Dumb Q #1: Benefits of Larger Turbo

[sdutton007]

Hi,

I realise that this sounds like an INCREDIBLY stupid question, but what difference does a bigger turbo actually make? I know it does, but I don't understand why...

Assuming your car is running the standard 9psi (on high):

As I understand it, a bigger turbo will be able to push out more air than a smaller one simply because it has more room, but surely the actuator is still going to open the wastegate once the pressure gets past 9psi?

Or does the turbo pressurise it by 9psi, then forces it into a smaller space (i.e. the inlet manifold) which makes the air higher pressure, then it is allowed back into a bigger space (the exhaust side of the turbo), so that the actuator only "sees" 9psi still, even though the inlet manifold could be seeing 10 or 11psi?
^^^ bear in mind that this is just a random idea which probably doesnt even make sense lol

so confused lol

 

[sdutton007]

oh yeah, i made a weird picture to help explain the b*ll*cks that im talking lol:

 

[GTti]

The MAP sensor takes it's reading from the plenum (Usually the neck) after the throttle body, just before it makes its way into the cylinders. The pressure reading here will be used to fuel the car (Also where a boost gauge is attached).

Prior to this there are many things that can create a pressure drop, intercoolers, hose, widths, lengths, angles etc.

So what you're saying works the other way round, more pressure will be seen by the actuator. You could be pushing 14psi at the turbo's compressor nipple that feeds the actuator yet only 10psi in the plenum due to pressure drops in the system which is to be expected.


A larger compressor will mean the turbos efficiency range will change and shift the power band up in the rev range, it will take more exhaust velocity to spool thus also creating more lag and less torque. It will most likely generate less heat when compressing air making the charge more efficient and cooler which is probably where the extra power comes from.


Pressure doesn't change regardless of turbo 9psi is 9psi.

Temperature, density are most likely to be where changes are.

 

[villen25]

so a larger turbo gives you less torque... heres another "dumb" question what exactly what is the exact difference between hp and torque(uses)?

 

[sdutton007]

so a larger turbo gives you less torque... heres another "dumb" question what exactly what is the exact difference between hp and torque(uses)?

people always say torque is to do with acceleration, but theyre talking out their a$$ in my opinion.... u try doing 0-60 in a typical diesel. let me kno when u get there if u dont die of boredom first....


torque is basically how well it can pull weight. Thats why they often use diesel's for towing caravans and trailers.
HP is the power of the engine - HP/weight affects ur acceleration, HP affects your top speed.


There is an equation that links HP and torque, but its not really worth getting into.

(This should cause a nice debate lol)

 

[sdutton007]

The MAP sensor takes it's reading from the plenum (Usually the neck) after the throttle body, just before it makes its way into the cylinders. The pressure reading here will be used to fuel the car (Also where a boost gauge is attached).

Prior to this there are many things that can create a pressure drop, intercoolers, hose, widths, lengths, angles etc.

So what you're saying works the other way round, more pressure will be seen by the actuator. You could be pushing 14psi at the turbo's compressor nipple that feeds the actuator yet only 10psi in the plenum due to pressure drops in the system which is to be expected.

A larger compressor will mean the turbos efficiency range will change and shift the power band up in the rev range, it will take more exhaust velocity to spool thus also creating more lag and less torque. It will most likely generate less heat when compressing air making the charge more efficient and cooler which is probably where the extra power comes from.

Pressure doesn't change regardless of turbo 9psi is 9psi.

Temperature, density are most likely to be where changes are.

So when my boost gauge (which is run from the same pipe as the factory boost gauge is) reads 9psi, what is at 9psi? the plenum or turbo or an arbitrary point inbetween?

 

[Rory]

Bigger turbo = bigger volume of air = More power

The more air and fuel you get into the cylinders the more power you will make.
With a bigger turbo @ 9psi, you will geta lot more volume of air into the cylinders, than you would with a small turbo @ 9psi.

Torque is what pulls you along the road, not horsepower.
Torque is what makes your car feel punchy.

 

[GTti]

The more air and fuel you get into the cylinders the more power you will make.
With a bigger turbo @ 9psi, you will geta lot more volume of air into the cylinders, than you would with a small turbo @ 9psi.

Can you explain this, as I don't think it's possible and is a misconception people have about bigger turbos.

