Q: Why doesn't turbo make boost w/o load?
 

I remember seeing something in a previous post, but don't remember when or the title. anywho, why exactly is it that a car doesn't build boost under load? I would assume that since the engine is still running at the entire RPM band, it would still create the same amount of exhaust? Like with a turbo, when you run it without load (i.e. charge pipes disconnected) it spools the ---- outta it. why not the engine reving all crazy like and spooling the turbo?

(yah i know my grammar sucks. :P)

Re:Q: Why doesn't turbo make boost w/o load?
 

not as much exhaust is created, when not under a load. fre revving your engine doesnt use as much gas as when you put a load on it.

Re:Q: Why doesn't turbo make boost w/o load?
 

Ok your at part throttle... The B-fly valve is just cracked. Its creating a HUGE pressure drop lets say 12psi drop (thats its job to restrict the ammount of air passing by making a pressure drop.). So now you have pretty much atmospheric pressure on the upstream of it and only 2psig (atm-12psi) on the manifold side. So know when that piston moves down on the intake stroke its only drawing from 2psig. Thats not alot of air molecules and not a lot of exhaust to spin the turbine....translates into no boost.

At WOT the B-fly is wide open and makes almost no drop so at worst you have atm pressure and under boost you have full boost pressure being pushed into your cylinders...a lot more air...a lot more exaust...BOOST.

Re:Q: Why doesn't turbo make boost w/o load?
 

So then you could have it in neutral and hit wot (really quick on/off the throttle), will this create boost?
(i'm not boosted yet just wondering)

Re:Q: Why doesn't turbo make boost w/o load?
 

that original post was here:

https://www.homemadeturbo.com/forum/...threadid=16880

Re:Q: Why doesn't turbo make boost w/o load?
 

If your creating boost while not under a load is a sure sign that your turbine housing is too small

Re:Q: Why doesn't turbo make boost w/o load?
 

why? because of the overlap between the end of the exhaust stroke and the beginning of the intake stroke? you're saying the pressure is actually coming from the turbo manifold propagating all the way to the other side of the turbo? please explain i'm curious.

Re:Q: Why doesn't turbo make boost w/o load?
 

Also Curious?

Re:Q: Why doesn't turbo make boost w/o load?
 

no, it must be under load to boost

Re:Q: Why doesn't turbo make boost w/o load?
 

thats a damn good question why it dosent.. i thought about it for a while and dont know why it does.. wheres the engineering major in here to straighten this out

Re:Q: Why doesn't turbo make boost w/o load?
 

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.

Re:Q: Why doesn't turbo make boost w/o load?
 

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?

Re:Q: Why doesn't turbo make boost w/o load?
 

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.

Re:Q: Why doesn't turbo make boost w/o load?
 

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....

Re:Q: Why doesn't turbo make boost w/o load?
 

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. ;D

Re:Q: Why doesn't turbo make boost w/o load?
 

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 ;)

Re:Q: Why doesn't turbo make boost w/o load?
 

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!

Re:Q: Why doesn't turbo make boost w/o load?
 

I hate it when I think Iam smart ;)

Re:Q: Why doesn't turbo make boost w/o load?
 

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.
;)