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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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There's no definitive answer to that.

Even with straight drag pipes the length is determined by what your trying to achieve and what the engine is built to do. Cams and intakes have to be taken into consideration and when you throw in a muffler, all bets are off.

If you want to go nuts and spend the time on a dyno you can but most people will just pick a length that "suits the bike" or what they think looks good and go with that.

Some where around the stock length set by HD would be a good starting point.
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36-40 inch total length from head pipe inlet to end of muffler for example?
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Originally Posted by Ferrous Head View Post
...
Some where around the stock length set by HD would be a good starting point.
That's exactly what i'm looking for. the stock HD length.

Were the pipes equal?
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  #34  
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Make it as long as you can stand.
packaging becomes an issue.
My fave 2:1 brings the rear pipe forward around the cone to gain some length on the primary tube.
Very difficult to build to a tuned length pipes will be unfashionably long.
It's just as difficult to build equal length.
My next 2:1 creation will have a shorter collector (imitation muffler) so it can have longer primaries.
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It's a question of time.

Shorter the pipe, the shorter the time interval between propagation of the pressure wave and when it arrives back at the valve head.

Ideally, the pressure wave arrives back at the exhaust valve head just as the valve is opening. It will reverse direction and travel back down he pipe helping to pull exhaust gases with it. This time interval of course e is dependent on the RPM. Higher the RPM the shorter the time interval between valve events.

And what this really means is that the length only works or 1, ONE RPM.

So while 5,000 RPM may be perfect, at 4,900 or 5,100 it will be less than ideal.

When the pressure waves are 180 degrees out you have the pressure waves attempting to push the mixture back into the chamber when the exhaust valve opens. This is what produces the "sag" in your performance curve.

If your primary pipes aren't an equal length on cylinder will be "tuned" for one particular RPM and the other cylinder for a different RPM.

This is not a dreadful thing in some cases. In effect it helps spread the gains and losses over a wider RPM range. Not as high as possible, but not as low as it could be.

This is the last 2 into 1 I built.




You'll note the primary tubes are not equal length. While the engine is "tuned" for 5,000 RPM I need/would like a wider power band for road racing than for drag racing or Land Speed Record attempts.

This particular pipe is perfect for deafening your passengers.
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In my experience you cant cut drag pipes down and add mufflers as the drag pipes are bent to exit together, you have to spread them apart to make room for mufflers to stack.

The original header pipes were awful short. Only a couple inches of pipe after the last bend.

You can cut a piece out of the rear pipe to raise the rear muffler to make room, but that is adding a weld and in the middle of a pipe....
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Quote:
Originally Posted by ryder rick View Post
In my experience you cant cut drag pipes down and add mufflers as the drag pipes are bent to exit together, you have to spread them apart to make room for mufflers to stack.

The original header pipes were awful short. Only a couple inches of pipe after the last bend.

You can cut a piece out of the rear pipe to raise the rear muffler to make room, but that is adding a weld and in the middle of a pipe....
mine are staggered, so there's room for mufflers.

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