Density is calculated from temperature and pressure
Mass is calculated from density and volume


Pressure is a variable as is temperature. There is no extra air mass flowing into the engine. The mass of air can be cooler or hotter but will remain the same regardless of the turbo.

Cooler mass at high range is where the power comes from, but you're also changing the VE of the engine - which the ECU doesn't know about.

 

[Adam_Glanza]

yeah if you think of it as pressure, the psi is the pressure of the air.

so amagine it like a fan, a bigger fan at the same pressure as fan smaller than it will always push alot more air because of its size

 

[Dan3SGTE]

exactly what rory said.....

The pressure is still the same "9PSI" but the volume of air is increased with the larger inlet/compressor wheel..... the more air forced in the engine, the more fuel your car wants so you compensate by adding the extra fuel your car is desiring then your Power goes up.....



The bigger turbo doesnt make the air more dense as appose to a small one, just gathers "more" air at the same pressure..... a larger volume of air....

 

[Gee]

If a bigger turbo does not take in more air at the same PSI as a smaller turbo, then how does it make more power at the same boost?

 

[Toplap]

so a larger turbo gives you less torque... heres another "dumb" question what exactly what is the exact difference between hp and torque(uses)?

a torque (τ) is a vector that measures the tendency of a force to rotate an object about some axis

1 hp ≡ 33,000 ft·lbf per work minute

how to derive HP from TQ

P/hp = [r/(ft .lbf)][w/(r/min)] / 5252

 

[Dan3SGTE]

If a bigger turbo does not take in more air at the same PSI as a smaller turbo, then how does it make more power at the same boost?

It does produce more air, at the same pressure.... There is no other explanation to it....

Its larger allowing it to flow a wider area of air through the compressor wheel, but the pressure does not have to change...

 

[Gee]

Makes sense to me.

 

[Rory]

Can you explain this, as I don't think it's possible and is a misconception people have about bigger turbos.

Density is calculated from temperature and pressure
Mass is calculated from density and volume


Pressure is a variable as is temperature. There is no extra air mass flowing into the engine. The mass of air can be cooler or hotter but will remain the same regardless of the turbo.

Cooler mass at high range is where the power comes from, but you're also changing the VE of the engine - which the ECU doesn't know about.

I'll do my best here,

A bike tyre at 30 psi will hold less air than a car tyre at 30 psi.
Its all about area, the bigger the area, the more air.

Same as a turbo, bigger turbo, more air.

This is my thinking on the matter.

 

[GTti]

It doesn't! It flows the same!

Air mass is calculated from density & volume and density is calculated from temperature and pressure... HOW can it flow more air at the same pressure. It can't!

"The manifold absolute pressure measurement is used to meter fuel. The amount of fuel required is directly related to the mass of air entering the engine."

"The mass of air is proportional to the air density, which is proportional to the absolute pressure. Engine speed determines the frequency, or rate, at which air mass is leaving the intake manifold and entering the cylinders (Volume)"

So if the pressure is the same (9psi) with an intake temperature of 20c that means the density of any turbo at 9psi is the same. Mass is calculated from density and volume (engine capacity/rpm). It will be the same mass of air entering the engine - it's impossible for there to be more.

So it makes the same power as long as both are in their efficiency range.

I doubt a TD04 at 9psi would make more power than a CT9 at 9psi.

A TD04 will make more power than a CT9 at 15psi due to aditional heat and its efficiency.

 

[Dan3SGTE]

rory your spot on mate..... Fact, that is the anwser.

 

[GTti]

Remember also - a cooler charge will create more power even if the pressure is the same.

Better combustion = more power.

 

[GTti]

A bike tyre at 30 psi will hold less air than a car tyre at 30 psi.

No because in this instance the capacity of the engine is the same (1300cc).

30 psi in 1300cc = x mass
30 psi in 1300cc = the same mass

(Assuming temperature is the same)

On your example the capacity of two tyres is different...

 

[Dan3SGTE]

Ok... abbotts old set up.. was running a HKSGT25 making 194BHP @10PSI.... CT9's make what at 10PSI? 150-160BHP.....?? a TD04 would make more power then a CT9 at 9psi... im sure someone on here made 160hp @8psi on a TD04.....