Turbo Spooling

I've always wondered why turbos will only produce boost when the engine is under a load and not when the engine is revved in neutral. I would think that they would produce equal levels of boost no matter what, as the amount of exhaust flowing through the engine for any given throttle position would be the same regardless of engine load. If someone could explain this i would really appreciate it.

I've always wondered why turbos will only produce boost when the engine is under a load and not when the engine is revved in neutral. I would think that they would produce equal levels of boost no matter what, as the amount of exhaust flowing through the engine for any given throttle position would be the same regardless of engine load. If someone could explain this i would really appreciate it.

I believe it has to do with the difference in the amount of fuel being used depending on the loading of the engine. For any given engine speed less throttle is required for less engine load and vice versa.

The amount of exhaust energy is going to be directly proportional to the amount of fuel burned. So less fuel used implies less energy (in the forms of temperature and pressure) to drive the turbocharger.

A while back I stumbled across this post from another forum in regards to the same topic:

http://www.eng-tips.com/viewthread.cfm?qid=99579

When you're revving the engine in neutral, you're only using a small amount of throttle, therefore only a small amount of air/fuel consumed. When its in gear and you're flooring it through to redline, the amount of air/fuel is much greater, creating a huge difference in the volume of exhaust going through the turbo.

The amount that the turbo spins is a result of the position of your foot. If your foot is to the floor and you are past the boost threshold RPM, you'll make much more boost than if your foot is only a 1/4 of the way down in neutral.

Excellent answers; thanks !!!!!!

When you're revving the engine in neutral, you're only using a small amount of throttle, therefore only a small amount of air/fuel consumed. When its in gear and you're flooring it through to redline, the amount of air/fuel is much greater, creating a huge difference in the volume of exhaust going through the turbo.

The amount that the turbo spins is a result of the position of your foot. If your foot is to the floor and you are past the boost threshold RPM, you'll make much more boost than if your foot is only a 1/4 of the way down in neutral.


turbos run on heat energy. Revving it in neutral even up to redline isnt going to make as much energy as opposed to taking it up to redline while driving the car because because it isnt under load.

There is also the component of TIME. Just revving the engine up in neutral takes maybe a second. With a cool exhaust, the turbo may need a couple seconds or so to spool up high enough to generate any boost at all. By this time you've already shut the throttle down. The turbo IS responding to the increased gas flow of the engine, but it needs some time.

I have floored my engine to rev it before and made zero boost. Why? The law of inertia.

does this mean that the ECU detects the load on the engine and then decides on the amount of fuel to be injected ?

The ECU(on EFI equipped cars) detects many things to determine how much fuel to inject.

There are two operating modes the ECU runs in:

1) Closed loop
2) Open loop

Closed loop is for drivability, fuel economy, emissions, ect. It uses input from the mass air flow sensor(MAFS), the O2 sensor, the coolant temp sensor, the throttle position sensor(TPS), some cars use a manifold absolute pressure(MAP) sensor, a knock sensor, and I'm sure I'm forgetting a couple. But this operating mode aims at daily driving. You know, your highway cruising, driving miss daisy, just everyday driving. It typically uses O2 sensor and TPS to determine what AF ratio to run at. It aims for 14.7:1 Air/Fuel(AF) ratio, but if you ever watch it on an AF gauge, it is constantly adding and subtracting fuel depending on signal from the O2 sensor. The AF ratio jumps back and forth between 14:1 and 15:1. Timing is generally run on a map based on air flow vs tps.

Open loop only uses the TPS and the MAFS. It determines how much fuel to use based on preset fuel and timing maps. For X amount of airflow at X throttle position, there should be X pulsewidth and X timing. The tables on paper look like multiplcation tables or charts. O2 sensor is disregarded here along with all other sensors.

oh, sorry but i didn't understand much, im not being rude or anything but does that mean the ecu detects load an controls boost ?

The ECU doesn't control boost on most cars AFAIK. Most of the time, a line is just run from a boost source(compressor housing, hotpipe, even IM) to teh wastegate. When you reach a certian boost level, the wastegate begins to bypass exhaust gas.

The more air you get in the engine, the more fuel is required. The ECU DOES control the amount of fuel.

oh, sorry but i didn't understand much, im not being rude or anything but does that mean the ecu detects load an controls boost ?

