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What's the truth about exhaust and back pressure?


cltdba

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What's the consensus on "back pressure" in our diesels? I hear they don't need back pressure and it makes no difference. It seems my 2007 has more low end torque with the cat installed when on stock tune than when I remove the cat. It seems very sluggish on take off as it the turbo needs time to spool in order to get the truck rolling. In higher RPMs it seems stronger though? In order to gain back that low end torque I have to tune it. Why is this so? I need to replace my exhaust and as much as I want to go with 5" for lower EGTs and better sound, I'm afraid it's going to hurt my performance. My truck is stock and I don't want to throw a tune on it and put other parts in danger, just to get the off the line power. Thought I'd pose this question here as I've gotten tons of mixed reviews elsewhere. :)

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I think on a basically stock truck, 5" is over kill and much tighter to install. I have the smaller outlet turbo (auto trans) & went with 4" full length. Just going to a full length kit is an upgrade as the OEM reduces beyond the muffler. I bought a SS kit off ebay. The muffler is just a straight through tube. I think a resonator or cat will quiet it down a bunch. I have not got the swirl resonator in yet. It's fine as it running light but the drone is pretty HD towing. I'm running a Quadzilla XZT+ in the econo mode, getting better pep empty & slightly better mpg. I'm also running a Intake Air Temp Fooler... in cold weather.

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That's kind've how I feel about the 5", but everyone raves about it. Perhaps it's that bigger is better mentality? In my 2007, I have plenty of room for the 5", but I still think it's overkill. But then I don't want to buy a new 4" kit only to later upgrade it to 5".My current setup is 4" aftermarket that was installed by previous owner. It's falling to pieces and needs to be replaced. Seems going back with 4" gives me the most options. I suppose if at some point I decide to make tons of power out of this truck, I could upgrade to 5". I don't see that being the route I take anytime soon though. I hate the drone I have now so I'm definitely contemplating a resonator or leaving the cat in place. Was thinking about doing a 4" kit with muffler and leaving my cat in place. Also thought about doing a 4" kit, removing the cat and adding 30" FTE resonator/muffler. But before I decide on any route to go I'd like to understand back pressure and how it affects a diesel engine.

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Let's say straight piped 3" is doing just fine for me. I can avg 21-22 MPG and haul my travel trailer without any problems with EGT's or anything else. So I've not really seen a really good reason to jump to even 4" yet. If my EGT's are in check nad MPG's are up why mess up a good thing right?? :shrug:

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Not trying to hijack but I pull alot of weight an I notice with my stock exhaust on the 06 my egts stay around the 900 mark running 70 mph with average gcw of bout 30 k Sent from my DROIDX using Tapatalk

I'm right about 800-900*F on flat ground with 16K# (GCW) running 55-60 MPH in the windy roads of Idaho..
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i am running 750 to 825 on flat ground in 6th gear(70 mph) and grossing around 21k. 4" turbo back. seems i am runnig a little cooler mike and mr towmamn. but i am alot lighter than towman. mike and i also have differant tuners that would make a differance i would think. i dont know what mine ran stock because i had no gauges. i was less learned at the time.

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There's no such thing as a hijacked thread in my opinion. If something I posted causes more questions then that means I probably didn't think of everything. :) All of it is good info. Between truck and trailer I'm only averaging about 12-13k. On occasion I will hit the 20K mark but it's rare. I haven't installed my EGT probe yet. I've got 1,000 more miles before the warranty runs out and I've been avoiding it until then. Anyone still have the cat on that can speak to EGTs?What about the "back pressure" bit, anyone know anything?

