Here is a picture of my situation:

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The red line is an exact drawing of my exhaust port. The black lines show the ID of 2.25", 2.125", and 2.00" header tubing. So, should I use the larger tubing so I don't block exhaust flow out of my heads? Or should I intentionally inhibit the flow and work with smaller tubing so the velocity stays high?

This engine is a big displacement BBF (528 - 546 cid) and my long term goal is ~1,500 hp.

For reference:
2.25" vs. 2.125": 12.9% larger
2.25" vs. 2.000": 28.5% larger
2.25" vs. 1.785": 63.9% larger

If more information is needed, just let me know.

Have a good day!
Michael

 

I would never go smaller than the port. Same size or a little bigger is what is required.

 

yep, always bigger, a 1mm or 2mm larger dia all around helps prevent reversion, the higher the exhaust to intake PR, the more the reversion becomes a problem.

 

Don't block the port.
Use the size that fits or swedge the smaller pipe to match the proper size header flange.

 

I thought I would come back to this thread and update it. Here is what I ended up doing:

Old flange (at bottom) versus new flange (at top):
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Cut a single slit in the 1-7/8" tubing:
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Weld a little. Form the tubing a little. Lather, rinse, repeat...Weld a bead on the inside of the header tube to fill in (like bondo) any surface irregularities.
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Grind inside smooth:
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Cut a small "V" shape to fill in the gap:
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Grind smooth again:
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Finished product:
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Have a good day!
Michael