Just finished exhaust modification today.Cut factory Y and welded into 4" straight pipe to 36" magna flow 12773 4" muffler 2-45 degree elbows and out the side in front of wheel.Took truck for drive and it seems like i lost power especially low end.This doesnt make sense.I read plenty before deciding on this.What should i do?Do i need some restriction?Planned to do ram air and turning pump up once i did exhaust.Truck is 89' dump truck 7.3, E4OD,5.33 gears, weighs 9000 lbs empty.Any help would be appreciated.Lot of time and money invested in this,very disappointed!

 

My 4 inch system kicked up my trucks responsiveness noticeably, my muffler is a 4" look right thru type. Only thing I can think of, is your muffler model a see through type or does it actually have baffles and restriction built into it?? If you haven't done the intake mods yet, that may be what is holding back the benifit of a bigger exhaust.

 

Muffler is a straight thru.Did soup bowl mod just didnt run 4" tubing to grille yet.This is a non-turbo engine in case that matters.Either way it shouldnt have hurt performance?

 

In naturally aspirated trucks, pipes too big can cause a loss of low end response.

Google "exhaust scavenging." Look for an article by David Vizard. I don't have time for a long post to explain everything but in a simple nutshell, scavenging is making use of the of the exhaust pulses to create a partial vacuum at the exhaust valve to help suck more exhaust out of the cylinder and more clean air in. It's kinda like a siphoning effect. This only applies to NA trucks, BTW, which uses only atmospheric pressure (14.7 PSI) to fill the cylinder. If the pipe is too large for any given flow, the "inertia" of the exhaust flow is lost and the engine has to "push" the exhaust down the pipe. At higher speeds, there may be enough exhaust flowing for the scavenging effect to start working but if you only have 3000 rpm to play with, a 4-inch system may be too big for the effect to occur. The rule of thumb for naturally aspirated exhaust flow is 2.2 cfm per horsepower produced. If your diesel makes 175 hp, it needs about 385 cfm of exhaust flow for adequate performance and no loss of scavenging. To see how much overkill you have, note that a piece of straight 4-inch exhaust 3-feet long will flow 2,200 cfm. A longer piece will flow less, but not a lot. Those Magnaflows will probably flow 1,500 cfm at least. You lose something flow-wise at the elbows but still, you have way more flow than you need. Straight 2.5-in flows 900 cfm, straight 3-in. flows 1400 cfm. Lots more to this story but I'll leave it to you and your keyboard to discover the details.

I'm not saying the 4-inch is your problem, or your only problem, but cause and effect seem to indicate that. Loss of low end is typical. You may have been another victim of the "bigger is always better" myth.

 

Could i just reduce the pipe size in one area or will it need to be all the way back.The Y adapter is only 8" section that i band clamped to 3' sect. pipe to 3' muffler.

 

Again, your problem may or may not be due to the big pipe, but you should be 3-inch all the way, IMHO, or at least most of the way. Some systems actually go smaller after the muffler because the exhaust has cooled and contracted, and you see that in some cases.

Here's an idea that may work for you with a minimum of buying lots more stuff. You might try putting the muffler at the very end of the system, with a short 4-inch outlet for the "look." Run 3-inch all the way to the muffler and use a 3 into 4-inch adapter. And here's a tip that some guys are doing... extend the 3-inch pipe 3-4 inches INTO the muffler (only a straight-thru type muffler). That will help prevent reversion (the exhaust trying to suck back down the pipe during the period gaps in the pulses). I just did that on my truck, going from a 3-inch into a 4-inch muffler. My truck is turbocharged, by the way, so it's not a good comparison to your NA truck. It's hard to say if it's doing me any good, I just did it, but it's not hurting (look for my Dynomax 4-inch post).

In some cases, a certain size pipe is used to make sure the engine doesn't lose bottom end, but the mufflers for that size pipe don't flow enough, so a larger muffler is adapted, say a 3-inch muffler on 2.5-inch pipe. On the inlet side, it doesn't hurt to have the pipe stick into the muffler, as I said, because it's in the direction of flow, but not on the outlet. On the outlet, it would really restrict flow.

Anyway, if you make these changes, or any others, be a good Guinea Pig and let us know how it works, one way other another, huh?

 

I may be off base since I haven't actually seen your system, but it sounds to me like you have a couple abrupt changes in size that are not too far apart. If I had to guess, you've induced a bunch of turbulence in the exhaust, AND when it hits the 4" section it loses a lot of velocity.

N/a engines like exhaust velocity as it increases scavenging.

 

I will be turning up pump and ram air scoop.Dont know if i should wait till other mods before i (Guinea Pig) my exhaust.

 

I think you have moved the max torque RPM to a higher RPM.

I would do your intake and fuel changes before I changed anything on the exhaust.

 

Ditto ... Increasing your available air for intake can increase your airmass for exhaust ... This would probably increase your exhaust pressure to a more acceptable range ... Not a very technical explaination but I am not very technical ...

 

Like Dave said, improve your intake and turn up your pump a flat before changing the exhaust. 4" shouldn't result in loss of power.

From what I've read, flow testing has shown significant power and MPG improvements on NA trucks up to a single 3" exhaust, but after that the improvement is very small - but it should still result in some improvement - certainly not a power loss.

Turboed trucks reach that point at 4" - up to that size the gains are significant, but drop off rapidly as the pipe gets bigger...

 

Most of you are still thinking high rpms here (relative to this engine), not off idle where the trouble appears to be.

We're not really talking a "power loss" here so much as a change in the torque band (moving it higher) that feels like a power loss at low speeds. I've see it on the dyno many times.

You have to have the volume of flow in order to get the scavenging effect working. Increasing the air intake will certainly help... but how much it helps down low will depend on how restrictive the intake system is. Most are not so restrictive that they inhibit low rpm performance significantly. I can't speak to whatever intake system F-Super Duty's truck has but I doubt it's restrictive enough to have much negative effect below 1500 rpm.

Turning up the fuel may help because it will increase the temperature and the volume of the exhaust gasses slightly at low rpms, but again, if the air isn't there, the potential power just goes out the tailpipe as smoke.

When you lose the scavenging effect, you are effectively reducing airflow at low speeds. If you have that effect working for you, you have a little more air to which you can add a little more fuel and get a little more power. It varies from engine to engine, but it's not unusual to gain 5-20 percent airflow at low rpms with the scavenging effect working. Headers really help with scavenging.

I contend that increasing airflow and turning up the pump may help (a lot) with peak torque and power, but may not do much for his problems at the low end. On a heavy truck like his, even with those crawler gears, low end performance is very important. Anyway, gotta go.

 

I got my ram air set up and it helped alot.Maybe turn fuel up tonight if i get time.Thanks for all the input guys.Ill try to get some pics posted when i get time to figure it out.

 

FSUPERDUTY, I just looked at the pic of your Y-pipe in the gallery. WOW, no wonder you're feeling a loss of performance, you've probably got tons of turbulence going on in that Y-pipe along with a major drop in velocity. You really can't just have that big of a change in size that abruptly. And, it almost looks like it comes together in more of a "T" shape than a Y now.

 

Got some pics in gallery if anyone is interested.Im pretty happy with my intake mod.It was cheap and air flows thru plastic better than dryer vent i was going to use.Got Fuel turned up and shes got pretty good overall power.I didnt want to do it twice so i probably went about half a turn on fuel.Blows some smoke at half throttle or better.Nothing extreme.I want get a pyro & trans temp gauge when i find a good place to get them.Now i cant help but wonder if i got the full potential out of her.Thanks again for input.Any other thoughts would be helpfull.