EvilBevel

Bob, John, I admit this is going over my head now a bit.

Simple question: starting from a standard car with standard intake system. Catless downpipe, free flowing backbox... Car A runs JECS and fits catless midpipe, car B runs Link and does the same ( I know this is a pretty academic situation).

Which system would compensate best for the VE change ? Or would they both compensate in the same way ?

Not after slagging X vs Y, just trying to get my head around this. I assumed until now that the MAF based car would be OK-ish, Link car would "need" a remap. (need is a bad/strong word)

Wrong assumption ? Or bad example ?

Bob Rawle

Both would "need" a remap, the VE issue is the key and its what would screw up a maf based map as easily as the map based, both systems run off a fixed map on boost, a proper Link map tailored to the individual car would need going over to restore maximum effectiveness. the only way to avoid this is to run from compensated maps all the time but then you risk something going badly wrong if a sensor fails ... for example a wide band sesnor running and adjusting a map across all load points would seem ideal ... even wide band sensors get tired and they are also prone to failure ... woops.

There is no way to shortcut the fact that a tailored map is tailored, change the cloth in some way and it needs to be re-tailored, that applies universally.

Bob

john banks

Like Theo I am trying to learn also so bear with me - if PV=nRT then n/t=PV/RTt when n/t is mass airflow, P is manifold absolute pressure, V/t is based on RPM and volumetric efficiency (ve) at that point, T is the temperature of that air and R is a constant. In other words MAF is proportional to MAP, RPM and ve and inversely proportional to the absolute temperature of the air. So if you measure temperature before the turbo, then you don't know the mass airflow after the turbo, but if you measure MAF more directly with an airflow sensor you would? So the temperature sensor or the need to know VE is not required for MAF, but the temperature could be used to trim say timing for detonation control in different conditions. I can see however, if the turbo inlet temperature is known and you then map the car, then at a given boost the turbo efficiency will remain the same so the inlet temperature is a proxy for the charge temperature? But the above was my reasoning for thinking that MAF copes particularly well with breathing mods. It is not one ECU vs another honest! Just want to know if this is correct.

Andy.F

Excuse me but how is a MAF based system changed by a change in VE ?

Lets assume we are talking an increase in VE ie more air flow at the same rpm, then the MAF will report this and the ECU will fuel accordingly, the load reference will change and a new (safer) ignition timing will apply.
A MAP based system may not detect any change, manifold pressure remains the same as VE has increased therefore increased air flow, more load but no increase to fuel or change to ignition timing .......remap required.

Am I missing something ?

Edited to say question for Bob not John

[Edited by Andy.F - 8/26/2002 7:52:54 PM]

Moles Dad

Bye bye.

Andy.F

I can see how ignition timing may not be optimum as the VE will change the knock threshhold of the engine but the fueling will be compensated for IMO !!

ustolemyname??stevieturbo

iTrader: (1)

Andy, part of the problem seems to be that some Air filter kits just completely screw up the airflow throufh the Airflow meter, causing it to give false readings, so relying on it to add fuel when necessary isnt going to work. It may even give much less fuel, as in the case of the APS.
Therefore re-mapping is required to ensure that the actual airflow seen by the MAF will see the car being fuelled etc correctly.
As for all this PV/rt=APS etc etc....This aint algebra, I left school some time ago, so Im ignoring it all, and reading the bits that make sense to me.

Andy.F

I realise that the APS is the main topic here and agree with you on that Stevie.
I was asking the question more from a general point of view ie an increase in VE due to an exhaust or a bigger turbo or head porting etc IMHO the MAF based system would cope well where the MAP would not.

ustolemyname??stevieturbo

iTrader: (1)

I suppose in theory it should cope well ( and I guess it does coz plenty of people up the boost, and fit exhaust and the car fuel it ok ). But perhaps once you go so far, the MAF system is only mapped up to a point as it would only ever expect to see a certain max airflow, and above this it may overfuel as a safety measure, and possibly retard the timing. If it sees too much and the MAF Vout rises to high, maybe the ecu might think there is a fault somewhere, and cause strange things to happen ( safe mode )
Never measured voltages, but just thinking.

