Really stupid question about dual exhaust pipe sizes

[Redneckgeriatric]

Anyone who says you need exhaust backpressure on a modern fuel injected engine is lying to you, directly or indirectly. No reason to regret going with a larger pipe than you really need, you're not going to lose any power/torque or hurt anything.

no one wants backpressure. it is exactly what is says.....pressure. what you dont want is reversion which is allowed when using too large/short an exhaust.

 

[L31MaxExpress]

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.

 

[df2x4]

no one wants backpressure. it is exactly what is says.....pressure. what you dont want is reversion which is allowed when using too large/short an exhaust.

But what about... no exhaust? (The fun starts around 2:20)

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For the record I'm definitely not endorsing this as anything close to a good idea, I just can't help but think of this video whenever these types of discussions come up.

 

[L31MaxExpress]

But what about... no exhaust? (The fun starts around 2:20)

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For the record I'm definitely not endorsing this as anything close to a good idea, I just can't help but think of this video whenever these types of discussions come up.

Reminds me of the non turbocharged V12 warbirds.

 

[upper_tanker]

Thank you all for your input. This definitely answers my concerns about possibly going to big. I've read before that the back pressure argument thing was bullshit, but I never knew how true it was.

I guess I'll keep on the hunt for better mufflers. LOL

 

[Redneckgeriatric]

But what about... no exhaust? (The fun starts around 2:20)

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For the record I'm definitely not endorsing this as anything close to a good idea, I just can't help but think of this video whenever these types of discussions come up.

lol. the idiot that says "THAT is sweet!" is a perfect example of the quality of information to be found these days with a youtube search. proves beyond a doubt one needs no intelligence or integrity to make money.

 

[Erik the Awful]

A while back I hand-built a pair of headers for the 500cid Cadillac motor in my Jaguar. The cam was .522 lift and 268 duration, and the header flanges I started with were cut for 2" primaries. I built tight block-huggers in a tri-Y configuration. The pipes started 2", but at the first Y went to 2.5". At the second Y they went to 3". Thus the pipe diameter kept expanding. A 3" exhaust was bigger than any calculator said I needed, and all the old-skool builders will tell you the expanding tubing diameter kills exhaust velocity. I'd had a pair of professionally built headers on the motor previously (in a different chassis), and it consistently ran rich. With my home-brew headers I had to rejet the same Edelbrock 725 to the largest jets I had. That's more fuel being burned, and more power to the ground. I think preventing reversion is more important than exhaust velocity.

 

[evilunclegrimace]

In my opinion, the only reason for backpressure is if you're running weak valves and you're afraid the engine might suck one. I tend to go big on exhaust pipes. If you're hoping to hit the 400hp mark, I wouldn't go any smaller than 2.5"

Back pressure is not going to prevent an engine from dropping a valve due to weak valve springs.

 

[Rick Reid]

I've got 89 GMC Sierra, nothings stock on her anymore. It's got a GM crate Vortec 350 truck engine, Edelbrock on top, Hooker Super Comp headers, Flowmaster mufflers and it's all 3", wouldn't change any of it. Sounds great and runs like a bat out of hell !

 

[L31MaxExpress]

A while back I hand-built a pair of headers for the 500cid Cadillac motor in my Jaguar. The cam was .522 lift and 268 duration, and the header flanges I started with were cut for 2" primaries. I built tight block-huggers in a tri-Y configuration. The pipes started 2", but at the first Y went to 2.5". At the second Y they went to 3". Thus the pipe diameter kept expanding. A 3" exhaust was bigger than any calculator said I needed, and all the old-skool builders will tell you the expanding tubing diameter kills exhaust velocity. I'd had a pair of professionally built headers on the motor previously (in a different chassis), and it consistently ran rich. With my home-brew headers I had to rejet the same Edelbrock 725 to the largest jets I had. That's more fuel being burned, and more power to the ground. I think preventing reversion is more important than exhaust velocity.

Or you weakened the vacuum signal on the carbs booster.

However the big cadillac probably made more torque everywhere with the tri-ys.