Nissan 200SX Club - South Africa

Ok Here is some more info from a engine rebuild libary.

http://www.aa1car.com/library/map_sensors.htm

HOW A MAP SENSOR WORKS

MAP sensors are called manifold absolute pressure sensors rather than intake vacuum sensors because they measure the difference in pressure between the outside atmosphere and the vacuum level inside the intake manifold.

Ambient air pressure typically varies from 28 to 31 inches of Mercury (Hg) depending on your location and climate conditions. Higher elevations have lower air pressure than areas next to the ocean or someplace like Death Valley, California, which is actually below sea level. In pounds per square inch, the atmosphere exerts 14.7 PSI at sea level on average.

The vacuum inside an engine's intake manifold, by comparison, can range from zero up to 22 inches Hg or more depending on operating conditions. Vacuum at idle is always high and typically ranges from 16 to 20 inches Hg in most vehicles. The highest level of vacuum occurs when decelerating with the throttle closed. The pistons are trying to suck in air but the closed throttle chokes off the air supply creating a high vacuum inside the intake manifold (typically four to five inches Hg higher than at idle). When the throttle is suddenly opened, as when accelerating hard, the engine sucks in a big gulp of air and vacuum plummets to zero. Vacuum then slowly climbs back up as the throttle closes.

The reason why MAP sensors measure pressure differential rather than vacuum alone is because atmospheric pressure changes with the weather and elevation. Since this affects the balance of the air/fuel mixture, the computer needs a way to detect the changes so it can compensate. Some vehicles use a "baro" sensor to measure barometric pressure (that's meteorologist lingo for atmospheric air pressure) and a vacuum sensor connected to the intake manifold to measure intake vacuum. The computer compares the readings, calculates the difference and makes the necessary fuel mixture and timing adjustments. But it's easier to let the MAP sensor measure the difference. On some vehicles, the MAP sensor is also used to check barometric pressure when the ignition is first switched on. This is done as a sort of baseline calibration check.

On turbocharged and supercharged engines, the situation is a little more complicated because under boost there may actually be positive pressure in the intake manifold. But the MAP sensor doesn't care because it just monitors the difference in pressure.

On engines with a "speed-density" electronic fuel injection system, airflow is estimated rather than measured directly with an airflow sensor. The computer looks at the MAP sensor signal along with engine rpm, throttle position, coolant temperature and ambient air temperature to estimate how much air is entering the engine. The computer may also take into account the oxygen sensor rich/lean signal and the position of the EGR valve, too, before making the required air/fuel mixture corrections to keep everything in balance. This approach to fuel management isn't as precise as systems that use a vane or mass airflow sensor to measure actual airflow, but it isn't as complex or as costly either.

Another advantage of speed-density EFI systems is that they are less sensitive to vacuum leaks. Any air that leaks into an engine on the back side an airflow sensor is "un-metered" air and really messes up the fine balance that's needed to maintain an accurate air/fuel mixture. In a speed-density system, the MAP sensor will detect the slight drop in vacuum caused by the air leak and the computer will compensate by adding more fuel.

On many GM engines that have a mass airflow sensor (MAF), a MAP sensor is also used as a backup in case the airflow signal is lost, and to monitor the operation of the EGR valve. No change in the MAP sensor signal when the EGR valve is commanded to open would indicate a problem with the EGR system and set a fault code[/b]

i'm with Gary on this one, I run a dicktator and all it cares about is the actual (absolute) pressure/vacuum in the intake manifold. if i have 1bar pressure in the manifold its going to respond the same to that 1bar regardless of where i am.

I think everyone is getting confused between absolute pressure and atmospheric pressure.

Atmospheric pressure at the coast is 1.01Bar Absolute pressure. 0Bar Absolute is a 100% vacuum (what is in space). These sensors are calibrated to Absolute pressure so yes at the coast it can reference atmospheric pressure. Now ALOT of tuners/people calibrate there sensors to atmospheric pressure (I am guilty of this one), the just set the sensor to 0Bar boost when the sensor in the car - this IS wrong. You need to plug a pump onto the sensor and calibrate it to Absolute pressure, this will keep the running the saem no matter where you are.

Sorry to say and this doesn't happen a lot but Gary is correct!

