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Greetings all! I know this forum is not as busy as it once was, but was always a great source of information. my question is specifically regarding exhaust size. Starting at the primary tubes and ending all the way at the mufflers. My current set up detuned makes just over 700 wheel horsepower. If I pulley it down and run it on kill it makes just under 800 rear wheel horsepower. This is through 1 5/8 Mac headers with a 2 1/2 inch Mac H pipe ending in 2 1/2 two chamber flow masters with full tailpipes. I’m wondering how much more power it might make if I switched to a 1 3/4 or even a 1 7/8 header with a 3 into 2 1/2 X pipe and straight through bullet type mufflers. Has anyone run both? I’m wondering if the power potential might be worth the cost? Any input would be greatly appreciated. Thanks in advance!

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I don't recall any controlled A:B comparisons for everything you are working with, but I do seem to remember Richard Holdner doing some dyno tests for header tubing size in his 4.6/5.4L Ford HP on the Dyno book. I believe he did make a comparison of 1 5/8 vs 1 3/4 headers on a 550 FWHP (or so) Terminator. The 1 5/8 header showed a minimal improvement in TQ compared to the 1 3/4 header. The minimal improvement in TQ was about 8ft/lbs at the 500 ft/lb level, so you would be looking at a 1 to 2% gain at best. At higher rpm, if I remember correctly, the larger tube header outperformed the smaller tube header, but marginally so, just as the smaller tube header marginally outperformed the big tube header around 500 ft/lbs of TQ.

A couple of relevant considerations;
  • That was FWHP on a dyno,
  • Run-of-the-mill street Terminators easily eclipse 700 RWHP today,
  • 700 RWHP approximates 800 FWHP,
  • An 800 FWHP Big Block would use 2" or larger primary tubes,
  • Your. engine produces a whisker over 900 FWHP,
  • 100 HP, irrespective of engine displacement, requires 10 lbs of air per minute,
  • You are moving a lot of air no matter how you look at it,
  • The biggest primary tube you can buy would probably be pretty close to just right.
For the most part, a muffled exhaust loses the tuning effect you would experience from an open competition exhaust. The larger tubing and muffling, however, produce a more free-flowing exhaust that lets the engine work easier and produces fewer pumping losses to diminish final power output.

Forced induction at only 14.7 psi makes a 281-inch Terminator look like a 562 cubic inch n/a engine — you are even bigger! Ask yourself what size header you would put on a 562-inch engine making around 800 HP, and you have a pretty good answer (and likely the largest header available) for a Terminator.
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