4" vs. 5" Exhaust- Definitive Performance Difference?
 

Looking to buy a new exhaust. I have read several threads stating that a 5" exhaust kills low end torque, slows spool and that a 5" exhaust is overkill for a mildly modified truck.

Does anyone have dyno results showing the impact of 4" vs. 5" exhaust on a mild setup(less than 600HP)? How many have experienced slow spool and reduced lowend?

Cost is basically the same if I am installing a new exhaust on my '03.
I don't want it loud, so I'll probably put in a Donaldson with whichever size setup I go with.

I went from a 4" TBE to a 5" TBE and didn't notice a single bit of difference in the way the truck ran, the power output, or the fuel economy. Just a different tone to the exhaust and a bigger tailpipe. The turbo provides all the backpressure the engine needs so I don't think you can lose anything by going to a larger exhaust. Shoot, if i could find one of those Bully Dog 6" cat-backs used on the cheap, i'd throw it on there just for the aesthetics.

You cant beat the sound of a 5" I dont think IMO. Sounds mean. I Love it. Didnt loose anything going from stock to 5" Silverline.

HotRod

I just put the 4" elbow and 4" exhaust on my truck. I used a 04.5 dodge downpipe and my stock tailpipe. I put on a Donaldson m090544, a 51" piece of pipe and a few clamps. I cut off brackets off the old exhaust and welded onto the new stuff to mount it. $345CDN from down pipe clamp back.

I got my 5" diamondeye turbo back no muffler not tip for $336 brand new.

I think the sound difference has got to you guys, I will tell you my STOCK truck had the OEM 4" stuff and I just went to 5" off the turbo and I lost some 1500-1800 RPM low end grunt. It takes more throttle to keep up with traffic now. I will be going back to a 4" downpipe and keep the 5" tail section for looks and noise. Keep in mind my truck is STOCK.

You dont lose anything going from 4" to 5" and anybody that says they do is full of it. If, IF, there was a loss, it would be so small that you wouldnt hardly notice it. It doesnt take more pedal to keep up with traffic with a 5" [duhhh]

Whatever you say strokethis, I guess an honest seat of the pants opinion from a guy w/ a bachelors degree in Automotive Technology and works for an OEM doesn't know anything about powertrains.

Bottom line, before the swap the same accelerator pedal input would net getting up to speed quicker. I will not disagree the difference is nominal, but someone that is very "in-tune" with their stock truck will notice a difference. Take it for what it's worth.

Keep in mind my swap was to a 5" straight setup, no muffler.

there is no detectable difference in 4" vs 5". had pressure gauges on while testing mpg mods and was done on dyno. weather you go 4"turbo back or 5" turbo back i would use the donaldson muffler the m090535 five inch is only $90 because it is used on many trucks oem i do not care about pretty just quiet and performance if you order a straight pipe 5" from fbd for about $300 you can install both in about four hours by your self with a minimum of tools and half of that time is getting the old one off. all of our trucks are duallys and i use the delete piping to extend the exhaust out further

Thanks for all the replies, but this is the type of response I was looking for: dyno results and quantitative results. In my experience, the SOTP dyno is pretty subjective and inaccurate regardless of how in tune one is to a given vehicle.

Anyone else with definitive dyno results one way or another? I thought I read in one of the threads, that Diesel Power put a 5" Magnaflow system on a 3rd gen and got increased power on the Dyno. Anyone got a link to a scan or at least know the date of the article?

I found this info, for a stock (03-07 24V) Cummins motor...

RPM:
2900

Boost:
29 PSI

Intake:
800 CFM

Exhaust:
2370 CFM


Straight Pipe Flow By Diameter:
5" 2200 CFM
4" 1800 CFM
3.5" 1400 CFM
3" 1200 CFM
2.5" 900 CFM
2.25" 600 CFM


Now, take into consideration aftermarket air intakes, manifold elbows, turbos, etc......

Carl, let's see those numbers, all I see are possible "quantitative" results. Just b/c someone sees a gain on a dyno doesn't mean the gain is where the owner wants it to be w/in the powerband. A dyno operator can display the charts over one another and I would be willing to bet you a donut you will NOT see a gain, if any (4" vs. 5") on a stock truck until over 2000 RPM.

Prove me wrong and I'll [tapdshut]

There was a thread years ago, where the owner of a 500 h.p. truck went from the factory 3 1/2" to 4" exhaust and gained something like 8 h.p. peak. It was also said that by going from a stock .3 muffler to a 4" straight through gained 8 h.p. on the dyno. Do the math, a 4" is all that is needed, and anything more will not gain or loose anything. 1000 c.i. over the road trucks only use 5".

Yep, "peak" is the all important word people need to ignore when it comes to gaining usable torque and power. The increase needs to be evenly distributed throughout the powerband to get my vote.