Andy.F

How much higher do you think I'll need to go to reach 'safe mode'

john banks

"So the temperature sensor or the need to know VE is not required for MAF"

I was kind of saying the same thing as you guys in a round about way Just referring to gas laws since they do seem quite relevant - ie MAF is what you actually need to know to add the right amount of fuel, so if you change the airflow it will adjust, whereas with speed density if you change a manifold or exhaust or turbo you need to redo all the fuelling?
Hence why Trout is always adding fuel and I am always taking it away

[Edited by john banks - 8/26/2002 8:24:28 PM]

Andy.F

Certainly that's my understanding I'm just not sure if/how the MAF actually compensates for inlet temp ie lots of warm air will cool the wire same amount as a little cold air.

Nah, Trout is just trying to justify his BIG injectors

[Edited by Andy.F - 8/26/2002 8:30:53 PM]

john banks

Hadn't thought of that bit... edit .. the warm/cold bit nothing to do with Trout

[Edited by john banks - 8/26/2002 8:32:29 PM]

ustolemyname??stevieturbo

iTrader: (1)

I dont think your car can go into safe mode, as it doesnt retain the ecu boost control. It probably dumping loads of fuel in, trying to stop the engine, not realizing your engine is loving all the extra fuel its giving it.
What is the std subaru MAP sensors limit?? and what would happen when you go beyond its calibration limit?

Andy.F

What would I be doing with a MAP sensor now ?

Did you mean MAF ? as you hit fuel cut before maxing out the MAP

[Edited by Andy.F - 8/26/2002 8:36:49 PM]

john banks

Saves weight

ustolemyname??stevieturbo

iTrader: (1)

I meant MAP, at 1.8 bar, you should have hit the fuel cut ages ago. I think I also read somewhere that around 1.7/1.8 bar was the calibration limit for the std sensor.

Andy.F

Yes, the signal (pneumatic) is bypassed. What I meant was, if you stop it reaching fuel cut either via electrics or pneumatics then the ECU will never actually see the sensor max out so the suituation cannot arise......i think ?

Bob Rawle

The important point is that any map is only as good as the values set in it, the jecs doesn't measure air mass, calculate load point and then CALCULATE fueling, it is just a means of finding a point on the map, now the main difference is that different combinations of load and tps could give rise to the same load point ie say 1.2 bar at 30% throttle could be the same load point as 0.8 bar at wot (probably a bad example but you see my meaning). in a map based system the map value will always pick the point so in the example two different points would always be picked, if you install an exhaust or induction or whatever and alter the system characteuristic of the air flow system thro the engine then the maf could be reading in a different way to that which the map was expected to see, therefore the ecu could end up picking the wrong points ... certainly the case with the APS induction, same airflow (possibly more) but maf is not measuring it, it thinks there is less so picks the wrong map point. Also the maf can only read the small amount of air that actually passes thro its sampling port, what goes on around it is of no effect, thats why there are complex induction "bits" tacked onto the front of intake systems to ensure that consistent flow takes place. Otherwise if non laminar flow with a side biased velocity centre is present the maf reads low, same air flow but different output.

Its not a question of gas laws etc, its just common sense, the maf samples a very small amount of air from one place in the cross section ... its what happens around it that determines how much actual air mass is entering and if there is a velocity gradient across the maf tube x section then it will be reading with an offset on reality.

On that basis the more consistent method could be considered to be map based although its not as easy to set up correctly and yes, may require more adjustment when something is changed.

The map will be set for a fixed set of VE conditions, change that and the requirenment for fuel changes, air flow may remain the same but map will still need changing to suit.

Edited to say that 1.8 bar is the top limit of the linear part of the std (MY97 onwards) map sensor.




[Edited by Bob Rawle - 8/26/2002 8:56:19 PM]

john banks

So IF you could measure mass airflow accurately and reliably it would be a very good system, but we are let down by sensor artefacts which spoil what theoretically would be an ideal system?

Why do the OEMs tend to use MAF when there are so many issues with it? Apparently loads of other manufacturers suffer MAF nightmares - seems to be an Achilles heel?

what would scooby do

Just a quick and simple question - I take it that a Unichip still uses the MAF as it "piggybacks" the original ECU. Cost-wise, which is the next cheapest ECU upgrade I should go for for my car fitted with the APS induction kit that dos wothout the MAF. I want to remap for safety not for any real extra power (yet!!)

john banks

Link?

what would scooby do

ok, obviously I'll check it out when me engines back in one piece - cant blame the APS for a busted main bearing....

