Headers vs manifolds on a stock 454, observations
I always wondered how much torque was added by switching from cast exhaust manifolds to headers, never had a car without headers already on. I recently did the change on my 79 C-30, and it far exceeded my expectations. These engines had a ~184/194 at 0.410 cam and 7.9:1 compression ratio, super mild and embarrassingly slow in stock form. I got really tired of the lack of torque and power, but was too lazy to pull the engine. I started by recurving the distributor, setting the Qjet correctly for best power, and yanking the smog parts. This was barely noticeable on the butt dyno, but helped. Next was ditching the OEM style mufflers, went with 20" Walker 2 1/2" turbos on the stock crimped up dual head pipes (dumps in front of the axle). I could feel the effect mid range, definitely noticeable but not really anything major elsewhere.
A few months later I noticed an exhaust manifold leak, so I removed them and installed a set of Summit headers and custom mandrel bent pipes. BIG TIME difference. Major improvement across the RPM range, off idle all the way up. I can now cook the tires from a stop, none of that before. WOT is like night and day, and the 3000 RPM cliff is no longer there. And, I went from 9 to 11.5 MPG on the freeway on 3.73 rear gears at 75 MPH. This is still with the stock cam, and the iron pancake intake. Next up is an aluminum intake, then 1.8:1 stainless roller rockers.
I'm very pleased with the bolt ons, the truck went from a gutless pig to drivable. I had seen some dyno pulls with manifolds on the old LS5 and 6 engines, you could tell there was some restriction and I guess that was it.