With a new less restrictive exhaust you should get a bit more power, adding a header may or may not help. My original headers were the old-old style square port with 3" collectors. The header ports matched the head ports reasonably closely but still needed a bit of port matching. I noticed a slight power increase, mostly on the low end, but then I never drive at very high RPMs. When the original headers finally rusted through at the collector the new set had the square ports but a 2-1/2" collector. There was no noticeably difference even with no port matching & the smaller collector. The newer Pacesetters are basically the same header as those for the 2.3 & 2.6 Isuzu engines but without the smog fittings. They have round ports instead of square to match the head, so there's no way to completely port match, there's not enough material in either the head or header flange, they also have 2-1/2" collectors. I think the header opening may overlap the head opening just a bit in the head port corners, if so the header flange probably needs a bit of grinding in that area. Pacesetter claims the new round port design is no more restrictive than the old square port, but I'm skeptical without at least some port matching. Either way a header is less restrictive than the stock exhaust manifold, but not a great deal less restrictive. Downsides: The header paint will burn off almost immediately. The provided flange & collector gaskets are just fiber & may not last long. Headers dump a lot more heat into the engine compartment than the stock manifold. Upsides: slightly less restrictive exhaust, coolness factor The header is not a bolt-up to the exhaust, so getting the new exhaust mated to the header is the way to go- it'll be cheaper to include the header now than to add it later. If future plans include things like a cam or larger valves or more compression then including headers in a new exhaust system now makes more sense.
_________________ '78 Chevy LUV,1.9,4sp,headers,31's,SAS Dana 30,Dana 20,SOA rear w/Aussie Locker, rough body, bought new 12/4/78. '87 Trooper,2.3,5sp,headers,31's,ball joint flip & spacer,Aussie Locker rear,Superwinch hubs,brush guard w/5.5K winch,more to come.
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