T680 Exhaust Questions...

I of course don't mean to sound condescending nor tell you how to spend your money. I'm just speaking from experience irl running these trucks.

In theory yes running lower rpms does net you better mileage but from my experience there is too low of an rpm to run at which is causing the engine to work hard just to maintain speed and that doesn't help economy. This has to do with engine load. Say at 62mph turning 1200rpm and engine load is at 30% vs the same road speed but higher rpm of say 1400rpm and engine load drops to 20%. That lower engine load should return you better economy because the engine isn't working as hard.

Its always a trade off between pulling power and economy.

If it were me I'd be looking for a truck with 3.55s. I'd hate for you to buy one with high rears only to wish 6 months down the road you'd went with a lower rear gear for better pulling power.

Like I said this is all my personal experience with these engines, the auto in my current truck should in theory get the most mileage possible but its rare I get above 5.4mpg which to me isn't all that great imo considering I've got an 01 international with a 370hp n14 10 speed 3.73 rears and 11r22.5s that gets the same mileage and a friend with a 98 w900 with a 450hp 3406e cat 18 speed 3.42 rears on 11r24.5s that turns 6mpg. Hand calculated. I understand the engineering theory and in perfect conditions they'll get great mileage and all that but in the real world working every day they dont do that great. Which also depends on driving style and all that but the auto is supposed to shift like the best driver on his best day every shift and its just not that simple. If it were these auto trucks with the high ratio rears should be getting astronomical mpg but in my experience they don't.

Just food for thought.

 

Your best course of action would be to ask around. Talk to guys that run similar setups to what you're looking for and see what they say.

 

This is actually the best economy I've been able to get.

 

The term we're discussing is driveability. I run an Mx13 with 3.25's in the PNW with up to 87K GVW. It works about perfect. It needs to be in that 13 to 1500 RPM power band to pull hills with weight on. On flat ground, you can lug it around, but driveability is poor. Reaction to giving it the pedal is freakishly slow.

 

I run a 680 with the mx-13 and a 13 speed. I can tell you unequivocally that 1000 or 1100 or even 1200 rpm... its NOT gonna cut it going down the road. I have had this engine at both factory 455hp/1650tq and now at 540hp/1800tq. Come to your own conclusion how that came to be. Doesn't matter which power setting tho, the MX13 does not drive well at all in low r's. I've got a 3.25 ratio and run LP22.5. At 1450rpm I am running about 70mph. At 1400 it's about 67-68. I'll run it down to about 63 in 13th but then split down because I'm running under 1200rpm if I slow any further. This engine pulls the strongest from about 1350-1600. That's my opinion and my experience.

As far as exhaust. I had a factory weed burner. At factory settings, you couldn't even hear the exhaust hiss... just sounded "windy" when revved. When we "trialed" the straight exhaust from 5" to 7", it makes a noticeable difference. It is not loud tho... like not at all. Under heavy acceleration, it growls really nicely. At cruise you hear a lot of turbo hiss. Jake's sound like a closed-mouth burp. Originally we intended to put on vertical stacks behind the cab, but with the trial of sound factors, we elected to add in a 36" x 7" SS tip under the frame to replace the burner box. These trucks aren't big hood rigs... being "cool" or "stylish" is a rather unlikely feat to conquer. I went for personalization and preference. I don't draw attention, but I can out pull any 'similar looking' mx13 with ease. These engines are mini power houses. I have always been a Cat and Detroit fan, but these engines are starting to gain my respect.

 

I have a T680, Cummins, Allison tranny, back window, and dual stacks. Coffin sleeper.