I totally fell in love with the power and smoothness of the 6.0L when I first drove it, but what bugged me was that the cabin was so quiet, that ...

I totally fell in love with the power and smoothness of the 6.0L when I first drove it, but what bugged me was that the cabin was so quiet, that I couldn't hear the revs easily. Hearing the revs is pretty important in the snow- there's never any time to look down at the tach. Anyway, when I put my Flowmaster on this winter, I was glad the the motor was plenty loud. I also noticed that on the freeway, at about 60 mph in 4th gear, the motor sounds like it's Jake braking when it drops down to 3rd after I let off the gas. It's always from 4th-3rd, never 3rd-2nd. It sounds awesome (especially under bridges), but you can feel the truck slow down when it downshifts into 3rd. Is this normal? Is there some backpressure issue that the muffler created? Do most trucks slow down a tad when they downshift on the freeway?
I don't mind it at all; it's nice to know exactly when I'm in 3rd, plus it's easier to drive more efficiently- I use the brakes less often now on the freeway.

I'm not exactly sure what you are saying. Assuming you are driving at 60 MPH, why does it downshift to 3rd on a closed throttle? It should stay in 4th till you get down to maybe 40 to 42 MPH. I do agree it would downshift if you accelerated but............. Can you give us more information please?

I will tell you this, on modern GM vehicles, the calibration of the fuel injection WILL shut off the fuel to the engine on certain closed throttle, decel conditions. Is that what you are talking about? Under those conditions, your instant fuel mileage will show 99 Miles to the gallon and stay fixed. Take a look.

What I'm trying to say is, when I'm on the freeway, doing 60 or higher, and I let off the gas completely, it will downshift at about 50 mph into 3rd- and the motor starts acting like a Jake brake. I'm just wondering if this is normal.

And yes, the instant econ. reading will shoot way up- sometimes it does flash 99 mpg.

No additional thoughts. I hope you figure out your problem.

OK. Thanks for your help, Gordon.

Are you sure it's downshifting, or is the torque converter just unlocking? What does the tach say when it does this? If it was downshifting, the rpms would raise quite a bit.....

I haven't watched the tach, but when I jump on it after it downshifts, it's always faster- like it's in 3rd; then it will upshift again into 4th once I get going around 60.
But wow, what a sound- it's just like a big-rig, except with more burble and pop.

I have the exact same issue as what your are explaining. (2002 Chevvy 2500 HD). I have Flowmaster duals. I have 2 seperate mufflers. I talked to Flowmaster and they advised me that this is because the mufflers should be balanced with and X pipe or and H pipe. They told me the exhaust should have had one muffler with a 2 in and 2 out on it instead of having two mufflers. This would fix the uneven back pressure which sounds like jake braking. I have not got a chance to mess with doing anything with it yet but when I do I will let you know.

Thanks, acollins. Did Flowmaster say that this messes with the performance at all? As far as I'm concerned, it sounds great, but if the exhaust system needs more breathing room, than it becomes a problem.
P.S Have you ever timed it so it happens under a bridge? It sounds like a flippin' earth quake!

mine does the same thing its the gears they make the truck slow itself down well thats what my dad told me and why r u complaing about the sounds that makes it sound awesome