Pypes make 2.5" mandrel bent headpipes for the exhaust logs that we've used quite a few times. They do neck down at the flange to 2.25" i.d., but that is still larger than the 2.0" and 2.1" log manifold openings. You can grind the openings to 2.20" to 2.25" for a little hp gain, but don't remove all your flange seat. you want to keep at least a solid 1/16" minimum to seal well (no donuts required).
Dennis
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