Generally, a tube header gives best power, with good torque. If torque is important, a divided cast iron header can be made to match and eclipse a tube header if you spend a few hundred dollars and a half a days fabricating. Group A racers in the 80's found that if you were limited to stock parts, you could make them do specialized part performance duty if you had the funds to test the effect of the changes made.
In my opionion, the later oxidizing cat exhast is a
real good piece of kit. It allows the splitting of a pretty bland system into two exhasts. The later Fox body sump allows space to transfer the exhast into two seperate pipes like in the Cologne and Essex V6's.
If you put the Clifford exhast divider in the log head, you then can extend a plate down into the football catalyst, and then make a four bolt adaptor to run two pipes out. One goes left, the other stays right. Like the 79-81 carby Turbo 2.3, you can run the pipe either under the trans or through the double kick in the sump. So you could run a V8 style exhast with a just some extra effort. You can even use the common 4afc Toyota Corolla/Chev Nova flex join to accomodate exhast flex, and have the whole shooting match solid linked without any concerns.The joiner could be welded to the sump, or you could even shove a 4" tube pipe right through a stock Falcon sump, and keep the oil pick-up where it is.
Basically, separating an exhast adds lots of torque, and changes the sound to a more invigorating 60 degree V-six/Jag XK/ Aston Martin pre DBS sound. Holden in Australia experimented with headers like Fords 1978-1984 item like this, but with dual outlets back in 1964 with the Holden 179 cube X2, and then the first 161 GTR. The 1980 XT5 3.3 Holden Commodore continued with a variant of this dual out system, and the the last EFI Commodore 3.3 got a tubular header version which tired to turn it into a tri y design, and it got even better torque. Each had a special sound signature. Same with the 4.9 EFI F150/E-line engine.
Dyno runs show that keeping the cast iron headers short tends to help power...they don't have to be long to suit, but it pays to keep two branches of an I6 divided. It's all getting back to Wain style dual out headers that used to be used in the 50's and 60's on Blue Flame and GMC in liners. Here are some figures on what can happen when a back to back comparison is done with really good tube headers verses common dual out iron
For more reading, its best to use the search function from Hardcore forum posts, where it has all been covered before, but see Case 1 in How Aussies Make Power
viewtopic.php?f=5&t=5554&p=38262&hilit=How+Aussies+make+power#p38262
(local server is
http://fordsix.com/forum/, then viewtopic.php?f=5&t=5554&p=38262&hilit=How+Aussies+make+power#p38262)
My suggestion to many has been to do something really odd. In cases where you've got access to the brillant tube headers we have here, if you won't damage the coating and engine bay, rip the headers off and place an 82 pallet converter header on like peugoet bills, and then make a dual out exhast headers. Do a dyno test with a 4 inch tall splitter going back up the exhast manifold. That might give you 20 hp extra like in case 1 above.
