I am assuming the standard 1.5" pimaries will be sufficient for my stockish config, ie will be '95 360ci, mopar pcm, 3" I/O magnaflow cat, 3" cat back, 1.7rr, home ported throttle body, maybe a ram air later down the road.

Keeping in mind my application is not drag racing but being a jacked up 4X4 which needs a little more grunt hauling a load through mountain passes.

My question/s: What type of performance changes would I expect with my setup if using the 1.5" primary headers but varying the secondaries diameter before the Y pipe/cat? Would the 2.5" trade off low end but add more revs? would it add more throughout the RPM band? Would I perhaps not even notice a difference between the 2.5" and a well routed 2" secondaries with my config? If there is a difference how big is it and what type?

Thanks.

 

I would go the 2.5 secondaries on the headers for scalability. You will have a better performace gain with those

 

With what you have & thinking about in the future, your almost into 1 3/4" & 3" territory

1 1/2" primary ok for the 318 mostly stock
1 5/8" primarty better for the 360 mostly stock
1 3/4" primary for a modified 360 or bigger

2 1/4" secondary for 318
2 1/2" secondary for 360
3" secondary modfied

 

Somehow I knew you all were going to say that. You can only get the cheap chinese stainless ebay headers in 1.5" primary/2" secondary configs. The stuff I need is all over 2X the cost. Is this an offshoot of Murphy's law in effect here?

So assuming this is the future (no future cams, blowers etc planned) how much of a tradeoff would the 1.5" primaries with a 2.5" secondary be aka the Summit headers? I could get them Jet Hotted locally and maybe be into a set for ~$300.

If the tradeoff the tradeoff between 1.5" and 1.625" primaries is all high RPM performance at the expense of low RPM performance then maybe I don't care so much. One of my main goals would be retaining low end performance and see high reving performance as good only as long as it doesn't sacrifice too much on the low end. Keep in mind my 4X4 camper hauler non drag racer status to see help understand my philosphy here.

Unfortunately there are no longtube headers available for my model or I'd be looking at them. I'm stuck with the shorties. The Thorley Tri Y's wont even fit my 4X4 either.

Thanks for the posts. What hurts even more than bucking up for the good stuff the first time is having to rework any half assed first attempts with the good stuff later.