help on understanding turbo sizes - GT-R Register - Official Nissan Skyline and GTR Owners Club forum

D4T · 29th October 2005, 11:40 AM

Hi all,

I wonder if someone can answer my hypothetical question?

If you fitted a T67 on a GTS-T and set it to produce 1.1bar of boost, would it produce less or equal amounts of torque and bhp at the top end as say a T78 at 1.1bar?

Reason i ask is because someone told me the T78 would produce more power but i dont understand how (they did a bad job of explaining!), you still have the same volume of air space within the intake pipework and intake manifold etc. so how can it? essentially the air is still pressurised the same amount right?

Many thanks in advance,

D4T

Ajax · 29th October 2005, 02:52 PM

The way i see it is that the T78 will move more air at the same presure as the smaller T67
As an example on mine, a pair of HKS GTRS turbo's will pump more air (volume) than the standard turbo's set at the same boost presure.

NISFAN · 29th October 2005, 05:02 PM

Inlet boost pressure is only one half of the deal. Pressure differential is the real value.
A bigger Turbo will produce more power on the same engine at the same boost level, because the Exhaust turbine will be less restrictive. This means exhaust manifold pressure will be less, and consequently the cylinder chamber pressure will be less too, allowing more fuel air mixture into the engine, thus more engine power.

drewby · 29th October 2005, 05:11 PM

nisfan baby yu well, as this will help both of us

do you kno the hp increase per psi drop in the exh manifold eg % as you kno a am guna measure my drop on my new tubs, cheers m8y, D

NISFAN · 29th October 2005, 05:33 PM

Hi drew,
It is not a straight line value. A smaller housing becomes increasingly more restrictive with higher flow in a non linear way. So the initial boost pressure would have to be referenced.
Also, as you can't do much to influence the back pressure for a given exhaust wheel/housing combo, so there's not much point in the values in any case.
You can fit a less restrictive exhaust side, but that usually means the spool up characteristics will change too, so you just have to experiment with what is best suited to your needs. (Yeah right, like we all have 20-30 Turbo specs lying around to play with

)

We can all only dream of positive pressure differentials like the F1 cars had.

I still find it hard to compute that you can generate more inlet manifold pressure than you have in the exhaust manifold. Seems like your getting something for nothing some how.

drewby · 29th October 2005, 05:44 PM

well when i measuse my new tubs, say at 7k at max boost, if there is may be 3 -5 psi drop from the std tubs, this must give an extra % increase in power, .D

D4T · 30th October 2005, 12:29 PM

ah i see! thanks for that Nisfan

it makes perfect sense now (except the F1 bit)

29th October 2005, 11:40 AM	#1 (permalink)
D4T GTR Register Member Join Date: Jun 2004 Location: Sussex Posts: 427	help on understanding turbo sizes Hi all, I wonder if someone can answer my hypothetical question? If you fitted a T67 on a GTS-T and set it to produce 1.1bar of boost, would it produce less or equal amounts of torque and bhp at the top end as say a T78 at 1.1bar? Reason i ask is because someone told me the T78 would produce more power but i dont understand how (they did a bad job of explaining!), you still have the same volume of air space within the intake pipework and intake manifold etc. so how can it? essentially the air is still pressurised the same amount right? Many thanks in advance, D4T

29th October 2005, 05:11 PM	#4 (permalink)
drewby GTR Register Member Join Date: Jan 2004 Location: sheffield Posts: 587	psi nisfan baby yu well, as this will help both of us do you kno the hp increase per psi drop in the exh manifold eg % as you kno a am guna measure my drop on my new tubs, cheers m8y, D

29th October 2005, 05:44 PM	#6 (permalink)
drewby GTR Register Member Join Date: Jan 2004 Location: sheffield Posts: 587	psi & hp well when i measuse my new tubs, say at 7k at max boost, if there is may be 3 -5 psi drop from the std tubs, this must give an extra % increase in power, .D

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29th October 2005, 02:52 PM	#2 (permalink)
Ajax GTR Register Member Join Date: Jun 2005 Location: Plymouth Posts: 420	The way i see it is that the T78 will move more air at the same presure as the smaller T67 As an example on mine, a pair of HKS GTRS turbo's will pump more air (volume) than the standard turbo's set at the same boost presure.

29th October 2005, 05:02 PM	#3 (permalink)
NISFAN GTR Register Member Join Date: Oct 2003 Location: Herts Posts: 1,696	Inlet boost pressure is only one half of the deal. Pressure differential is the real value. A bigger Turbo will produce more power on the same engine at the same boost level, because the Exhaust turbine will be less restrictive. This means exhaust manifold pressure will be less, and consequently the cylinder chamber pressure will be less too, allowing more fuel air mixture into the engine, thus more engine power.

29th October 2005, 05:33 PM	#5 (permalink)
NISFAN GTR Register Member Join Date: Oct 2003 Location: Herts Posts: 1,696	Hi drew, It is not a straight line value. A smaller housing becomes increasingly more restrictive with higher flow in a non linear way. So the initial boost pressure would have to be referenced. Also, as you can't do much to influence the back pressure for a given exhaust wheel/housing combo, so there's not much point in the values in any case. You can fit a less restrictive exhaust side, but that usually means the spool up characteristics will change too, so you just have to experiment with what is best suited to your needs. (Yeah right, like we all have 20-30 Turbo specs lying around to play with ) We can all only dream of positive pressure differentials like the F1 cars had. I still find it hard to compute that you can generate more inlet manifold pressure than you have in the exhaust manifold. Seems like your getting something for nothing some how.

30th October 2005, 12:29 PM	#7 (permalink)
D4T GTR Register Member Join Date: Jun 2004 Location: Sussex Posts: 427	ah i see! thanks for that Nisfan it makes perfect sense now (except the F1 bit)