Understanding what kPa is for tuning

[Steve Cole]

Since it seems some would like to believe that tuning requires things to be done in TPS versus a kPa measurement I think people need to learn and understand what kPa measurement really is. Who out there understands what kPa is and how it's measured?

[glens]

(k)Pa is an SI unit of pressure measurement.  It's usually measured with a pressure transducer.

[Tsani]

As I understanding it on my bike, it is a derived reading using various sensors like the Map, CKP, Tp and possibly the ET and IAT sensors. Not sure if the VSS is involved. Used help set fuel requirements at certain rpms for the derived value.

[Steve Cole]

It is a measure of absolute pressure and it does not matter where you are it will give a reading of what the absolute pressure at that point in time. It is from a MAP sensor on a HD motor cycle that is placed in the intake manifold after the throttle blade.

Now with that understanding what is the difference in say 50 kPa manifold pressure if the throttle blade is 1/2 open, 3/4 open or WOT?

[Tsani]

Engine is working harder at 1/2 compared to 3/4 or WOT to develop the at pressue or vacuum and require more fuel compared to less at a higher opening, but wouldn't RPM play into it as well? I don't think it's linear either, more like a bell curve function.

[Tsani]

Lets just go with a curve. Definately not linear.

[Tsani]

And since we are talking about it, excuse my ignorance on the subject. But it's my understanding that the Map sensor doesn't directly the pressure in the manifold on a Harley, that there is another type of sensor in use on cars that do that or very similar job. Or am I just wrongly informed? EFI is new to me so I am going to enjoy this.

[Steve Cole]

All the MAP does is measure absolute pressure, nothing more or nothing less. MAP stands for Manifold Absolute Pressure.  It has no idea nor does it care what position the throttle is in, it just reads the pressure that is in the manifold at any given time. The units that it gives to represent that pressure in this case are kPa.

So again, I ask what is the difference in say 50 kPa manifold pressure if the throttle blade is 1/2 open, 3/4 open or WOT?

[Tsani]

As far as the pressure is concern, nothing. It's pressure.

[glens]

So again, I ask what is the difference in say 50 kPa manifold pressure if the throttle blade is 1/2 open, 3/4 open or WOT?

In this case the pressure outside the manifold is nominally twice that of the pressure inside.  The outside pressure will be aggressively trying to fill the void and hopefully will only be able to do so through the throttle plate opening.  If the engine is pulling half the atmospheric pressure from behind a 1/2-open throttle plate, it will be moving less air than when it's pulling that same pressure at 3/4 throttle, which will be allowing the pressure to (try to) equalize at a more rapid rate than 1/2 throttle will.

But I think what you're getting at is that the MAP sensor doesn't know or care how much air is moving through the manifold, what speed the engine is turning, what position the throttle plate is, or even whether/where the sensor's mounted.  The sensor itself is merely seeing and reporting the absolute pressure at it's location.

[mayor]

So again, I ask what is the difference in say 50 kPa manifold pressure if the throttle blade is 1/2 open, 3/4 open or WOT?

it's highly unlikely that you will be able to be at 50 kPa in all those throttle positions...in fact,  I doubt that keeping the map to 50 kPa at 3/4 or wide open could even happen unless you found a really really steep hill to ride down. 

From what I've seen, steady state Map is very coralate ble to throttle position...at least with in a reasonable range.  I just don't think that range is +/-  50 kPa. 

[saddle tramp]

All the MAP does is measure absolute pressure, nothing more or nothing less. MAP stands for Manifold Absolute Pressure.  It has no idea nor does it care what position the throttle is in, it just reads the pressure that is in the manifold at any given time. The units that it gives to represent that pressure in this case are kPa.

So again, I ask what is the difference in say 50 kPa manifold pressure if the throttle blade is 1/2 open, 3/4 open or WOT?

the KPA would be lower at the larger (wfo) opening than at the 1/2 open throttle position. I'm still trying to learn this stuff so I could be wrong, but I think thats how it works ? Am I close Steve ?

[Steve Cole]

The correct answer is that it is just PRESSURE and there is no difference in the pressure regardless of where the throttle blade position is as long as the reading is fixed at 50 kPa. So once you know the manifold pressure it no longer matters where the throttle blade is. I'm not looking for what can and cannot happen here, but yet to get people to understand the basics of how and why first, then we can move from there. There is just to many of you that do not seem to understand/know the basics.

