Hi guys, GTti mentioned this thread a few days ago and I thought it was worth posting to clear up some misunderstandings. The idea that a bigger turbo gives you more power at the same manifold pressure is as common as the torque/horsepower debate, pod filters and the advertised versus 0.050" nonsense.
As a way of intoducing myself, I can tell you I'm probably old enough to be your father, have modified engines for about 36 years and spent countless hours testing various combinations to see what effect they have.
The problem always faced with these arguments is the determination to adhere to preconceptions and inuendo. The internet is a great birthing place for myths and invariably an idea takes on a life of it's own, culminating in copious validation without any real substance.
The problem that GTti has been faced with is trying to get around the "my cousin's friend's brother put a bigger turbo on and it got more power" and he may well have, but that isn't the principle that was raised. The query as I understand it, is will 9psi in the manifold give more power using a bigger turbo than the stock unit?
Well fortunately for us there have been a lot of gyrogearlooses over the centuries who have had nothing better to do than muck around with physics and one of the pertinent laws that applies here is the Ideal Gas Law.
Siimply put the equation tells the story. PV =nRT. As one variable changes so must another or other variables.
In this argument we know the engine can only displace so much volume (V) during the suck stroke. So the volume is fixed per stroke.
The pressure (P) has already been contrained to 9psi. So the left hand side of the equation is constant.
The ideal gas constant (R) is also fixed as long as you don't introduce something other than what we breath.
For the argument we can also say the temperature (T) is the same and constant.
That leaves the value of "n" nowhere to go, but to also stay constant. The "n" is the number of gas mols present.
If the number of mols is the same then there won't be any more oxygen molecules and thus the combustion will be the same. If the combustion is the same the power per stroke will also be the same.... Myth busted.
A larger turbo may well be a better match for the engine, but the only variable it will change with a constant 9psi will be the temperature and thus the number of moles (density). The adiabatic efficiency curves on the turbocharger map clearly illustrate whether a compressor will elevate the gas temperature, by either choking or surging.
The key factor to remember is that the larger turbo will require more exhaust gas flow at the turbine, which means it will need more strokes per unit of time (rpm). It may accelerate the air flow better too, but at a constant engine speed 9 psi in the manifold is 9 psi with the same amount of oxygen (temperature being held constant) regardless of whether it's a turbo charger or a space shuttle compressor connected to the pipe.
As a way of intoducing myself, I can tell you I'm probably old enough to be your father, have modified engines for about 36 years and spent countless hours testing various combinations to see what effect they have.
The problem always faced with these arguments is the determination to adhere to preconceptions and inuendo. The internet is a great birthing place for myths and invariably an idea takes on a life of it's own, culminating in copious validation without any real substance.
The problem that GTti has been faced with is trying to get around the "my cousin's friend's brother put a bigger turbo on and it got more power" and he may well have, but that isn't the principle that was raised. The query as I understand it, is will 9psi in the manifold give more power using a bigger turbo than the stock unit?
Well fortunately for us there have been a lot of gyrogearlooses over the centuries who have had nothing better to do than muck around with physics and one of the pertinent laws that applies here is the Ideal Gas Law.
Siimply put the equation tells the story. PV =nRT. As one variable changes so must another or other variables.
In this argument we know the engine can only displace so much volume (V) during the suck stroke. So the volume is fixed per stroke.
The pressure (P) has already been contrained to 9psi. So the left hand side of the equation is constant.
The ideal gas constant (R) is also fixed as long as you don't introduce something other than what we breath.
For the argument we can also say the temperature (T) is the same and constant.
That leaves the value of "n" nowhere to go, but to also stay constant. The "n" is the number of gas mols present.
If the number of mols is the same then there won't be any more oxygen molecules and thus the combustion will be the same. If the combustion is the same the power per stroke will also be the same.... Myth busted.
A larger turbo may well be a better match for the engine, but the only variable it will change with a constant 9psi will be the temperature and thus the number of moles (density). The adiabatic efficiency curves on the turbocharger map clearly illustrate whether a compressor will elevate the gas temperature, by either choking or surging.
The key factor to remember is that the larger turbo will require more exhaust gas flow at the turbine, which means it will need more strokes per unit of time (rpm). It may accelerate the air flow better too, but at a constant engine speed 9 psi in the manifold is 9 psi with the same amount of oxygen (temperature being held constant) regardless of whether it's a turbo charger or a space shuttle compressor connected to the pipe.
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