Even most SBC's utilize a 1 5/8” id header. It will help in the bottom but your stumble is most likey a tune...

 

I had the same issue when I bought my '06 Fatboy a couple of years ago. It has a V&H FulePak on it, and V&H worked with me on mapping which helped a lot but didn't rid the problem completely. Last winter I installed an Andrews 21N cam, while doing the tensioners. V&H sent me a new map, bike runs flawlessly now. Super smooth, no hesitation anywhere.

I'm not sure exactly what my issue was considering cam change, etc... I'd say the original map was off. Probably your issue too. Doubt the header pipes are the problem, because lots of bikes run that size.

When cruising through the neighborhood, what gear/rpm are you at? TC's like being wound a up a bit compared to EVO's or Shovels.

 

The comment about most SBC's using a 1 5/8" header, not true for those engines making 1 horsepower per cubic inch, those engine are using 2" - 2 1/4" headers.

 

Back in the day with cast iron cylinder heads, 1 5/8 and 1 3/4 was the choice. And by the way most Harley's do only make 1 hp per cubic inch or less.....

 

Exhaust gas velocity is a critical parameter to engine tuning.

Exhaust gas velocity is one of the two controlling factors of the exhaust reversion wave. Smaller diameter tube has higher velocity given same exhaust gas volume. Length of exhaust tube is the other factor.

Exhaust reversion wave will scavenge combustion chamber IF reversion wave arrives at exhaust valve at correct time.

Exhaust reversion wave can be controlled by exhaust gas velocity and exhaust tube length.

Larger diameter exhaust tube comes into "tune" at a higher rpm than smaller diameter tube of the same length.

Practical applications are Ford "high-velocity" exhaust ports (read smaller area but higher gas velocity) and "long-tube headers" (again smaller diameter but longer length). Both applications increase low rpm torque values.

(We did this fifty years ago on the Can-Am cars)

 

One thing I have noticed while measuring exhaust headers. The inlet on a lot of the aftermarket headers are 1.5 to 1.6. This is dependent on how the flange is attached. The welded on flanges or the cone shaped that are formed into pipe seemed to be the smallest. A 3/4 initial headpipe design that has the flange lip just welded on the outside diameter of the pipe is what is needed for a true inner diameters entry of the exhaust to be 1.7.

 

It could be either second or third gear. It's not as noticeable in second gear, but still there. I don't have a tach, so maybe between 1800 and 2200rpm. Open it up even just a little and the "burbling" disappears.

Here's the thing. I've been drag racing for a long long time. I have experimented with different header tube diameters......on a RACE application. When you're buzzin' a motor up to 7, 8 or even 9000 rpm, bigger is better.

But on a STREET application, how critical is it? With the plethora of different header configurations out there, is there any real research for anything other than max performance? As I said in my initial post, this is a low rpm deal that never sees the other side of 4000 RPM. My thinking is that a smaller primary tube diameter might be beneficial, even maybe at the sacrifice of top end power.

So, maybe it could be tuned out. Maybe not. I have richened and leaned the thing on either side of where it's at now. It's always there.

When it really comes down to it, it's something I can live with.

 

Yeah, that. Well stated.

All that said, I think the general consensus is correct - what you really need is a decent dyno tune by a competent tuner.

 

If this helps. The two dyno sheets are identical build and dyno. The only difference is the pipes. The RB is 1 3/4 2-1 and the Python staggered duals are 1 5/8.

 

Interesting little dip in torque with the RB header at about 2700 rpm. Also interesting that the 2-1 header reach peak torque a lot sooner, but is a lot "peakier."