Exhaust system size, 3.5" or 4"

[Craig M]

Thought I would bring up an old topic. Optimum exhaust system size for a modified 6.5. Many appear happy with the upgrade to a 3" system. Current thinking seams to suggest a 3.5 " system. A few have gone to the 4" system. I am starting new with my exhaust so can go 3.5" or 4". Any hard data to justify the 4" system? I will be without the cat and with a muffler in the system.

[patrick m.]

i got the 4inch just for the looks, a 3inch pipe looks very small, and with the Dodges runnin around with 5inch, 3 looks tiny. :D
i guess its exhaust envy

[Tony J.]

This is like 8 inches, right?

(------------------------)


:D

[Andy Chesek]

I'm going to change my exhaust soon as well, and I've decided to go 4" with a 5" tip. May as well get good looks with the extra performance.

[Kennedy]

My 6.5 ran 3.5" pipes for along time. One day I decided I'd like a 4" tail, so I redid it and installed. Looks was all that I gained IMHO. That seems to hold true even with the big turbo...

[Billman]

I went from stock 2.75 to 4".

I came across some clearance problems through the crossmembers and over the rear.

If I had to do it again, I would go with 3.5".

Other than that, no hard data, just fitment problems...

[David Brady]

I recently installed a 3.5" system with a straight through muffler from Donaldson, and it sounds great on the low end but over 2500 rpm and with heavy boost it's got a rapping sound that I think means restriction. Maybe it's my muffler. Let me know when you do yours and I'd like to compare. My experience is that the boost comes in much faster. If I did it again I'd go 4". It was just hard throwing away a perfectly functional exhaust system.

[Scooby]

A few weeks ago I installed John K's 3.5 inch system. It works great, sounds great, looks great. I honestly dont think you would see an improvement going to 4" . The downpipe from the turbo was the only place I had difficulty, and it is a 3" downpipe. It is a matter of getting it positioned just right so it clears both the frame and the bottom lip of the firewall. I did cut off some of the old hanger studs and weld them to the new system in order to get everything to sit the way I wanted. Getting rid of the old downpipe (krinkle pipe) IMHO does a lot for boosting performance.

[markelectric]

I have a 3 1/2 and a 4. There is s noticeable power difference. Also a noticeable noise difference. If you dont need the extra power you may save yourself the cost of hearing aids in later life......

[S\W Off Road]

If you have to ask then go BIG. And if you like the sound of your turbo then remove the cat and muffler.

[Movin Air]

Craig,

What I have:
I had the same dilemma a month ago. I removed the stock exhaust and went with the 3.5 from the turbo to tail pipe, removed the cat, and put on a flow through muffler. I also added a mandrel bend crossover pipe.

I really wanted the added power and i wanted to hear my turbo. I thought the 3" to small and the 4" more than i needed. So i went with the 3.5".
The power difference is very noticable and the turbo spools in a hurry and sounds great, you will be satisfied!

The 3.5" fit nice but noticed that if i put a 4" on i would have had to do some adjusting. If you plan to add a mandrel bent crossover pipe and a 4" downpipe you will have to bend the inner fender wall to get it to fit. I wouldn't be surprised if both of these add ons together didn't fit. You also may have trouble getting 4" over the frame cross bar.

So, if the cost from 3.5" to 4" is considerable, i don't think it is worth it becasue a 6.5 does not have the boost of Cummins and Navistar and the room to install it will be tight. If you are not worried about price or room, what the heck go bigger.

[gmenor]

Been following what you all saying. I plan to upgrade my exhaust probably to 3.5 in the next couple months. North Carolina has just started emission testing (don't know if diesels are excempt or the if 95 will be excempt - have to research it). Is there a catalytic converter and muffler that is 3.5? What is the recommended brand - JK? Ant suggestions would be appreciated.

[Scooby]

From what I understand, some of the 6.5's had Cats, and some didnt. JK's system comes with enough pipe, and some suggestions, so you can do what you want with the CAT (aka- soot trap). I wonder how much they do on a Diesel anyway. Mine is in the attic of the garage now. I see less smoke since I put JK's system on. I think maybe the cat was partially plugged, and keeping the turbo from spooling up properly. Less smoke= more environmentally friendly- right ?? :D Turbo sounds great too.

[Fredeberle]

Just installed jk 3.5 exhaust& downpipe this past weekend. Had the old downpipe flange welded onto the new one and it worked out fine. smile.gif
Found it very easy working if you open the wheel well and remove the air filter unit.
Of course this was all lying on my back on the ground ! No Fun :(
Way to old for this foolishness! :eek:

[99gmccrew]

I too had the delema of what size exhaust to go to. I followed the advise of all the experienced wrenchers of this great website. Not everyone agrees on the 4" exhaust but I went with it. Got an awesome deal on ebay. 3.5 downpipe from the turbo, turns to 4" on the way down. Installed it myself, no problems with clearence except by the one crossmember. But I worked it out. No Cat! This system was worth it just for the sound alone, not to mention the performance difference and lower egt's. I'm getting ready to tow a fifth wheel this summer 14,000#. Thats why I started doing the suggested mods from the members. Thanks to every body and does anybody think I'll have any trouble towing this 34' fifth. My truck is stock except for what I have listed.
Steve

[Craig M]

Hey 99GMCCREW;

What make was that 3.5" down pipe and 4" system. Have not seen any for the late 90's GM/Chevrolet. Seems like all I see is Duramax and Fords with the 4". Weather I go 3.5 or 4 would like a 3.5 down pipe.

