T680 Exhaust Questions...

Discussion in 'Kenworth Forum' started by 7milesout, Jun 3, 2020.

  1. 7milesout

    7milesout Bobtail Member

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    Guys,

    A soon to be OTR owner operator here. A question about aero T680 stock exhaust systems on line haul / OTR trucks with a sleeper. The T680 is the leading candidate of the trucks I'm considering buying. Any truck I buy would be along the lines of a raised roof 76" sleeper truck (most likely used). Some T680's can be seen with rear of the cab exhaust stack(s). I know there is a factory option for such, at least on the 40" and 52" sleepers. But in my experience, most of the T680's with 76" sleepers I've seen, there are no stacks to be seen. I assumed the exhaust exited near the roof but with the stacks hidden behind the sleeper's fairings somehow. However, now I'm wondering if I'm wrong. Kenworth refers to "exhaust cut-out covers," and seems they are referring to them near the chassis fairings.

    Question 1: Are these 76" sleepers with no stacks visible … are they all ground dump exhaust systems?

    If so, I can see a couple advantages there maybe. One, it may be lighter in weight than a stack system, and two, a ground dump system may be more aerodynamic than a stack system.

    Also, I have NEVER heard a loud T680. If I buy a used T680, I will be targeting a 2018 with an 18 speed and MX-13. I'll be honest, the first trouble it runs into with the stock emissions system, to the scrap heap it would go. At least I would prefer to send it to the scrap heap and run a straight exhaust. But I've never seen a T680 with a straight (loud) exhaust.

    Question 2: Is there something I don't know (I'm sure there's plenty) about deleting the T680's emission system that I've not heard about? Or would it be like deleting any other system. I understand the emissions system is a Cummins design. So it should be common.

    Lastly - I know about most of the complaints about the MX-13. The thought of stepping into this industry as an owner operator only scares me in the aspect of failing as a business. The day to day work, and documentation doesn't scare me. It's just plain failing to provide for my family that concerns me. So, I don't care to be the coolest trucker on asphalt. The truck I buy will just be a tool. The MX-13 is an efficient choice. A manual transmission in my opinion should last longer and cost me less in the long run than any automatic or automated. Maybe in a few years and knowing all the ins and out of being an o/o, and when I need a truck upgrade, I might be willing to move to a cooler truck. But as of now, it's all about the dollars. I'm open to your feedback, opinions, etc on both the MX-13 and the T680.

    Thanks.


    7milesout
     
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  3. Lysdexis

    Lysdexis Road Train Member

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    1. Yes. They are a weed burner. Same as my pete 579 I drive. I'm with you on preferring a stack, especially when the truck is doing a regen. That of course doesn't prevent you from routing it up. Just have to buy the appropriate brackets to mount to the frame and the pipe to make it happen. They don't have a muffler just the dpf which muffles it plenty.

    2. Talking about deletes on this forum is a no no so youll not get any answers to that question here.
     
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  4. 7milesout

    7milesout Bobtail Member

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    Oh, ok thanks.

    I would never delete it. I only ask that in relation to why I've never heard a loud T680. I guess that would be why. I also think that of all the loud trucks I've heard, approximately 100% of them had stacks. If I heard a loud ground dump, I didn't know it. Perhaps that's also why I've never heard a loud T680's. To add stacks would require all of the supporting hardware to brace them … which leads to more expense.

    If you or anyone want to shoot holes in my T680 with MX-13 with 18 speed direction … or maybe confirm I'm going down the right path, that would be appreciated.

    It would be nice to find an example of such a truck with about 250k on it. But most have right at the warranty expiration miles … of course, when the big fleets dump them. And I figure I'll keep it 4 to 5 years as long as it is not crapping out weekly, and then move on.
     
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  5. Lysdexis

    Lysdexis Road Train Member

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    I've got an n14 in an 01 international that runs local and I drive a 17 pete 579 with the mx13 and an auto 10.

    I have zero complaints from the mx13 but the aftertreatment will give you issues. If i bought a truck with it id delete it. The truck im in currently has 320k miles on it. No warranty, ive already had to have the dpf removed and cleaned out. If it happens again I'll probably delete it.

    There's a Facebook group that just deals with info pertaining to the mx13 and they sound very similar to a 12.7 detroit.

    Overall not a bad engine, look for an epa17 because those can be turned up to 510hp. Factor that into the negotiations when you buy a truck because its about 3k to do it after the fact.

    What do you plan to pull? A van? Reefer?
     
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  6. 7milesout

    7milesout Bobtail Member

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    Lysdexis - Thanks for the info. I believe (I could be wrong), that the 2018 T680's and beyond are epa17. Correct me if I'm wrong. I've been looking at the 455MT, if not, just the 455. I like the idea of that engine making max torque down to 900 rpm. I don't plan on setting any land speed records. The 455MT makes the same 1850 lb-ft at 900 rpm as the 510 does at 1,000 rpm.

