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Bonneville Black 2007 865
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I would be curious to see if there was a power difference between the two exhaust header sizes. Afraid I don't know enough about the subject. However I would think that if an exhaust is designed for a 865cc stock engine, then fitted to a 904 making more power. I would think some changes would need to be made to get the most out of it but then again I don't know enough about the subject really.
I suppose testing is the only way to find out.
 

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Discussion Starter · #62 ·
As you may recall I touched on the various nuances of exhaust design and priority of criteria for ones exhaust earlier in this thread.

But this snippet of info may help guide you and Harrison exhausts in designing your exhaust,
Although mikeinva managed to make good power with the 1-3/4" (44.5mm) diameter D+D , my experience with my exhaust is that he was probably at its limit for flow.
The reason I say this is because the underbelly silencers and mini-cone silencers on my exhaust are the straight though perforated tube type baffle, they offer no restriction in exhaust flow. and although my exhaust is very loud they do take the edge off the volume for shorter rides, but for a trip to the TT with my son back in 2015 which meant many hours on motorway and riding around the isle of man, I decided that I'd fit a couple of dB killers in the mini-cones to try and reduce the volume even more.

The result was that 4" long dB killer reduced the noise marginally but knocked about 10-15 seat of the pants hp off at the upper rpm, so I cut down the dB killers half an inch at a time until they were about 1-1/2 long. At 1-1/2" I reckon I was still losing a few hp in the upper rpm but there was virtually no noise reduction.......so since then I've never bothered with dB killers 馃槒馃ぃ.

What this told me was that 46mm is about as small a diameter as you'd want to use without losing power in the upper rpm (on my system anyway!),
but don't think using tube alot bigger than 46mm is an easy way to more peak power, exhausts need some back pressure to help produce power at lower/mid rpm. So with bigger diameter tube you could end up in a similar situation as the D+D i.e. you might find an extra 1-2hp at peak but have a big dip in your torque curve at low/mid rpm that even tapered headers can't fix.
 

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Bonneville Black 2007 865
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It's such a mine field really, without alot of dyno time it's hard to tell anything. If you up the headers to 46mm from 44.5mm, then what do you up the secondary's too?
I think best thing I can do is to dyno the D&D 2-1 and then see what Harrison Performance thinks. He said he has calculations for different displacements etc. I'm torn whether I mess with a known good recipe or try to make it better and potentially fail. I'm already looking to change some of the bends to make the system fit a bit better.

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If the horizontal section was moved forwards an inch and the kickup before the Y was flattened and the bend placed after the Y piece. The horizontal section could sit parallel to the frame tubes giving better ground clearance and a nicer look. On this system the stainless clamp in the middle is where the secondary pipes start. They are not much bigger than the primarys. I should really take all the sizes down. Inner diameter and outer. Then I can give the info to HP.
 

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Bonneville Black 2007 865
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Found this calculator based on Graham bells calculations from his exhaust book.
https://www.mezporting.com/exhaust_length.html

The calculator with 813 cam says 47mm primarys!

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I found this last bit very interesting.

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This makes me think the D&D 2-1 really was designed primarily for the stock motor. Maybe a decent improvement can be had.
 

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Just for reference, D&D 2-1 is a 42mm internal header size pipe. It has a 38mm short tube from the head which runs for 40mm before going out to 42mm.
MikeTT what was your headers internal diameter?
 

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Outer diameter is 44.5mm. crazy eh!

Which makes the measurements from that website even more interesting.
 

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Head adaptors are 38mm for 40mm
Primarys are 42mm
Secondarys are 44.5mm
Post Y link pipe 47.5mm
Then silencer cones out to 52mm
 

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I've sent these sizes to Harrison performance. He will get back to me what he thinks of the diameters and lengths.
 
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