I agree that on a performance oriented engine where the exhaust really acts as a restriction, you will see noticable gains. But on these trucks I just don't see it happening. If you do see any improvement in horsepower, you will probably see a dropoff in low end torque, which in my opinion is what you want in a truck or blazer. When you have too large of an exhaust on a vehicle low end torque drops off.

I would imagine Roush doesn't really spend much time on stock 4.3 V6 engines. Yes they are all engines alike, but I imagine they spend most of their time on performance oriented engines, running above the 5000 rpm range where you would actually need a larger exhaust and feel noticable gains on the track. The 4.3's heads and camshaft don't allow power to really be made past that point.

I personally had a single exhaust split out into two pipes on my 4.3 powered full size truck. It really didn't make a difference in power or fuel mileage. The only difference was it made a lot of noise. I really just bought the kit for looks since I think two pipes sticking out the back looks cooler than one.

I think if there were any power to gain in the blazer's exhaust system it would be before the catalytic converter. A good y pipe would probably help free up exhaust restriction, along with a set of headers. I think that modifying some of the most restrictive cylinder heads on a chevy engine would change things quite a bit. Anything from the cat back isn't really going to change much.

 

I've got personal proof that a 4.3 doesn't benefit from a dual exhaust. Guy who my gf works with has a 4.3 in his S10 and he had dual exhaust put on that truck about 2 months ago with 2 1/4" Magnaflow mufflers (rest of the system was 2 1/4" as well). Before adding duals he had a Magnaflow catback, single in/dual out. Back then his truck was faster than mine, but the duals made him lose a good amount of torque and now I put some distance on him everytime. In addition to the duals he has a K&N intake and Hypertech Programmer III. He thought it was the programmer slowing him down, so when he unprogrammed the tune his truck was even slower. All I have is a custom intake with a k&n cone filter and Hypertech III and me with my stock exhaust smoked him. 4.3s do not respond well to dual exhaust. Exhaust mods don't do much for 4.3s in general. Brett added headers on his old truck and he said they didn't do anything besides change the tone. To kind of pick up on what 91Chevywt said though, I think a better y-pipe would give the same benefits as headers. Not that I have the cash to put into a y-pipe.

 

A single exhaust can flow just as well as a dual outlet, single 3" will flow as well as dual 2.25" pipes, and it's cheaper to use one pipe. Honestly with just an intake and a tune, you won't need any larger than 2.5" single all the way back, it's a lower rpm motor and you need low end torque so unless you plan on actually changing motor parts and racing with it, any larger and you'll just lose torque and get worse mileage. My Sierra with a 305 does well with a 3" single system, I felt no difference when I put on the 2.5" dual outlets and I have it out the side in front of the axle Ford Lightning style. The 4.3 flows less air, a free flowing 2.5" single system is best, and it'll sound nice if you use a decent muffler. If you like the look of duals, I suggest 2" tailpipes to keep exhaust velocity up then use 2.5" tips or whatever size you want since all tip size does it make a deeper tone, doesn't affect flow.