Sorry to say and this doesn't happen a lot but Gary is correct!

What you mean, I AM ALWAYS RIGHT. Even when Im wrong I am right, there is no other way but my way .

Owwwww....

Sorry read the other posts now, your working on the 'Ideal Gas Law" calculation:

There are still somthing about this I dont understand like how does a car maker work out Moles? (no the kind that live in the ground) beuase you need that to do volume.

An ideal gas can be characterized by three state variables: absolute pressure (P), volume (V), and absolute temperature (T).

PV=nRT

n = number of moles
R = universal gas constant = 8.3145 J/mol K
N = number of molecules
k = Boltzmann constant = 1.38066 x 10-23 J/K = 8.617385 x 10-5 eV/K
k = R/NA
NA = Avogadro's number = 6.0221 x 1023 /mol

But I still don’t understand because for gases, pressure is sometimes measured not as an absolute pressure, but relative to atmospheric pressure; such measurements are called gauge pressure (also sometimes spelled gage pressure).[1] An example of this is the air pressure in an automobile tire, which might be said to be "220 kPa", but is actually 220 kPa above atmospheric pressure. Since atmospheric pressure at sea level is about 100 kPa, the absolute pressure in the tire is therefore about 320 kPa. In technical work, this is written "a gauge pressure of 220 kPa". Where space is limited, such as on pressure gauges, name plates, graph labels, and table headings, the use of a modifier in parentheses, such as "kPa (gauge)" or "kPa (absolute)", is permitted. In non-SI technical work, a gauge pressure of 32 psi is sometimes written as "32 psig", though the other methods explained above that avoid attaching characters to the unit of pressure are preferred.[2] densities or changes in densities, must be calculated, pressures must be expressed in terms of their absolute values. For instance, if the atmospheric pressure is 100 kPa, a gas (such as helium) at 200 kPa (gauge) (300 kPa [absolute]) is 50 % more dense than the same gas at 100 kPa (gauge) (200 kPa [absolute]). Focusing on gauge values, one might erroneously conclude the first sample had twice the density of the second one.

Vacuum comparison
Vacuum is the difference between the absolute pressures of the intake manifold and atmosphere. Vacuum is a "gauge" pressure, since gauges by nature measure a pressure difference, not an absolute pressure. The engine fundamentally responds to air mass, not vacuum, and absolute pressure is necessary to calculate mass. The mass of air entering the engine is directly proportional to the air density, which is proportional to the absolute pressure, and inversely proportional to the absolute temperature.




Am I missing something Gary , Look Im not clued up in everything and if I am wrong no problem I will admit it and learn


*** UPDATE***

Ok I get it.

Great glad you understand now, this is what the forum is here for hey.

[/img]

Cool removed the inaccuracy tag from the main post.

so in layman's terms my gotech should be fine if i drive the car to the coast???

Yes if you use the internal MAP sensor.

widowmaker wrote:[/img]

LOL

My last ten cent from www.autospeed.com

Every engine management system uses a load sensors - either an airflow meter or MAP sensor. But which is better to have when you're modifying an engine? In this article we'll look into some of the pros and cons of each.

Types of Load Sensors
A load sensor is an essential part of any production car EFI system. The load sensor serves to inform the ECU of the amount of air being consumed by the engine. This enables the correct quantity of fuel to be injected and helps determine the appropriate ignition timing for each situation.

There are two different types of load sensors - airflow meters and MAP (Manifold Absolute Pressure) sensors. Leon Vincenzi of Adelaide's Awesome Automotive suggests that today's car manufacturers favour airflow meters because they offer greater tuning accuracy.

"This enables tighter control of emissions," he says.

There are three different types of airflow meters.


The most commonly used airflow meter in late-model EFI cars is the hot-wire airflow meter. The hot-wire airflow meter incorporates a thin platinum wire mounted in the intake system prior to the throttle. The mass airflow into the engine is calculated by the amount of current required to heat the wire to a predetermined temperature.


Many older EFI systems - such as the BMW Bosch system seen here - employ a vane-type airflow meter. In this arrangement a pivoting flap is mounted in the intake system prior to the throttle. This pivoting flap opens as engine airflow increases; the ECU receives information on the flap angle. Mass airflow into the engine is determined by the flap angle and the input from a separate intake air temperature sensor.