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I think the backpressure bit is a buncha gossip. I don't see how making the exhaust stroke work harder makes the engine more efficient. As for size, the turbine housing is hardly 3", it's a bottleneck. I think there are some physics involved that show a bigger exhaust to have better flow characteristics. If it is 3" for an inch then 5" the rest of the way and the engine is capable of flowing more than the 3" can handle, maybe at WOT per say, then I think the 5" would show it's abilities. Everyone on here is telling about level ground pulling loads at 55-70mph which, is nothing if I must say. It might be fine with 2.5" doing that. But what about dead mans hill with the same trailer? You're going up it and EGT's are spiking and you're wondering when your pistons are going to melt.. I want to see proof of this and hopefully that diesel power mag shows that 12V in my thread dynoing after they change the exhaust. THracing has a dyno and he probably knows the answer to all of this. My belief is that a bigger exhaust assists in other mods, but does nothing by itself. I put a 5" straight pipe on mine but really don't think much changed. I changed a lot of other things around and now I can pull our trailer up dead mans hill at 70 and have a hard time reaching 1000F.

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Next load I pull I'll try to remember to see what my egts are pulling hills! I live on the side of a big ridge so I'll be pulling some hills! Now on the old green dually of mine that has 5" straight exhaust I hardly ever see egts much higher than about 800 pulling the same weights!

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I think the whole point of the back pressure debacle is related to the amount of time it takes to build spool. More back-pressure = faster spool? I'm assuming that's why the truck feels more responsive on take-off with the cat in-line. At WOT I imagine having an unrestricted exhaust would be more beneficial. However, I know nadda dang thing about anything of this and am just guessing. :P

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I wonder if it has kind of the effect you get when you put your hand over one side of an enclosed fan, it speeds way up because it's just spinning the air inside it rather than pulling air through it. If you have back pressure I believe it does the same thing. I think this causes an overspeed condition under extreme circumstances.

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I think the backpressure bit is a buncha gossip. I don't see how making the exhaust stroke work harder makes the engine more efficient. As for size, the turbine housing is hardly 3", it's a bottleneck. I think there are some physics involved that show a bigger exhaust to have better flow characteristics. If it is 3" for an inch then 5" the rest of the way and the engine is capable of flowing more than the 3" can handle, maybe at WOT per say, then I think the 5" would show it's abilities. Everyone on here is telling about level ground pulling loads at 55-70mph which, is nothing if I must say. It might be fine with 2.5" doing that. But what about dead mans hill with the same trailer? You're going up it and EGT's are spiking and you're wondering when your pistons are going to melt.. I want to see proof of this and hopefully that diesel power mag shows that 12V in my thread dynoing after they change the exhaust. THracing has a dyno and he probably knows the answer to all of this. My belief is that a bigger exhaust assists in other mods, but does nothing by itself. I put a 5" straight pipe on mine but really don't think much changed. I changed a lot of other things around and now I can pull our trailer up dead mans hill at 70 and have a hard time reaching 1000F.

Actually, the only place you want back pressure is between your exhaust and your turbo, once it exits the turbine side you want NO back pressure. And to the OP, the reason you have a sluggish low end is because it is a common rail, most of the older mechanical diesel had something "like" on demand torque being they spool up quickly, ' Add a smarty and add fuel on the low end and you will notice a huge difference, the stock programming was designed to save your transmission. I drive a large international 4400 around at work, and with a DT560 inside it i found the best ondemand torque you get out of that CR's is when you have a load on it
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This condition, where boost is higher than backpressure, is called crossover and crossover is what ever turbo system designer strives for. In crossover, VE percentages as high as 110 percent are not unheard of. Unfortunately, some of the design features that can create a free-flowing turbo can also contribute to turbo lag, something that is not desirable in a street-driven car that needs a wide dynamic power band.

Excellent thread! This explains why my truck feels laggy without the cat. With the cat my VE percentage is likely lessened, but my "light to light" response is better. In theory, if I'm in the city and have the cat on, I should save fuel because the truck won't need to supply as much fuel to build boost to get the truck moving.

As for increasing the exhaust size...Assuming my math is correct, the 4" exhaust is way bigger than the turbo's exhaust outlet, so there shouldn't be any flow issues. There's not really any chance for an overspeed condition, even with the cat on. The only valid point for upgrade to a 5" that I can see is if I were running a larger turbo. I don't know the exact turbo in my truck, but I'm pretty sure it's not a VG. I assume HY35? Anyways, even if it were a VG it's not going to be fully open most of the time so 4" would still be sufficient.

