What is boost?

[fivebob]

Boost is a measure of restriction at a given flow rate. That is all, it is not a measure of potential HP. Having more boost does not always mean you'll make more HP.

In fact having higher boost at the same flow usually means you'll make less HP than an engine that achieves the same flow at a lower pressure.

Why is this?

It is because increased pressure comes with a downside... Increased temperature and lower charge density.

pV=nRT is one of the few things you learned in school that comes in handy when trying to extract HP from an engine. Though unfortunately it seems a lot of people on car forums weren't paying attention that day

[KinLoud]

Yes
Had a big "discussion" about boost vs power
Tried to explain that some of the pressure (boost) increase came from non adiabatic heating of the air by the turbo due to inefficiency of the compressor.
So by winding the boost up on his factory (small) skyline turbo he was adding a lot of heat but not much more air (mass) due to operating in an inefficient part of the boost map.
He's like "boost is power"
I'm like "mass is power"
LOL

[fivebob]

Exactly, some people just don't get it. You can make more HP at lower boost with a more efficient turbo. A lot of people can't read compressor maps

Not only can you make more HP at lower boost, but you get a more reliable motor because it's not so prone to heat soak which leads to detonation.

Of course it's all swings and roundabouts, more efficient turbos tend to be bigger and slower to spool, so you lose low end power, unless you use a small turbine, which in turn creates restriction at high rpm and reduces the overall flow of the system.

[Grrrrrrr!]

Boost is a measure of restriction at a given flow rate. That is all, it is not a measure of potential HP. Having more boost does not always mean you'll make more HP.

While this is true, if the only change is increased boost, then the result is more power. If the intercooler is capable of keeping the temps under control then more pressure = more power, assuming fuelling and knock don't become limiting factors.

[fivebob]

Boost is a measure of restriction at a given flow rate. That is all, it is not a measure of potential HP. Having more boost does not always mean you'll make more HP.

While this is true, if the only change is increased boost, then the result is more power.

No, if the only change is increased boost then there is a power decrease.

pV=nRT. (V)olume is fixed, R is a constant, so an increase (p)ressure without an increase in (n)umber of molecules results in an increase in (T)emperature only.

If however flow is increased along with boost then there may be a power increase, providing that the increase in temperature is not too great.

It's all about flow, boost is just easier to observe

[Grrrrrrr!]

Unless the engine was at choked flow for the entire intake cycle, more pressure @ same temp is more flow.

[strx7]

While this is true, if the only change is increased boost, then the result is more power. If the intercooler is capable of keeping the temps under control then more pressure = more power, assuming fuelling and knock don't become limiting factors.

but you are talking about increasing VOLUME aswell as boost, which is how you make more power.


Another good way to look at "what is boost?" - its the stress you put your engine under and how much pressure is in your inlet manifold, nothing more, nothing less.

[Grrrrrrr!]

but you are talking about increasing VOLUME aswell as boost, which is how you make more power.
.

No, no change in volume.

[strx7]

No, no change in volume.

then how are you expecting it to make more power???



at the end of the day, CFM is what makes power weather its from increased engine capacity, or increased breathing capacity

[Grrrrrrr!]

pV=nRT. (V)olume is fixed, R is a constant, so an increase (p)ressure without an increase in (T)emperature results in an increase in (n)umber of molecules.

Increased charge density, not increased volume. Manifold still same size, bore and stroke still same size.

But yes, the result is an increase in free air CFM, (which is a rate, not a volume)

[siren676]

I could use you guys on another forum im on, someone is planning of hooking up a scuba tank to an intake on a n/a engine thinking that 3000psi=more power

[iOnic]

For what it's worth. One of the top tuners in the country uses the same equation and strategy when approaching his tuning. As it turns out, he's also a member of this forum.

But 2jayzgte may be able to teach him something new since he clearly knows something that everyone else missed in physics lectures.

Or just throw out a snide remark and contribute 2/5ths of $&#$% to a tech discussion as usual.

[Grrrrrrr!]

It would be,, for the few seconds that i'd take to empty the tank (assuming one big arse valve.. and it not freezing up).. oh, and the fact that half the air would go out the airfilter.

[Dell'Orto]

I could use you guys on another forum im on, someone is planning of hooking up a scuba tank to an intake on a n/a engine thinking that 3000psi=more power

Holy shit I want to see this in action. Imagine if he plumbed it up behind the throttle plates

[siren676]

I could use you guys on another forum im on, someone is planning of hooking up a scuba tank to an intake on a n/a engine thinking that 3000psi=more power

Holy shit I want to see this in action. Imagine if he plumbed it up behind the throttle plates

It was really a video suggestion for the mighty car mods guys to do, i have given up after 3 pages of trying to tell them that it wont work.
Have a read if you dont mind facepalming through your head
http://forums.mightycarmods.com/showthread.php?26840-Supercharge-a-car-with-a-SCUBA-tank-%28SCUBAcharge-it%29

I made this diagram to try show them that it wont work.(Please ignore any physics failures, i never took physics at school and am only going off common sense and knowledge of how motors work)

[Flannelman]

Increasing molecular density is the technical term for pressure.
One way to measure pressure is in PSI, or Pounds per Square Inch.

