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Losses with 3" exhaust?

2K views 14 replies 5 participants last post by  yjstl 
#1 ·
I recently had a 3" in/out Magnaflow muffler installed on my '06 4.7L. I purchased the muffler because everything I read lead me to believe the stock piping on my Jeep was 2.75". Turns out it's 2.5". I have a shop that will bend me a 3" exhaust to compliment my new muffler, however I'm wondering if increasing the diameter that much will result in any performance losses with the lack of back pressure? I'm considering adding a resonator before the tailpipe as I'm getting some drone on the freeway (I'd like to avoid adding a resonator tip), and I don't see adding a 3" system will quiet anything down.

Any advice would be great, thanks for the help.
 
#2 ·
I'm going to move forward with the 3" piping and adding a resonator. I love the sound of the Jeep on startup and idle as it is now, but want to dial it back a little bit on the freeway. I'll post pics when I get 'em.
 
#3 ·
Don't do it with a 4.7 engine.....you will lose power. The 2.75 piping is stock on the 5.7's The 3 in. piping is good for a 5.7 with headers, but even a bit marginal with the stock restrictive 5.7 exhaust log manifolds.
 
#5 ·
What size is the piping from the "Y" pipe on back to the exhaust tip? If it's truly 3 in. all the way back (with no size reductions along the way) on a 3.7 I'll guarantee you're losing power. Even before I did the header back system on my 5.7, the diameter of the stock exhaust log collector was 1 7/8 in. and the necks of the "Y" pipe (where it connected to the log) were about the same size, but the rest of the "Y" pipe was 3 in. Everything else, IIRC, was 2 3/4 in. Now everything from the header collector to the tailpipe tip is all 3 in. A few of the biggest issues with running too large a pipe is that you won't get adequate scavenging and flow velocity through the system, and your throttle response may also suffer. All of this will adversely affect the overall volumetric efficiency of the engine. Pipe size also should be chosen depending on where you want the power gains to occur.....low end, mid-range, or top end. There is a "sweet spot" pipe size that will give you the best of all three, but not optimal for any of them.

So, there is a lot involved in selecting a performance exhaust for a given vehicle. It can be said best by this quote from an outfit knowledgeable in exhaust tuning:

"The main purpose of an exhaust system is undoubtedly to route the spent exhaust gas out of the car's engine. Along the way the exhaust gasses may be used to drive a turbocharger and now-a-days it will most definitely incorporate a catalyst converter to reduce carbon dioxide emissions. But on a high performance car, such as a modified street car, or a modified race car, the performance exhaust system is much more important as it has a direct affect on engine performance and engine power. As a result, the exhaust system, and particularly the exhaust header design, plays an important part in both engine tuning and car tuning.

In general terms, an exhaust system consists of an exhaust manifold (which is sometimes called an exhaust header), a front pipe, a catalyst converter, a main muffler or silencer, and a tail pipe with an exhaust tip. In terms of tuning the exhaust system, the muffler is the easiest to deal with it's simply a matter of replacing the stock muffler with a free-flow or high performance muffler, such as a Flowmaster muffler. The result is a free flow exhaust system. However, the performance muffler must have an inlet and an outlet pipe that is the same size (diameter) as your front pipe and your tail pipe. Your front pipe and your tail pipe should also have the same diameter. The rest of the exhaust system is much more complicated as you need consider exhaust back pressure, your engine's power band, and your engine's maximum usable RPM.

The amount of back pressure produced by the performance exhaust system is crucial, as too much back pressure will have a negative effect on your engine's top-end performance because it will restrict the flow rate of the exhaust gasses at high RPM. The result would be the engine not being able to expel the spent exhaust gasses fast enough to prevent spent exhaust gasses from contaminating the fresh air/fuel mixture that is drawn into the engine on the next intake stroke. Ultimately, this will result in reduced engine power! Therefore, attaching a little 1-inch pea-shooter to your engine instead of an performance exhaust system is not such a good idea! But then neither is fitting a 10-inch sewage pipe. If the exhaust pipe is too large, you will get reduced flow velocity of the exhaust gasses. The flow velocity of the exhaust gasses assists with the scavenging of the spent exhaust gasses as well as the amount of air/fuel mixture that can be drawn into the combustion chamber on the next intake stroke. This is because the flow velocity of the exhaust creates a low pressure immediately behind it that sucks more gasses out of the combustion chamber. The trick is thus to get the exhaust back pressure just right."

Hope this helps explain it a little.
 
#6 ·
Hope this helps explain it a little.
It does thank you, I appreciate that info. I'm going in to talk to the guy doing the exhaust on Thursday. I'll rethink the 3" piping but am open to explore other options. Worst case I'll get an off the shelf exhaust and call it a day.
 
