SIXSLUG

^^^makes sense.

I re read the article I was thinking of in Diesel Power, and they checked all the stuff mentioned when on a dyno. Sorry for the mis-info.

KATOOM

rightwinger, let me see if I can help explain.....

First off, as simply put as I can, boost is a byproduct of heat and air flow. As air enters the cylinder its of a specific mass per the volume of the cylinder size. Basically the cylinder works like a vacuum but can only suck in so much air based on its dimensions. This is why naturally aspirated engines can only work within the confines of the engines displacement and why "charged" engines can make more HP than their given displacement. The actual displacement of these engines is 5.9 liters but while under boost their volumetric displacement is much greater.

In a diesel engine the piston compresses that trapped air to a much smaller volume and then injects the fuel at a set timed point of the piston. The injected fuel mixes with all the hot air and combustion takes place. That combustion becomes rapid expansion of air which is taking up more volume that the space provided, therefor forces the cylinder down to make room. Even with the cylinder down as far as it will go there's literally more in the cylinder exiting than came in. Upon the exhaust stroke that air (exhaust) leaves the cylinder very hot and enters the turbo turbine side whereby it turns the turbo by not simply moving through it like wind but more by the velocity it generates within the impeller because the hot exhaust air is still expanding. This is why turbos mounted farther from the engine dont work as well as turbos mounted closer to the engine.

So now what happens is the accelerating turning turbine spins the shaft which thus turns the compressor side of the turbo which then sucks in ambient outside air and compresses it making it more dense and then shoves all that cool dense air into the engine. The increase in boost is a result of more air entering the engine than can be ingested by the cylinders so the turbo is literally cramming or pressurizing air into intake side of the engine. As the cylinder intake valves open, the cylinder area becomes engulfed in pressurized air which results in more volume of air overall in the cylinder than the actual space provided.....and that larger amount of air means a larger combustion when the fuel is injected. Therefor the amount of exhaust coming out of the cylinder becomes greater and hotter and turns the turbine even faster.....and the whole process just keeps going. That is until the amount of fuel levels off and there is no more fuel entering the cylinder than there was during the last combustion process so the amount of boost eventually levels off as well. This is why boost is a direct relation to the amount of fuel entering the cylinder. The more fuel you shove in there, generally the larger the boom, equaling more heat and air and more boost. Just noting too that what I explained in simplistic concepts is for a diesel engine. Gasoline engines will perform in similar fashion but not exactly the same.

Boost is a hard concept to understand and explain because the two variables of fuel and boost go hand in hand. But in a turbo diesel engine you cant have boost without fuel although you can have fuel without boost. Thats just when EGT's get too high and therefor boost then plays another role whereby it shoves the excess heat out of the cylinder so that the combustion process can get bigger and hotter without meltdown.

So why will boost not go up revving in neutral? Well because the load on the engine is very low and therefor there is very little air and heat passing through the turbo. Only enough to make about 1-2 psi of boost.

I left out a lot of details so I hope I explained it well enough.

SIXSLUG

That was a great explanation!

KATOOM

Thanks SIX. I do better with pictures though.....