I think I'm going to go for the 4" exhaust so I can experiment with the cat and how it affects EGTs. It's also a bit cheaper than the 5". If at some point I find the exhaust is somehow limiting me from creating more power then I can spend the money on the 5" I guess.

--- Update to the previous post...

Actually, the only place you want back pressure is between your exhaust and your turbo, once it exits the turbine side you want NO back pressure.

And to the OP, the reason you have a sluggish low end is because it is a common rail, most of the older mechanical diesel had something "like" on demand torque being they spool up quickly, '

Add a smarty and add fuel on the low end and you will notice a huge difference, the stock programming was designed to save your transmission.

I drive a large international 4400 around at work, and with a DT560 inside it i found the best ondemand torque you get out of that CR's is when you have a load on it

So what's your opinion on 4" vs 5" on a stock truck? I will be adding the smarty jr and will likely run it on 40 HP while towing and 70HP unloaded. (Having some issues with reading the ECM, but once I get that fixed I'll be buying the smarty.) Might even opt for a SR or SSR. Still more research to do on each.
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As for increasing the exhaust size...Assuming my math is correct, the 4" exhaust is way bigger than the turbo's exhaust outlet, so there shouldn't be any flow issues. There's not really any chance for an overspeed condition, even with the cat on. The only valid point for upgrade to a 5" that I can see is if I were running a larger turbo. I don't know the exact turbo in my truck, but I'm pretty sure it's not a VG. I assume HY35? Anyways, even if it were a VG it's not going to be fully open most of the time so 4" would still be sufficient.

I think I'm going to go for the 4" exhaust so I can experiment with the cat and how it affects EGTs. It's also a bit cheaper than the 5". If at some point I find the exhaust is somehow limiting me from creating more power then I can spend the money on the 5" I guess.

--- Update to the previous post...

So what's your opinion on 4" vs 5" on a stock truck? I will be adding the smarty jr and will likely run it on 40 HP while towing and 70HP unloaded. (Having some issues with reading the ECM, but once I get that fixed I'll be buying the smarty.) Might even opt for a SR or SSR. Still more research to do on each.

i do believe that your turbo's flange is 4inch. My thoughts are 4 inch will be the best for a stock truck.

Anything over 4 inch is useless unless your trying or aiming for anything over 500HP, and even then 4inch will handle it.

What i found is when i took out my stock exhaust and put in a 4 inch exhaust my EGT's were lower, way lower then they ever were stock. IMO a 4 inch exhaust would be the best bet for a stock truck, it lowest the back pressure the most once you leave the turbo flange, which some degree of heat is desired to allow for the exhaust the pull more air out.

The ideal setup for a 2nd gen truck (with the HX-35) is a pac elbow and then a 4inch exhaust, the is no restriction from the turbo's turbine all the way to the tip.

If set up right, between BHAF and a good 4 inch exhaust you should see slightly better spoolup times, but once again the best spool up ill be seen with some more fuel on the low end.

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Thanks everyone for your help! I feel the 4" kit is the way to go. Now where to buy it? I'm leaning towards TV diesel. (Found out about them from a post here on this forum) They seem to have the best price around and it's a Diamond Eye kit. I like the DE kit because of the u-bolt brackets with hangers. I have yet to wire 220 to my shop so I can't use my welder and this would make things easier. TV Diesel Diamond Eye 4" Aluminized Kit with Muffler And I'll be ditching the u-bolt clamps and going with some of the EZ seal band clamps. Those seem to do a better job and make things easier if i want to change it up later down the road. As far as installation, obviously I would put the cat just after the downpipe. Would you put the muffler directly after the cat or put pipe in between it and the cat?

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Johnfak has a "4" Bullydog SS TB exhaust" and I must say, that thing is niceeee. Polished chrome.. I might have to get one one of these days. Of course then I would have to chrome plate everything else to cover the ungodly cab as seen from underneath but until then yeah a stainless chrome exhaust is definitely incredible looking. You always think of exhausts as being rusty or just dull but his really turns that around.

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