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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #31  
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And the later peak pressure occurs
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  #32  
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Originally Posted by bustert View Post
the ironhead was prone to detonation largely due to small bore.
I'm no expert here so I won't say that this is wrong, but it contradicts my understanding of what makes an engine prone to detonation. The areas in the combustion chamber furthest from the point of ignition are where detonation usually starts. If so wouldn't a larger bore be more susceptible to detonation? I've been able to find writings from so called engine experts that say so. Haven't seen one that said the smaller bore was more prone to detonation. The consensus is that the larger bore is.


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Originally Posted by bustert View Post
..... when compared to the bt, the smaller bore detonated more....
It could be, but was it due to the size of the bore?
I've never heard that the ironhead detonated more than a bt. (assuming shovelhead?) but I'll take your word for it. It sure is possible. But wouldn't other factors besides the bore size be the more likely reason? Two possibilities being an aluminum head vs a cast iron head and shovel timing advance of 35° vs 40-45° of an ironhead.

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Originally Posted by ryder rick View Post
I don't think the Ironhead bore size makes it prone to detonation,....
I'm thinking the same

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Originally Posted by bustert View Post
why more prone to detonation over the shovel?
well there are spaces that you cannot get rid of and these are furthest from the point of ignition and mostly appear at the circumference of the piston and cylinder wall bore. these are potential hot spots and can lead to detonation.
The area just above the top ring is what is called crevice volume and yes, it is a source of detonation. But the shovelhead has that area too, and it's volume would be a bit more due to the larger bore.

Putting the top ring groove higher on the piston can reduce this crevice volume with a small increase in power but at the expense of some top land strength. Some pistons are being made with anti-detonation grooves on the top land to help protect the top ring from detonation waves. Wiseco's ironhead pistons have them.

Just a comment on the original post #1 question. Would it run better at 40°? You say "last 4 years runs great". Try it. If you can't feel a difference change it back to 36. You don't want to run any more advance than what is needed to make it run well.

I've been enjoying this discussion.
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  #33  
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Originally Posted by Iron Mike View Post
Post like this are what brings me here. You guys look much farther into these than most. Good discussion with experiences to back most of it up.
Not saying all are correct, or incorrect, but leads the reader into something he/she could research and make their own distinctions.
Well done guys.
man this is a great thread and i'm digging some of the discussion
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Hypothetical question- Would a smaller bore draw in less fuel, thus there would be a smaller concentration of fuel in one area of the combustion chamber than the other side of the chamber, making the less charged side hotter? <<Did I just post that?
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dam,, rick ur good,,, too good,,,,,butt can u speak English,, luv u bro,,,,,,
sorry cazz I wanted to address this yesterday but post#28 distracted me.HC =hydrocarbons.O2=oxygen.
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Originally Posted by Toejam503 View Post
Hypothetical question- Would a smaller bore draw in less fuel, thus there would be a smaller concentration of fuel in one area of the combustion chamber than the other side of the chamber, making the less charged side hotter? <<Did I just post that?
No.
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Originally Posted by Toejam503 View Post
Hypothetical question- Would a smaller bore draw in less fuel, thus there would be a smaller concentration of fuel in one area of the combustion chamber than the other side of the chamber, making the less charged side hotter? <<Did I just post that?
Well now, We have this thing called volumetric efficiency.
VE= 34546 X CFM / CID X RPM.
With which you calculate corrected Mass Air Intake.

I use the optimal formulation myself a lot to figure Carburator size for idiots I do work for ( NO PUTTING A Dominator 1000 ON IT WON'T MAKE IT GO FASTER )
... CFM needed =MAX-RPM X CID / 3456 ...
No matter how big the valves or cams, this is as big as the engine can use physically...
Can you tell I slept at a Holliday Inn Express..
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Quote:
Originally Posted by Toejam503 View Post
Hypothetical question- Would a smaller bore draw in less fuel, thus there would be a smaller concentration of fuel in one area of the combustion chamber than the other side of the chamber, making the less charged side hotter? <<Did I just post that?
Again, any "hot spots" are a result of incorrect combustion (detonation). Any hot spots would be a symptom not a cause.

The air/fuel mix in a cylinder is mostly the same throughout. Technically this is not absolute and there are some minor variations.

In the sense of what is being discussed, the hot spots actually begin as cold spots, pockets of fuel that are too far or shielded from the flame front. When these spots finally light the cylinder pressure is too high for them to burn in a controlled fashion and they burn explosively.

You have to slow things way down, super slow motion, to visualize what is happening. The Air/fuel mix does not all light at once or instantly. From the spark "kernel" a flame front passes through the combustion chamber as a wave of sorts. Ideally this is timed to cause peak cylinder pressure to occur ATDC as the piston is moving down so it can do work. If the timing is too late the fuel does little work and may even be still burning when the exhaust valve opens. If the spark is too early the cylinder pressure rises above what the fuel can tolerate and it burns explosively.
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