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# Link G4/Vi-PEC Vxx Master Fuel setting

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Hi everyone,

The Help files in the Link/Vi-PEC software mention that the Master Fuel should be set to allow for values of 50 in the middle of the fuel map.

If we consider that injector pulse width is loosely the result of the product between the Master Fuel, the value in the fuel map/100 (which would be some kind of relative VE) and some other coefficients, is there a way to figure out the minimum value of the Master Fuel in function of injector and engine size to get the engine to idle without stalling?

I was surprised to notice that decreasing the Master Fuel by just 1ms, even after adding the % of fuel in the fuel map to compensate, would still cause my engine to stall, and I can't help thinking that's because there may be a point of diminishing returns regarding reducing the Master Fuel which may be directly related to injector size.

Any thoughts?

Hi TTM,

The master fuel value is a multiplication factor to give a relative pulse width when referencing the numbers in the fuel table barring all corrections. The G4/Vxx ECU s do not run VE based, it is basic traditional fuel mapping so multiplying the fuel table by the same amount as you do the Master Fuel value will not work to give you the same values.

Hi Chris,

On some forum someone mentioned that the fuel equation was about something like this, for what it's worth :

Base pulse width= Master Fuel x MAP/100 x %VE/100

This means for example that if we substract 25% from, or multiply by 3/4, the Master fuel we therefore need to add 33% to, or multiply by 4/3, the values in the Main Fuel table (%VE) to get the same Lambda correction. I did just that and was able to confirm from checking in the Real Time values table that the % of Lambda correction was exactly the same as before. Since 100 is the maximum value that can be entered in the Main Fuel table the maths simply say that reducing the Master Fuel "will allow" or "require", depending which way you look at it, a higher resolution/finesse of tune from the Fuel Table and logically result in a "smoother" Fuel Table without peaks and valleys that some folks occasionally comment about. Driving around also showed more stable AFR variations at low and medium loads in load ranges where I have Closed Loop Lambda enabled.

However I noticed from further reducing the Master Fuel that there seems to be some choking point below which the injectors may not deliver fuel smoothly enough to avoid stalling the engine, and this is where my question comes from - is there a way to evaluate the "lowest best" Master Fuel value for a given engine size and injector size to avoid stalling?

I'd recommend watching this webinar which deals with correctly adjusting the master pulsewidth: https://www.hpacademy.com/previous-webinars/fuel-table-resolution/

If you're running in traditional fuel mode, the numbers in the fuel table represent a percentage of the master pulse width before background compensations are applied. So if your master pulse width is 8 ms and you're operating at 100 kPa and the table value in the fuel table is 50% then the pulse width supplied to the injectors is 50% of 8 = 4 ms. Most of the time we would be using the 'Load = MAP' fuel equation too which works in the background and adjusts the pulse width based on manifold pressure. This just works on the principle that if we double the pressure, we need double the fuel to maintain a consistent AFR.

The aim with choosing the master pulse width is to give good resolution so that you can make finer changes to your air fuel ratio. I'm not sure that this however has any bearing on your stalling issue. For example with the master pulse width set to the smallest allowable value of 4 ms, we could still enter a table value of 0.1% which would give us a requested pulse width of 0.004 ms. That's pretty fine control over the injector output.

My guess is that your issue is related to a non linear area of injector flow or perhaps your trying to command a pulse width that is below the minimum the injector can provide. What injectors are you using?

Hi Andre,

Thanks for the link. If I had noticed this webinar earlier I would probably have figured out a better resolution of my Main Fuel table without feeling the need mention it on here!

The engine is a 3L I4 fed with unleaded by Bosch EV14 1200cc injectors, with a Master Fuel set at 6ms with a peak value in the Main Fuel table of 70%. Theoretically I should be able to lower the Master Fuel further than 6ms but then the engine stalls, even after compensating in the Fuel Main table. Would I need smaller injectors to be able to lower the Master Fuel? Not that I need this as the engine idles fine with MS=6ms, but just want to understand what's going on.

If your peak numbers are around 70% then you're not doing badly and I wouldn't be too concerned to be honest.

I'm a little confused about the stalling though as dropping the master fuel to perhaps 5 ms and compensating the fuel table accordingly should have the engine run exactly the same. I'd log the injector pulse width at idle and see how this is being effected when you change the master fuel number. I imagine this should make the problem pretty obvious.

Hi Andre,

I was able to lower the Master Fuel further, the initial issue was caused by me not re-multiplying the Cold Start Fuel Table...

All is well now. Thanks again for your input.

I'm not sure I see how that would effect your stalling to be honest? The cold start tables are all percentage adjustments that act on top of the current injector pulse width and hence they shouldn't need to be manipulated if you change the master fuel number and then correct the numbers in the main fuel table.

Regardless if you now have everything operating as you want then I'm happy for you.

Really?

The AFR on cold start up really was spot on after adjusting the Cold start table... Will check again and let you know.

Hello Andre,

After checking again with the engine fully cold I confirm that I needed to multiply the values in the Cold Star Enrichment table by what I had divided the Master Fuel by in order to get the same AFR reading when warming up the engine from cold.

That's interesting. I'm going to run this past a tech at Link as it shouldn't act in that way and hasn't been my experience with the ECU to date. I'll come back to you.

might be firmware related? interested in how this turns out.

I already use the latest soft/firmware versions.

Andre, have you heard back from Link?

Hi Andre,

Were you able to get info from Link about the cold start enrichment and how Master Fuel affects it?

Hi Andre,