relation of boost to power

[carl0s]

Is running higher boost the only way people gain power on their cars?

With the obvious exception of lighter and more free-moving parts of course.

 

If so, does this mean a single-turbo T78 Supra running 18 psi makes the same amount of power as a stock (okay, Hybrid) car running 18 psi?

 

I understand that power delivery would be different, but my question refers to where power comes from. With turbos, it's all just PSI, right?

[Scoboblio]

I think that a large turbo running 18psi will produce a lot more power than a smaller one running the same boost, because the larger turbo will be generating less heat and will be running in the middle of it's efficiency range, rather than being pushed.

 

I imagine that different cams could produce more power, but maybe at the expense of low down torque?

 

Wow.... that nearly DIDN'T sound like a load of muppet-mechanic bull

[carl0s]

I think that a large turbo running 18psi will produce a lot more power than a smaller one running the same boost, because the larger turbo will be generating less heat and will be running in the middle of it's efficiency range, rather than being pushed.

 

I imagine that different cams could produce more power, but maybe at the expense of low down torque?

 

Wow.... that nearly DIDN'T sound like a load of muppet-mechanic bull

 

Yes but the heat thing isn't really relavent, is it?

I mean, if it's hotter, then the pressure is less, if it's cooler, then the pressure is more, but I'm comparing 18psi to 18psi.

[carl0s]

I imagine that different cams could produce more power, but maybe at the expense of low down torque?

 

I think you're right, or I suppose it could go either way depending on the specifics of the cam designs.

 

I'd like not to dwell on ways to get power other than by raising boost though, as what I really wanted to know was does 18psi on a big single = 18psi on small hydrids.

 

Guess I asked it in a roundabout way It's just that there's a lot of modded cars out there, and when you look at it, they don't really do much other than raise boost, add intercoolers to keep that boost safe and increase fuel flow to fuel the added air. So it all just looks like 'run as much boost as possible and try to keep the other parameters in line with said boost'.

[Miguel]

A larger example 71 at 18 psi will create more power than stocks at the same psi due to it flowing more air . More air in the chambers with more fuel = more power. A 71 will compress more volume of air at 18 psi than the stocks will . Its about getting as much air in the cylinder as possible.

[Jake]

A larger example 71 at 18 psi will create more power than stocks at the same psi due to it flowing more air . More air in the chambers with more fuel = more power. A 71 will compress more volume of air at 18 psi than the stocks will . Its about getting as much air in the cylinder as possible.

I don't understand. Wouldn't compressing more air simply equate to more pressure?

[Miguel]

whats gonna fill a baloon faster 2 small fans at full chat or one big one that has more left in it ?. A larger single or even twins can compress more air due to them being able to move more volume of air at the same psi as smaller variants at the same psi.

[Jake]

whats gonna fill a baloon faster 2 small fans at full chat or one big one that has more left in it ?.

I honestly don't know.

How is 18psi from a single turbo better than 18psi from a pair of smaller turbos? (Apart from temperature matters)

[Miguel]

I honestly don't know.

How is 18psi from a single turbo better than 18psi from a pair of smaller turbos? (Apart from temperature matters)

 

Ok take a 61 at 18 psi then swap it to a 67 at the same psi and then jump up to a 76 at the same psi ..why do they produce more BHP ...becasue they can shift more volume of air at the given psi .

[Jake]

but if they can move more air it would result in more pressure, wouldn't it?

I mean, 18psi of pressure from a single isn't going to put anymore air into the cylinders than 18psi from stockers, is it?

[Miguel]

but if they can move more air it would result in more pressure, wouldn't it?

I mean, 18psi of pressure from a single isn't going to put anymore air into the cylinders than 18psi from stockers, is it?

 

Think water theory . A small 1 inch pipe at 6psi will fill up your sink from your tank in the atic. Now swap that pipe for a 10 inch one from the tank and wap it up to the same psi and how quick will your sink fill?

[Miguel]

But it will take longer to create the pressure in the 10 inch pipe with the same pump (lag)

[carl0s]

LOL I didn't know there'd been any replies here.

 

I had a think about what I said, and it didn't quite make sense. Hotter air is higher-pressure, that's why aerosol cans go pop when you heat them!

 

So when an aerosol can goes pop, 'cause it's been on top of the gas fire, well... it never got any more air molecules in it, but it's pressure went higher.

 

With the turbos, we want more air molecules in the chamber, not necessarily more air pressure. At a constant temperature, 18psi would give the same power whether it was from stockers or a T78, but the reality is that the temperatures of the air are different - so the 18psi from stockers is in reality 'not very much air, but at high pressure 'cause it's really hot', while the 18psi from a big T78 is.. a regular 'not overheated' 18psi.

