turbo on an IDI, 3" or 4" exhaust?

[OB_WAN]

I have a banks non-wastegated turbo ready to install and was curious how much I need to open up the exhaust for the best benefit. the banks has a 3" downpipe which is nice but is 4" beneficial at all to 3" behind the downpipe?

regardless I plan on removing the muffler so it'll be free-flow. Just don't know enough about back pressure and the IDIs.

Thanks.

 

[85f250dually]

im running a 3" straight pipe on mine and it acts great. thinking of going to a 4" soon

 

[Diesel JD]

The 3" is enough for the turbo, but the 4" looks and sounds cooler. However you probably won't see much actual change in performance in going from 3-4" pipe and the 4" is usually way more expensive. I got cheap 4" pipe in a 10 foot section from NAPA then got their version of the Dynomax tailpipe to route it over the differential and exit near the stock location.

 

[forcefed]

I have 4" from the 3" downpipe and its all about sound not performance. The 4" gives you more of the diesel sound than a gasser sound when going down the road. You can buy a replacement 4" tailpipe for a powerstroke on internet for $100 shipped to the house. Them you can get yourself a straight piece of 4" pipe and install yourself for less than $150 in all I would say. The price for the tailpipe might be a little different now, been awile since I priced it.

 

[icanfixall]

I have a 4 inch run al the way from the down pipe to the end of my crewcab. Thats a long piece of 4 inch pipe. Also I'll bet your Banks down pipe is 2 7/8 diameter. Mine sure was but that can easily be fixed at any good muffler shop

 

[forcefed]

http://www.summitracing.com/parts/SUM-640040/ Summit racing has 4' sections of pipe for $22 dollars. I sure you can probaly find it a little cheaper than that if you shop around. http://cgi.ebay.com/MagnaFlow-15463...r_Truck_Parts_Accessories?hash=item2c52e85149 This a tailpipe but its stainless steel and you will have to mount your hangers on it. But you get the idea.Or you can just buy a whole kit that has hangers and all http://cgi.ebay.com/Diamond-Eye-4-E...r_Truck_Parts_Accessories?hash=item3ef9a50ade. I bought this kit. This is what is on my truck now. I sold the powerstroke downpipe for $80 dollars to make some of my money back.

 

[OB_WAN]

thanks guys!

yeah mine does have a 2 7/8 ID as well, thanks for clarifying.

so it looks like the benefit of a 4" pipe over a 3" pipe is sound and unless the change includes sound decrease(well, I'm competing with the sound of the engine anyway so prob doesn't matter unless getting on the throttle), I might just be inclined to stick with a 3" unless as previously stated I can swing a good deal on 4". I have a crewcab LB as well and yes, it's a lotta pipe!

 

[RLDSL]

I might disagree with the only benefit being sound in a 4". I could care less about sounding cool, but I went to 4" after the downpipe because that is the size they used to put on the buses with these things ( on some of the real long units they would even go to a larger size pipe ), and I'm quite certain they weren't concerned with sounding cool for a bunch of third graders. Diesel exhaust continues a certain amount of expansion on the way down the pipe. I assume they used the 4" to allow for that and decided a proven combination is good enough for me. A 4" pipe with a straight through big truck muffler just like on the buses, and it runs great, no restriction so the turbo can spool right up and sound level is civilized.

 

[resqzuzu]

I have the banks down pipe as well and I went with 4" straight pipe from there. I was able to get the dynomax tailpipe 54387 from Advance for 65 shipped and a section of 10' 4" pipe and 5" tip from a local supplier for 58. A couple a stainless band clamps and hangers and a 12 pack and it was a easy sat afternoon. The sound is noticeable on full acceleration but nothing obnoxious. I also noticed a reduction in EGTs and the turbo spools much better. Makes the truck much more fun to drive.

 

[OB_WAN]

Diesel exhaust continues a certain amount of expansion on the way down the pipe. I assume they used the 4" to allow for that and decided a proven combination is good enough for me.

I was always curious about this. If I understand properly, heat=expansion and cooling=contraction. That's why wrapping the Y and up-pipe will allow more boost, because the exhaust gasses are not cooling/contracting before hitting the turbo = more pressure to spin the turbo. So aside from potential turbulence as the gas has to route around bends, when it's heading out the straight pipe it should be cooling and contracting. I don't see how it would expand. But again, I don't know enough about diesel exhaust and it's characteristics to comprehend.

I have the banks down pipe as well and I went with 4" straight pipe from there. I was able to get the dynomax tailpipe 54387 from Advance for 65 shipped and a section of 10' 4" pipe and 5" tip from a local supplier for 58. A couple a stainless band clamps and hangers and a 12 pack and it was a easy sat afternoon. The sound is noticeable on full acceleration but nothing obnoxious. I also noticed a reduction in EGTs and the turbo spools much better. Makes the truck much more fun to drive.

These are great prices and would definitely be in my price range for 4" exhaust. Did you go from factory exhaust with a muffler straight to 4" or was the reduction of EGTs from 3" straight pipe to 4" straight pipe?

 

[dansvan]

Please explain how diesel exhaust expands more as it travels down a pipe. Hot gasses expand, cool gasses contract. The farther away from the heat source the cooler and thus smaller the pipe needs to be. Also the gasses slow down when cooler. Nothing wrong with 4 inch pipe but it's over kill for our low airflow motors.

 

[resqzuzu]

I went from factory exhaust to the Banks DP and 4". I want to say it was in the neighborhood of a 200 degree drop. For the pipe and tip I went with these guys : http://performance-curve.com/ They are just up the road from me and are amazingly awesome to deal with no BS with them. Not sure how much shipping to you would be.

 

[RLDSL]

All the theory is correct...for complete combustion, but under load, these things do not often complete combustion within the combustion chamber ( i.e. that black smoke from unburnt fuel ) there is a certain amount of hydrocarbons that are completing ( or not fully completing )their oxidation cycle while in the pipe which leads to further expansion in volume of the gasses while in the pipe. Longer the pipe, the more expansion has time to take place within its confines.

This is the reason that they put catalytic converters on later diesels was to enhance complete combustion of the unburnt fuel in the exhaust gasses in the pipe before they exit the pipe ( that is the basic function of a cat, exhaust passes over a superheated grid on the way out to assure a complete burn)

 

[dansvan]

Unburned fuel can't make heat therefore
can't expand. And it's not a theory. According to your idea you could drill a hole in the pipe after the down pipe and observe fire. Doesn't happen.

 

[dansvan]

Also as a side note. The opning around the exhaust turbine wheel is somewhere around 2-2 1/4 inches. Same as the opening out of the exhaust manifold around 2 1/4.