TheSamba.com :: View topic - Why does a turbo boost only when the engine is under load?

Sat Jul 03, 2010 6:43 am

dlund wrote: The full explanation is much too detailed to post here...The power comes from the fact that the combustion process results in more gas molecules than are in the intake gas/air mixture.
This is patently false; the ratio of molecules before and after the reaction have absolutely no relationship to power.

Example 1: The combustion of hydrogen
2 H2 + O2 ---> HEAT + 2 H2O
You start with 3 molecules, and you release energy and are left with 2 molecules.

Example 2 (since you were talking out of your arse): The combustion of methane
CH4 + 2 O2 ---> HEAT + 2 H2O + CO2
You start with 3 molecules, and you release energy and are left with 3 molecules.

juki48 wrote: go take a thermodymamics class.
And you the same. Had you taken a thermodynamics class, you would be eating your Q's and W's right now. Half of you guys arguing against heat are arguing on the basis of TEMPERATURE; and the other half of us arguing for heat are arguing on the basis of ENERGY.

hydrocarbons + air ---> HEAT + H2O + CO2 +CO + NOx

Do you remember this from Thermodynamics class? How about from Chemistry class? What does HEAT mean in this basic representation of the combustion of hydrocarbons? This is the HEAT we're talking about. The HEAT OF COMBUSTION. This is the energy released during combustion.

Heat is simply a measure of the amount of energy in a substance or the amount of energy released by a reaction or combustion. When you subtract the useful work that has been performed (such as the expanding gases exerting pressure onto, and pushing down, the piston), you are left with heat <--- this is the leftover energy from combustion that has done no useful work yet.

This is the HEAT that drives the turbine; a representation of the energy leftover from combustion that did not impart any work on the piston. This HEAT is what causes gases to expand, increase in pressure, flow, and eventually perform work on the turbine. It does not mean that pressure and gas flow is not driving the turbine; it means that the HEAT is the sole determinant of the amount of work performed in this system.

Sat Jul 03, 2010 7:07 am

I agree that heat = energy. I also agrree that this energy helps spool a turbo. you need the energy released by combustion and you need the mass of the gasses to exhert a force on the turbine. i guess my point of view is heat energy alone will not do it but gas velocity and pressure drop across the turbo will. when you have both it works even better.

Sat Jul 03, 2010 9:00 am

juki48 wrote: i guess my point of view is heat energy alone will not do it but gas velocity and pressure drop across the turbo will.

Again, where does gas velocity and pressure come from? The heat released during combustion.

This is the crux of the problem; the combustion of hydrocarbons creates heat. All subsequent work that is performed, whether on the piston or on the impeller of the turbo, results from this heat.

Sat Jul 03, 2010 5:06 pm

When you have a bunch of hot molecules in a small contained space (combustion chamber/high pressure) and you release them into a larger less confined space (exhaust pipes) those molecules want to spread apart to a comfortable distance (expansion). As they expand they create pressure on the surrounding area (exhaust pipe) and those pipes direct those pressures to an even less confined space (atmosphere/low pressure). On the way there they go thru the turbo and spool it up. Higher heat means greater expansion which means more pressure which means more boost.

Sat Jul 03, 2010 5:22 pm

Boost is a function of load. If you're sitting at a light, idling, there is no boost. Stomp on the gas and there is, minus the spool up time. :D

Sat Jul 03, 2010 5:26 pm

chrisflstf wrote: Boost is a function of load. If you're sitting at a light, idling, there is no boost. Stomp and the gas and there is, minus the spool up time. :D
thanks for clearing that up :lol:

Sun Jul 04, 2010 8:22 am

Its not heat that drives the turbo. It is the volume of exiting gasses that turn the impeller. Hotter is not necessarly better. Hotter will provide more pressure but not an increase in air mass. You need a good balance. The exhaust gasses are not expanding very much after the exhaust valve opens. It is the volume of exhaust gasses created during combustion and the force at which the piston pushes out the gasses that spool up the turbine. A shorter stroke motor of the same size as a longer stroke motor will spool up the turbine faster.

Sun Jul 04, 2010 2:00 pm

Indeed, cfm and pressure drop.
I think we got it pretty well covered, I shall leave the combustion chemistry to you guys it's not my cup of tea.
How bout this for an creative example:
Weld a washer with, say, a 3/4 hole in it to the end of your tailpipe.
Now this washer is like some kinda rocket nozzle; it restricts flow, you'll have pressure drop, and what it does is accelerate the exhaust gas to high velocity.
You could put a gauge right before it and measure the pressure drop.
Ya get it? washer is like rocket nozzle. No pressure drop-no rocket action.
What the turbine does is harness this washer induced velocity, and convert it into work.
For a given washer size there will be a window of cfm it will work in. Not enough cfm and it won't create enough pressure drop or velocity. To much cfm and well, washer is too small if you hit the speed of sound! After that it's a horrible restriction probably.
Hahaha, get it!

Sat Jul 31, 2010 12:04 am

Okay, it appears my last post in this thread may have been overlooked.

Anyway....

Under what conditions would the turbo make 10 pounds of boost when there isn't any load? Is this a sign of a problem? Because that is exactly what I've been experiencing. I have a diagnostic vacuum/boost gauge (handheld type, not in the dash) which I hooked up to a port on the manifold.

