What does the boost guage tell me?

[bigbob7777]

Aside from the obvious answer, What am I supposed to be getting from my boost guage?



I have a BD X monitor digital pyro, trans, and boost guage setup. I understand the pyro and the trans temp, but what am I supposed to be reading from the boost? When I accelerate, the boost increases; decelerate, the boost decreases.



So what is normal? I am completely in the dark. Sorry. I am really a novice (1st diesel).



I have a '05 3500 5. 9L, 3. 73, auto and pull a 13k# trailer.



Thanks in advance.

bob

 

[Kirwin]

Boost pressure meaning

Its basically a rough measure of the engines power output. If the engine is able to achieve rated boost pressure it will be producing its rated power output. Pyro temperature is the most limiting at low boost pressures and high fuel rates as the combustion temperatures can get hot enough to melt pistons and cause catastrophic engine failure. This won't normally happen on a stock truck. I'm not sure what the max boost number for your truck is but I'd suspect between 23-26 psi. Note that this will only be achieved at high engine RPM and full power. Hope this helps. Ken Irwin

 

[GECCO]

The boost tells you the amount of pressure inside the air intake system that is being generated by the turbocharger. In a normally aspirated (ie, no turbo or supercharger) the air (in a diesel) or air/fuel mixture (in a gasser) is "sucked" into the cylinder by the low pressure created when the piston travels down in the cylinder in the intake stroke. This creates a vaccuum (negative boost) condition inside the intake system. Think about what happens when you expand your lungs to create negative pressure when you drink from a straw.



Naturally, you can put more air into the cylinder in the same amount of time if the air is already under pressure when the intake valve opens. This is the purpose of the turbocharger. The turbo is basically two fans with a solid axle between them. One "fan" is in the exhaust stream, the other is in the intake stream. As the engine runs the exhaust rushing past the blades spins it faster, which in turn spins the blades in the intake stream faster. When you open the throttle, more air goes into the engine. When more air goes in, it spins the exhaust blades faster, which pressurizes the intake side, which means more air to spin the exhaust blades, which creates more intake pressure... . and the cycle builds on itself. So, instead of the engine sucking in it's air supply it is being "force fed". Now, instead of sucking on a straw you are drinking from a fire hose. The amount of time that elapses as the cycle builds is referred to as "turbo lag".



So, the gauge is telling you how much pressure is inside the intake system compared to the atmospheric pressure. If you are seeing a reading of 15 psi that means that the pressure inside the intake manifold is 15 psi higher than the pressure of the air you are breathing. It's all relative, because the pressure of the atmosphere around you varies constantly depending on weather, temperature and altitude. Just like when you put air in your tires, the pressure gauge is telling you the difference between the pressure inside the tire and outside the tire.



Lastly, a supercharger is basically a turbo charger that is driven mechanically off of the crankshaft, instead of being driven by the exhaust gases like a turbocharger.



The debate about which is better has gone on since the inception of the internal combustion engine. Each has it's strengths and weaknesses and there's no way I'm opening that can of worms. :-laf

 

[rbattelle]

When you open the throttle, more air goes into the engine.

I agree with what you've said except this sentence. There is no intake air throttle mechanism in these engines. They run maximum air conditions at all times and vary fuel flow to achieve demanded output.



-Ryan

 

[bigbob7777]

Ok, thanks.

So, what reading should I be looking for? At idle, it reads 1-2. When accelerating to a freeway, it reads about 18, then drops when spped reached.



Haven't tried under a tow load yet.



It's nice to look at, but I still have no idea what the info is telling me.



bob

 

[TSpecht]

It tells you the relative amount of load that's on the engine. In my experience experimenting recently, I have found that if I keep boost pressure under about 6-7psig (i. e. not working the engine too hard) I get the optimum fuel mileage. That is one thing the boost guage does for you.

When you see the boost at idle around 0-2psig, that is because the engine is not using much fuel so it isn't turning the turbocharger much if at all. When you floor it to get on the expressway, the engine is burning a lot of fuel and therefore the turbocharger spins faster and pressurizes the intake which in turn rams more air into the cylinders. Your boost will be higher driving into a stiff head wind than it would with a stiff tail wind. Boost will be higher pulling a trailer.

The boost guage can also indicate problems, such as leaking intercooler boots, clogged exhaust, etc. To use it in this manner, simply get used to what the guage does under normal conditions, and if it starts doing something different, it might be time to look a little further into it...

 

[GECCO]

I agree with what you've said except this sentence. There is no intake air throttle mechanism in these engines. They run maximum air conditions at all times and vary fuel flow to achieve demanded output.



-Ryan

You're correct, I was just trying to be more generic covering gassers and diesels

 

[RMalone]

The basics of what has been discussed is true, however the amount of fuel has a huge effect on boost. Rpm and airflow are very instrumental in determining boost, but it is certainly feasable to see more boost at 1800rpm than 2800rpm. If fueling and/or load (which increases fueling) are increased the boost will increase. Boost is a function of rpm AND fuel. I think.



I believe the turbine/compressor wheel always turns if the engine is running, perhaps not enough to register boost on a guage, but short of some serious back pressure or a barking condition I think the wheels are turning.