Anyone have any opinions on which exhaust helps retain the most low end torque. Right now my factory muffler has a hole rusted through it and now this thing is a dog on any sort of incline. I'm going to replace it over my Thanksgiving break but I want an exhaust that will help retain the low end torque. The 4.7 doesn't have as much low end torque and the 5.2 so I really want to keep as much as possible.

I read up on some, and Playmaker said he lost quite a but with his Hooker. Anyone (w/ a 4.7) do an aftermarket exhaust and notice that it kept a lot of the low end?

I was thinking about going with the flowmaster because they aren't a straight through muffler. It would probably create more backpressure than a magnaflow or others that are straight through. Originally I was going to go with the Magnaflow, but I'm reconsidering now...

Any suggestions would be cool. Thanks!

 

There are a couple things to consider when deciding on exhaust. First of which... backpressure does not help power or torque at any rpm, never has, never will. That idea has always been a myth due to a misconception by people who don't understand the properties of flow through a pipe. The second thing is that you cannot set up an exhaust that will maximize both the low and high end, its a compromise.

What you need to know about a good exhaust system is that it is all about the velocity of the exhaust through the pipes. High velocity = low backpressure = more power. Low velocity = high backpressure = lack of power. When you are in the low rpm range, the exhaust gasses are already traveling slower than in the high rpm range, and there is less overall exhaust. If you are looking for low rpm power, you want a smaller pipe to keep the exhaust's velocity up, cause a large diameter pipe will slow it down. If you are looking for high rpm power, you want a larger diameter pipe because the smaller one will "choke" the engine due to the increase in exhaust gas trying to get out. If you go too big, even the exhaust gasses in the high rpm range will slow down and hurt power all around. So the compromise is, do you want all low end, all high end, or the best possible mix of both?

If you don't believe that, consider this... No one that I know of has ever complained that headers hurt power at any rpm over factory manifolds, right? It's because the factory manifolds are set up where the heads just dump the exhaust into an open chamber without any real direction. This slows the exhuast gasses waaaay down causing high backpressure. Headers have individual tubes that direct the exhaust gasses to help keep the velocity up and reduce backpressure. Tuned headers are actually set up to create a vacuum in the collector in a certain rpm range to suck the exhaust gasses out of the cylinders, creating vitually zero backpressure.

And if thats not enough, think about the term "backpressure." The term itself says that the exhaust gas puts a pressure on the engine itself, making the pistons have to work harder to push the exhaust out... work that could be going into pushing the truck forward.

Anyway, now that I'm done ranting, I have no idea what the best setups are for the 4.7, but hopefully now you have some useful info to help you decide!

 

Hey, thanks alot for the info. I did not know a lot of that and its very useful info.

Im really looking for the low end power because I don't race around in my truck. There are a lot of steep grades on the highways up here in Vermont and I really like the low end. Plus I have 3:55's in the rear so the r's are pretty low on the high way.

I was going to go 2.5" in and out. Would this kind of be the compromise of both low end and top end? What about the difference in SI/SO vs. SI/DO. Im guessing it doesn't make a big difference since its already after the muffler, but I could be wrong.

So do I want the most freeflowing exhaust there is? Why do I lose power with a hole in my exhaust?

Thanks again for that info. Never really knew a lot of that.

 

I hear that! My truck doesn't hit 2000 rpm until I'm just over 70 mph!

I don't know what the factory exhaust size is on the 4.7, but the 5.2 is 2.5". Just saying that my suggestion is to go with a 2.125-2.25 for the range you're looking for, but like I said before, I don't know enough about the engine to say for sure. Sometimes its just a trial and error situation!

Now the difference between splitting or running one and one is an interesting question. I'll start by saying that going from 1 2.5" pipe to 2 2.5" pipes is a terrible idea as far as performance goes... may sound better though! You're basically splitting the exhaust in half to go through the same sized pipe all the exhaust gas was just going through, and that leads back to the idea of the pipe being too big for the amount of exhaust gas. Now after saying that, I should now say that splitting the exhaust up into two pipes whos cross-sectional area added together equals the cross-sectional area of the original pipe shouldn't harm performance. Say for example you were to go with a single 2.5" pipe. If you do the math to split that cross-sectional area in two and run duals, you get 2 1.75" pipes, which would keep the velocity higher than the single 2.5 inch pipe, while still maintaining the ability to handle the same amount of exhaust gas... I hope I'm making sense here!

IMO, the best possible exhaust system to have next to tuned headers is one where there is never a collector, and each cylinder has its own pipe from the heads to the tips, but that is very impractical!

Yes, you want the most freeflowing exhaust there is, but like I said before, it depends on the rpm range you're looking to get power out of to determine how freeflowing the exhaust really is for any given pipe. A big pipe is actually not as freeflowing for lower amounts of exhaust gasses as most people think!

Thats an easy one! Any disruptions in flow cause chaos/turbulance, and chaos/turbulance disrupts the flow, slowing the velocity. A hole for some of the exhaust to escape from is a big disruption!!!

 

Wow thanks again. Thats like, everything I've ever wanted to know about the science behind exhausts. The stock piping is 2.5" on a 4.7 like the 5.2. So i guess I plan on going SI/SO. Probably will end up going with the magnaflow.

 

My son got a FlowSound (Flowmaster knockoff), stock in/out. He not only likes the sound, but there is no noticeable loss in power.

 

Yes SI and SO are the best way to go when doing a exhaust. I did duals cause Id much rather have the sound plus the 5.2 isnt all that fast to begin with. Anyway I do love my magnaflow it sounded good when I put it on and sounds even better after some time went by.

 

Lol, good, cause thats all I know about exhaust!

Thats a good idea. I still say if you were willing to re-do the entire exhaust setup, that it would be a good idea to go down to maybe 2.25", but if you were happy with the low end performance of the 2.5" exhaust before the hole was created, then I guess it's not necessary. Also, if you really want a good low to midrange increase, get a set of headers! They don't have to be expensive ones... I have a $150 pair of summit branded (made by pacesetter) headers on my truck and I noticed a huge low to midrange increase with them.

 

I might look into that next summer when I have some cash flow (college sucks, no time for a job with 30 hours worth of class). I've heard the 4.7 is pretty easy when it comes to headers...

Would there be a good increase in low end even with a set of shorties? I know they don't do as much as the LT's but they are cheaper and easier to instal...

 

The manifolds on the 4.7 are already as much free flowing as they can be and they say its not really worth getting headers for it.