why does a turbo spool faster with more load

I understand how the exhaust gets the turbo spining, but why is more load such as lower numercally gears like 3.42 better than 4.10 gears. I figure built up exhaust would help get the turbo spooling, is this what happens when the car has more load?

Originally posted by D James
I understand how the exhaust gets the turbo spining, but why is more load such as lower numercally gears like 3.42 better than 4.10 gears. I figure built up exhaust would help get the turbo spooling, is this what happens when the car has more load?

Yes, more load = more exhaust gasses produced.

Rich krause

More heat generated under load.

Heat get it spinning faster like stated. The more load, the more heat produced.

Originally posted by blueta
Heat get it spinning faster like stated. The more load, the more heat produced.

Agreed.

Forget everything you've heard about exhaust pressure spinning the turbo. HEAT is what drives the turbo. Exhaust backpressure is a by-product.

Mike

thanks guys-I finally understand it now:)

When You floor it you get more more exhaust gas from more air and fuel being let in the engine and burned.

When under load the motor is doing more work. The resulting extra waste energy moving out of the exhaust creates more potential energy to move the impeller.

Think of it this way. If the back wheel of a bicycle is in the air you can pedal and get the tire moving easily. Now make yourself do more work by putting the back tire on the ground. You exert more energy to do the same amount of work when you're under load. The difference is that the extra heat energy you exert as a byproduct of that work radiates into the air instead of moving through a pipe.

-Chris

It isn't heat per se. It's the volume of the exhaust gasses. Which will spin a turbine faster, a 383ci V-8 at WOT or a 2L 4 cyl? The temperature of the exhaust will be the same. Heat is what causes the expansion of the exhaust gasses, but it is not the heat per se that drives the turbine.

Rich Krause

I'm gonna state that is not a true comparo, almost like 'apples to oranges' here. I believe a better comparo, would be an engine operating at 'X' rpm on a level road, compared to an engine operating at the same 'X' rpm, but going up a hill, while pulling a trailer. Which scenario will generate a higher exhaust gas temp? Which scenario will generate more/faster responding boost from turbo?

If you really want the science of it, here goes. The power output of a turbine (exhaust side of a turbocharger) is:

Wdot = mdot X (H1 - H2) X n

mdot = mass flow rate of exhaust through the turbine.
H1, H2 = Enthalpy of the exhaust gas, a function of temperature.
n = efficiency of the turbine.

Mike