or working the combustion chamber shape.

This motor's design screams for dual plugs, I'm sure the MoCo did not dual plug them in production because they were already faster than the flagship model.
This is the most simple and economical mod you can do to an Ironhead and provides the biggest bang for the buck improvement.

Any single fire ignition will do dual plugs using 2 stock style EI coils
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Does this affect spark advance? What happens if you foul one plug and do nothing about it?
Both plugs same heat range?

(dial tone)
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probably not,nothing,yep why not
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If it’s tuned correctly for dual plugs, it would most likely feel like a single plug tuned a tad retarded and slightly rich.
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OJ, I'll answer.

es, the duel plugs makes for a shorter burn time. You are starting two different flame fronts in separate locations. Flame propagation is exponential which means is starts slowly and accelerates as it increases in size. in theory if the flame consumes all of the available fuel in 60 degrees half of the fuel will burn in just the last few degrees.

What this all comes down to is getting maximum pressure created by the burn to happen when it is most advantageous to the engine.

On my current race engine with single plugs it likes about 45 degrees BTDC. That's using Methanol with about 15 to 1 compression and V9 Andres cams along with porting work by Dan Baisley. All of that and the ignition point remained virtually the same as a stock engine.
That did surprise me by the way.

But with single fire and dual plug ignition it wants around 41 - 42 BTDC.

The engine definitely idle better with single fire. They start easier with single fire.

Those are two big pluses on any "high performance engine".

Although you do lose the potato,,,potato,,,cough, potato sound we all live so much.

What happens when you foul a plug ?

Don't know about fouling one but if a plug lead falls off, surprise ! The engine just continues to run on the other plug.
Will that cylinder then have poor ignition timing ?
Yes, but it will only be retarded a few degrees and it won't hurt the engine.

This is what works for me.




It's not the answer for most people. You can get more advantages by running an electronic ignition and the price is dramatically lower.

The magneto does not allow me to vary timing between cylinders.
The magneto has NO advance curve, Fixed timing.
The magneto prevents me from running my $500 Scitsu electronic tach.
It has no rev limiter function.

On the street I would opt for an electronic ignition system and use it to it's full advantage.
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The size and shape of the dome, its intrusion into the chamber, and the masking effect it has on spark propagation and cross-flow during overlap are all greatly affected by engine design including included angle, bore and stroke, and rod ratio.
In an ironhead, longer stroke allows a smaller dome for the same CR.
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