I am not an expert on these trucks, nor am I a certified mechanic - so factor that into my statements here. I am a trained observer with AE and a sharp eye on my gauges while doing specific things on the same 45-mile drive to/from work every day.

I'm still collecting AE data after installing a 4" turbo-back exhaust with a Flo-Pro 6192 muffler, but I have one stark observation to share (80e Tune, red line disconnected, mods in sig.)

• Stock WOT - boost leaps to 20 PSI and slowly climbs to 27 PSI as RPMs build (27 is max because wastegate bleeds excess).
• 4" WOT - boost leaps to 27 PSI right freaking now and stays there.

My understanding of this - The stock exhaust was engineered for a peak boost of 17 PSI, so free flow up to 20 PSI boost makes sense. After 20 PSI boost, it takes a lot of pressure to overcome the flow restrictions imposed on the turbo drive turbine.

Increasing the exhaust diameter from 3.5" to 4" may make one feel like saying "Big whoop - only a 1/2 inch increase. I want more". Our friend pi reveals how much change 1/2" really is. The formula for measuring area in a circle is pi R squared. In casual terms, the area in a 4" pipe is 31% larger than a 3.5" pipe. Add that 31% to 20 PSI and you end up very close to 27 PSI. The exhaust math really applies to the drive side of the turbo, but the boost side is the one with the gauge - so I'm using that as a reference.

The 4" exhaust (with the correct non-restricing muffler) is "tuned" to give my truck the maximum performance, without being overkill. The only reason I can see for going bigger than 4" exhaust is if I wanted more than 27 PSI boost, but I would need to mega-mod the truck. Bigger sticks, more intake air than the AIS provides, bigger turbo, eliminate the wastegate altogether, bigger HPOP, more tuning, BTS or JW transmission, and the list goes on. That ain't gunna happen over here.

If you're looking for a 5" exhaust, why stop there? Go 12"... if size matters. If sound is the thing, ask who likes the sound of the 5". If towing EGTs are a concern, I don't have that info. If performance is the goal, the extra inch won't do a bit more good until you really pony up - in more ways than one.

$0.02

 

The turbo will always be a restriction in the exhaust system. Exhaust backpressure is what drives the turbo. An increase in size on the outlet side of the turbo would benefit with acceleration of the turbine wheel due to better free flow. Boost is allowed to build faster. Basicly saying, "more in, more out". I have the larger 1.15 turbine housing on my 38r. The larger housing reduces backpressure/drive pressure at lower speeds, but wakes up the acceleration curve of the boost as fuel is applied. It is sort of a trade off, but the larger volume of the compressor side adds more air at the lower speeds and helps offset the lower drive pressure. Once you get past the point of lag, the benefits become more apparent.

 

I wonder the difference between stock exhaust with muffler delete Vs a 4'' turbo back.

 

Now I don't know if there is any truth to this but I have read that on a stock set up of guys going to a 5" thinking if 4" is good that 5" is better and ending up creating alittle turbo lag. Or making it a alittle doggy. And from reading what Roland just said maybe there is some truth to it. I don't know? Would this apply "reduces backpressure and hurts drive pressure at lower speeds" ??
Now I myself went with a 4" MBRP turbo back with flow thru muffler and I am very pleased with the sound and at that time I did see mileage improve about .5 to 1.0 mpg. And that was checking mileage by gallons used not the computer. Now my stock exhaust had a cat and muffler.

 

Tug, with the info you keep posting, your sure making it easy for us to decide which "exhaust" to go with...I as well, pissed in the wind with a few items, and now just collecting dust on the shelf, case in point, stupid K/N FIPK system

 

Zip - nada - bupkis. The stock muffler doesn't do anything to restrict flow (beyond what a 3 1/2" pipe does). I tried the muffler delete (it's in a box now) days before I jumped to the 4". I hated the muffler delete so much because of the sound, I pulled the trigger on the 4" immediately. Somebody else might like the sound (I really liked the whistle), I think I have a buyer for the kit.

I read material from a lot of experts that getting rid of the cat is where the gains occur. If you stayed stock and deleted the cat alone, you would see an MPG/power gain. Going to 4" gives you a performance gain if you have mild mods (I consider mine mild), but I don't have figures on 4" MPG on my system yet.

Side "note": I really wanted the whistle without the raspy "brack-brack-brack", but I was told only the straight pipe would give me whistle. Not so! I have a throaty rumble with a whistly accompaniment. The stomp gives me a full-on crescendo that rivals Pixar Cars character "Snotrod".

 

One additional factor in the back pressure issue is temperature. Since volume increases linearly with temperature, you don't want your exhaust temps to drop off too dramatically before exiting the tailpipe or you'll loose some of the necessary back pressure just because the gases are cooling too quickly.

Back before Pinnacle Exhaust went out of business a year or two ago (not sure why that happened, since they had been so successful for decades), I had several long discussions with them over exhaust sizes and their impact on performance as related to engine horsepower. Essentially, they insisted on maintaining a transitional down pipe which began at the stock size and transitioned up to 4" about halfway down, and the reason they reached taht design feature was due to measured differences in back pressure and it's impact on engine performance. If they went with 4" all the way down, the temperature would cool too quickly and they would loose "performance". Additionally, they had measured repeatedly that for our diesel engines, 4" provided the ideal performance range until you crossed the 500 hp barrier, at which point a 5 inch system made sense. The impact of putting a 5 inch system on a lower hp rated engine was a loss of low end performance under launch conditions.

All of this is related to the dynamics of managing the exhaust temperature (volume) and it's related flow velocity through the system.

 

I am so friggin' there with you bro. I have a veritable warehouse of perfectly good products for someone else's tastes, I just need to sit down and start making a catalog for the brotherhood.

 

Thank you for posting this info up so there is truth to them statements. Now from my own experiences I did coat & wrap my down pipe 30" and in my research on these diesel engines I was told you don't want to hold the heat in too long either to help exhaust savaging. And now that I will be running a 1.0 housing and I am hoping it will help alittle with the lag.

 

That completely substantiates my observations/conclusions.

• No mods? Only the cat delete or bigger cat offers any gains.
• Heavy towing or Mild mods? 4" offers the gains needed.
• Mega-mods? The 5" would then be justified.

I'm excited to learn that I'm not too far off the mark with my thinking. Thank you.

I tried to rep you for that post, but I'm still suffering the side effects of the Rep Exchange. Great time, but nasty hangover.

 

Tugly I took care of F250_ for ya. I also went with 4" exhaust and so far loving what the truck has become for what it was when it was stock. When I get a chance I will take pictures of my DIY 4" set up and post them in my album.

 

Back Pressure and Drive Pressure are the same thing...

 

There is NO POINT where a 5" exhaust will cause any benifit... The outlet of MOST turbos is less then 4"...

1000hp 7.3 powerstroke runs a 4" down pipe...

 

Joe is there a point you are trying to make by pointing this out??? I may not be the sharpest tool in the shed but that was a QUESTION and a quote from what Roland had posted.

 

I never said they weren't.