Turbo Spool Up - 4" vs. 5"

[carl48]

i like quiet so i used a donaldson m090535 5"and a fbd 5" turbo back same price as 4" kit

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06 3500 dually, 4X4 qc big horn g56, dynatrac free spin, fitch fuel catalyst, bd exhaust brake, donaldson m90535 muffler, no cat, cfm+,winspeed heater spacer, smarty jr, taylor wings, 9750 luberfiner bypass filter, psm cold air, cool hose, tag, air tab's, pdr cam

[lukeduke]

I don't understand how having back pressure would be a good thing with a turbo. It seems like back pressure would prevent the turbo from spooling as quick as it could if there were no back pressure. I say go with the 5". That is what I would have bought if my MBRP 4" was not already on there when I bought it.

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2006 Dodge Ram MEGA Cab 2500 4x4, leveling kit, 295/70 R17 Toyo M/T's, Edge Juice w/ Attitude, MBRP 4" exhaust, AFE stage 2 intake w/ProGuard 7 filter.

[carl48]

you can buy a Donaldson m90535 muffler for $90 and the 5" turbo back piping from fbd for less than $300 this is cheaper than 4", quieter , and no restriction . spool up is a non issue

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06 3500 dually, 4X4 qc big horn g56, dynatrac free spin, fitch fuel catalyst, bd exhaust brake, donaldson m90535 muffler, no cat, cfm+,winspeed heater spacer, smarty jr, taylor wings, 9750 luberfiner bypass filter, psm cold air, cool hose, tag, air tab's, pdr cam

[terminaltsi]

any engine with a turbo wil lperform best with the largest possible, or no exhaust. the turbo itself provides tons of backrpessure; after the turbo, you want the largest size piping you can fit/afford based on your HP goals.

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Lexus LS400 19" wheels, coilovers, etc.
'99 Dodge 2500 CTD 5 speed, 4x4, Greddy EGT, Edge EZ, BHAF, 4" exhaust, AirDog, running 80-100% biodiesel.

[Rollin Coal]

Originally Posted by terminaltsi

any engine with a turbo wil lperform best with the largest possible, or no exhaust. the turbo itself provides tons of backrpessure; after the turbo, you want the largest size piping you can fit/afford based on your HP goals.

completly. you want the exhaust to be as free as possible after the turbo, but that brings up another topic of discussion, the outlet in the turbo is 2.75", so when does making the exhaust bigger have no increase on flow. itd be nice to measure the pressure in a 3, 4, and 5" exhaust at WOT.

as far as the 4" down pipe into 5" muffler back, its pointless for performance, if you were going to make a step in sizes you'd want the 5" down pipe into 4" muffler back, as the exhaust gets further from the engine it gets cooler, more dense, basically smaller, so you dont need the larger exhaust toward the end.

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'01 - 2500, 47re 2wd, 53 Block 70k miles and counting.... no cracks
4" turbo-back, 5" single stack - Intake w/ heat shield - Traction bars - AirDog 150 - Edge Juice with Attitude HOT - HX-40 making 35psi
14.2 @ 100 6psi boosted lanuch(hx-40 making 35psi)

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Last edited by Rollin Coal : 05-11-2008 at 09:24 AM.

[Ops]

My rig had a 5" straight pipe on it when I got it. It has a nice deep snarl under throttle, sounds like an 18 wheeler pulling a hill. The down side is a nasty drone at certain throttle settings. I'm considering a 5" resonator in place of the muffler delete pipe.

As far as the theory, less backpressure is better, and yes the pipe can get smaller as the exhaust gases cool.

Ops

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01 QC, tow package, 4X4, 33" BFG's, Edge EZ, 4:10s with LS, 5" straight pipe. Updated May 07: Holley Black on the frame rail, ISSPRO fuel pressure guage. Modified heachache rack, brush guard, 100w driving lights.

[199eightcummins]

Exhaust size is only as good as the smallest part it has to go through..

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1998 Dodge 2500 Cummins 5 Spd SB clutch
MODS: AEM intake, duel 5in. stacks, gauges,3in intake horn ,bd-boostfooler, Superchips, Aurora 3000 FASS 95, and Vulcan draw straw II

[Cummins 429]

Yeah go with the 5'', you'll be happier with in the long run.. I think I would leave the downpipe a 4'' and then 5'' the rest of the way back. good luck

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Too many cars to list. Drag truck is in the plans for next winter as my project.

[witchdoctor]

I'll say this. The size of the pipe on a diesel dictates the TONE that comes out of the end. Larger pipes have a quieter db level and have a deep mellow tone while a 3.5 inch like I have tends to be loud and raspy. 199eightcummins also hit the nail on the head. only as good as the smallest point to travel through.

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04 quad cab 3/4 ton 2 wheel drive long bed. 6 spd tranny with a straight pipe.

[ILLINOISRAM]

Boy this topic gets kicked around alot. It is all personal prefrence to looks and cost. My 2002 peterbilt c12 435 horse runs a 4 inch downpipe until under the cab. As far as exhaust being only as good as the smallest point, (in refrence to actual turbo outlet) i partly agree with but as the exhaust cools, it also looses velocity which is helped by the free flowing pipe all the way out being the same size. The only way you could see if one was slower to spool than the other is a controled enviroment as a dyno and datalogger to actually know exactly waht rpms spoolup happens. Tim

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98 qc/dually 4x4 12 valve built
Video of truck http://www.youtube.com/watch?v=J_cpLtX_CWs

[terminaltsi]

i actually thought that the piping needed to be larger as it got farther from the turbo, because the heat helps with exhaust velocity, so it can be smaller up front, but as it cools, velocity slows, so it needs larger piping near the rear to keep the flow the same.

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Lexus LS400 19" wheels, coilovers, etc.
'99 Dodge 2500 CTD 5 speed, 4x4, Greddy EGT, Edge EZ, BHAF, 4" exhaust, AirDog, running 80-100% biodiesel.