bigger exhaust worth it?

[dgrund61]

Just wondering what your thoughts are on going from stock exhaust w c/c and going to 4" exhaust with or without c/c? How much improvement is there in mileage and power. I know it helps it breathe easier but is the payback worth it. What about the crossover, change to 2.5" help any either? . I trying to justify it on My 98 1500 td. Any thoughts ?

[6.5 Detroit Diesel]

Stock 6.5 exhaust is rather restrictive. Going to a bigger exhaust will help, but don't forget to upgrade the stock downpipe to a proper 3" freeflowing one. If you don't do that, I don't think that there is much benifit in making the rest of the exhaust bigger. On a side note, you will hear the turbo a lot more with a bigger exhaust!

[DaveNY]

Just finished 3" from downpipe flange back with high flow (aero) muffler and removed clogged cat. Will probably change the downpipe when I get some time.
The truck has significant improvement in throttle response and performance. Does'nt seem to help mileage much but need a steady trip to check on Hwy, but sure its not enough to justify the cost of the exhaust as to savings in fuel. I'm sure you will enjoy the way the truck runs and the way it seems to me it will definately contribute to the longevity of the engine. Dave

[sturgeon-phish]

My first step in freeing the exhaust was removing and gutting the cat. I saw an improvment in fuel mileage and sensed an improvment in performance. I did not have gauges at this point to evaluate changes in egt temps. I then upgraded to a complete 3 1/2 kennedy exhaust, (3" DP) again there was a noticable improvment in fuel mileage and performance. If I would of gone from stock with cat to the Kennedy exhaust the improvment in performance would of been very pronounced. Fuel mileage data from original exhaust with Cat to Kennedy showed a consistant 2 mpg improvment with the same driving style. Key point; with the upgraded exhaust it is very tempting to accelerate harder, because the truck runs so much better and thus negate the improvment in fuel mileage.
From what I have read changing the crossover has a minimal effect, and cost per improvment hardly makes it worth it unless the pipe needs replaced.
Jim

[Robyn]

My DaHooooley has the Banks Stinger setup on it and uses 3-1/2" exhaust all the way with the 3 inch down pipe.

I think that the ability of the 6.5 to move air is about maxed out with a 3-1/2 system.
At this point the cast turbo wastegate housing is the bottleneck and will really limit any further improvements.

If one stumbles onto or otherwise gets a good deal on a 4" system it would not hurt but I dont think much more improvement is in the offing.

My truck has the 3-1/2 system with the air snorkel mods and a performance chip from one of our vendors.

The truck will get 19 on the Hwy (If I drive it in a sane manner)
As was mentioned the urge to let the horse run is hard to resist :D

Having a 94 Burb that is bone stock and original as far as the fuel and exhaust system to compare it with it is like night and day.

My 94 Burb is an early one and the programing is one of the doggiest they produced.
It gets 21 MPG on the Hwy and about 17 around town though.
(2500 4X4) But it is a slug compared to the DaHooooley 4X4

A 5 inch system however cool it looks and the sound is awesome is overkill on these little engines.

Have fun

Robyn

[DA BIG ONE]

Larger exhaust is always better. You will get to a point that the exhaust size increase does nothing if you allow the turbo to superheat the air charge by over boost situation resulting in fuel cutback. Or, if your turbo turbine housing (bottle neck) just can't flow the cfm needed for performance increase.

Being that the down pipe is limited to 3" diameter w/3.5"/4.0" or whatever you have another bottleneck the limited 3" down pipe. However, stock GM turbine housings appear to have smaller than 3" outlets', so a turbine w/3" outlet would be a nice option to go w/3" and larger diameter exhaust. Some have gone to the bigger turbine and compressor and no gate control and are happy.

I'd like to have a 3.5" or maybe a 4.0" down pipe w/system matched to it!

[Robyn]

The biggest issue is the limitations of the chassis to allow a really nice high flow turbo.
If a completely modified setup that allowed for a system with gentle bends coming out of the turbo was utilized this would be a different arena all together.
The flow limit should be the volumetric efficiency of the cylinder capacity along with the limits of the heads to flow air and not the exhaust system.

A turbo charged engine can usually flow more air than an N/A but there are limits even then.

The intake system on the 6.5 as well as the heads are not too bad and could do far better but the design contraints of the chassis held things in check.

The setup as it is will allow the 6.5 to produce a power level that will take it up to its thermal limits in a truck chassis. (About 300 HP)

Heat and the ability to get rid of it is the limiting factor in these applications.
A Marine setup with a limitless supply of cold water to cool the savage beast makes a whole different story out of it.

