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Discussion Starter #1
Just had flowmaster 70 series, dual in dual out cat back system installed on '04 2500HD Sierra. The guy installed w/ 2 1/4" pipes and 3" tips out the back. Sounds great but seems to me I've lost HP and torque. The guy told me " you went from one 3" out to two 2 1/4"s, you should have less backpressure" Is he full of ...... or would I get the perfomance gains I expected w/ a 2 1/2" or 3" system? Or is there something else going on here to cause this "Imagined" loss of power? :guns: :pissed: :bawl:
 

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less backpressure can cause torque loss, though the diameter of the pipe doesn't matter as much as the fact that there's 2 of them. you have 4.5" of pipe instead of 3" and that combined with the new mufflers means less backpressure. i don't think 2.5" or 3" is going to help any.
 

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Actually, 2 2.25" pipes will flow just slightly better than 1 3" pipe...and i mean its not much of an increase at all...i guess that it could make a slight difference in havin 2 mufflers if the actaully tip dia. is 3". less backpressure can cause less torque, but it shouldn't be that much honestly...should be able to tell a lil gain in top end tho. I wouldn't mess with it anymore if it was me.
 

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HOT AIR BALLOON OWNER
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I thought HD gassers came with 2.75-3" duals stock, nay?

Or are you talking about 2.25" tailpipes?
 

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I know my 8600GVWR 97 3/4 ton has 3" pipes stock. I bought 2 1/4 flowmaster 40's in hopes of keeping some lowend and had meineke make some adaptors and hangars, and dump it right out at the tire. I have noticed a touch less lowend, but it makes up for it higher up (and the sound is neat, but I wish it was deeper).

The truth of it is, you really don't need dual exhaust unless your running like 900+ horsepower. It's more of a showy thing started from the muscle car wars...A properly bent single pipe setup can accomodate most any motor better then a dual exhaust. Not to say you won't gain horsepower from a dual setup (I probably gained 5hp and another 5ft. lbs up higher with my flows, but the lowend torque isnt quite as strong) as compared to a stock...but the most efficient way is just one pipe unless your making some SERIOUS power.
 

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I wouldn't go with anything bigger than 2.25 on true duels. U might have lost a touch of low end like everyone said, but i couldn't really notice it in my setup. U are running one muffler though, i dont know if that has anything to do with it. I ran two 40's and i can tell the difference in the mid to upper RPM's. I did ditch my cats though so that could have a lot to do with it. 2.5 or 3 will kill your backpressure considerably. In the case you have some serious HP like ernie said, then go with three. My auto parts guy runs 2in primaries on his headers and feels he wants to go a little bigger cuz of the extensive amount of headwork done. u can prop fit your fist in the intake ports!!!!. Anyways i bet u gained some over hp and may have lost low end like these guys said and that why u feel u lost some power. i'd stick with the setup you got.
 

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Discussion Starter #7
Hit Man X said:
I thought HD gassers came with 2.75-3" duals stock, nay?

Or are you talking about 2.25" tailpipes?
Yeah. they ran two 2 1/4' tailpipes. And yes, the stock HD does have two 2.75's from manifold, through two cats into one muffler w/ one 3" out.
 

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Discussion Starter #8
Thanks Gent's, That explains the "feel" for a loss of power. And yeah, feel it especially @ the lo end. Just to clarify though, I've just got one muffler. A 70 series w/ two in and two out.
So I guess I won't go chew out the "Guy"
 
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Kilborg said:
and the sound is neat, but I wish it was deeper
If you want it deeper run tailpiipes with tips...completely changes the exhaust note and it sounds awesome.
 

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On my 96 1500 with the 5.7L im running 2 1/4" from the manifolds back to the muffler where the muffler shop expanded it to 2.5" (inlet/outlet size) and from the 2.5" outlet it goes to 2 3/4" all the way back to the turndowns.

Sounds tough as hell to me, and even though this same setup was on the truck already...it seems to pickup alot more too.
 

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Dual 2.25" and single 3" flow about the same. Dual 2.25" pipe has slightly more area. With that said I would have been kinda pissed. I would want dual 2.5" pipes with that setup. You would have been surprised at how much low-end you would not have lost.

I have driven a stock 283 SBC car with the stock setup and then again with headers and dual 3" exhaust. That car hardly lost any power down low and made more everywhere.

And backpressure isn't the problem, it's the scavaging (sp?) effect of the entire exhaust system...
 

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Discussion Starter #12
brodystruck said:
Dual 2.25" and single 3" flow about the same. Dual 2.25" pipe has slightly more area. With that said I would have been kinda pissed. I would want dual 2.5" pipes with that setup. You would have been surprised at how much low-end you would not have lost.

I have driven a stock 283 SBC car with the stock setup and then again with headers and dual 3" exhaust. That car hardly lost any power down low and made more everywhere.

And backpressure isn't the problem, it's the scavaging (sp?) effect of the entire exhaust system...
So, what you're telling me is, I'd notice the gains if I went to 2.5's. Cuz right now I'm not feeling any, anywhere. And from all I've read everywhere these cat backs are supposed to increase HP and torque all around. The idea, I thought, was to let the system "breath' better. I've also Got K&N filter and hypertech 93 octane power tune.
 

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Do you have any other mods done to the truck? I know when I changed out my exhaust and had dumped right after the muffler, I felt a little loss of low end power. After a little while it felt as though it picked up a little. Also, when I added my cold air intake though it really woke up with the exhaust. Give it some time. I agree, I don't think you'd want to go much larger than 2.25" pipe on a true dual setup. You should be fine.
 

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pvine said:
So, what you're telling me is, I'd notice the gains if I went to 2.5's. Cuz right now I'm not feeling any, anywhere. And from all I've read everywhere these cat backs are supposed to increase HP and torque all around. The idea, I thought, was to let the system "breath' better. I've also Got K&N filter and hypertech 93 octane power tune.


Well Flowmaster mufflers aren't know for big power either. The stock HD exhaust flows very well.
 

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Respending the money for dual 2.5" might not be the best decision either ya know....

And it's true flowmasters arent the best flowing out there, but i dont think its hurting you right now.

im just saying if it would have been dual 2.5" i am betting you would have been much happier.
 
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