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2.5" vs. 3" cat or Y back exhaust?

20301 Views 30 Replies 11 Participants Last post by  Strategy9
In a previous post I asked about quiet performance mufflers and pretty much narrowed it down to Dynomax super turbo or Walker quiet flowSS in a large size can, single outlet. My exhaust guy says he'll do either 3" or 2.5" tailpipe w/muffler for $170 but if I want him to wack off the end of the Y and put on a better 2" to 3" collector then do 3" all the way it'd run about $250. Both sound pretty reasonable to me, here's the question for an '04 4.7L 4x4 DAk QC:

With only mods planned being a set of HO cams and moddified TB plumbing to modded air filter box lid(larger) & an sct tune is there really anything to be gained or lost by going to 3" ex. pipe all the way with my few mods? Bottom to midrange is what I would hope to increase. My impression from searching is the general concensus is that the 4.7L engine likes 3" plumbing, is this in all cases, regardless of mods or lack of? Thanks, Mark
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There is everything to gain from the 3" diameter. I would honestly look into something like a flowmaster y-pipe (or other brand, ~$40-45) that is pre-fabricated. The pre-fabbed y-pipes will have a much better fit/finish/design, and will cut down on the overall system losses due to restrictions/etc.
Magnaflow makes a cheap 2.5 dual to single 3" prefabricated Y-pipe that works great. The stock 4.7 Rams had a single 3" exhaust, hemi sized catalytic converters, and the larger, but still restrictive hemi Y-pipe.
More info on a 2.5 to 3" y pipe. Our trucks only have 2" before the Y. A Y pipe from a Ram will fit our trucks? What year Ram? What about the cats?
On my brothers 03 Dakota we installed JBA headers that have ~2.25" collector openings on them. We borrowed a set of spare 2.25" flanges that came off my junkyard 5.7 hemi. We stayed 2.25" for about 2" on each side of the truck, the piping was bumped to 2 1/2" mandrel bent, into a Magnaflow pre-made 2.5" dual in single 3" out Y into a single 3" exhaust.

I am almost certain the Ram Y will not fit, but I was using it for reference that some 4.7s use a 3" exhaust stock.
I have two of the Magnaflow Ys and they work better than the Flowmaster version, not to mention the Magnaflow is stainless.

This is the Ram Y-pipe and the Dakota 4.7 is even worse!!


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Are you certain that they work better? I'd beg to differ based on actual design theory and the fact that the high-quality guys make them in the same fashion as the flowmaster.

Don't you think if I have run them both, that I have track times that show the Magnaflow did marginally, but measuerably better.
Over how many runs? Were they consistent enough to actually show a difference created from the power delivery, and not your driving? I cannot imagine changing such a minimal (and singular) variable being changed and documented over a series of track runs. It's hard enough to get verifiable data on a dyno.
Considering it was a 300 HP 305 pulling 5,500 lbs, 3.08 gears, 1,600 rpm converter, and had 275/50/R17 tires on it, shifting in drive @ 6,000 rpm it was dead consistent. I made dozens of passes in it at that time. The whole exhaust system was identical, Doug Thorley Tri-Y into 2 1/2" pipes into 2.5-3" y, into single 3" high flow cat, 3" piping, into a 3" single in/single out "turbo" muffler, dual 2 1/4" stock tailpipes.

This is what fed the exhaust

I always ran the 1/8 mile and ran very consistant 9.80s fully warmed up in cool weather, the Flowmaster piece cracked and started to leak, so I replaced it with a Magnaflow. Ran 9.70s consistantly after that. I run true duals now, but the very same magnaflow Y is installed on the Dakota.

This is my very best time slip from the 305 TPI in that configuration. The internals were all stock GM parts.

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is that a van?
I dont get it?
Yes G-series van. 3.08 + posi + low stall converter+ low hp/tq and good weight transfer means that the thing NEVER would spin off the line just smacking the throttle and once heat soaked ran the same time, run after run after run. The ETs were VERY consistent. So much so that if I had known how to sandbag/get the tree right I would have done very well in bracket racing.
Sounds good, but I'm not completely convinced.
Ultimately it is your truck, run what you want, but having owned both, the magnaflow may look like it is a worse design, but the quality is much better. The magnaflow piece has a large open space that works much like a resonator. The Ram owners that have installed them on stock 5.7 hemi engines have seen 8 RWHP.
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