    A couple years ago, I put an AFR (air fuel ratio) gauge on an old muscle-ish car I have. A 72 Plymouth Scamp with a LA360 V8. This not exactly apples to apples I know. But since watching the AFR gauge, learning and tuning, my eyes have really been opened to what is going on inside an internal combustion engine. The idea of an engine that can run down to 900 rpm and not be lugging is an eye opener. Not that I'd want to cruise along at that rpm. But I would like to cruise along at about 1,140 - 1,150 rpm, and when pulling some hills, if the rpm drops down to, you know, a skosh above 900 rpm, no need to shift … and should be making substantial torque at that rpm. The fact that a smaller 13L engine will flow that much less air (vs a 15L running at 1,350 rpm) should do wonders for mpg. Watching the AFR on my little 360 gas engine, I have been able to tune it for really decent mpg on the highway, and absolute maximum power at full throttle (considering it is only a mildly modified engine).

    I've started doing some calculations with rpm, differential gearing, 18 speed ratios (only 7H, 8L and 8H), and effective tire radius in a spreadsheet to determine highway rpm and cruise speeds, based on tire size, and gearing. Pretty eye opening. I totally get the low differential gears and direct drive stuff they're doing with in some of the auto transmissions. But buying a used truck, with the plan to run it for 5 years, I don't trust an auto. I'm plugging in the differential gearing for some of the trucks I'm looking at to judge their cruising rpm in 7H, 8L and 8H. Getting a little ahead of myself maybe, but have some time to kill. And it helps me understand and make good decisions.

    One of the trucks I'm looking at has a 3.08 diff, and 18 speed. With 11R24.5 tires, it would turn about 1,160 rpm at 62 mph in 8H. 1,360 rpm at 62 mph in 8L, and 1,540 rpm in 7H at 62 mph.

    1,160 at 62 mph in 8H sounds perfect to me. And if a grade pulls it down 10 mph, it would be turning 940 rpm. Still OK for that 455 engine, especially the 455MT. Unless I'm smoking crack, that sounds ideal to me.


    7milesout
     
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  7. 7milesout

    7milesout Bobtail Member

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    Oh, and reefer...
     
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  8. Upinsmoke

    Upinsmoke Medium Load Member

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  9. Upinsmoke

    Upinsmoke Medium Load Member

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    James has a T680 with dual stacks and a back window.
    Cummins and an automatic.
    They are out there.
     
  10. Lysdexis

    Lysdexis Road Train Member

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    I can tell you from years of driving paccar equipped trucks that they don't pull at 900 rpm like they claim they do. Im downshifting at about 1300 to keep in the power.

    3.55 rears would be your better option. You're not going to see that great of fuel mileage out of it lugging it all over the place.

    My truck pulls usually around 5mpg and its got a 10 speed smart advantage auto that is junk and something like 2.70 rears on lp22.5s. At 80 in 10th it turns 1500 rpm.

    You can run 1000rpm down flat road all day long but you come to a hill you might as well drop a gear before you even start up it because you'll loose more speed waiting to down shift at 900 than you would dropping down and starting to pull the hill at 1500.

    My 17 model is an epa13 engine so while it should be there not exactly a guarantee that it will be without looking.
     
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  11. Lysdexis

    Lysdexis Road Train Member

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    Also i should add that 62mph isn't going to get you much if any fuel savings vs running 70mph. If you're pinching penny's tight enough that .2 or .3 mpg is going to break you you need to park it and not run cheap freight.

    THESE FIGURES DONT REFLECT A TRUCK/TRAILER PAYMENT NOR INSURANCE BROKER FEES ETC ETC ETC. This is merely a for instance and is just a generic generalized breakdown. Your cost will be different and will vary wildly from week to week.


    Figure out your running cost based on 5mpg and how much it cost in wear and tear per mile. Plus profit for yourself.

    For 1000 miles you'll burn 200 gallons. That equals roughly 700 bucks in fuel at 3.50 a gallon. Youre at about 1.42 per mile in fuel over those 1000 miles

    Most tires are advertised last 100k under optimal conditions so assume 75k miles per tire. One tire is in the neighborhood of 500 bucks. So 18 tires 9000 bucks. Divide that by 75k miles and you end up with tires costing you about .12 per mile. Over 1000 miles thats 120 bucks.

    Oil changes are around 350 and paccar says 50k mile intervals but I keep it around 30k intervals so about .05 per mile in routine maintenance. Over 1000 miles thats 50 bucks.

    So youre going to spend a total of 870 over 1000 miles which equals out to 1.15 per mile break even money. Now assuming you as an o/o want to make more than your average joe company driver does per mile you're looking at paying yourself at least .75 to 1.00 per mile.

    You're looking at a cost of 1.90ish to 2.15 a mile to run your truck depending how much you want to make in profit.

    Referring those numbers at 5.5mpg youre looking at saving yourself about 65 bucks. Or about .06 per mile.

    So moral of the story is yes fuel is your largest expense but unless you can turn that into like 7 or 8mpg youre not going to see any major benefit to a dismal .2 or .4 mpg increase.

    If you run 3k miles in a week youre looking at
    2100 in fuel
    360 in tires
    150 in oil changes and general maintenance

    2610 bucks in expenses per 3k week plus 2250 to 3000 for your pay.

    5600 dollars for a 3k mile week equals out to 1.86 per mile.

    There's other methods of calculating that might work better but this works for me and keeps me in the black. Of course you won't know how much it cost to run your truck until you actually put your specifics into it including truck payments, brokers fees, insurance, accounting fees, etc.

    This is also not me telling you how much you should bid on loads. No one likes hauling cheap freight but freight is cheap because people can't budget and set a minimum per mile theyll pull for and they'll haul cheap which drives the prices down for the rest of us
     
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