Hi all,
I've been slowly modding my '99 4x4 5.9 to make it a little less of a slug. So far the list of mods include
-14x3 filter
-Superchips 3815
-Flowmaster 50 muffler single in/out
-Magnaflow high flow cat
-Modified throttle body
-180* Tstat
I think that's it. My truck screams as soon as it reaches 3000rpms (seriously, it's fast) but getting there is another story. I have no low end power whatsoever. Tires (stock size) won't even chirp unless I'm making a sharp turn. I have trouble towing even the lightest loads, especially up hills. My plenum gasket is fine (hughes kit) so I know that isn't the problem. I have the factory 3.55 gears, I know 4.10s would help, but I don't have the money for that right now. How can I move my power band down to a more usable rpm? Thanks in advance.

 

Are you using the 91 performance setting on the Superchips?

 

I thought the round cleaner decreased your low end torque, while a typical cold air intake increased it. Then, higher in the powerband, the cold air was not as efficient as the round cleaner... It could be possible

 

Improving airflow into an engine will not move the power curve. More likely is that by opening up the exhaust flow you lessened the back pressure needed for good low rpm performance. Engine mods need to be tailored to the desired results. Some mods are better suited to high rpm performance than low, but improvements to intake airflow are generally across the entire rpm range. Exhaust improvements are typically better for the high end.

 

Miami Son, so what your saying is that if you open up your exhaust too much (i dropped the muffler in favor of a straight through y-pipe and put shorty headers on) that you will lose torque? if i got a magnaflow would it lower it even more? well how about if i got a magnaflow and re-did the exhaust from the headers back with a 2.25" pipe? i remember steve05ram360 (can't remember his name) said that he lost torque when he opened it up but got it back when he did a 2.25" pipe. if this is the case, would it have to be from the headers back or could i bottleneck it with a 2.25 pipe from the cat back to my y-pipe (same distance from the cat back to the oem muffler, i think its about 3 1/2') and get the same result so i wouldn't have to redo the whole system?

 

Opening up the exhaust too much will hurt low end torque and power. Some back pressure is needed for proper scavenging and evacuation of the cylinder. Exhaust design is often a compromise because there is no way to build a system that works optimally across the entire rpm range. You have to sacrifice one for the other. What is important to deciding what mods to do is to know where you need to make the most power. If you do a lot of low rpm rock-crawling then there is little benefit to a free-flowing exhaust setup. Conversely, if most of your time is spent in the mud in low gear with the tires spinning and the engine cranking above 3500 rpm, then as little restriction as possible is your goal. Unfortunately, most of us need our trucks to perform well in both situations and on the drive to get to them. That means we have to make tradeoffs between the two and find something in the middle or biased to the type of driving we do most.

Look at top fuel funny cars that scream down the track at 10k+ rpm. They will have short, stubby exhaust pipes whose only consideration is to get the exhaust out as quickly as possible. Back pressure is not needed at that rpm. We need torque, not horsepower, to do the work of what we do in our trucks. That means making our engines scream at high rpm has little benefit. A free flowing muffler will often be better than stock because the factory puts very little into the design of a muffler. But too much of a good thing can hurt performance. You will often see an engine builder put a free flow muffler in and then reduce the size of the exhaust pipe in order to maintain balance and back pressure. The new muffler reduces turbulence and restriction, which are bad compnents of an exhaust system and where stock mufflers and manifolds often fall flat. Headers and free flow mufflers not only increase flow by reducing restriction, but they smooth out the exhaust flow and reduce turbulence. Back pressure is now tuned by flow rather than restriction.

 

so if i were to step the section between the stock muffler and the cat to 2.25, wouldn't that reduction kick the exhaust back toward the collector on the headers enough for scavenging to take place and give me back the torque i had pre-headers?

 

Scavenging happens right at the exhaust port. How long is that section of pipe? 12-18"? You might be better off reducing the size of the pipe from the muffler to the tip. There really is no way to know for sure without some experimentation. I've done extensive testing with motorcycles and, although the principles of the ICE are the same, the practical solutions are very different. You don't want to just throw in some restriction. Most people think of exhaust as just simple tubing, but there is definitely some science involved in making it perform optimally.

 

well i'm gonna try it and see what happens, i'll put a post up if it does any good. i believe that section of pipe is about 3 1/2' long. and i know where scavenging happens, i was just thinking that putting that bottle neck right after the cat would make the next series of exhaust pulses hit the previous ones because of the reduced pipe size and come back to the exhaust port for the scavenging to take place. i believe the pipe reduction all depends on distance from the engine and since i haven't seen anything talking about where the reduction (ie: muffler/cat) needs to be in relation to the manifolds/headers when you have headers, no muffler and a high flow cat then i guess i'm just going to have to experiment.

 

Should be interesting. I look forward to seeing your results.