Boost gauge readings?

[Texasjack]

Been trying to sort out the motor in my Model 10 ,has a 6v92 turbo,740 trans.I know I need the rack run. Had the turbo rebuilt,should the gauge(boost) show anything at idle? At rat chewed a hole in my airline, so now after fixing it and testing the gauge,( had my wife suck on the other end!) it wont show anything at idle or high idle.?

[Hartley]

Boost is Pressure not vacuum... -0 to 30 psi ? or so ...

Bet the wife wasn't too thrilled with that idea..

Try blowing some air in there with a low pressure air hose. You may
still have a leak somewhere.

I may be wrong but boost pressure only comes up good at higher rpm than idle.

Good Luck....

[Eagle]

No boost pressure until you put a load on the engine.

[larryh]

You will not have any readings unless moving and working the motor.

LarryH

[lostagain]

Could someone explain this to me, please.

How come there is only boost pressure when there is a load on the engine? Isn't the turbo always pushing air pressure into the intake when it is spinning?

And why do some set ups have a waste gate and some not?

The answer might be simple, but I can't picture it now.

Thanks.

JC

[DrivingMissLazy]

Could someone explain this to me, please.

How come there is only boost pressure when there is a load on the engine? Isn't the turbo always pushing air pressure into the intake when it is spinning?

And why do some set ups have a waste gate and some not?

The answer might be simple, but I can't picture it now.

Thanks.

JC

I'm gonna guess:
Turbo must be up to higher speeds (rpm) before any boost. It is not spinning fast enough at idle speeds.

 I do not believe load on the engine has any effect.

Waste gate is to maintain maximum boost on the engine. If the boost gets too high, the waste gate opens to reduce amount of boost.

OK, you experts, how did I do?
Richard

[Tim Strommen]

Pretty Good Richard,

The boost of a turbo depends on the ratio of the compressor turbine (intake/cool side) to the power take-off turbine (exhaust/hot side).

The output side is sized for the output volume and velocity of the engine's exhaust so that at a certain torque and speed is created on the turbo drive shaft.  The input turbine is sized so that it creates a certain pressure of boost for a given RPM/torque of the turbo drive shaft.  This may be a different (steeper) curve than the output turbine - and if this is the case, without a wastegate, the turbo would cause a run away boost situation.

A mechanical wastegate actuator's "signal line' (air pressure) is connected to the boosted output of the compressor stage and has a diaphragm acting against a spring that moves a shaft attached to a bypass valve on the output turbine's input and output.  When a preset boost pressure is reached, the shaft pushes on the waste valve which begins to allow exhaust gasses to escape without pushing on the exhaust turbine (util it reaches equilibrium).  This allows an exhaust turbine that is efficient at a lower air volume/velocity to be used in order to spool up a turbo sooner (by maintaining turbo RPMs without adding boost or lots of exhaust turbine load).  Too much boost will cause the piston to overcompress and you could push a piston right out the bottom of the mill during combustion - or blow the head right off the block (both very bad).

A common second turbocharger valve is the blow-off or recirculation valve on the intake side of a turbo.  This allows the exhaust turbine to continue to take power from the exhaust as the engine slows down without adding boost to the intake side of the engine (it either dumps the boost into the open air with a “wooosh” or lets air pressure go back to the intake of the turbine).  This allows for faster engine RPM transitions and reduces the turbo lag upon the re-application of throttle.

Cheers!

-Tim

[lostagain]

Thanks guys.

I have a boost pressure gauge in my Ford Powerstroke pick-up and the reading is significantly up for increased load, given the same RPM. Say going down the highway on cruise control and reading 5 psi of boost, then up a hill, same RPM and speed, and watch the boost go up to 12 or so. Low 20' pulling a loaded trailer. Where is that boost measured? Does that mean there is more intake air pressure at a heavier load than light load, given the same RPM? The turbo is spinning at the same RPM, or is it? Is there more exhaust gases to spin the turbo faster at bigger load? More fuel burned=more air? Would be nice to picture the fonctionning of that system.

Thanks again fellows for any insight.

JC

[Tim Strommen]

JC,

    Basic compressed gas theory (the name of which I can't recall).  When a specific volume of gas is raised in temperature, the pressure increases.

With the contribution of both temperature and volume to the intake air mass, due to the combustion of fuel, you get a higher post combustion in-cylinder pressure.  A lower engine load requires less burn time and thus fuel to over-come the resistance of the piston/drivetrain for the power stroke.  A higher engine load requires a longer burn time and thus a bit more fuel to over-come the resistance of the piston/drivetrain for the power stroke.  The longer the burn, the higher the in-cylinder temperature at the end of the power stroke (beginning of exhaust).  A higher pressure (due to high-load) being vented within the same time period (same engine RPM should have same exhaust valve opening time) as would a lower pressure (due to low-load) equates to a higher velocity of pressure equalization (high in cylinder, versus low post-turbine).  If you push the throttle harder like you would under a load - the in cylinder temps will increase and push harder on the exhaust turbine during the exhaust cycle (forcing more air into the manifold via the intake compressor due to the increased torque/RPM injected to the turbo's drive shaft by the power take-off turbine).  This will force more air into the intake manifold making the longer (more fueled) combustion more efficient producing more power (and this is why we use turbochargers).

