2006 Sierra Lq9, TV2 cammed, D1SC @ 10psi and 1 3/4 LT's. Looking to change over my single 3" header back to a single 4" header back, and have it run over the axle, and out the back. Will be replacing the current Y merge from dual 3" into singe 3" to a true dual 3" to 4" merge also. My concern after getting the magnaflow muffler, and the flowmaster 3-4" Y that this **** is huge. Maybe overkill even? Whats your guys opinion?

 

My 5.3 putting out 11-12psi I ran AI 1 7/8 LT headers, dual 3" into 4" FM merge, 4" catback with dual mufflers and It was fine. I actually just removed my 4" catback style exhaust and looking to sell it. 4" was quieter than I thought at idle prob due to the dual mufflers but gets pretty loud as you get on it. I went back to a 3" catback. Mainly because 3" catbacks dont start to become a restriction till like 450hp and even then a 4" wont give you much unless your making considerable more HP. I'm only putting out 500-550hp so a 3" will suit me better as I spend most of my time below 3-4k and under 450hp. I'm actually looking to upgrade to an Lq9. I'm sure I'm going to loose alittle higher rpm hp/tq but its all always a compromise. Look at it this way my 2014 ram 5.7 Hemi puts out 395hp stock with a very restrictive ypipe and cats and the OEM catback is 3" with dual 2.25" axle back pipes. A larger High flow catback 3" with 2.5" dual axle back pipes only nets 5hp gain. replacing the Ypipe and manifold with LT headers and high flow Y you gain like 35rwhp plus intake mods with the same style 3" catback. Thats a 450+hp with a 3" catback (if they put out 395 from the factory which they normally are higher). Upgrading to a dual 3" exhaust nets some more power but at the cost of noise and its not "alot" like 15-20hp. I would rather keep the single 3" and enjoy alittle more low end at the cost of 15-20 hp at the top end. So it depends on your goals... if you dont care about power below 4k then go with the 4" exhaust, if you care about alittle more low end around town stick with a 3" or a 3.5".

4" pipe is huge lol going from a 4" to 3" is crazy to see. With the 4" I had to run a PHB relocation tho which causes undesireable rear suspension issues which I didn't appreciate and contributed to the 3" conversion.

 

I was pretty surprised at the 4" stuff, didn't expect it to be so big lol. The truck currently makes 593 @ 7ish lbs. Added meth and dropped the pulley, expecting 650+at this point. I know the 3" is restrictive, but I felt like going to 3.5 would be a waste of money, so I jumped straight to 4"

 

I would expect single 3" to 4" to free up 20ish hp on your setup

 

I agree with jumping from 3" to 4"... as that's what I did. Gain 20hp in the upper ranges vs the loss of how much under the curve? Exhaust needs savaging and velocity to be optimal, the issue with a larger size is the loss of velocity and therefore scavaging at low to mid RPM affects hp/tq in DD driving. You could gain 20hp at 6000rpms but you could loose 20hp below 3500rpms. Exhaust is always a compromise and should be designed with the majority of your driving style in mind. I spend alot of time below 3500rpms and very little at 5500rpms so hence the downgrade of the 4" to 3". But I'm at 100-150hp less than you so a 4" is better suited for you. An idea I just conjured up is to run a electric cutout and hook it up to a boost pressure switch, so it would open only when in Boost... seems like you could have your quietness/velocity for DD and then it will open up when flow is needed. I might do something like this down the road depending on how the car feels/sounds.

 

I actually do have a cutout on there now, and picked up 2mph through it, but it's so damn obnoxious. Also it looks like I'm going to have to taper down to a 3.5" pipe over the axle t clear, as the truck is a little low. Either that or beat in the 4" to make room.

 

Yea thats the prob with cutouts... open pipes are obnoxious. I vote to beat the 4" for clearance. Not worth the full reduction in DIA and the added reducers etc. Not to mention it only flows as much as the smallest bottleneck. Beat the 4" for the clearance.

I was gona say the truck in your sig is low lol.

 

I have yet to see one example of gaining substantial power in the upper rpm's and loosing substantial power in the lower rpm's.

