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

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  #801  
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Found some old S&S instructions in ebay listings.

This one shows 1" spacer when used with Sportster manifold:


This one says that the 1" spacer is needed for magneto equipped Sportsters:


I may try the 1" spacer if I have appropriate bolts, though I don't have any clearance problem with the magneto since I have taller cylinders and stroker plates.
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  #802  
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Well you know what they say about the word "assume".

Trust me , it's a plenum. There's no size limit on the definition of a plenum.

I don't know who Denish is but publishing information does not make it correct.

So if you believe it's still tunable ,which cylinder are you tuning it for since the firing order is uneven? Front or rear ,you can't do both. So which gives the greatest benefit in your experience?

The reason spacers work , and they do work, has to do with what we refer to as air quality. In the airflow world there is air quantity and there's air quality.

When an air stream encounters a throttle plate and shaft it is torn into 2 separate air streams that will then have to merge back together after passing the object. This creates a lot of turbulence. It's of great help to this column of air if it has a chance to get re-established as a single non-turbulent column before being asked to deal with the next task , turning a 45 degree corner .

The spacer just provides a little extra distance for that to happen.

A flow bench will clearly show a 5cfm increase with a 1" spacer on every configuration of ironhead port I have ever tested. A stream of smoke flowing through the port makes it very visible what is happening. No theory necessary.

I have made insulating spacers for street bikes but couldn't tell any difference between them and aluminum and never bothered measuring actual carb temps.
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Denish actually gives a graph of intake manifold pressures showing definite waves of pressure inside. So I guess it's possible some effect is still there. He discusses using different harmonics of the pulse as well.
I have a mate, Trevor Liversidge, who designs racing two-stroke exhaust systems. He also designs violins. I can't completely understand his theories on these things but what I do know is that his deigns work better than anyone else.
But I have always believed the effect is really only fully realized with WOT at one specific RPM/ Smooth bore carburetors also help here I know.

In practical terms what most people use is what physically fits. A 28 inch long intake tract is not really practical on a bike. On my sidecar I can't run a spacer as the carb would foul the top frame rail.

You also run into the problems with tuning for such a narrow powerband. The Ducati singles I used to race only made torque between 7 and 10 grand. That's an (nearly) impossible bike to ride on the street.

I don;t really know if a pheonlic spacer is necessary on a Tillotson. I do know my Corvette has a heat shield under the carb to help with the problem.

Heat really is the enemy with these old cast iron engines. The more power you try to extract from a heat pump the more it's going to get hot and cause you problems.
My old sidecar ran on race gas and got seriously hot. Frying pan stuff. And that was with stock bore and stroke.

If you didn't have to kick start this beast I would say run it on alcohol. As long as you drain the carby and tank every night and regularly replace the gas lines it's easy.
And also much better for the environment.
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Quote:
Originally Posted by Ferrous Head View Post
I don;t really know if a pheonlic spacer is necessary on a Tillotson.....
Probably an aluminum one would work just as well. When harley changed from a linkert to a tillotson they used the same manifold, so they needed an adaptor to change the manifold's vertical bolt pattern to the tillotson's horizontal pattern.

Could it be that was the primary reason for the spacer and making it out of a heat insulating material just seemed like a good idea but wasn't really necessary?

I've always thought that an over heated carb was more of a problem when the engine was shut down and then restarted a short time later. When running, the carb has relatively cool fuel running through it and the tillotson doesn't hold and heat the fuel as much when running because there isn't a float bowl.

But I'm sure the Harley engineers know more than me.
Just thinking.
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or it could be the cost,wood vs.aluminum?
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I don't think HD ever compromised on manufacturing costs. They would reduce expenditure on advertising long before considering the price of raw materials.
I understand they would even reduce their line of clothing if required.
/SARC
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Enjoying this discussion. Same old problem with carbs, as in generally are cold on start up, could use a bit of heat from the head, then when red hot from a beasting, could use an insulating medium between head and carb to keep things cool. As in all compromises its never right.
On intakes, I have always thought the V twin manifolds are too short on the engines I have knowledge of whether JAP, Harley, Matchless etc. Just a carb then a stubby T.
Most tuned engines have longer runners than their lowly road brethren. My own logic says look at the 500 singles and replicate their years of race tuning. So you really want an Amal per pot and 7" of intake for each carb. If you can arrange it, also looks feckin cool.
But of course you then have to tune the zorst lenghth and live with a razor sharp power band....but hey
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