Question #2

If you increase the load on the engine at a fixed throttle opening what do you think the MAP reading will be provided you hold the RPM fixed?

[FSG]

As far as the pressure is concern, nothing. It's pressure.

  but it's a loaded question

[mayor]

Question #2

If you increase the load on the engine at a fixed throttle opening what do you think the MAP reading will be provided you hold the RPM fixed?

if the load increases, the Map increases.

[rbabos]

The correct answer is that it is just PRESSURE and there is no difference in the pressure regardless of where the throttle blade position is as long as the reading is fixed at 50 kPa. So once you know the manifold pressure it no longer matters where the throttle blade is. I'm not looking for what can and cannot happen here, but yet to get people to understand the basics of how and why first, then we can move from there. There is just to many of you that do not seem to understand/know the basics.

Question #2

If you increase the load on the engine at a fixed throttle opening what do you think the MAP reading will be provided you hold the RPM fixed?

You can't keep a fixed rpm without opening the throttle if the load is increased. Once the load is applied and throttle opened to keep the fixed rpm the kpa reading will increase due to less engine vacume. The higher the kpa reading the less vacume inside the manifold. 0 vacume would be 100kpa at sea level or whatever your air pressure is at elevation or atmospheric highs and lows would be in a given day.
Ron

[glens]

He didn't say a fixed engine speed.  He said fixed throttle position.  The MAP goes up because the engine speed decreases with more load, thus giving more time for the pressure outside the manifold to get 'round the throttle plate and better-equalize the differential pressure.

[FSG]

Seems to me he did. Read the question again, taking note of the  provided you hold the RPM fixed.

[rbabos]

He didn't say a fixed engine speed.  He said fixed throttle position.  The MAP goes up because the engine speed decreases with more load, thus giving more time for the pressure outside the manifold to get 'round the throttle plate and better-equalize the differential pressure.

The way I read it he said both at the same time in the same load condition. No wonder it's so hard to figure out stuff.
Ron

[glens]

Damn!  How'd I miss that?  Well, then, it's an unanswerable question, because if you increase the load you have to either lose engine speed or increase throttle position.  MAP is the man behind the curtain there.

[mayor]

interesting, was thinking the same...but didn't think he meant to word it the way he did.  I would suspect that Map would increase just before rpm would drop though, since the throttle position is fixed. 

[rbabos]

Damn!  How'd I miss that?  Well, then, it's an unanswerable question, because if you increase the load you have to either lose engine speed or increase throttle position.  MAP is the man behind the curtain there.

I thought it was a test to see if we could read accurately. A form of a trick question. I'd hope so or I'll be doomed following this directions.
Ron

[Tsani]

Question #2

If you increase the load on the engine at a fixed throttle opening what do you think the MAP reading will be provided you hold the RPM fixed?

If it was possible to acheive this, then the map would not change at all is the way I see it.  However, I do not see how this can be done in a real world engine. Kind of how we were taught stuff in the mil. Theory is theroy, real world is another matter. I think the point here is map is map no matter what. It is just simply a reading of pressure or lack of.

[hotroadking]

sure you can, going from a flat road to a hill, why couldn't you hold the  throttle steady and rpm steady, you'll just slow down
like using the old throttle lock, the engine speed (rpm) won't change but the vehicle speed reduces.  it's why cruise advances the throttle
to maintain speed, question wasn't how do you maintain speed when load increases - ie more throttle.

[Steve Cole]

BINGO, MAYOR is a winner! The MAP will increase as soon as the load changes when throttle position is held constant and RPM stays fixed.

So MAP or kPa is much quicker responding to load changes than throttle position and is much more accurate! This is why it is used in near ALL automotive applications. Take a look at any automotive application and you will find a MAP sensor to measure the kPa along with any other system. The reason a MAP is used is due to accuracy and response at low engine speed in a MAF application.

So now that we understand how the MAP shows what is happening in the manifold let's move on.

Question #3

What is the difference if the manifold measures 100 kPa at 1/2 throttle or 100 kPa at WOT in sea level conditions? Sea Level conditions means a Baro reading of 100 kPa for this case.