MAP sampling: what's your timing ?
Turbo Mini Forums

Hi,

This is my first post here.

I wonder when do your ECUs do the MAP sampling. Because I observe a strange behaviour described below, sorry for the long explanation.

I have a wasted spark ignition with a HIF38 carb on my 998 A+ (SW5 cam, 998 head, CR 9.3:1).
The ECU is connected to a MAP and a MAF (hot film), the MAP is used for the ignition, the MAF is just displayed. There is also a wide-band lambda.

The MAP gets the pressure from the hole which was originally connected to the distributor.
The MAP measure is sampled once per 180 degrees (once per cylinder) at 106 and 106+180 degrees, where the cam lift is at max.

I observe that MAP does not reflect the actual air flow. The max pressure is reached at about 90% of the WOT flow. Then lambda and MAF show that the flow increases, while the pressure does not change any longer.

I suspect high pressure oscillations in the intake path, so I'm going to sample in 3 points per cylinder to see if they give different values.
What's your experience ?

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

Be carefull, the HIF38 Vac advance take off is most likely to be what is called 'Throttle Edge Depression' therefore at small throttle openings the 'vacuum' is not the manifold depression. You will need to find a way to provide the ECU with a manifold depression.

You will usualy see the MAP hit 100% before throttle reaches 100%, if it didnt, your intake tract is far too restrictive. You will find MAP reach 100% at lower rpm with less TPS compared to higher rpm.

The SW5 cam is a nice soft cam that works well on the injection because of the small valve overlap, this prevents pulsing of the manifold depression, where it could cause a poor MAP signal (not representative of actual depression) This can also happen if you have a bad valve seal. It can be severe to the point that not even a standard Rover engine/MEMS sytem will work (i have personaly experienced this)

Not sure about sampling rates, I suspect thats a specific to your ECU. Whos ECU are you using?

Using MAPas air flow measurement will depend on how well your perticular ECU has modled the speed Density method of air measurement, you should also have an air temp sensor? if not, MAP will not be representative of air flow, but rather just an indicator of engine load.

Edited by Sprocket on 19th Nov, 2007.

One dodge that may help is to fit a retrictor and a damping chamber in the vac line from the manifold.
A mig nozzle 0.6mm and an in line fuel filter often work to damp any pulses when fitted in the vac line.
regards
Dave

Yeah of course I have to use the air Temp for injection.
But before that I have to solve this currious problem.
Remember I have a MAF, so I can observe the flow. And I see that when going from say 90% WOT to 100% the flow goes up but the pressure does not change.
So either the measure point is wrong (which is certainly right for small opening but not for full), or the flow does not change at the instant of the measure but changes for example at the valve closing.

That's why my question about when to sample.

I'm using a Bosch Motronic ECU with my own firmware, so I control everything.

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

danboy: actually I'm not sure to want to restrict. Because it would always filter too much in some particular conditions. I'd prefer to sample at the right instant (may be closer to T/BDC or twice per cycle ?), because there are lots of chances the measure remains the same through all air pressure and temperature.
May be I'm wrong.

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

I would say that it could well be a peculiarity of your ECU, not sure about the sampling rates or the timing of it in relation to valves, first ive heard of that. Can you double the sampling rate?, it may be that you are only measuring the MAP at a slight dip in actual MAP ??

MAF sensors can get confused, as it doesnt know which way the air if flowing, slight pulsing in the inlet tract caused by valve overlap could cause this???

Not many use MAF, it will either be Speed Density (MAP/ MAT) or Alpha N (TPS based)

Yes I'm going to sample 3 times per intake cycle and display the 3 values.
MAF is here only to see the VE, which BTW is never higher than 75%. Actually the measure is not stable even if the MAF is ahead the carb air filter, there are too much pulsation.

I'll probably use the MAF later to auto-adjust the MAP/engine parameters. But only once I'll switch to an ARM CPU, the ECU I use is 8052 based, not enough CPU power for all these extras.

Edited by alpa on 19th Nov, 2007.

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

could this be connected with the su using a constant vacuum principal to operate ,so there is a slight vacuum in the manifold all the time ?the piston on the carb rises and flow increases but a slight vacuum to raise the piston could show on the map ?

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

I'm not surprised that you see no change in MAP for 90% to 100% WOT.

At 90% WOT the throttle disc is virtually out the way and the pressure drop is minimal. I would expect very little change in MAP above 75% throttle at say 4000rpm and a 50mm throttle. Smaller throttle and higher revs then that percentage would go higher.

As for sampling the MAP, I would suggest that if you are using a standard automotive MAP sensor, then it will be self damping and you would not see any variation in MAP through the engine cycle.

I think most ECUs have code that adjusts the ignition timing ever few revs. It just looks at the MAP figure and revs, then picks an advance value from a table.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

robert: what's strange is that in the 3000-4500 RPM range the WOT MAP is slightly above the ambient pressure, while it's 5% below at 2000 RPM.

