Does turbo rob engine power too?

[Mario]

I was just thinking about this: a supercharger is conected to a pulley on the engine which spins the shaft blah blah blah... well, with a turbo you have a turbine that must be spin from the exhaust gases... wouldn't this create a backpressure that could theoretically hold the engine back from power as well intil you hit full boost?

[fodo]

yes

[David]

yea, but its not as bad as a supercharger, not even close.

[Mario]

Thanks guys. Any actual comparison's... technical data or otherwise...?

[john]

It's a dynamic relationship. The larger the turbine housing, the more flow & less backpressure are present. And the larger the turbine housing, the more lag experienced.

A comparison between the 2 is pretty impossible, IMNSHO. Selecting a turbo for a given motor requires a lot of homework.

[HONDA GHANDI]

what john said. I would also like to add that a turbo does not operate simply by the ex gasses just "blowing" past the turbine blades causing them to spin. Heat expansion is the real driving force behind the turbo. On a small, restrictive turbo it is forcing most of the heat energy to be spent in the turbo. This explains the quick boost times and lower RPM efficiency range.

[exciv2000]

God dammit, once again I can't find that fucking article from sport compact car. Basically what it said was that a turbo uses up something like 15 HP at idle, due to the backpressure, but uses about 1 HP at WOT at 6000 rpm. A JRSC, on the other hand uses about 1/2 of 1 HP at idle, and 9 HP at WOT at 6000 RPM. The numbers probably aren't exact, but that's the closest my memory allows right now.

[HONDA GHANDI]

I got a buddy with a dynamic flowbench. For about $300 an hour we can test the theory and find results that are definite.


Maybe not.

[ryanman]

haha

[Blackice]

geee only 300$

[saabracr]

Due to pressure differential between the intake and exhaust manifolds, the turbo hurts the engine's volumetric efficiency. It does not directly rob power in the manner that a supercharger does. It basically puts constraints on the engine's performance envelope due to the extra heat and pressure in the exhaust manifold. This is why having a downpipe etc. that promotes flow creates so much more power and consistency than what is available when relying on a stock turbo discharge setup.

Check out my solution to this at the next meet if you want a good laugh.

Andy
91 SAAB SPG
some ghetto-functional shit going on

[HONDA GHANDI]

But it overcomes the loss of VE by increasing the air density. The gains compensate by far for the loss.

[MadMax95]

Basically what it said was that a turbo uses up something like 15 HP at idle, due to the backpressure, but uses about 1 HP at WOT at 6000 rpm.

That doesn't make sense. At idle, you aren't even flowing enough gas through the turbo to spin it effectively. If the turbine isn't spinning, there isn't back pressure. How can you be losing 15hp at idle?

[exciv2000]

That doesn't make sense. At idle, you aren't even flowing enough gas through the turbo to spin it effectively. If the turbine isn't spinning, there isn't back pressure. How can you be losing 15hp at idle?

at idle the turbo is the backpressure, particularly if the turbo isn't spinning at idle.

[MadMax95]

at idle the turbo is the backpressure, particularly if the turbo isn't spinning at idle.

You need back pressure to spin a turbine. If the turbine isn't spinning... no back pressure.

[exciv2000]

no, you need exhaust gases and heat to spin a turbine. I say again, the turbo is the backpressure.

[MadMax95]

no, you need exhaust gases and heat to spin a turbine. I say again, the turbo is the backpressure.

What's the difference between "Drive Pressure" and what you're calling "Back Pressure"?

[phil]

not sure what you mean by "drive pressure" but back pressure is when air hits an obsticle and is forced back. Think of your throttle body. when you let off the gas air still flows through the intake to the throttle body, but the throttle body is closed and blocks the flow of air, therefore it creates backpressure. the same would happen if a turbo is not spinning much.

[12seccivy]

You need back pressure to spin a turbine. If the turbine isn't spinning... no back pressure.

I think he was refering more to blockage of exhaust flow..
It is easier to block exhaust at idle than at higher Rpm.
I cant imagine a turbo losing that much at idle, but then I dont know much about turbos to begin with..

[HONDA GHANDI]

What's the difference between "Drive Pressure" and what you're calling "Back Pressure"?


Back pressure is air flow resistance causing slight pressure increases in exhaust systems.

Drive pressure is the pressure created by heat expansion of the still burning gasses of the exhaust itself. This is what causes a turbo to work. You can have all the airflow and pressure you want in a turbo but if you have no heat expansion (drive pressure) it will not work efficiently. If at all.