I just turned 15,000 miles on my 2005 and curiousity has gotten the best of me. When does the turbo begin to build pressure? I've been watching my gauge and it seems to increase psi when I exceed 1500 rpm, does it "kick in" at a set rpm or is it always producing pressure even at low levels below the factory boost gauges abilities?

Why does the gauge barely move (it does, but barely noticable) when I put the truck in neutral and rev the motor to 2000-2500 rpm? Is this part of the computer control?

I read a post a while back and someone stated "the higher the psi, the lower the fuel mileage." Is this true simply because you are working the motor harder like any other combustion engine, or is there more to it?

 

http://www.turbos.bwauto.com/en/prod...gn_turbine.asp

Check out the info in this article. Your turbo is the variable vane type and has no wastegate as are depicted in several of these schematics, but it's two pages of good basics on how your system basically works.

When you asked if the higher the psi the lower the mileage??? yes--the more air you put in the more fuel will be put in correspondingly. This is generally true. The programming on these trucks works to optimize performance and fuel economy (i know, i know--what fuel economy). You'll learn lots reading that article.

 

Another good, basic write-up on turbos:

http://auto.howstuffworks.com/turbo.htm

In a nutshell, a turbo has two sides--an exhaust & an intake. The exhaust side of the turbo takes advantage of the exhaust gases that are being expelled by the motor to spin a turbine. (Much like a waterwheel in the old days.) As the exhaust-side turbine spins up, so does the intake-side turbine. The turbine design on the intake side makes it pull in more fresh air & compress it. This compressed fresh air is then fed into the motor. In our case, the air is first passed through the intercooler to cool it down (the cooler the air, the more dense it is), and then it is fed into the motor. With more air being forced into the engine, it can then be fed more fuel while still maintaining a safe & efficient fuel-air mixture.

This is actually kind of a tricky question to answer.

To over simplify it, a smaller turbo will generate boost quicker & at lower RPMs, but small turbos limit the amount of air flow. A larger turbo will take longer to spin up, but they can have significantly higher air flow rates.

The variable vane turbo was designed to try & take the better aspects of both a large & small turbo, but not be limited with their drawbacks. It will spin up quicker than a similarly sized turbo with fixed vanes, and it does not suffer the same flow rate issues of a smaller turbo.

The lag in creating boost is also due to the inherent laws of physics (energy transference...blah blah blah). Think of it like blowing on a pinwheel to get it moving. At first it barely moves, and then it is off & running. The same basic concept applies with a turbo.

In a no load situation (i.e. you rev the motor with the tranny in P or N), the turbocharger is going to spin up, but the vanes won't be set in a position wherein they'll provide much in the way of boost, as the computer knows that there is no load/work being done. It knows there is no need to provide boost & increase the fuel delivery, as it would just be a wasted effort.


That isn't entirely true in the strictest since, but there is definitely a fair amount of truth in it. It takes work to do work.

Keep in mind that every motor/machine has a "sweet spot" wherein it is at a mechanical advantage. This is why you will see so many members reference the 2,000 RPM threshold when talking about the 6.0L motor. Given a thousand factors (and probably more), the 6.0L is at its best mechanical advantage when running right around 2,000 RPM.

 

My 2007 is exactly the same way.

Not many people know this but engines and compressors are driven by heat not pressure. The net effect of the heat is a creation of pressure, so they are similar. But as the above post indicates, there is no load so no heat.

Yes and no. More PSI means you are moving more air which should have a more complete burn as each fuel molicule has an easier time of finding the correct amount of air to mix with to burn proper. Back to para 2, you need more heat to get that increased boost, so how are you being more effeicient unless you can get more boost given the exact same conditions (ie something for free, but it is possible).

Back to your first question why does it kick in around 1500rpm? Couple of thoughts to that, port sizing and cam profile are optimized for 2000rpm, below that in an unboosted condition the diesel just cant flow enough air to build up the heat to spool up the turbo (remember these dont smoke (stock), so the computer waits to inject more fuel until the air is there for a stoichiometric burn, adding a tuner will make it smoke because it doesnt wait). i have heard others claim the cummins and duramax spool up faster, I believe this is because thier peak torques come in at 1600 instead of 2000.

 

The point of a turbocharger is to recover energy from the exhaust stream and transfer it mechanically to the intake stream. Higher energy streams have high pressures and temperatures.

Engines are supposed to take energy released from combusting fuel and transform it into shaft work (torque) that we can in turn use for other purposes. Since they aren't perfectly efficient, a lot of this energy never goes into spinning the crankshaft. It has to leave the engine some other way, and one of the ways it does this is leaving you with a hot exhaust gas stream that's at a higher pressure than what you had in the intake stream. The turbo puts an obstacle in the way of this escaping gas, so the gas has to go around it. That spins the turbine blades, which takes away some of the energy of that exhaust stream. The turbo outlet stream is always cooler and at a lower pressure than the turbo inlet.

When you "load" the engine, you are demanding that it produce more shaft work for you. The engine must burn more fuel to meet demand, but that also means that more energy is wasted and handed over to the exhaust stream. The turbo will spool faster because it has more energy to reclaim. There just isn't that much energy to reclaim when you don't have a big load, because there's not as much fuel going in which leads to less energy being rejected from the engine as exhaust.