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I have a 69 Mach I with a rebuilt 351w 4v. It's got an RV torque cam and a new 600 cfm holley carb. Everything else is stock. My exhaust system is about to fall apart so I'm going to change it. So I've come here for advice.

Reading a magazine, I saw that the original setup is like the one from a 390, GT 350/500, 68 428cj, 69 boss 302 & 429, or at least that's what the NPD parts catalog shows. It looks like a complicated exhaust system, going through 2 resonators into one muffler and then 2 tips coming out with lots of bends. So my question is which is better? The original setup or would it be better to have it like a 70 boss 302 where it's just an H-pipe, two separate mufflers, and then coming out to the tips, not that many sharp bends. Which is better? And also what size pipes should I go for: 2.25 or 2.5 in.?
 

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How about half an answer? As far as the 2.25 vs 2.5 inch pipe, you will definately want to go with the smaller pipe on a mild motor to avoid loss of low end torque. By the time you get to the RPM range where the larger pipe makes a difference, your cam, heads, intake and 600 cfm carb will already be choking your engine anyway.

As far as the stock vs boss setup, my intuition says the boss setup is better, but I have no hard facts to back that up.

Good luck,

-Rory
 

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Several of the car magazines ran dyno tests on various exhaust system components, including the 2.25 versus 2.5 versus 3 inch pipes. The thermodynamic concept is that a certain amount of back pressure is necessary to maintain the velocity of the existing exhaust gases. Thus, running a header with about 18 inches of pipe downstream of the collector is typically more efficient than open headers, per the dyno results. This test, however, is very different from comparing a 2.25 versus 2.50 piping for the length of the car. My recollection of the dyno test was that on their 5.0 EFI test mule, the 2.5 inch system on a mild build improved torque across the rpm band, as well as the torque peak (also for hp). 3 adn 4 inch systems become problematic on essentially stock motors, but 2.5 dual exhaust with headers will definitely help even a stock engine, particularly when displacing 351 cubic inches. You might want to do a google search on dyno tests of exhaust systems and look for these magazine articles. BTW, the comparison of mufflers showed that a Hooker muffler, the modle with cutouts, yielded the best torque/hp and was also one of the top 3 in terms of lower decible rating.
 
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