The third type of airflow meter is the Karman Vortex, as used on many Mitsubishi engines. The Karman Vortex meter operates by producing internal vortices. An ultrasonic transducer and sender measure the frequency of these vortices. Unlike most hot-wire and all vane airflow meters, a Karman Vortex meter sends a frequency output to the ECU.


A MAP load sensor operates on a completely different principle to an airflow meter. A MAP sensor is a pressure sensor that's connected via a hose to the intake manifold downstream of the throttle. In the case of a naturally aspirated engine, the MAP sensor reads manifold vacuum only, while those fitted to forced induction engines measure vacuum and boost. The output of the MAP sensor is fed into the ECU where it is referenced against revs and intake air temperature. These inputs allow the ECU to calculate the engine's mass intake flow, enabling it to provide the appropriate fuelling and ignition timing.

Pros And Cons of Both Types of Load Sensors
The airflow meter is widely regarded as the most accurate type of load sensor. To achieve the optimal air-fuel ratio, it's the mass of air entering the engine that it is critical to determine.

"The airflow meter gives a grams of air per second measurement of intake flow, which is exactly what the computer needs to deliver the right air-fuel ratio," says Leon Vincenzi. "It's not like a MAP system where it arrives at an indirectly calculated value."

Hot-wire meters provide superior transient response to vane-type meters. Vane-type airflow meters are better damped and offer greater smoothness.


But Leon Vincenzi says the MAP sensor offers even more advantages in terms of transient response.

"You can feel that the earlier MAP-sensed V6 Commodores are a lot snappier than the late airflow metered V6s," he says.

Further advantages of MAP sensors include compactness, no requirement for maintenance and potentially greater reliability. Another important point is since the induction air doesn't have to flow through a MAP sensor (as it does with an airflow meter), it poses no intake airflow restriction, allowing optimal torque and power to be generated.

An Aftermarket Perspective
When modifying an EFI car (with extractors, an exhaust and air intake, for example) the MAP load sensor arrangement typically gives the biggest power gain. But this does come with some drivability trade-offs.

Leon Vincenzi uses the example of Holden Commodore V6 to illustrate the characteristics of a MAP-sensed vehicle with breathing enhancements.

"You can do extractors and exhaust on a late VS-onward Commodore [which uses an airflow meter] and the management system will know what's going on and it'll maintain about the standard air-fuel ratio.

"But the earlier cars [with MAP sensors] will be running off the same base program - oblivious to the effects of the exhaust changes. That means there'll be a mixture variation at high load - it goes leaner. This leaner mixture helps to make power in itself, but you'll often end up with flat spots."

Note that, in addition to giving leaner air-fuel ratios at high load, the MAP sensor allows the engine to breathe without restriction. In contrast, as the engine's airflow capacity increases, so does the restriction of the airflow meter.

According to David Alexander of Sydney's Silverwater Automotive, MAP sensors also have advantages in high power turbo applications.

"Some modified turbo engines that run airflow meters can suffer mass flow reading issues. When you've fitted a big turbo you can get a lot of turbulence that affects the airflow meter at idle and during low speed operation. This can be very tough to fix if you aren't aware of exactly what's going on. When you make a speed-density calculation [as you do with a MAP sensor arrangement] intake turbulence isn't an issue," he says.

Yet another advantage of a MAP sensor induction system is its resistance to engine backfires. Airflow meters, particularly vane-type airflow meters, are very susceptible to backfire damage.


Various programmable management systems are available with external 1, 2 or 3 Bar MAP sensors while other aftermarket ECUs come with an in-built MAP sensor. In the latter case, note that running a vacuum/boost hose from the engine bay through to the cabin (where the ECU is typically mounted) is illegal in some areas. MAP sensors in production cars are invariably mounted on the firewall or directly on the manifold.

Having said all this, the airflow meter has a few advantages up its sleeve...


"An airflow meter system gives more modification flexibility before you have to re-chip the ECU - it's measuring the actual airflow of the engine," says David Alexander.

"If you make an exhaust change, for example, it won't upset the ECU's operating characteristics. There may be other hidden functions in the ECU that override that advantage, but that's another whole issue...

