How come BHP figures vary so much with same boost?

[Ian C]

Apparently, a lot of men have trouble finding the plenum.

 

Isn't it the bit between the arse and the cock?

 

So in a sales rep's BMW when he's got his manager out with him, it's the gearstick.

[Dash Rendar]

This has been an interesting thread, and I think I've learned a lot.

 

BTW, Wikipedia has a nice page on turbos which has a surprising amount of detail. I found that useful too, as well as the pages about volumetric efficiency (which I'd never even heard of before).

 

Thanks everyone!

[JohnA]

As said, static pressure (boost) doesn't necessarily translate to power.

Power needs oxygen and fuel molecules working together.

 

Pressure you can get simply by heating things up. Sure, you'd get lots of boost, but not many molecules bouncing around (so power would ultimately be low)

[Dash Rendar]

John, I'm not sure I'm in agreement with this argument. (Again, I'm happy to be told I'm being an idiot!) Surely you're talking about chemical kinetics here?

 

I agree that increased pressure need not translate in a proportional manner to power for a number of reasons. You then say: pressure you can get "simply by heating things up". This is certainly true, given the volume is a constant. But saying you get "more boost" but "not many molecules bouncing around" seems like a non sequitur to me.

 

Increased pressure, by definition, means increased collision of molecules. It's the increased number of collisions, and collisions with higher speed (and therefore energy) that causes the physical manifestation of pressure.

 

But, as you say, to get a bigger bang, it's not simply the pressure. Ultimately, it's the reaction between oxygen and gasoline. Increasing the pressure of the air gives us increased concentration of oxygen (since increasing the pressure of a gas amounts to the same thing), which, according to collision theory, will increase the rate of reaction.

 

It's been a while and my chemistry is a bit rusty, but if I recall correctly, the limiting factors will be the steric factor, the diffusion coefficient and the activation energy for the reaction. We can't do much with the steric factor, but increasing the pressure certainly helps with the other two.

 

I would have thought that aside from increasing boost, the only way to mix the molecules more effectively would be to change engine timing, alter injector design, or introduce a catalyst into the mix. But this is where I go quiet, because I don't know a hell of a lot about engines. (But I'm trying to learn!)

 

If only I'd stayed awake in a few more of those chemistry lectures so long, long ago, then I might actually know what I'm talking about... Rather than just pretending to!

[JohnA]

.. But saying you get "more boost" but "not many molecules bouncing around" seems like a non sequitur to me.

boost is static pressure.

There is also total pressure, which is more representative of power potential (it includes static pressure)

Increased pressure, by definition, means increased collision of molecules. It's the increased number of collisions, and collisions with higher speed (and therefore energy) that causes the physical manifestation of pressure.

Yes. the kinetic energy of molecules registers as pressure.

But for power you need more of these molecules, not necessarily more bouncy ones.

 

Increasing the pressure of the air gives us increased concentration of oxygen

yes, if temperature is kept unchanged.

Doesn't happen easily in real life though.

Ever heard of terms like isothermic, adiabatic?

 

 

For more power you need to pair more oxygen molecules to (suitably) more fuel molecules. Higher readings on a boost gauge don't guarantee that.

[Ewen]

Isn't it the bit between the arse and the cock?

So in a sales rep's BMW when he's got his manager out with him, it's the gearstick.

(if I'm allowed one in technical)