Other Boost issues

[Alex]

When I'm driving along at speed and I lift off the throttle the gauge drops to 500-760+ mm/hg which is fine. On full boost it swings round to .85bar about 13-14PSI.

In between is 0 which frankly confused me the first time I read the gauge. What I need to know is when am I "on boost" and forcing induction and when am I off boost.

I hear alot of whistling before I hit the 0 but after the 0 it changes to a whoosh. But before the needle gets to 0 if I lift the throttle the dump valve activates therefore releasing pressure. So if I've got pressure before the 0 am I boosting before I get there?

 

So would I be getting forced induction before the boost gauge reads 0 bar?

 

Hope you can see what I'm driving at (no pun intended)

 

[Guest Ash]

 

 

When you consider a pressure measurement, you have to first ask the basic question: Is this gauge measuring absolute pressure (psia) or gauge pressure (psig)?

 

An absolute pressure-gauge references its reading to zero pressure, and ordinary gauge pressure is referenced to atmospheric pressure. Therefore, in an instance where a meter that is reading gauge pressure shows zero, another meater reading absolute pressure will show 15psi (or thereabouts).

 

What you are seeing on your meter is ordinary gauge pressure. Therefore, the zero reading is the transition between filling the cylinders under normal atmospheric pressure and forcing it in using the turbos.  

 

Yours,

J

[Doughie]

OK Ash. i'd always wondered about this too.

So when the boost guage reads +15psi, for example, that means it's actually force-feeding cylinders with 15psi ABOVE atmospheric pressure (about 14.7psi), thus total pressure = 29.7psi.

 

is that right ?  

 

So at about 1 bar (14.5psi), the charge is actually about 2 atmospheres, and there effectively transforming your 3 litre engine into a 6 litre engine due to the volume of fuel/air being squashed into the cylnders.     This is why compression ratio of turbo engines is much lower than highly-tuned atmo engines as otherwise at +ve boost pressure you would get pre-ignition.

 

At idle rpm (800rpm-ish) when my boost guage reads -0.65bar (= -9.5psi) are the cylinders being filled with fuel/air mixture at 14.5 - 9.5 = 5psi ??  

 

[Guest Ash]

Look, when you say "boost gauge" in your first paragraph, you have to *first* ask the basic question: Is this gauge measuring absolute pressure (psia) or gauge pressure (psig)?

 

Now, the term "gauge pressure" can sound confusing. Because, in the past, people have come back to me saying, "Of course it is gauge pressure, as that's the reading on the flipping gauge!"

 

But it is the word "gauge" that is unfortunate.

 

Basically, you can read a measurement of an amount of pressure off a pressure gauge. However, that same "pressure gauge" could be reading either "gauge" pressure or "absolute" pressure. You see, the actual word "gauge" sounds and is spelt exactly the same in each instance... but has an *entirely* different meaning depending upon the context in which it is used.  

 

Okay, so if you had a gauge that referenced to "absolute pressure" and was measuring 15psi of turbo boost, then that gauge would be reading 30psi. (Or thereabouts, as some people call atmospheric pressure 14.5 psi, or 14.7psi or 15 psi. Personally, I use 15psi.)

 

If you had a gauge that referenced to "gauge pressure" and was measuring 15psi of turbo boost, then that gauge would be reading 15psi.

 

______________________________

 

So at about 1 bar (14.5psi), the charge is actually about 2 atmospheres, and there effectively transforming your 3 litre engine into a 6 litre engine due to the volume of fuel/air being squashed into the cylnders.  

_______________________________

 

 

Er, on a very basic theoretical level, yes.

 

But by using a device to compress the intake air, means there is a "compression-device efficiency" calculation to perform. Plus, the more that air is compressed, the more it is heated and the less dense it becomes.  

 

The upshot of which means that IN THEORY what you say is absolutely correct. However, in practice, typical compression efficiencies mean you end up with an overall increase in power of around 60%.

 

So your 3 litre engine becomes more like an engine 60% bigger (4.8 litre) at 2 atmospheres of pressure, as opposed to double the size.  

 

 

_____________________________

This is why compression ratio of turbo engines is much lower than highly-tuned atmo engines as otherwise at +ve boost pressure you would get pre-ignition.

_____________________________

 

Yes, because there is a "static" element and a "dynamic" element to compression ratio.

 

The static compression ratio is simply the combustion chamber volume plus the cylinder volume divided by the combustion chamber volume.

 

But with a turbo engine, dynamically, the compression ratio rises once under boost.

 

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At idle rpm (800rpm-ish) when my boost guage reads -0.65bar (= -9.5psi) are the cylinders being filled with fuel/air mixture at 14.5 - 9.5 = 5psi ??  

___________________________

 

Again, I'm not sure whether you are saying this gauge references absolute pressure or gauge pressure.

 

Yours,

J

[Alex]

So despite the fact that the SQV dumps pressure before I get round to the 0 on the gauge I'm not building the pressure using the turbo's to anything over normal atmospheric pressure.... assuming that the gauge is right and that the 0 reflect absolute pressure PSIA.

 

Yey or Ney?

 

Cheers,