Dual pipes with X pipe muffler

[OutlawDrifter]

Factory 3/4t/1 ton pipes are 2.75" size wise, between 2.5 and 3". The single tailpipe is also 2.75".

True dual 3" is a waste of money on something that doesn't need it...all it will do is make noise. If that's what you're after, more power to ya!

If you want "power" from exhaust you need to do some serious math/science to keep the velocities where they need to be to keep the gas moving fast, which helps with scavenging. The exhaust cools as it gets further from the engine, you need to find the happy medium between "free flowing, and keeping the velocity(temp) up." If you're going through all that trouble, you also need to make custom headers, tri-y's or equal length primaries...

Too big of exhaust pipe and too much cam are common mistakes made when people start modifying a vehicle.

 

[Erik the Awful]

I believe exhaust velocity doesn't matter as much as preventing reversion. I plan my exhaust so the size increases at it goes out. Unfortunately, every full-length header for our trucks has a 3" outlet. I had to put a set of 3" to 2.5" reducers on my exhaust. Ugh. Sure, the exhaust gases speed up as they go through that reducer, but there's backpressure that's pushing it.

Edit: Doh! Every CHEAP header has 3" outlets. There are full-length headers with 2.5" outlets, but they start at $300.

 

[L31MaxExpress]

That’s weird because I have a 96 2500 hd and it only has 2.5” pipe. It looks to be stock however it’s cut off right at the bed.

The 3/4 and 1-ton trucks got 2.75" OD pipes. The Express vans got 3" OD pipes. Not sure why GM used 3" dual but they did.

 

[Pinger]

I believe exhaust velocity doesn't matter as much as preventing reversion. I plan my exhaust so the size increases at it goes out. Unfortunately, every full-length header for our trucks has a 3" outlet. I had to put a set of 3" to 2.5" reducers on my exhaust. Ugh. Sure, the exhaust gases speed up as they go through that reducer, but there's backpressure that's pushing it.

Edit:Doh! Every CHEAP header has 3" outlets. There are full-length headers with 2.5" outlets, but they start at $300.

Is that true for shorties also?

With full length headers - is the O2 sensor on one of the branches or the pipe that follows on from the collector?

 

[OutlawDrifter]

I had to put a set of 3" to 2.5" reducers on my exhaust. Ugh. Sure, the exhaust gases speed up as they go through that reducer, but there's backpressure that's pushing it.

Edit: Doh! Every CHEAP header has 3" outlets. There are full-length headers with 2.5" outlets, but they start at $300.

I don't agree there is significant backpressure created, quite honestly, if any at all...you'd have to have some serious engine in front of a 2.5" reducer + some large primaries.

$300 for a set of headers is still in the bottom end for a "quality" header. Hell, the Chinese stainless knockoff LS swap headers can run $275-$299 based on application.

 

[Erik the Awful]

I didn't do enough research when I bought my headers, or I'd likely have spend the extra hundred on headers with a 2.5" outlet.

OutlawDrifter, if your headers dump into a 3" chamber and then you reduce it from 3" to 2.5", you're going from a 7 sq/in cross section to a 5 sq/in cross section (Area = Pi x R^2). That's a 30% reduction in size. It's going to negatively impact your exhaust flow.

 

[Schurkey]

Kinda depends on HOW you reduce the diameter. "Merge" collectors deliberately create a venturi effect with a "restriction" at the tightest point of the venturi.

Wild Guess: Doesn't make a bit of difference on a street-driven pickup. The exhaust is going to be non-optimal in so many different ways that it's kinda pointless to worry over one item while neglecting ten others.

 

[OutlawDrifter]

OutlawDrifter, if your headers dump into a 3" chamber and then you reduce it from 3" to 2.5", you're going from a 7 sq/in cross section to a 5 sq/in cross section (Area = Pi x R^2). That's a 30% reduction in size. It's going to negatively impact your exhaust flow.

Kinda depends on HOW you reduce the diameter. "Merge" collectors deliberately create a venturi effect with a "restriction" at the tightest point of the venturi.

Wild Guess: Doesn't make a bit of difference on a street-driven pickup. The exhaust is going to be non-optimal in so many different ways that it's kinda pointless to worry over one item while neglecting ten others.

Erik, I agree with Schurkey, and you're not dumping all 4 primary tubes into the collector at once...keep that in mind. You're not going to see any negative impact unless you're turning crazy high rpm or have lots of cubes where you're moving tons of air...and at that point running 2.5" collectors isn't likely anyways. On a mild 350, or even 383, you'll never notice the difference. I understand your math, and don't disagree that you're bottlenecking it down...it's not enough on a DD to make a difference.

 

[CalebAdams]

So based on all of this, for a mild performance build I plan to eventually do to the engine: shorty headers, 2.5” down pipes with individual 2.5” high flow cats or a 2.5” dual in/ 3” single out high flow cat, 3” in/ dual 2.5” out muffler, two 2.5” pipes out the rear. Really the only reason to split it back in two after the muffler is to mimic how the exhaust is right now on the truck, but I’m open to any suggestions on any of it

 

[PlayingWithTBI]

Just an FYI, dual 2-1/4" and single 3" flow about the same which is plenty good for anything up to 400 HP.