Need help understanding boost. I plan on adding gauges to my 05 CTD 3500 4x4. I get watching EGT's and tranny temps, but not sure where boost comes in at. I tow pulling cattle/horse trailer and Jeep to the mountains. Is this something I need to watch with a stock motor? What is normal and does boost change?

 

boost is air going into your motor via the turbo. the turbo uses exhaust gas to force cold air into your motor.

a 0-35 gauge is probably pretty good for you, with a stock turbo.

the gauge you want to watch when towing is your pyrometer, which tells you how hot your motor is.

hope this helps

 

i thought thats what the coolant temp was for? .JK... man im bored

 

The boost is simply the intake manifold pressure. It is constantly varying according to the horsepower the engine is making. Higher boost means more air is forced into the engine and that allows more fuel which makes more power. On a stock truck you do not need to worry about it or monitor it. The EGT is simply the temperature of the exhaust. The concern is if it's too hot a piston may get melted. Again, you do not need to worry on a stock truck as long as your air cleaner is clean and you are not deliberately lugging it at full load for a long time. Stock trucks should not be able to hurt themselves with a little common sense, and especially with an automatic. It's the trucks that get extra fuel, beyond stock settings, that must be watched while towing.

These engines are durable workhorses. No need to be worried with stock horsepower. Run it and enjoy that roar.


John

 

[QUOTE=WUnderwood;1578219]

the gauge you want to watch when towing is your pyrometer, which tells you how hot your motor is.




Not quite. The Pyrometer tells you the temp of the exhaust gas, not the engine temp.


Wetspirit

 

One thing I have always wondered is this:
How does the turbo know that the engine is under a load so it will build boost? If you sit there in neutral and rev the engine it will not build boost but if you put it in gear and rev it the boost builds. I just don't get it......

 

it takes more fuel (skinny pedal) to rev it when its in gear. more fuel = more exhaust gas = more turbo spin = higher boost

 

35 psi boost gauge won't do. A stock my '03 hits 34 psi under load. An '05 can go higher. Get a 0-60 psi gauge.

 

On the newer 3rd gens with the zero-throttle launch feature, the ECM fuels the engine to keep the RPMs from dropping below a certain point (like idle).

When we were mapping the coolant pump pressure map on the '05, 3500 RPM in neutral produced 12psi of boost.

 

It's not about RPM, it's about HP output. While idling, the engine uses very little fuel so there's not much more exhaust gas exiting the engine than air entering the engine. But there is a large excess of air available for combustion in the cylinders, as with all diesel engines. A lot of air moving through. Remember, the turbo only responds to the power of the exiting exhaust. At idle or reving in neutral there is not enough exhaust gas to spin the turbo hard enough to make much boost. Reving is not horsepower, and no boost is needed under those conditions. But when more fuel is added to the already excess air available, more exhaust is made, much more. This happens with the engine under load. So now there is a far greater amount of exhaust than intake air and the turbo begins to rev up. This begins to compress the intake air or raise it above the atmospheric pressure of 14.7 PSI (zero boost is really 14.7 PSI boost in absolute pressure). Now with even more air available in the cylinders, even more fuel can be added, which, in turn, makes even more exhaust volume and higher exhaust temperature. So the turbo spins faster, and makes higher intake pressure. Etc., etc. There is always a large amount of air moving through the engine. Boost means we are simply pumping more air into the intake than it would normally draw in by atmospheric pressure. All driven by the exhaust.


The turbo only responds to the exhaust temp and volume. We control those factors with the loud pedal. And the loud pedal only controls the amount of fuel delivered through the injectors.


Producing power is all about getting more air into the engine and then adding more fuel. If an engine could make 100 HP at atmospheric pressure only (no turbo). It is really running on a boost of 14.7. So now we add a turbo and boost it to a gauge pressure of 14.7, or 29.4 absolute pressure (atmospheric 14.7 plus 14.7 more) and the engine should make about 200 HP. If higher boost cannot be acheived, then adding more fuel just makes the exhaust darker and darker. No more power is made but more smoke is made. So, smoke is when there is too much fuel for the amount of air in the cylinder. It's when the turbo cannot feed a higher pressure or when too much fuel was added before the turbo had a chance to spool up and produce boost.



John

 

Wetsprit you worked hard on your explanation but much of your answer did not address the reason why the turbo spins faster and why boost is generated.

The turbo spins faster when you drive the truck under load because the exhaust gas exiting the cylinders have left over energy not extracted by the piston and crank. The energy is expressed as expanding gas and it generates a pressure to spin the turbo and it is called drive pressure.

The reason boost is built, is due to the compressor on the turbo is making more air volume and then the engine can take in for a given rpm. Thus pressure builds.

This is an ever increasing cycle as more fuel equals more expanding exhaust gas and that equals a faster spinning turbo and that equals more volume pushed by the compressor on the turbo and more boost pressure means you can add more fuel to start the whole thing over again.

 

Camper,

I think that's exactly what I said, in so many words. The question was "how does the turbo know the engine is under load so it will build boost?".

Re read my first paragaph.

John

 

So I guess I will either get a EGT gauge or a tranny temp gauge. I bought a SRT 10 A pillar. Now just have to get the right gauge to put in it. Sounds like I should get the EGT.

 

Definitely get the EGT gauge first!