I start the engine and the gauge reads vacuum until it gets up around 5 or 6K then the needle 'climbs' from vacuum to boost...as it transitions to boost the engine starts to run like shit...and then the needle slams into boost all the way into 10 territory...all the while the engine is sputtering and spewing blue smoke and struggling to keep rpms up. I'll hold the throttle at WOT but RPMs will fall back and hold around 4K rpm, vacuum/boost gauge needle flickers around 10 boost, blue smoke stinking up the garage.

Anyone care to take a stab at that?

2276 EFI w/ T3 Turbo, 009, Link fuel only efi.

Sat Jul 31, 2010 6:58 am

FireBug1966 wrote: Okay, it appears my last post in this thread may have been overlooked.

Anyway....

Under what conditions would the turbo make 10 pounds of boost when there isn't any load? Is this a sign of a problem? Because that is exactly what I've been experiencing. I have a diagnostic vacuum/boost gauge (handheld type, not in the dash) which I hooked up to a port on the manifold.

Where is the port on the manifold? Is it before the throttle plate? At the throttle plate? Or, well after the throttle plate? Only a port well after the throttle plate should be used for vacuum/boost reference. Where is the reference for your MAP sensor/fuel pressure regulator? Are you disconnecting the MAP sensor/FPR reference to hook up your vacuum gauge?

Quote:
I start the engine and the gauge reads vacuum until it gets up around 5 or 6K then the needle 'climbs' from vacuum to boost...as it transitions to boost the engine starts to run like shit...and then the needle slams into boost all the way into 10 territory...

Three general thoughts: too rich, not enough timing, or turbo is too small. Rich AFR and low ignition advance would contribute to combustion continuing into the header and effectively spooling up the turbo. This is a strategy used, in part, to build boost on the line for drag racing. I don't know anybody that tunes an engine around load-free revving to 6000 RPM in the driveway, so it's possible that your EFI tune could be causing this. Also, if your turbo is too small, it could spool up under these conditions at higher RPMs...certainly not outside of the range of possibilities.

Quote:
all the while the engine is sputtering and spewing blue smoke and struggling to keep rpms up. I'll hold the throttle at WOT but RPMs will fall back and hold around 4K rpm, vacuum/boost gauge needle flickers around 10 boost, blue smoke stinking up the garage.

That would be an oiling problem. You either have a bad oil seal in the turbo, your turbo is not draining properly (mounted too low, or not using a large enough drain line), or your oil is cold and you're forcing 80+ PSI of thick oil into your turbo and it simply cannot drain fast enough for the seals to prevent the oil from entering the compressor or turbine. You need to take care of the oiling problem before you can take care of the tuning problem.

Quote:
Anyone care to take a stab at that?

Aside from questioning why in the world you even care about how the engine revs in your driveway, there are a whole host of problems that can contribute to what you've described, and we don't know the specifics to tell you one way or another: ignition timing, air/fuel ratio, vacuum/boost reference location, turbo size, turbo location, drain line size, oil viscosity, oil temp when you are flogging this poor engine in your driveway, etc.

And I really REALLY hate to consider that you are doing this to a cold engine. Clues you've written in your post suggests this, and the simple fact that you're unable to equate blue smoke to an oiling problem completely baffles me.

Sat Jul 31, 2010 8:59 pm

Bro, put the turbo down- it's kicking your ass.

Sun Aug 01, 2010 4:23 pm

FireBug1966 wrote: Okay, it appears my last post in this thread may have been overlooked.

Anyway....

Under what conditions would the turbo make 10 pounds of boost when there isn't any load? Is this a sign of a problem? Because that is exactly what I've been experiencing. I have a diagnostic vacuum/boost gauge (handheld type, not in the dash) which I hooked up to a port on the manifold.

I start the engine and the gauge reads vacuum until it gets up around 5 or 6K then the needle 'climbs' from vacuum to boost...as it transitions to boost the engine starts to run like shit...and then the needle slams into boost all the way into 10 territory...all the while the engine is sputtering and spewing blue smoke and struggling to keep rpms up. I'll hold the throttle at WOT but RPMs will fall back and hold around 4K rpm, vacuum/boost gauge needle flickers around 10 boost, blue smoke stinking up the garage.

Anyone care to take a stab at that?

2276 EFI w/ T3 Turbo, 009, Link fuel only efi.

Didnt read any of the other posts. But, do you have a wideband? anywhere in this setup? What do the plugs look like?

Sun Aug 01, 2010 6:18 pm

FireBug1966 wrote: Okay, it appears my last post in this thread may have been overlooked.

Anyway....

Under what conditions would the turbo make 10 pounds of boost when there isn't any load? Is this a sign of a problem? Because that is exactly what I've been experiencing. I have a diagnostic vacuum/boost gauge (handheld type, not in the dash) which I hooked up to a port on the manifold.

I start the engine and the gauge reads vacuum until it gets up around 5 or 6K then the needle 'climbs' from vacuum to boost...as it transitions to boost the engine starts to run like shit...and then the needle slams into boost all the way into 10 territory...all the while the engine is sputtering and spewing blue smoke and struggling to keep rpms up. I'll hold the throttle at WOT but RPMs will fall back and hold around 4K rpm, vacuum/boost gauge needle flickers around 10 boost, blue smoke stinking up the garage.

Anyone care to take a stab at that?

2276 EFI w/ T3 Turbo, 009, Link fuel only efi.

when you have inadequate ignition advance and an inoperable wastegate. If you rev it to about 9800 rpm, these issues will go away.