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

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  #11  
Old 1 Week Ago
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the ironhead was prone to detonation largely due to small bore.
you can set the ultima to run like a mechanical adv. and alter settings just like changing flyweight springs.
i run a compu-fire dual fire unit with(mechanical) max adv. 2.5>2.8k range. the 2k was problematic. i always dynamic check the timing although, the static on the electronic units are pretty much spot on.
basically the ironhead is a glorified B&S engine and does fine for street level and maybe a tad more preformance. it really does not require a fancy curve.
can the curve be mapped, why sure it can with accuracy.
as for as the voes, it was basically a epa thingy. i do not run it, i run rpm base adv., the redeeming feature of it , once again due to big brother, is it will roll back lead at wot where serious engine heating can occur unless you are tuned for it.
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  #12  
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There is NOTHING extra ordinary about the Ironhead combustion chamber. Many older Brit bikes used the Hemi chamber. True the piston has a bigger dome than normal but it compares also with later Brit bikes with high compression pistons. If you use it for touring as I do then setting up a more modern inspired ignition curve is going to gain you both torque and fuel economy. Might even help the longevity too. I always wondered if there may be gains to be had on a standard motor by putting in a less domed piston and skimming the head to regain the compression. If engine cooling is a problem then a modern solution would probably be to ceramic coat the ports and chambers but I probably won't go that far........yet. As for the Ironhead being a glorified Briggs & Stratton, that is quite simply crap. The B&S is a sidevalve engine made in aluminium and if I remember correctly the bore and stroke migh even be square.
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  #13  
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satirical jest my friend satirical jest.
briggs made many engines types.
the one you pushed around in the grass was just the tip of the ice burg.
ironhead heads did not flow that well and the hump was a necessary evil.
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Any idea if anybody has done dyno test with lower comp pistons to compare. I remember a test that showed a 650 Triumph motor with 11/1 pistons produced almost no practical gain over standard 9/1 pistons. It was a long tome ago now but it encouraged me not to use high comp pistons when I built my Triton 750 motor. I have pondered the idea of another set of pistons modified to improve combustion without dropping the C/R too much. Thinking along the lines of turning off the sides of the domes to allow the flame so spread better through the chamber. Somebody has surely tried it over the years? Not good at humour. Good at anger and indignation these days. Comes with age and nobody listening to what I tell them. Am world class at "I told you so" though! Have a pleasant evening Mr Bustert.
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  #15  
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I don't want to pick apart your post, I'm not going Old Skool on you, so no popcorn needed.

I don't think the Ironhead bore size makes it prone to detonation, I think any detonation susceptibility it may have is due to it requiring so much advance to give time for the flame to wrap around the big knob in the middle of the chamber. Combine that with the air/oil cooled motor huge temp variation and the motorcyclists tendency for prolonged or frequent WFO and also the complete lack of squish.

If bore size was an issue a 307 would have been a grenade since the bore was smaller than a 283 (which was previously the smallest bore of the V8 era)? To meet EPA standards Engineers went to a small bore and longish stroke to prolong the available burn time and concentrate the charge.

The Ironhead chamber design came from a "Flathead world" of different fuel and different roads (low speed 2 lane pre-freeway), mostly a different era of knowledge. The XL models update to overhead valves was actually very late to the game. The XL's position in the lineup remained because it was a sturdy design proven by the K (don't fix it if it ain't broke) and a mainstay as an low cost entry level to the brand. I think K model sales fell way off as they were not sexy. And the Ironhead design was an old one they already had on the books pre-planned and board approved (I'm sure management waited until they milked every last drop from the K). HD never dreamed/intended them still to be running 50 years later! I think HD really blew it by not making a dual plug head standard issue, the technology was common knowledge and cost would have been minimal. But there was no incentive to make a change due to the economics of the era and the model was positioned as an entry level machine. I assume they did not want to make it any faster than it was, as it already (in good hands) could beat a big twin. You can't have your entry level (Cheap ass girls bike) model be faster than the big boy bike.

The Ironhead finally got an upgrade (from the steam era) when the new ownership turned things around. A nudge from the EPA regarding noise pollution came in the form of hand fit gears to quiet the cam chest. (Baisley says HD figured out some way to hand fit the gears creating matched sets that ran quieter, they knew of this before but did not implement it until the EVO)(He noted that the aftermarket was unable to figure out (bother with) how they matched the gears, so he recommends regrinding OEM cams to maintain that gear match, so your EVO doesn't sound like an Ironhead)

A VOES maybe was an EPA thing but it is not like EGR or AIR, a VOES actually makes the motor run better more efficient and start easier, it is a positive not a negative.
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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.
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  #17  
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you cannot compare an auto engine to the ironhead, first and foremost, it is liquid cooled and runs a stabil temperature. when compared to the bt, the smaller bore detonated more.
as far as timing, it is ball park with most engines. what hurts is that like all strictly mechanical systems, a compromise is made to get the most of what is there. as time marched on, mechanical got married with vacuum adv and when that marriage fell apart, the new boy in town was electronic control and in the beginning, they were not that great.
after owning a side valve, it was hands down the worst one i ever owned, leaked like a sieve, noisy as heck and total lack luster performance. now if you wanted to dump tons of money into it, yep, can improve it but face it, machines WAYYYYY smaller in size would smoke it, hd saw the writing on the wall and if they wanted to excel at the track, ohv engine was a ticket.
what does cam noise do with anything, tires make more noise. epa noise requirement were aimed at induction and exhaust. my 440 sounded like a tornado at wot, nowdays, the stereo has to make the car sound muscle.
where does an engine make the most pollution?? at idle and low speeds. at startup, the voes is in full retard mode, excellent for starting and when idling and low speed, the lead is advanced to give more burn time and in a crude way reduce emissions. this is totally a epa thingy, you think for one moment, hd on its own would do such a thing?? more so today than yester years.
the thing about american engineering is that they always try to hang something on an existing piece of out-dated shinola instead of redesign, our competition has more smarts than us. we hung smog pumps, more weight, more things to go wrong and power robbing, the japanese made it a part of engine function. we hung egr to reduce emission, the japanese once again take care of it as an engine function, the list can go on, hd any better, hardly, that is why rice is nice.
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  #18  
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if we look at the piston, we can see a large included valve angle. this requires a high dome piston top and thusly makes for large piston dome and chamber surface areas.
the ironhead and shovel have large hemi like combustion chambers which made for high surface to volume ratio. along with this surface area came increased heat loss. as you said, the dome is an interference and impedes flame travel and air flow especially at valve over-lap. this dome also contributes to a heavier piston and the need for more ignition advance.
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.
why??
well portions of the intake charge can be trapped here and the heat is more concentrated which can cause spontaneously ignition before the normal flame front can reach this area. when this happens, detonation raises its ugly head as the two fronts collide. it stands to reason, smaller bore equals less heat sink area.
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The way I understand it is that they are not hot spots, they are pockets of unburned fuel that burn uncontrolled (detonate) as the cylinder pressure has increased beyond limits before it lights.

The detonation causes the heat (hot spots).

This is all fixed if the motor is dual plugged.
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The way I understand it is the pressure wave through the gas effectively diesels these unburnt corner gasses which are also being heated by radiated heat. Twin plugs allow the spark to be triggered nearer TDC so there is less time to heat the end gasses. Something like that anyway.
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