Paul: I use the GM 1bar MAP.

Tomorrow I'll do measures with another sampling point between 106 and 180 degrees.

Thanks guys.

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

I am not sure that you will gain anything by picking a specific sampling point.

Your MAP sensor will be a very simple strain gauge device that will not react quick enough to give you a precise trace of MAP through the engine cycle.

If you are picking up some variation, then it is possible vibration induced.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

In theory the MAP sensor is fast enough up to say 6000 RPM. The time constant is of few ms. If you connect a scope you'll see oscillations during revolution. Many automotive papers describe their MAP sampling policy (with intake plenum) and they often sample once or even twice per induction.

However I'm not sure (as already noted in posts) the carb is the right place to connect the MAP as it's too close to the throttle plate. As long as I'm only managing ignition it's not very important as the Mini engine is not very sensitive to the advance, but injection will need a more precise pressure estimation.

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

OK, I am surprised at that.

If you want that sort of accuracy then you will need to mount the sensor direct to the manifold as any length of interconnecting pipe will dampen the signal.

Some aftermarket ECUs, such as Megasquirt, have a MAP sensor on the PCB and are connected to the manifold by several meters of vaccum tube. The signal is very stable after such damping.

Are you sure that you need that level of precision in measuring MAP?

Your biggest problem is going to be getting equal mixture distribution due to the siamesed ports.

Edited by Paul S on 20th Nov, 2007.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

I'm not sure of anything :) .

Today I have the GM sensor 30cm from the intake and I'm able to see pressure difference between internal and external cylinders. I'd like to use that to balance the injection. For the moment these 4 MAP measures are just displayed, the ignition uses the average value.
If I was filtering pressure for 2000 RPM I would not see per-cylinder difference at 4000 RPM. At 6000 RPM it does not matter.

The problem with this plenum-less system is that it does not match the usual injection setup. Street engines use an intake plenum to simplify the management, while ITB race cars don't bother with partial loads. Only rare brands like BMW with M-Power engines had worked on these extreme cases, and I believe the M-Power ECUs are radically different from usual BMW Motronics. But as they did not have siam ports they used a dump air chamber to pick the MAP pressure.

Edited by alpa on 20th Nov, 2007.

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

Hi,

I added another sampling at around 150 and near 180 degrees (the initial point is at 106, max valve lift).
There are obviously high oscillations and like many others I found there is no way to easily exploit this information except by oversampling and making an average value through the intake cycle.
So I'll have to do a dumper between the manifold and the MAP as my current ECU is too slow to make extra work. And I'll define a per-cylinder trim to cope with VE dispersion.
I tried restrictors and long hoses but oscillations are still there, may be in the hoses.

On the other hand GM seems to have a small response time, so with 3 sampling points it's very easy to see how the intake is tunned. At 3500 RPM (with SW5) I have a huge spike near BDC which is higher than the ambient pressure.
I've never found the GM MAP spec. I have specs of two Marelli MAPs, their response time is 2 and 7 ms so there must be possible to find a very fast sensor.

So, how often do you guys sample the MAP ?

Edited by alpa on 21st Nov, 2007.

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

I'm not aware of anyone on here that uses an ECU that can be adjusted for MAP sampling.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Do you know how your ECU samples ?

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Ok, so it samples at the ignition reference position, likely once per cylinder.
What's the reference position ?

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

I'm guessing but I think that it senses the missing tooth from the VR sensor which is 90 degrees before TDC on number 1 cylinder, calculates when it needs to charge the coil from the dwell setting and then grounds the coil at the timing point.

So would the reference point be 90 deg before TDC on number 1 cylinder?

As the MS system uses wasted spark ignition, it fires once per engine revolution, rather than per cylinder.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

90 before TDC sounds good. So likely one sample per revolution. I'll check on the MS site.
Thanks

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

90 before TDC sounds good. So likely one sample per revolution. I'll check on the MS site.
Thanks

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

What you need to bear in mind it that you are dealing with a tuned system, so the ideal point to measure MAP changes with RPM.

The phenomenon that you describe of MAP reaching atmospheric before WOT is well known, and is worse the bigger your throttle. The easiest way to deal with it is to have a TPS term in the calculations to handle that. Otherwise you will be chasing your tail for no real gain.

I'll try to measure 3 times per induction phase. Or at a ms monotonic rate. I did that in the past, a 2ms rate sampling worked very well and per-cylinder measure was still possible. While a restricted MAP measure would filter too much at high RPM and would hide transients.

I'd like to avoid using TPS to keep extra ADC inputs for other sensors.

std 998 A+, g295, MD266, RHF4, 109hp @0.8bar/5400rpm

I have to say I wouldn't, but every setup is different.