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PW Vs VE base map?

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Hi, I have a Lotus Exige 350 and recently had an SCS Delta 900 ECU and TMAP sensor fitted (replacing stock MAF sensor) in order to get a custom map (stock ECU is locked/encrypted). This is my first step into custom tuning, never done anything like this before.

I've been getting to know the software it comes with, SXtune, and it is possible to use either injector pulse width or VE for the base map. The tuning company I used, used IPW for the base map (engine RPM against manifold pressure) and the AFR table is barely filled out (only two rows with target AFRs at 0 mBar and 816 mBar).

Am I right in thinking IPW gives better fueling accuracy but lacks taking into account environmental factors i.e. changes in air temp / pressure, and would not require an AFR table lookup. I presume it is fuelling specifically to the IPW table, so actual measured AFR will change depending on air temp, airflow/pressure etc, regardless of what the target AFR is set to?

However if I am correct this makes me think it is an inferior method to VE which would allow the ECU to correct the fuelling based on changing atmospheric conditions, as long as the AFR table is appropriately filled out. Again if I am correct I think the reason they may have used IPW instead is to have tighter control over combustion temps on track to avoid knock/detonation by adding extra fuelling since my car is prone to heatsoak and tends to retard timing heavily - presumably extra fuelling might sacrifice a bit of power but control temps better, compared to letting it heatsoak and losing even more power.

If someone could verify my understanding/logic above (equally please shoot me down if I am completely wrong) I would greatly appreciate it! All the info out there seems to be about VE, struggling to find anything in depth about IPW mapping.

Many thanks

Nick

Hi Nick

I am old school and think in terms of injector pulse width. As my current ECU only does VE (Motec M1) I have a tendency to have pulse width logged/displayed to get my head around VE and sense check the kind of numbers I am seeing.

The main benefits of VE compared to IPW, certainly on the M1, is you don't have to worry about injector size, engine capacity etc. As the injector width is derived from these and VE (and much more). So, in theory, for my startup map all I had to do with is enter injector details, engine capacity, make sure the critical sensor are correctly configured and the engine would pretty much just start and run, possibly with a bit of tweaking. Throw in a couple of lambda sensors and you probably have a drivable car with little extra effort.

This is all fine with a MAP sensor type setup. The second you end up on ITB's (mine is a V6 on Jenveys) it all goes out the window and might as well be on TPS vs RPM etc...which, fortunately, the M1 gives you a method option to use where you specify VE figure at each TPS/RPM site (actually works really well, especially when Marelli stop making the injector you use and you have to swap to a different type when you break one with the engine hoist!).

As regards compensations there is no real difference to what you can apply to VE or IPW, it is just easier with VE, especially if you start changing injectors.

If I was starting afresh I would go with VE, especially on an engine with a MAP sensor or similar. It just makes things easier not having to worry about the specifics of the injector and let the ECU deal with that. The end result of VE vs IPW mapping will depend far more on the quality of the tuner, the available sensors for compensations and to a lesser extent nowadays the ECU than the actual method used.

On the subject of the AFR table, that sounds a "limited". Even the base Motec M1 AFR map is massively more complex than that. Whilst you should NOT be relying on the AFR/lambda table as the rest of the ECU's compensations should get close to correcting the VE/IPW first, I would expect a little more effort to have been made.

Nick,

I'm not familiar with that exact ECU so there's a few things I suggest checking. Depending on the ECU (and settings in some cases), AFR target may have no impact on fueling at all. Sometimes it's only used to help you reference what you were wanting vs. monitored wideband data in logs, sometimes it's used for closed loop fuel trimming, but changing it doesn't alter base fuel delivered. In other cases it's an integral part of calculating how much fuel to deliver.

Whether you choose injector on time or VE based main fuel table, corrections for baro, intake temp, coolant temp are generally possible. Some VE systems integrate baro, some use it as an external correction. IAT and coolant temp corrections are generally independent of the main fuel table, regardless of the main table being IPW or VE based.

I suggest VE based fuel tuning any time it's an option, as long as the ECU allows you to properly characterize your injectors and the fuel you're running as well as engine displacement. With that data accounted for, the VE values actually mean something, and it allows you to tune an engine far more quickly, see how well the engine system is flowing, and it's more useful for diagnostics.

Any strategy related to altering fueling during an overheat condition is generally independent of the based fuel table being IPW or VE based as well, so that's not something you should have to forgo in a VE environment.

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