#9 ·
Unfortunately, I think all WK's are like this. There are no newer ones than mine and those little manifolds are certainly an issue.

The problem is much like trying to swap in SRT manifolds. The cost of the work to connect them to anything is just stupid money. Around here, 9 out of 10 shops would not touch this work at any price, I've asked around. The few "custom shops" that will, are so expensive you'd think I was asking for something trick on my AMG.

Hill is the Conejo grade heading south into T.O.

Before the Gibson, it would easily pull OD at 70-75 with the stock tires and lift. Added the AT 2's and could still barely due it in OD, but usually would bump it out of OD and it let shift back on it's own at the top. Now it lugs 4th and isn't really happy. The SC tuning doesn't know what to make of this combo. When it's pretty cold, it's snappy and responsive. The warmer it gets the more "boggy" it becomes in the midrange. At full operating temp it's not a stumble or anything, but it's obvious to a motor head.

Picking either the 91 or even 93 tunes and it's better when first installed, then within a few KO/KO cycles and driving it, it's the same. I will say it does go like stink when the pedal's on the floor, even after it "learns" a tune. The midrange is just not there anymore and going back to stock is not an option. My guess is I could benefit from custom tuning quite a bit and be much cheaper than fixing the manifold issue. Any ideas on a local shop that could tune this?
 
#10 ·
I moved away from the "rat race" about 25 years ago, so I can't help you with any local shops down there. I have a buddy that still lives in Camarillo and builds custom cars and he said there are fewer and fewer shops around that will do custom work. Back when I lived there is wasn't a problem though. I can also imagine that, if you can find someone to do the work, it would cost a fortune....but I guess everything down there does anymore??

What year/model is your AMG? Any mods. done to it? I was an old classic MB nut (mbbuff on the classic Benz forums), but have gotten rid of my "babies", as we've downsized and I no longer have the room, plus as I get older it's not as much fun to work on them anymore. I had a black '67 250S (my Dad bought it new) and a black '73 280C that I completely restored. I gave the 250S to one of the kids a few years back and just last year sold the 280C to an MB forum member in Portland. Here's a couple pix. Still kind of miss them though.

If you're going to get a custom tune, you'll have to purchase one of the Diablosport Tuners, as they are the only ones that allow for custom tuning. I wish the Superchips folks would do them, mostly because I love the Advanced Transmission Tuning Application they offer as an option. That's the biggest reason I run the SC tuner. Otherwise I'd be running custom DS tunes. There are numerous choices for custom tunes if you pick up a DS tuner though. Johann at DS is real good (I hear), as is HemiFever, to name a couple. Best thing to do there is ask around on the forums and go from there.
 

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#11 ·
He's not lying.

I moved here "temporarily" 25 years ago. It's lost it's small town feel over time and while I do like the coast it's also gotten expensive as you guessed. The fast is an '06 E55. Pulley, tuning, big heat exchanger and pump, exhaust mods, etc. When I was younger I had a 560SL I bought for turning a profit that I ended up missing a lot. But I lost my mind when I sold the 2002 E55 for the '06. Very rare Tektite grey with every option including nav and the built in flip phone with integration that was so trick in the day. The real trick was the "additional modifications". Notice the second coolant tank under the hood. ;)
 

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#12 ·
Sweet ride, Fast55! Looks like you like to take care of your vehicles too. Those aren't the factory AMG wheels, are they? That's the kind of wheels the guy that bought my old 280C put on it. He said he paid $1000 for the set with almost new tires which I guess was a good price. If they're not AMG's, do you know what they're off of?
 

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#13 ·
Those are the correct staggered fit W210 E55 wheels, they're just chrome instead of the silver painted finish 99.99% of them came with. The blower, tuning, injectors, exhaust, torque converter, and transmission were not stock. :D
 
#14 ·
Any thoughts? I bought a MF 12589 5x11 3"in/out from Kolak years ago. Took it to a few shops that said it wouldn't work. Bear with me. Got a 4x9 installed instead. Sat on the 5x11. Took my caliper out today and measured 2.70 from the adapter the in/out of the muffler is 2.70 and the tailpipe is 2.50. I can only assume the front pipe after the header is 2.70 although the article above says it should match the tailpipe. So will the 3" effect it negatively especially if there is already a .250 increase at the muffler assuming 2.5 before the cats
 
#15 ·
Okay so I think this will be alright to do. MF 12589 is 3" ID. The 2.70" OD from the adapter (which I think is more like 68mm) should fit in the muffler just fine. Then I will reduce out of the muffler to the 2.50" OD without problem. I hope
 
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