ECU's are complex things. I assume everyone knows how circuits and impedence works. An ECU detects signals from various sensors using one of two techniques. 1) voltage and no voltage (open and closed). 2) impedence amount. ECU's first off use impedence to detect the amount of air entering the engine via a MAF or MAP sensor. Next the ECU takes a reading from the throttle body, from the throttle-position sensor. The air enters the engine and is burned. Upon exiting the engine, the O2 sensor detects the level of impedence in due to the unburned or burned fuel. Other sensors are used by the ECU to control the engine also. These include the crank-position sensor, and in some cases a knock sensor (i'm pretty sure that most cars have these now).
The ECU controls the engine using sensor data in the following manner. Fuel is delivered into the engine based upon the MAF/MAP reading, and the throttle-body position sensor reading (and those gizmos that trick these sensors are useless, and i will explain why later). The reading from the O2 sensor is taken to determine if the sensors on the intake side are functioning properly. If the intake sensors have been tampered with (useless gizmos), or are faulty, the O2 sensor will allow the ECU to determine the correct amount of fuel needed for proper running. Next the crank-position sensor is used to detemine the piston location. This is used to determine spark timing and advance. The knock sensor is used as a precaution incase the CPS breaks or is tampered with, to prevent detenation.
Please note that i only mentioned the basic sensors, there are ALOT more. I hope that between this explaination, and nissianfanatics explaination, it will help you to understand how the ECU works.

Also note that when using forced induction, the set up you have is very depended upon what type of mass-air sensor you have on your vehicle. The reason is that a MAF reads the air passing by the vanes inside the sensor. Once the air has entered the system, the ECU expects it to stay there. Therefore the MAF must be located befoer the turbo (like the 1G talon)(to measure the amount of air being compressed), and your BOV MUST dump AFTER the MAF. This also means that without an ECU mod, you can't externally dump on most cars. If you have a MAP sensor, the MAP must be placed AFTER the compressor, but the BOV can dump externally, or before the compressor.
Hope the info helps.

The O2 sensor doesn't correct anything under WOT though. That is strictly maps based on, like Omega said, the MAFS and TPS. There are maps(they look like spreadsheets when on paper) with MAFS voltage and TPS voltage on perpendicular sides. Much like the multiplecation tables back in grade school where you take one number and another then follow the lines to find your product. Same thing, but when you follow the MAFS voltage(nuber 1) and TPS voltage(number 2) and follow each to find the product, the product is pulsewidth of the injectors or your timing value. Pretty simple once you understand it.

And like omega said, all sensors work off of resistance to the voltage applied. The ECU detects how much voltage was sent out as to how much came back. It then uses this figure to determine whats going on and where to make changes.

Most new turbocharged cars use electronic control of the boost pressure, it has infact been around since the early eighties.

Today engines tend to be equipped with a "torque demanding" control system. They work something like this:

The driver pushes the pedal, a sensor reads the location of the pedal and the ECU transforms this signal into a torque demand. The ECU recalculate this torque demand into an airmass/combustion and opens the throttle and control the boost so this airmass/combustion is reached by comparing with the actual value from MAF and/or speed density calculation (based on MAP, airtemp, engine speed and VE).
When the airmassflow is known the fuelflow can be calculated; fuel is added according to lambda 1 except at high load where the mixture is richer. This is done according to fuel maps. The ignition is controlled according to ignition maps, these maps will keep the ignition at optimum values when possible.

Modern system are also what is called "adaptive"; so if the ECU notes that the lambda isn't correct it can change a factor which multiplicates the basic fuel map, let's say that the factor is 1.01, then the fuel flow will be 1% higher than the basic map. Idle usually have a separate factor.

The cranksensor and phase sensor work together so ignition and fuel injection can be timed. The crank sensor is also used to calculate engine speed which is used to calculate the airflow when using speed density to mention one thing.
The cranksensor is also used for misfire detection, a misfire will be noted if there are speed irriegularities in the crankshaft speed. This can also be measured using ionization current.

A typical engine today uses one or more knock sensors and they can together with cam and cranksensors measure knock intensity on all induvidual cylinders. If knock is detected ignition timing is changed, if that isn't working the fueling is changed and if it still keeps knocking the boost is reduced. When the sensors note that the knocking has stopped the ECU starts going back again until it once again can sense knocking and then it all starts over. Ionization current can also be used to detect knocking, it can also replace the cam sensor (phase sensor).

There are of course lots of other functions in a modern ECU, I have just mentioned a few. There is also a large difference between older systems and what we have today. Most aftermarket system is also quite primitive compared to the latest systems used by the car manufacturers or the systems used by the racing industry.

And there are also knock sensors that retard timing a great degree once knock is present. The knock sensors on the KA24DE work like that. Only if they pick up very strong detonation will they retard timing, and it will be very drastic.

the KA has knock sensors?

Yep. Between #2 and #3 cylinder right below the head on the block.