 

So, i guess I answered my own question, with many thanks to HowStuffWorks

[carl0s]

Think water theory . A small 1 inch pipe at 6psi will fill up your sink from your tank in the atic. Now swap that pipe for a 10 inch one from the tank and wap it up to the same psi and how quick will your sink fill?

 

Oh yeah

 

So i guess what we're missing, is some kind of volume measurement.

 

This PSI/Bar measurement, without a temperature measurement to go with it, is surely pretty useless?

 

I'm visualising how much pressure one can create with the point of a needle, and how that same amount of power equates to fuck-all pressure when it's spread over something much larger.

[carl0s]

Okay, this is quite a headfuck, but I think the lesson I have learned here is that Pressure does not equal Volume.

[Jake]

Yeah but doesn't that just boil down to "A single turbo makes more power because it doesn't heat the air as much as stockers"? (at the same pressure)

 

I don't understand Mig's sink/pipes analogy because we measure boost pressure at the intake (the sink) rather than at the turbo (the pipe)

[Miguel]

Ok lets level the field and forget about the stocks. Lets say we work on a 1 turbo basis and the temps are controlled and set to a constant. So we have a 57 and run it at 1.6 bar would it produce the same power as say a 71 at 1.6 bar ? . No if this was the case we would only need to buy one turbo and just keep turning it up to the pressure we desire for the power. There comes a point on a turbo where it reaches it max flow and creates heat instead of flow ..but forget that for a second (which you have hit on ). Take a small straw and put a pressure gauge at the end of it and blow to get it to say 2 psi now take a 20mm copper pipe and do the same do you not have to blow more air down it it to hit the same pressure at the end . The same applies to turbos ,yes larger ones creates less heat at the constant psi due to them being able to shift more air which in turn helps to gain more power...but untimately its the amount of flow they can produce. I so hope this makes sence because im poo at typing it out and spelling.

[Miguel]

Yeah but doesn't that just boil down to "A single turbo makes more power because it doesn't heat the air as much as stockers"? (at the same pressure)

 

I don't understand Mig's sink/pipes analogy because we measure boost pressure at the intake (the sink) rather than at the turbo (the pipe)

 

Jake think about the water theory a bit harder. You dont meassure boost at the intake you meassure in at the plenum or the outlet of the turbo ...just like you would meassure it in the pipe from the pump from the tank to the end of the pipe to obtain 18psi of water pressure would take a little amount of water in a 1" pipe ( meassure in the midlle of the pipe beween the tank pump and sink) just like you meassure boost between the outlet of the turbo and the plenum ..then imagine how much water you would need to flow through the 12" pipe to obtain the same pressure. now the turbo IS the pipe and it needs to flow more air to produce the same psi....hence the engine will recieve more air at that given psi on a larger turbo . The water is the air flow and the pipe is the turbo ..it needs to move more air to hit the pressure. hence bigger turbos at the same psi will create more power.

[Jake]

Sorry Mig, too tired/pissed. I will try to get my head around it tomorrow. Thanks for your patience though.

[Miguel]

It will come to you in a Eurika moment

[normore1]

Engine power is directly related to air mass not pressure. Air pressure, temperature and volume are only important as they relate to air mass. The power generated by the explosion in the cylinder is determined by the mass of the fuel and air and efficiency of the burn. Assumming a proper fuel/air ratio somewhere close to stoichiometric (14.7:1) in both cases then power from a large single or twins will be equal if the mass is equal. For equal mass, pressures may be different if temperature is different. Conversely if temperature is different, mass will be different even if pressure is the same. This is the basic principles of Boyle's Law.

[JohnA]

This is a slightly advanced subject.

I have a page on my site named "Boost is Good" where it is tackled.

 

In a nutshell, boost by itself is nothing to be proud about, it is just a measure of the engine's reluctance to breathe (consume the air)

And pressure maps are based around airflow (cfm etc) because that's what's important (along with temps ofcourse)

 

On the same engine, at (say) 2 bar boost, a single 'big' turbo will have the potential for more power compared to a pair of 'smaller' ones, or a 'small' single.

Even if the other turbos are within their efficient range, the big one will still have an edge (potentially) because it is likely to have a larger A/R ratio in the exhaust, which is the main killer at this level of performance. "Potentially" means that the head and the cam timing (and mapping of course!) will have to take advantage of this A/R and shift the powerband accordingly.

 

(Just 2 Euro cents worth)

[neo1]

Go BIG.... If anything it puts a Grin on your face