Dear Mr. Crockett;
The whole purpose of headers on the 235/261 engines is to get the exhaust out ASAP.
Aside from the 'rap' sound It's most important during high RPM conditions (5500)in the case of these small CID engines with 3 cyl. going into each pipe.
Now; The Dyno. numbers/studies you speak of probably relate to the larger V-8s with 4 Cyl. going into each pipe running well over 6,000 RPM.
If these indicate that the (X type)crossover improves performance, fine. But that's their world & not applicapble to our 235/261s.
I know it sounds confusing, but as a matter of practice "pipes" are much louder W/O the crossover which is why it was introduced for "street machines" in the early 60s.
Having the exhaust pressure 'equalized' with this (X type) may aid some V type engines under some condition, but not the old 'sixes' because it slows things down (exhaust relief) and adds extra plumbing.
John M, #3370.....
