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

20261 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.
So you're thinking this http://www.summitracing.com/parts/MPE-10778/ Will weld up to stock stuff just fine? Sure the spacing and everything will be correct?

more like this one from flowmaster. The magnaflow y-pipe doesnt really have an efficient design
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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!!

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.
is that a van?

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 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.
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