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theres a page with some big block chevy or ford guys doing some header testing on full blown, fully built high compression large v8's in racecars.. on those large v8's they were making more power with 1.5 inch header primaries.
if a big block v8 can use 1.5 inch primaries and make the most power, you gotta wonder why they sell 5sfe headers with 1.75 primaries.. i think my 3sge header had 1 5/8th or around there, and its by far the best header ive ever had on an mr2.. my steel tubular modified camry header isnt too bad either, beats the hell out of an ssac. i think it would be interesting to see where a 1.5 puts your power.. i think you'd up response quite a bit and still flow a reasonable amount towards the stock cammed limit. remember, headers are made for an effective range, not just a peak figure. its better to gain 10hp for 3000rpm than gain 15 for 1000rpm.
tried a bunch of secondary lengths.. i think i was using 1.75 secondaries, maybe they were 2.0.. but the super short y-merge produced a significant bump in power..
also keep in mind that the stock b-pipe is 1.5 inches. its not even a smooth 1.5 inches, its just all over the place with lips and weird metal blocking the flow. you can have a great header and it'll be negated by the stock b-pipe.
i think a smooth, mandrel 2.0 exhaust and flex section would give the best gains with the stock cams. i went 2.25 just because its what i had laying around.. i think 2.0 would be better. for reference, a RSX-S makes 200hp with a 2.0 inch mandrel bpipe.. a big, long, front engined b-pipe at that, flowing to 8k. the mr2 has a shorter pipe and much lower rev range.
plus a well built b-pipe and header sounds SO much better than an ssac/chinese knockoff.
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