"Also, in most cases, an airflow meter system is more accurate and requires less computer brainpower - it doesn't have to make constant calculations like a speed-density system. Airflow meter systems generally also require a bit less tuning," he says.

Airflow meters are also more suitable in applications where hot cams are used. Big cams can cause pressure fluctuations in the intake manifold at light load. These fluctuations are known to confuse a MAP sensor.

"You find that vacuum drops at low rpm and the engine will run too rich," says Leon Vincenzi.

One of the biggest benefits associated with a MAP sensor is that is poses zero intake flow restriction. While this is a valid consideration, Leon Vincenzi points out that many people ignore that availability of 80mm airflow meters that flow extremely well - the Cobra Mustang SVT engine (pictured here) comes standard with a 80mm airflow meter.

"I know a lot of people rip out the screens or honeycomb in airflow meters," says Leon Vincenzi. "All that does is stuff up their intake flow readings, particularly in the case of airflow meters that have a thick internal honeycomb.

"Another thing people forget is that they can run twin airflow meters if airflow restriction is a concern."

Final Words From the Experts
David Alexander suggests in most instances the decision to go for an airflow meter or MAP sensor is not terribly important.

"Each approach does have some subtle advantages but so much comes back to how well it's tuned," he says.

"Each approach does have some subtle advantages but so much comes back to how well it's tuned," he says.

BRILLIANT

*** Update ***

***Just adding info***

There are some unique challenges when installing individual throttle bodies.

Most problems stem from the lack of a dependable vacuum source. Unlike a single throttle setup (which can generate a strong vacuum inside the manifold), there is minimal vacuum downstream of each individual throttle body. To make matters worse from a tuning point of view, this vacuum fluctuates dramatically with each intake stroke.

According to Mr Thomas, it is possible to run an engine with individual throttle bodies using a MAP sensor – but it’s a difficult and inaccurate way to do it.

“It is certainly possible to connect the vacuum side of each throttle to a shared vacuum chamber – this will improve the strength of the vacuum signal to the MAP sensor but there will still be big pressure fluctuations which make it very difficult to tune.


“Depending on the size of the throttles, you might also find that manifold pressure might not vary much after you’ve reached about 25 percent throttle. That makes it even harder to tune on the basis of MAP.

Mr Gischus has similar views in relation to tuning with individual throttle bodies. In the past, Mr Gischus has removed the factory individual throttle system on highly modified RB26DETTs because there’s not enough vacuum to accurately tune on the basis of MAP.



***Engine Specs***

The ’89 Nissan Skyline R32 GT-R is equipped with the company’s showpiece 2.6-litre straight-six engine boasting twin turbochargers and, of course, six individual throttle bodies. These throttles feed from a large volume plenum chamber which is pressurised by the twin turbocharger system. Twin airflow meters provide the ECU with a load input. Claimed output for the GT-R is conservatively quoted at 206kW at 6800 rpm and 355Nm at 4400 rpm.

The GT-R was followed in late ’91 by the N14 Pulsar GTi-R. The GTi-R uses the famous SR20DET engine equipped with high-spec internals, a top-mount intercooler, large capacity turbocharger and four individual throttle bodies – as seen here. Like the GT-R, the GTi-R’s intake system comprises an aluminium plenum chamber fed by the turbo system. Engine management is also airflow meter based. Power is 162kW at 6400 rpm and torque is 284Nm at 4800 rpm.


Why Not Get Rid of the Airflow Meter?
---------------------------------------------------

About now you may be wondering why the BRPM kit comes with an airflow meter upgrade – wouldn’t it be better to switch to a restriction-free MAP sensor system?

Chris Romano explains that the airflow meter gives much better tuning flexibility (which is important if you later decide to fit a bigger cam) and maintains more consistent mixtures at all load/rpm combinations. It also avoids the flat-spots that are found in some MAP sensor tunes.

Flow restriction using the VZ airflow meter is not a major concern in most tweaked LS1s. BRPM has a customer car which runs a VZ airflow meter and is comfortable making 400hp (around 300kW) at the wheels. Put simply, restriction shouldn’t be a problem unless you’re chasing monster power.