[99gmccrew]

Craig M, This system is made by pinnacle and comes with a magnaflow muffler (4" straight thru)non restrictive. It's 14 gauge aluminized tubing and comes with a lifetime warranty. Most systems I have seen are 16 gauge. This thing is heavy duty! Its not obnoxiously loud but more of a deep throaty sound. The seller on ebay is markoidaho and sells out of Idaho. Simple to install even for me and I usually make everything more difficult than It should be. I got the system for 280 plus 40 for shipping. I don't see how you can beat it. He sells alot of dodge and ford systems. I emailed him about the 6.5l system and he said he had them. I think you'll be satisfied with it I know I am. The 4" tubing looks very impressive.
My egt's used to be around 600 while cruizing now between 400 and 500. Good luck with whatever you decide.

[99gmccrew]

Craig M, I went to double check size of the down pipe. It looks like 3.5" but it may be 3". Very tough to measure it while it is installed. You may want to double check with markoidaho on ebay.

[Craig M]

99GMCCREW

Thanks for the information. Will check it out. Have a line on a 3.5" stainless setup, but the 4" with a drop pipe might be a better deal. The 3.5" stainless is not for the 6.5 so would require a bit more rework to fit into my Yukon. I will be going by your way Saturday. Going to a meeting in Campo, antique truck club I am a member of. You going to be around?

[Dart]

Just went with the BD exhaust for my 97 6.5. It starts out at 3", and retains the converter. Emissions here in the valley. I understand the turbo outlet is 3". After the converter, it opens up to 4".

Sounds good, but not too loud. The exhaust note was non-existent with the stock pipe. Now, you can hear a bit of v-8 rumble. I am no diesel technician, but I think the modified downpipe that came with the system really enhances the turbo performance. When the engine is running higher rpms, you can feel a bit of a surge when the truck shifts. Also, seems to move through the gears faster. Still waiting to see about fuel economy, but I do believe I have gained some horsepower and torque.

My main interest was fuel economy and some additional power. I plan on replacing the 3.73 gears for 3.43, to put the truck at highway speeds closer to the torque peak.

Not sure about the 5" chrome tip that came with it. Looks a little silly to me.

I will retain the catalyst. But, I am considering an aftermarket unit, that is substantially larger in size to cut backpressure to a minimum.

[S\W Off Road]

Originally posted by Dart:
Just went with the BD exhaust for my 97 6.5. It starts out at 3", and retains the converter. Emissions here in the valley. I understand the turbo outlet is 3". After the converter, it opens up to 4".



I got mine tested last year after installing a 4" system and I left out the cat, They did'nt say a thing. This year I removed the muffler and I wonder if they will say anything at all. I currently have the truck at the dealer for a new pump (118,000.0 miles) when I get it out I'll need to get it tested for this yeasrs tags. Hope they say that it passes.

[99gmccrew]

Craig M, Im in San Diego but Saturday is my 11th anniversary. However Sunday I'll be working in Imperial Beach at the firehouse on 9th & Imperial Beach Blvd. Drop by if your still in the area.
I'm thinking real hard on putting in an IC because of my recent 5th wheel purchase. Just have to break it gently to the little woman.
Steve

[DogDiesel]

Have a Gale Banks PowerPack exhaust system (3 1/2" - with 4" Chrome Tip) that is now for sale. It is in great shape - you get everything from engine to chrome tip - comes with hangers. Fits an extended cab-1993 chevy K2500/K3500 pickup which is all from 92-2K. Trucks shorter than Xcab remove section. I intend to clean it up- box it in my new exhaust kit box for shipment. smile.gif
Also have:
-Intercooler - air to air and the 93' K&N and box with Banks inner horn
-Eventually I'll be selling the entire 300HP engine, but not now. Everything 6.5 under the hood must eventually go.
Wayne

[CareyWeber]

Originally posted by DogDiesel:
Have a Gale Banks PowerPack exhaust system (3 1/2" - with 4" Chrome Tip) that is now for sale. It is in great shape - you get everything from engine to chrome tip - comes with hangers. Fits an extended cab-1993 chevy K2500/K3500 pickup which is all from 92-2K. Trucks shorter than Xcab remove section. I intend to clean it up- box it in my new exhaust kit box for shipment. smile.gif
Also have:
-Intercooler - air to air and the 93' K&N and box with Banks inner horn
-Eventually I'll be selling the entire 300HP engine, but not now. Everything 6.5 under the hood must eventually go.
Wayne Dogdiesel,

I'm at Ft Jackson please email me about the your exhaust system, and maybe some other parts.

carey.weber at us dot army dot mil

Carey

[Craig M]

DogDiesel

Please email me at ccccraigcrane@hotmail.com to discuss some of your 6.5 items.

Thanks

[DA BIG ONE]

Anyone know the formula to determine cfm flow of the 6.5td @ 100% VE? I have not seen any charts on the web showing cmf flow of the 6.5td

Need to know what cfm the motor flows @ 100% VE w/o boost, then under boost in 5lb spacing (5, 10, 15, 20, 25 etc.) then the best size turbo & exhaust is easy to determine.

Getting our 6.5td to 100% VE is another story all together, maybe some of you who are more in the know can do a write up on this!