T680 Exhaust Questions...

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

 

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.

 

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

 

Oh, and reefer...