I shoulda put this with all the info about diesel operations but I thought it might not be seen for a while. Explain for me this, Im crusing on the freeway 60 mph in 3rd not O/D up a 7% grade pedal down 1/2 or so, producing 12 lbs boost. I have the gas pedal kinda fixed in one place with my foot. I hit the O/D button to put O/D on and the boost shoots up to 16 psi and stays there with no more pedal?
Wouldent the drop in RPM make me loose boost?
It seems this truck will just run out of rpms to early to really get going compared to my other 93 with pump mods. Then I feel like im lugging it. If I remember I read that the pump will start to defuel at 2800 rpm? this is from the governer correct? I dont think I ever take either of mine over 2500 rpm. At least I dont think I have to worry about high EGTs on this one?

 

The issue that is producing more boost is the amount of load on the engine. Although the rpm's have dropped, by dropping into overdrive, you have put more load on the engine, which produces more boost. (Very simply put...)

 

That is correct.

Another way to look at it is:

To move the truck down the highway at a specific speed, grade, load, wind resistence, etc. requires a certain horsepower.

To produce a specific horsepower amount, your engine must burn a certain amount of fuel and air in a specific amount of time.

When you slow the engine down, but continue to require the same horsepower output, you have to burn the same amount of fuel/air per minute to generate that horspower, but there are fewer piston firings per minute. This means that more fuel and air must be forced into the piston at lower rpms than was required at higher rpms to maintain the same horspower output. (It also means that the torque output has to go up.) As you add fuel, the egt's go up, and the turbo spins faster, supplying the required air -- to get more air into each intake stroke, that air must be at a higher pressure.

This is why lugging the engine can cause EGT's to skyrocket and ruin the engine, and why lugging the engine is hard on the transmission (it raises the input torque applied to the transmission for a given load). However, running the engine at lower speeds normally increases its "breathing" efficiency, and since most of the energy loss in an internal combustion engine is due to losses in pumping the air in and out, this can have a big effect on overall efficiency.

While driving, you don't necessarily notice that you are adding more fuel when you upshift, because the governor on the trucks is actually a modified constant speed governor -- as the engine slows down, it automatically adds more fuel to try to keep the engine speed matched with the pedal position.

Probably more of a response than you wanted . . . hope it made sense.

 

Ive taken for granted how a diesel really works for years, it just went when you needed it to. Ive messed with gassers and bikes for a long time. but the diesel bug really has me. I wish I had a junker that I could expirement on. Thank s for sheding some light my question.

 

Holy crap.....asilitch...

Next we'll be seeing the laws of particle physics as they pertain to a fixed mass in a chaotic state while under extreme exitation.....kinda like my wife...ooppsss.. sorry...at least I'm gonna be if she see's this..

Pastor bob.

 

also, to prove the above, next time you do this, watch your pyro as well as your boost gauge, it'll be going up as well. As the temp goes up, the heat energy in the exhaust increases, spins the turbo faster, and viola, more boost.

 

Bushy -- the key is to excite her just enough so that she reaches an unstable state, which will collapse, releasing enough energy to bring her to a less excited state than she began. . . Just make sure that you have appropriate shielding to absorb the energy released