Increasing molecular density without increasing temperature is impossible. See, when molecular density increases, it is being compressed. No matter how small or large the compression, there is heat created.
Put this to the test in the real world and no doubt some one will try shoot me down saying "my mate with bla bla engine increased boost by bla ba and got bla bla bal gain." Hmm, really?
What I hopefully have shown that there are a group of individuals that when a visual change is made, in this case boost by the dash mounted gauge, and there is a power gain, then the conclusion is "boost goes up, power goes up". This group also associates noise with speed and excessively low cars handle better.
Back to the "my mate" fella. Why did he get a measured power increase with more boost? Could it be that the stock fuel system that overfuels the engine to keep things nice and cool was the cause? By adding hotter, higher pressure air to the port this would have helped vaporize alittle more fuel than with the cooler, less pressured air than before. To vaporize the fuel, the excessive heat would have been removed. Probably not all of it but enough to make a difference to keep detonation at bay.

The short answer to What Is Boost? Its the buildup of air molecules behind the greatest restriction of air flow. In turbo cars, its the exhaust housing. In N/A cars, its the intake valve.

[sergei]

if the restriction remains constant and pressure is increased, there will an increase in flow.

more mass more power. the original problem is that centrifugal type of compressors are very inefficient when demanded to output higher pressure to what they were designed. the extra heat it produces is combination of adiabatic heat and drag. Even if compressor was 100% efficient there would still be heat produced. due to thermodynamics the effect of increased flow is diminished when higher boost pressure is used.

Stating that increasing boost will not increase power is silly. as silly as stating that doubling boost will double power.


for example an engine that makes 160kw at 0.5bar can make 200kw at 1.2bar, nothing else changed. if same engine was subjected to 2.0bar then the power would probably be not far off the power made at 1.2bar (probably somewhere around 220kw), provided it holds up.


Think of boost as pressure in water hose with a restriction at the end, if that pressure is increased, the flow will increase.

[fielderz]

Boost is a measure of restriction at a given flow rate. That is all, it is not a measure of potential HP. Having more boost does not always mean you'll make more HP.

While this is true, if the only change is increased boost, then the result is more power.

No, if the only change is increased boost then there is a power decrease.

pV=nRT. (V)olume is fixed, R is a constant, so an increase (p)ressure without an increase in (n)umber of molecules results in an increase in (T)emperature only.

If however flow is increased along with boost then there may be a power increase, providing that the increase in temperature is not too great.

It's all about flow, boost is just easier to observe

Thats a pretty simplistic way of looking at it. You're not using the ideal gas equation properly. The assunmtions you have made are appropriate to a perfectly insulated cylinder (for example) at contant volume, where an increase in P must yield and increase in T or n or both, and vice versa.
In a fluid dynamical setting, ie an induction system, there is alot more going on. Yes there is near adiabatic heating from compression, and sure there is also heat added by the turbo, these will give you diminishing returns (decrease in the rate of increase of air mass flow rate) as the boost pressure is increased. But for a given tube size (port and valve area etc) and friction factor, an increase in boost pressure will increase flow, albeit in a non linear log function like curve.

If pV=nRT was all there was to thermal and fluid dymanics, then I know some guys who have wasted alot of time getting PhD's and all my text books would be a bit of a f**king waste of money.

[Lith]

Very nice to see some blooms of conversation exploring ideas going on here

[Grrrrrrr!]

Increasing molecular density is the technical term for pressure.

Its not as simple as that.
Put a cold gas in a container with a pressure sensor, seal it, then heat it. Pressure goes up, but density is the same, since no extra gas has entered the container and the container hasn't changed size.

Refer to the ideal gas equation: PV = nRT or PV = (m/M)RT. (P)ressure is a measure of how hard, and how often molecules are banging into things. More molecules (n) means more banging about, more (T)emperature means they have more energy, so they hit harder.

At constant temperature more density equals more pressure.
At constant density more temperature equals more pressure.

Increasing density without increasing temperature is impossible. See, when molecular density increases, it is being compressed. No matter how small or large the compression, there is heat created.

No, You cannot create heat, only transfer it. Conservation of energy and all that.

What you are describing is by compressing air you are packing the same amount of heat into a smaller volume, which increases the temperature. ( heat is energy, think of temperature as a indication of energy density.). Then you allow some heat to leave (via conduction through the sides of the vessel ie intercooler), and the temperature drops back to the original temperature. But you still have the same number of molecules in a smaller volume, therefore you can have a higher density at the same temperature