This coud get a little long, but this is a subject about which I was interested and did some digging and modifications.
Just a quick review of how the Jake ( compression ) brake works. It all happens on the compression stroke. On a normal compression stroke fuel has been draw in on the intake stroke and then that air fuel mixture is compressed on the compression stroke. Compressing a gas makes it very hot and the hot mixture combusts and with all the valves still closed that force pushes the piston down and makes your coach go. Ever notice how quickly the turbo spools up when you press on the go pedal - that's the result of those gases increasing tremendously in volvume due to their heating?
Now when the Jake comes into play you are off the "gas" pedal and no fuel should be flowing through the injectors. On the compression stroke there is no explosion since there is no fuel, but right at the top of the stroke the valves are suddenly opened. Now there is no compressed air to push the piston back down so all the work invovled in compressing that air is turned to heat and the coach slows. And yes, watch your turbo pressure when the Jake is on sometime, it jumps right up as the hot air rushes out the exhaust and spins it up. The sound your hear is the compressed air released at the top of each compression stroke. You have turned the engine into a huge air compressor and the energy to compress the air is recieved from the coach's motion. The higher the rpms, the more energy consumed.
How does all this affect mileage and when one should use their Jake. It is something each person must decide and I am only describing how I do it and why.
As has been pointed out if you desire to just coast and the Jake is on, you won't coast you will quickly slow and will have to apply throttle to attain your desired speed. That is not what I wanted. But if I don't have the Jake selected all the time what happens when I need to make a stop in a panic situation? Do you think I would be able to mash the service brake and remember to engage the Jake? I think there is little debate the Jake can help you stop the bus in a shorter distance. Well, don't know about you, but I would never be able to engage the Jake and panic brake at the same time without lots of practice and that just seem impractical. What I wanted was a Jake that functioned a bit differently. What if the Jake engaged when I pressed the service brake and stayed engaged until I touched the throttle? Well that is what I wanted and what I have. I think it works great. When cresting a hill on which I want to use the Jake I just touch the service brake, the Jake engages and all is good.
I did have someone point out that by having theirs engaged all the time that just as soon as they lifted from the throttle in a panic stop they would get compression braking. I agree that the part of a second it take me to move from the throttle to the brake they have compression braking and I don't. I hope it's never that close. Other than that, it works wonderful. If I lift from the throttle I coast. Other that leave the Jake on all the time apparantly learn to just hold enough pressure on the throttle to keep the Jake off but not use any fuel. I never could get that to work so well. Here is a company that seems to help with modifications if you want to use your Jake without having it slow you everytime you lift.
Brakeswitch, A better way to manage your diesel exhaust brake