guy above got it right.. it takes alot more power to get the car moving then to just rotate the motor. more gas.

if your in neutral u can rev pretty high without even pressing the gas very far.. but when in gear wont happen, because u have a a hell of a lot more friction, the road.

 

when i rev in neutral i can get almost 1.5 psi on my boost gauge which is referenced to the manifold. how does that happen?

 

This is becuase your turbine housing is too small. You probably have a very fast spool up and than it doesn't produce much power after that. A bigger turbine housing creates more power but needs more exhaust gases at a higher velocity to produce boost.

 

so the evos the eclipse's and the sti's have too small of a turbine housing, cuz boy oh boy do they boost at idle.
I do too, 1.5 lbs. thats with the td04hl turbo, yes its small, but doesnt die out til....i dunno never took it past 7.5 prm....

 

It is not the exhast flowing past the turbine that spins it, if it was the more you reved it the more boost you would see. It is the expansion of the exhast gasses (heat) created by engine load, not rpms.

 

Actually According to the first law of thermodynamics the work output of a turbine is equal the the mass x enthalpy change + the change in kinetic energy + change in potential energy. However the enthalpy(heat) change is very small compared to the significant change in kinetic energy(---- that's in motion). Therefore, the velocity of the exhaust gases is what causes all the work and creats boost.

Heat is a very BAD way to produce power. Heat is VERY unorganized and does not create too much power. Think of a heater compared to a stream of water, the stream of water would turn a fan blade ALOT faster than a shitload of moving heat.

I thought this was common knowledge we all learned in our Mechanical Engineering Thermodynamics classes :P

 

Thats why i let you guys think. I just read, memorize, and act like i know what i'm talking about :P j/k, thanks everyone!

 

I hate it when I think Iam smart

 

I think a couple of guys were on the right track, but nobody has hit it yet. Unless I misread someone's post, in which case I apologize. It has to do with the relation between revs and throttle position. You need to know two things.

1. Every revolution, the engine is going to displace a certain amount of air. At, higher revs, it is displacing it more quickly.

2. Throttle position determines how much air makes it thru the TB and into the intake manifold where it registers as vacuum or boost.

If your revs are high, but your throttle is mostly closed (what happens in neutral where you barely have to blip the throttle to rev it high), air has a hard time getting thru the TB and into the manifold to replace all the air being displaced by those high revs. Thus you have vacuum, turbo or no.

If your revs are low relative to throttle position (under a load), the air can easily fill the intake manifold faster than it is being emptied by the intake/combustion/exhaust process, so you have boost.

If you floor it, under a load at low rpms, you will quickly build boost (once the turbo spools), and then depending on the size of the turbo, the boost level may drop as you enter high rpms where it is having trouble keeping the flow high enough to supply the engine.

So, you only build boost under a load, because it is all about Throttle position vs rpms.