Dear Frenchtown;
These are the 'dyno tests' I refered to at the onset here.
As I recall; they were done on the Ford 351-C (big blck) engine which was used a lot in their 1/2 & 3/4 trucks and Station Wagons. This engine had the valve timing retarded too (for SMOG) around that time and this is one of the reasons for the testing.
Motor homes using it had to change the chain/gears to pull any hill.
My 1972 wagon had one with dual exhaust (separate) from the factory too. I got it from a "fleet manager" who had changed the chain/gears and drove it to/from Las Vegas at the time. By 1983 it had over a hundred thousand and was still running great.
No "H or X" pipes on wagons or motor homes (where needed/if worked) but some did appear on the Ranger trucks around 74. These also had dual cats as well.
This truly was a sight to see; A 2" pipe out of each header into a crossover-X pipe-crossover-cat on each pipe-then a muffler on each pipe-to tailpipe**.
I guess they (Ford) were trying to offset the horspower lost from the EPA garbage, forced upon them. There were "test results" published to save the muffler aftermarket then too.
Either way; none of this testing was done on the inline 6 in that era, or applies today for one on the streets.
Also; dual exhaust systems don't add horsepower, they just replace what the single system took away.
Happy trails.
** A friend cut his off (whole system) and hung it on the back fence for all to see. His truck (brand new) would barely run across an intersection if you 'flored' it from a dead stop.
