ITBs on alpha N - adding boost

[AndrewR]

Hi, I have a naturally aspirated Hilborn IR setup on a SB Chev in a road race car, controlled by a G4 xtreme running alpha N (main fuel table TPS vs RPM) with fuel equation load = MAP.  It is well tuned with 12.8 AFR at 100% TPS across the rev range.  I have an integrated Innovate WBO2 for datalogging and tuning but run open loop.  I am turbo-charging it using a custom plenum with the same intake set-up. I have enough injector capacity so all I need to do is add fuel once I go above atmospheric

Do I just maintain my current main fuel table as is, and add a 4D fuel table MAP vs RPM, setting all values below 100kpa to 0 and start by trimming it around 10% richer as I move into boost to achieve my target 11.5 AFR?  

Thanks for your help.

Cheers, Andrew.  Wellington NZ

[Adamw]

Provided the open loop lambda correction is already enabled this will just look after itself without a 4D table.  The equation load source = MAP takes care of the change in air density, the open loop lambda correction takes care of the richer target as boost increases.  

If the turbine is restrictive you may need to enable a 4D MAP or MGP Vs RPM table to pull a little fuel out at the top end if it goes richer than target as the turbine starts to effect efficiency.  But usually if you do, most of the table will be 0 with just some small negative values in the bottom right corner at high RPM/high boost.  

[AndrewR]

Thanks Adam, open loop AFR has not been enabled up until now.  I have however tuned the engine manually using datalogging against the AFR target table below, focusing mainly on what it needs for nice idle and light cruise, and wide open.  I have now added rows to that same table (below) for AFR in boost and leaned 100kpa back a tad. 

I take from reading the help info and your post that if I now enable open loop AFR it will add fuel based on the delta between stoich and the table value, meaning it would add fueling across the board, including the atmo areas that are currently tuned?  If so, is there a workaround for that or should I bite the bullet and take fuel out of the main table and let this add corrections everywhere?

Cheers, Andrew

[Adamw]

Correct, if you enable the open loop correction now your lambda will change by the difference between stoich and the new target.  So idle would end up about 10% richer etc. 

Since your NA fuel table wouldnt have had a large target variation "baked in" to could probably just do an initial crude adjustment such as remove 9% from all fuel table cells below say 50% TPS and say remove 13% from the 50-100% rows.  That should get you somewhere in the ballpark but you will likely have to tickle up the fuel table afterwards.  

[DerekAE86]

13 hours ago, Adamw said:

The equation load source = MAP takes care of the change in air density

Is this something that is only applicable to boosted engines. Or does it prove helpful on na engines that retain a usable amount of manifold pressure even with itbs?

[Adamw]

For an NA engine, Load = MAP is needed if there is an idle valve, vacuum assisted brakes, VVT or restrictive airbox etc, otherwise I generally would stick with Load = BAP as MAP will often just add noise to the inj PW with little other benefits.  BAP still takes care of the day to day density changes.