Stock-ish 5.7 Build...2.5" or 3" Exhaust?

[4speeds4me]

I'm putting a 2013 5.7 (395HP according to literature) in my 68. I have decided that I'm not going to mess with it internally at this point, though I will likely play with the tune some. I am trying to decide on exhaust size, and wondering if a fairly free flowing 2.5" system is sufficient, or is it worth stepping up to 3"?

Thanks!

 

[RockinRobin]

I think you'll be happy with the 2.5" system as long as the mufflers aren't too restrictive.

 

[70blackfish]

I agree on the 2.5 inch, the 3 is overkill, i think it only comes into play at High rpm and big cubes

 

[hemisedan]

Im for the 2 1/2" also, but maybe use the 3" for the end pipes. They tell me that they will add a little but looks is more what it will do. Just a clean, non restrictive muffled exhaust should do just fine. I'm looking that truck 395 hp 5.7 too. For me, in a street Valiant, I think that 485 hp is a bit much for the street. Maybe I'll regret it, but that's my thoughts. 400 hp in a 2900 lbs Valiant, that should suffice.

 

[4speeds4me]

Yeah...I was thinking something like Dynomax UltraFlos. I like the sound, and they seem to flow quite well...

hemisedan- Kinda my thoughts...I know guys who are in the 425-440HP range after reworking with the Diablo, and I started thinking that I'm pretty impressed with the performance of my 5400# truck, so dropping a ton, adding a stickshift, and a bit of tweaking isn't going to minimize the "giggle" factor when I mash the throttle!

 

[gregsdart]

about 2.2 cfm of flow per horsepower will get you 99 percent of the open exhaust power.
Straight pipe flows 115 cfm per square inch. A 2.5 inch pipe flows 560 cfm, but there will be some flow loss for bends and total length. Then you need to look at the mufflers. The pipes can't flow anymore than what will go through the mufflers, so they have to be matched to at least the minimum cfm you need for least power loss. According to one chart I looked at, if you only have 1.75 cfm per hp, then the loss will be about five percent of total power. So a 400 hp motor will lose 20 hp on the top end, and less in the mid range. Be sure and use a balance tube in the system, this will help with sound and total output as well.
I just installed a 2.5 inch system on my 65 street dart, for the next motor which should be 450 hp. The mufflers I used are 17733 Dynomax, which flow about 410 per side for about a max 4 or 5 percent loss in the 475 hp range. I can handle 22 less hp on the top end on a car I don't race.

 

[4speeds4me]

So, by my calculation, not counting loss for bends and total length, dual 2.5" should be good for ~515HP. Even if one allows a 10% loss over the system, we're supporting over 450HP. As well, the exhaust becomes denser as it cools through the system, so by the time the bends are really coming into play, the exhaust is taking up less volume.

I thank you for that indepth reply. It's nice to have hard numbers to look at/manipulate...2.5" it is!

 

[mcat]

Do a 3 inch.. so you only do it once... when is anyone every happy with their first attempt at horsepower....room to grow

 

[Hellcat Cuda]

2.5 is plenty for a stockish 5.7 3.0 would be overkill if not detrimental for a stockish 5.7

 

[joeboy]

I agree with the crowd…

I have 2.5 with borlas and a x-pipe on my 5.7 with a cam… sounds good and runs great. I have always heard big pipes for top-end power, but a bit smaller will help retain some mid range torque.

My 3" system cars always seemed to have more resonance too…

 

[72BBSwinger]

I'm doin 3" out the side, gonna kill weeds when driving down my alley. Megaphone tips FTW!

 

[swifter]

Been here and did 2.5 first THEN when we stroked the 5.7-2.5 wasnt enought and had to pay AGAIN for 3in-do it once with 3in and be done and pay for it once.casue you WILL want more .we have 518 to the rear wheels so somewhere around 600 at the crank, if you do the math 3in is perfect .- steve