"If" an engine can gain that much power (20hp) in the upper rpms, then that engine NEEDED that extra exhaust flow and is not going to suffer down low. It might not feel as snappy in terms of throttle response but the power is going to be there.

 

In this application its a no brainer that the OP needs the 4" piping

 

 

I was running pretty much the same thing in my SSS. LQ9, D1SC anywhere from 10-15psi, 1-7/8" ARH headers and their catless mid pipes running to a dual 3" to single 4" flowmaster merge with two of the small case borla XR-1 mufflers back to back (got the pair cheaper than buying one big one) then a 4" over the axle pipe dumped under the bed. It was by far the best sounding and performing exhaust I had on that truck and I dont even want to get into how many different exhaust setups I had on that truck over the years.

 

Larger primary headers will show gains on that setup.

 

I have 4 inch on my setup. Love it, other then space is tight.

 

The video you posted actually proves that I'm correct. Hard to see but the first 2.5 exhaust made more TQ below 3200 with the 2.5" exhaust than the open headers. Same goes for the larger 3" exhaust vs the 2". All hp didn't show any gains until 3200+rpms. The tq below 3200 in the 2.5 test easily showed a 20-30ftlb difference between 2.5" and open headers. Sure the larger DIA exhaust shows improvements above 3200 or starts to show (they meet at that point as being even performance wise) which is great if you plan on staying up that high. I drove to work today and spent 85-90% of my time below 4000rpms. I also have experience with my 2014 ram with the hemi that I did a muffler swap (OEM to a 18" magnaflow) stock sizing and all and reused the muffler back OEM system as thats popular on our trucks as the OEM system isn't really a restriction from the muffler back. I could definitely feel a loss in low end power/tq with the muffler upgrade. So much that I sold it with less than 100 miles on it. And went with a larger dynomax ultra SS muffler. In that truck I spend most of my time cruising below 2200 on the highway which is where it felt doggy after the muffler swap.

I mean your video literally confirmed what I stated lol

Here is a screenshot of what I'm talking about, look at the very beginning of the graph. Another thing to note in the 2nd screenshot and they talked about it was the fact that they didnt TUNE for the new exhaust! They left it rich as hell and used that for the comparison. They should have tuned the 2.5 and 3" for max power that was it was a actual comparison compared to the already tuned max power base with open headers. But they didn't do that... so I call bull **** on the actual comparison as a whole. Even tho it still proves the principle its not as accurate as what the test could have been.

While I agree that the OP needed the 4" exhaust I too believe I needed the 4" exhaust which is why I went that route. But I did end up selling mine after 100 miles due to the noise on constant street use. I went with a 3" catback and have felt NO difference in top end power and Still hitting 10psi at 5k. Sure am I leaving some power on the table yes but Its not very often that Im spinning to 5-5500k on the street. At least now I can spin that high and not feel like everyone in the town is looking at me (including cops).

 

Another video for you...

http://onthedyno.com/GM-LS-motor/art...s-1-78-sc-376/

 

Doubt it, 1 7/8" is prob bigger than needed. I only ran 2" primaries on a 521 which was specified from a very knowledgeable engine builder who had dyno'd my combo before many times and found that 2" primaries made more power than 2 1/8" and larger. You would think 2" was small for a 521. Smaller is usually better on header sizing due to velocity and scavenging.

 

Oh good, proof about header size. Gotta love proof.

 

Below 3200??? seriously? What scenario would you be wot at 3200 and how long is the rpm gonna be at 3200 in a performance environment??/ That makes absolutely no sense and the guys in the video point that out.

 

watch the video again and pay attention to 5:20

 

Already knew u were going to say that. I'm not even speaking WOT. The fact that this test shows WOT only means the gains will be less if not less than a good smaller DIA system with anything below WOT. My point being we spend 2-3s max at WOT unless your at the strip in which case 8-12s max. Then your off WOT and spend all your time cruising around below WOT. Why give up power everywhere minus WOT so you can have a few ponies at the top end. The OP never said anything about WOT max power only, were also discussing single exhaust systems and not dual 3". The fact is this video only proves WOT gains above 3200 on an untuned exhaust system running considerably rich compared to the original tune.