[GRN69CHV]

Ran Hedman 68198 1-3/4" on 469" motor down to consistent 11.50's at 116-117 through 3" closed exhaust. Fit was great, plug access with factory AC was as good as stock manifolds.

 

[GRN69CHV]

There was a good dyno article a few years back - They used a 540 CID with about 720 hp as the test engine - all were hooker super comp or hooker headers. Very similar to mine 10.8 to 1 compression, Dart 320s, cam was like 255/263 at .050 and like 660 lift big 4 barrel etc. They started at 1 3/4 and went up in 1/8 increments to 2 1/4. Basically, the big 25-30 hp jump was from 1 3/4 to 1 7/8 thereafter, every 1/8 of an inch after it was like 7-10 hp more except for 2 1/4 it was only like 5 hp. Now the 1 3/4 had a 3 inch collector while the others were 3.5 inches. The TQ was up like 20 or so with the 1 3/4 to 1 7/8 but after that all the engines had about the same TQ just at different rpms (all around 695 foot pounds) . This motor the peak TQ was like in the high 4000s/5000 and the peak hp was like 6,100 to 6,300 rpm. In that 540 combination the 2 inch or 2 1/8 would work fine and I would then base my decision on overall fitment. That all being said 500 hp a 1 3/4 would be fine but if you have better options and or fitment in the 1 7/8 that would work too. The 1 7/8 gives you room to grow also. I also wonder on the 1 3/4 do they block the port a little as the ID is really 1 5/8 but the ones I have seen seem to have a large tube/bubble coming out shrinking immediately to 1 3/4 so maybe that fixes that. However, on some aftermarket heads the 1 3/4 would probably block the port a little.

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That article was what steered me to just go with the 1-7/8" headers. Packaging with the factory AC box limited header selection, I just figured, it was a street car that I took to the track for fun, no roll cage, did it really matter if I gave up 10-15HP on the top end.