Good morning everyone. I was thinking the other day about dual exhaust tubing sizes and HP gain/loss. I've googled this a few times and mostly get people with race cars and open headers saying how the smaller the tubing size, the more restrictive it is. This is obviously true for high HP vehicles, but what about a vehicle with 250-400 HP? Would it be more beneficial to have 2" or 3" true dual exhaust, assuming routing it was not an issue?
I guess I'm REALLY asking this because I'm regretting going with 2.5" dual exhaust on my Tahoe (in sig). I have OBX stainless longtubes to true 2.5" pipes with knockoff Flowmaster mufflers (Xcelerator? I forgot). I'm wondering if stepping down to 2.25" would be a benefit or if I would lose power. I'm probably only making 300 +/- at the wheels, so I'm obviously not trying to break any records or win any races.
Most of me thinks that the larger exhaust, the merrier. However, another small part of me is thinking that some restriction would be a performance benefit. How? I have no idea.
Just a stupid random question of the day. LOL
Just FYI my stock Vortec 5.7 came from GM with 3" duals starting a few inches after the 2.25" outlets on the manifolds. 3" duals through the cats to the muffler and a small 2.75" tail pipe. Two of the first modifications I did years ago were shorty headers and a fual 3" in/single 4" out muffler with a 4" tailpipe. Woke it up noticably. Later I had a cat plug up and swapped a pair of 3" thunderbolt high flows in place of the factory cats. If you already have 2.5" dropping down to 2.25" will do nothing except make it more raspy. I later added Thorley Tri-Y headers to it. They came with head pipes that bumped up from the 2.5" collectors to the 3" GM piping. Just cut the factory pipe to fit and clamp the new headpipes into place. Tri-Ys on that existing system made it run so much better than with the shorties.
If your 2.5" pipes are routed side by side a 2.5" X-pipe would really change the tone and free up some TQ and HP.
My old 83 G20 had the factory catless duals from an 85 1-ton G30 van on it. Those vans had tubular manifolds and dual 2.5" pipes with dual mufflers. I swapped the mufflers for some long case magnaflows. To fill up the gap in the piping from the shorter mufflers I added a DR Gas X pipe where the pipes joined in the front just behind the driver seat immediately behind the front exhaust hanger. Added alot of torque and power compared to the stock manifolds, stock 2" to 2.5" Y-pipe, pellet box cat and factory dual out muffler. I liked that the 1-ton van system was stainless steel and had the stock look. At the time I had a performer rpm Q-Jet intake and a 1904 Q-Jet on it. Ran really well for being an internally stock 305.
If you are starting from scratch. For a stock 305 or 350, I would suggest 2.25" duals. With some breathing modifications they run well with 2.5" duals. Backpressure really only hurts. I found that my stock L31 ran well with headers and dual 3" piping even after removing the restrictive stock manifolds, stock cats and stock muffler. It just kept building torque the less restriction I had. All true dual systems will run better with some form of a crossover. H helps marginally better on the bottom end but a X-pipe generally sees gains over a broader RPM range. X-pipes usually work a little better with an even pipe length between the engine and the X.
Magnaflow even has a dual in/dual out muffler that has an internal X that worked really well on my cammed 4.7 Dakota and sounded wicked.