This loading/temperature can be observed by the use of a pyrometer (thermocouple) on the pre-turbo exhaust (with the lower pressure after a turbo, the temperature will be reduced a bit, so won't be as accurate).

I believe several people here on the board will tell you that they use a pyro to gauge how effectively they are driving their engine (not over-loading it causing the exhaust temps to shoot >1800F) - so it can be said that exhaust temperatures reflect engine load (it's also why it's said that one should drive a DD 2-stroke hard to get it to operating temperature, as combustion temperature directly contributes to the block/water temperature).  Most of the after-market boost kits for a diesel pickups (Banks Power comes to mind) will use a pyro to control the propane/diesel injection without blowing the engine up.

Your boost shold be measured after the compressor but before the the intake manifold (this will reduce pulsation artifacts in the boost measurement due to intake values opening).  With your boost pressure up this should indicate that the turbo is spinning faster (unless the boost gauge is a "decoration" but I'd doubt that a manufacturer would waste the effort...)

Cheers!

-Tim

Forgive me if I "f'd" up any sentences - I'm having some trouble typing today... (I've edited this post at least three times already ) -Tim

[Stan]

Let me give it a try: If you are going up a hill at the same engine speed as on the level then you are using more fuel. More fuel equals more exhaust heat and more heat equals more pressure to the turbine which makes it turn faster giving more boost.

That is one of the reasons people wrap the exhaust system before the turbo.

[lostagain]

Thank you Tim for the in-depth explanation, and Stan for the simple but clear one. It is all starting to make sense in my little brain now.

As I have said in other posts, I am installing a turbo on my 4-71 DD this winter. It is the AireSearch that is spected for that motor. It is going to give me 12 to 14 lb of boost. I asked the DD mechanics if I should have a pyrometer and they said don't bother, it won't overheat. Do you guys think I should have one anyway? What temp would it run at under load with the turbo? (My Ford Powerstroke EGT goes to 1200 F under heavy load). I have no idea what it should be for the 4-71T DD. I understand that I should wrap the exhaust manifold, turbine and pipe to insulate. Should I wrap the compressor side too?

Anyway, your info is much appreciated. Thanks.

JC

[gg04]

Wrapped exhaust side showed hp increase,wrapped compressor side showed loss,held too much heat in charge..supposed to be bad for bearings to..Only motor I have had dyno time on was in my old race car now street car gas turbo motor..built horse power with wrap after intercooler,kept charge air from heat gain from underhood. gg

[Stan]

When I had a bus with a 4-71N I talked to a highly qualified DD man and his comments were that he would not recommend a turbo if there was a hired driver. If I was going to be the only driver, install a pyrometer and back off on the hills when the temperature started to climb. Of coarse, this isn't an issue if you have low compression pistons

[buswarrior]

Hello.

Lots of misconceptions regarding how a diesel is fueled. The amount of fuel delivered for each stroke is different, depending on what the governor is calling for. If we could see the fuel rack while we were driving, it would all make sence. Under throttle, it is constantly changing according to how well the engine is maintaining or building RPM.

Rise in the road, rack opens in proportion to what fuel it takes to maintain RPM being called for by the throttle position. Road falls, no fuel injecting as we coast.

And on idle, and even revving the engine when idling, uses only smallest little drip of injection.
(That's why you can't get a good bark out of it with the muffler removed unless you drive it! Hardly any fuel delivery!)

So, velocity out the exhaust (caused by a heat induced volume change of the exhausted burned fuel/air) changes as carefully described in the earlier post, according to load and throttle positioning.

Heat on the incoming exhaust side of the turbo is good, speeds up the spin, wrap to keep heat in and not wasted by shedding to engine compartment air.

Heat on the outgoing pressurized intake air is bad, hot makes for less molecules of oxygen in there. Leave pipes exposed to air. That's also why popularly an aftercooler or intercooler is used after the turbo, and before the intake manifold to cool the charge before inhalation by the cylinder.

Of course, wrapping the turbo pipes is a hotrodding trick and all the cautions to unintended outcomes that go with that apply!

happy coaching!
buswarrior

[NJT 5573]

from the drivers seat. when the engine idols the turbo is slowly spun by the gasses coming from the exhaust manifolds. a turbo charger has two wheels connected by a shaft. one wheel is spun by the exhaust gasses. the other wheel picks up fresh air from the aircleaner. the exhaust wheel spins slowly at idol because there is very little exhaust gas to spin it, you dont burn fuel until you put your foot into it. when you put the foot to it the extra fuel spins the exhaust wheel or compressor wheel faster. this spins the wheel on the intake or aircleaner side faster making air pressure or boost and forces lots of air into the engine. this is called boost