Robyn

[Kennedy]

My first step in freeing the exhaust was removing and gutting the cat. I saw an improvment in fuel mileage and sensed an improvment in performance. I did not have gauges at this point to evaluate changes in egt temps. I then upgraded to a complete 3 1/2 kennedy exhaust, (3" DP) again there was a noticable improvment in fuel mileage and performance. If I would of gone from stock with cat to the Kennedy exhaust the improvment in performance would of been very pronounced. Fuel mileage data from original exhaust with Cat to Kennedy showed a consistant 2 mpg improvment with the same driving style. Key point; with the upgraded exhaust it is very tempting to accelerate harder, because the truck runs so much better and thus negate the improvment in fuel mileage.
From what I have read changing the crossover has a minimal effect, and cost per improvment hardly makes it worth it unless the pipe needs replaced.
Jim

That's basically what I found. I started out on the exact same path back in 1998? or so with my 19996 GMC. Started with a downpipe, then ditched the cat, then a muffler bypass. Then I decided to do the full system and designed my system. This is where the REAL gain started.

Oh, and I have dyno proven the gains over a cored out stock system.

[Robyn]

John
Do you have any documentation on the really big systems 4" +

I am of the opinion that anything over 3-1/2 is probably overkill as far as any real improvement???????????? So long as the rest of the system is in good shape.

Robyn

[sturgeon-phish]

There might be an ego factor to see a BIG tailpipe.

[DA BIG ONE]

I see the WRX Subaru even the BMW has an after market down pipe that is much larger the the rest of exhaust system. It is claimed that the larger down pipe allows for mush faster spoolup while the rest of the system which is actually smaller that aftermarket downpipe but bigger that stock diameter system reduces back pressure. How it works? I really don't know but there must be something to it.

[rustyk]

Diesels (naturally-aspirated or turbocharged) are more sensitive to back pressure than gaso engines. The less back pressure (bigger exhaust components), the better.

[dgrund61]

After reading all your thoughts on changing exhaust I will be getting a bigger (most likely 4" exhaust) after tax time. IN the meantime I decide to modify the cat a little bit. After spending 6 hours on what should have been a 2 hour job, I came to the conclusion that (as in the past) the people that design do not fix. Broke all but one stud off the flange on front side of the cat. Drilling them out ruined several bits and My patience. Will find out this week what effect "cleaning the cat" will do for mileage and performance. I do know You can hear the turbo spool up now which I am pretty sure is a good sign. Thanks for all your input; Will let know what the new exhaust does for the truck in the near future.

[tedtobler]

What is gutting the cat and how do you do it?

[cheyenne1500]

What is gutting the cat and how do you do it?

gutting a cut is removing all the filtration within the unit. its bad for the enviroment and very illegal. but there is some good that comes out of it...litterally

[sturgeon-phish]

To gut the cat Unbolt it from the downpipe and cut it at the straight section. You can weld it back in or sleeve it. Once removed, take a pipe, and beat out the contents. It is made like porceline or hard baked clay, but will come out.
Jim

[Truckie117]

Hey Guys
When gutting the cat how does that affect your smoke test in states like NY?

[ercaduceus]

I just recently put a new exhaust system on my truck. It is a 3in down to 4in out. Major difference over the factory with cat. The 4in is likely overkill but it sounds good:D. The truck seems to run much smoother and cleaner (rarely smokes). I don't think diesel cats are like gassers in that they get hot and clean themselves out. Over time the increased restriction will decrease airflow and lead to more smoke. So I would think that no cat is better for the environment than an old cat. As far as gutting the cat, for and older truck you should only have a visual emissions test. So if the case is intact you would pass. Im not sure about NY, but that is the case on the counties that have emissions in PA. My 98 is emissions exempt in PA.

[JohnC]

Hey Guys
When gutting the cat how does that affect your smoke test in states like NY?


What smoke test? ;)

[Mainous]

All,

I stumbled across this thread and wanted to throw in my $.02 worth after recent events with my truck. From a performance standpoint, I feel like my truck is pretty well maxxed out. I have been more than pleased with the perfromance. Especially, since installing the IC. This fall I started noticing a tick out of the left bank (drivers side). Expecting an exhaust leak, I crawled under the truck with it running and sure enough I could feel exhaust coming out just below the flange. No big deal, the pipe had cracked at the flange and the crack had eroded open over time. I ordered a cheap replacement crossover from an Ebay vendor and installed it. I could not believe the positive difference in performance. Initially, I just thought that the leak had been there for some time and given that the crossover affects boost, I just figured that the performance had diminished over that time. Then I stumbled across this photo:

http://www.heathdiesel.com/P/HDP1349/Gallery/

Prior to this, I saw no value in replacing a good crossover. Sure, a ceramic coated one would hold a bit more heat but when your boost pressures are where they are supposed to be, why bother? Anyway, I went back and cut my old one apart and found basically the same thing you see in the photo link. Given all this, and the fact that my truck did crack the block at 135K miles, I would be curious to hear from anyone out there with an engineering background regarding the loads this blocked crossover creates on these engines. The backpressure difference between the two banks has to put a serious load on the block itself. Has anyone else experienced this and thought the same thing?

No vendor endorsement intended...