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Originally Posted by youngkadafi
Did you forget about velocities? Plus hotter air flows faster.
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Heat loss is more the material and the coatings/wraps on the exhaust not the size.
Header Main and Secondary sizing is important cause you want to create a scavenge effect to pull the exhaust out of the cylinders. Making it flow faster then an open head. To small and it will restrict. To big and the gases will stall out negating any gain over open head.
After the header collector, bigger is better. As long as the exhaust stays the same size or steps bigger as it gets further. You should see gains up to a point. should never loose anything.
The gains from 2.5 to 3" for me was 4~5hp. Tq was the same. I also replaced the factory "fake" cat with a resonated test pipe. I think this is a bigger problem on the S14's cause the fake cat is a hollow case 6" across and 4" high. Big area for exhaust to stall out and possibly cause backpressure.
The gains being negligible 2.5" vs 3" on the KA. It comes down to there being more 3" exhausts on the market for the 240sx. IMHO and others that have heard my car with the 2.5" and the 3" said the 3" is lower toned and better sounding overall. I dunno about DBL arguments cause I don't care down here and both my exhausts were JASMA cert for sound level.
Exhaust leaks and obstructions will make you lose power. Make sure you get new gaskets and use coper sealant on the flanges. I would rip out S14 fake cats and replace them with (resonated) test pipes. Think it was stupid of them to place that there when the real cat is in the DP. Odd shape allows for turbulence and going from a larger diameter to a smaller creates a pressure differential which results in back pressure.
Cliffs: It's all about flow. You want to get the exhaust out as fast as possible.