Andy.F

Cheers Bob, certainly agree with that As John has said previously, the JECS is so rich to start with that there is a fairly big safety net there to allow some changes in relative safety.

ustolemyname??stevieturbo

iTrader: (1)

Andy, I think someoe mentione somewhere that you ditched your knock sensor. If so, did you just remove it from the block, and re-earth it, or did you replace it with something, and if so, what?
I want to get rid of mine, to see if it makes any difference, as I just cant help thinking its retarding the ign timing all the time for no real reason.

Andy.F

Yes I removed it Steve, when I was tuning I was forever having to reset the ECU if I got slightly OTT. It's just unplugged and I run with a CEL. I believe you can wire in a resistor to clear the CEL but I don't know the value ? Either that or just bolt it to something quiet like the battery tray !

I only done this AFTER fitting a knocklink

ustolemyname??stevieturbo

iTrader: (1)

I had thought that maybe with the CEL on, that it may have caused the engine to run on a different map, bearing in mind that the ecu is seeing a fault. Maybe this is one of the reasons why your car goes so well. I had thought that the ecu may have retarded the ign timing when no knock sesnor is present for safety reasons. It seems this may not be the case then? I'll try the quiet method first, as I dont really want the CEL annoying me

[Edited by ustolemyname??stevieturbo - 8/26/2002 11:59:16 PM]

APS

Guys,

It would be particularly helpful if people checked facts with the manufacturer before posting here.

(1) We work on 3 generations of WRX. 97/98, 99/00 and 01/02. Issues frequently vary with the generations and this is one of those instances.

(2) With the 97/98 and 99/00, our HFA is attached to the entry flange of the standard MAF meter assembly. With the 01/02 only, our HFA replaces the standard MAF sensor mounting tube (which is a moulded part of the standard air cleaner housing) and the standard MAF sensor is re-fitted to our HFA.

(3) Provided that the standard honeycomb airflow laminating screen is retained in the standard MAF meter housing, installation of our HFA to 97/98 and 99/00 vehicles has the following worst case effects:

(a) Possible slight lean surge at high speed part throttle cruise. Irritating to some people, however certainly not dangerous.

(b) Alteration of the WOT A/F ratio over 3,000 rpm of less than 0.5:1. Given that the cars run awesomely rich standard, we have never seen this as a problem, even in our climate.

Please remember, I said worst case. We have seen many installs with no issues at all. Remember also, we have sold literally 1,000’s of these over the past six years.



(4) With the 01/02’s, it is an entirely different situation. For increased performance, we chose to increase the ID of our HFA which replaces the standard duct. This certainly alters the MAF signal output relative to air flow. This then requires aftermarket engine management or the Unichip. Any of our dealers are well aware of this requirement and would only supply accordingly for this generation vehicle.

(5) It is also important to remember that we take emissions compliance seriously. All the preceding statements are based on running either the standard exhaust system or, an appropriate high flow catalytic converter equipped, low restriction exhaust system.

I trust the preceding clarifies this issue without the histrionics.

Dave – APS.

john banks

If users of this filter are getting much more than 0.5 AFR points leaner as seems to be the case, then does it not merit more investigation rather than saying it doesn't happen and calling them histrionic? Did all these lean UK cars have the MAF screen removed or not?

As it is I cancelled my order for the APS induction kit to go with the APS FMIC until I have more information on UK cars.

[Edited by john banks - 8/27/2002 9:54:52 AM]

Pavlo

Bob,

I am not convinced the ECU uses throttle position to determine load. So I would be interested to hear how you determined it did. Not say you are wrong, but maybe my testing was flawed.

The basic testing I did just looked up load and airflow on the select monitor port. At idle they are the same, a 2000rpm with a little throttle they are the same. If I got WOT very quickly, as the revs rise, they are the same.

I have yet to remove the TPS sensor and move it independantly of the butterfly, but I will try it soon.

Paul