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Lean spot rb26


turbodailydan

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Have had this annoying lean spot between 2000-3000rpm on my rb26 running on the factory itb's. My mapper couldnt figure it out so he added fuel in there to lose it however it still has a slight flat there. Had another mapper look at the file and he also mentioned the fuel map looked rushed and should be alot smoother. (Pics below of my current fuel map to show the spike of extra fueling and the example my other mapper showed me as a comparison of what it should be like)

 

have checked over everything mechanically and cannot fault. Fuel pressure is ok when it runs lean, as is the timing. also factory o2 sensors not being used at the moment. 

 

I can attach the map file if needed

 

 Make of it what you will, any advice apreciated as always, cheers dan 

 

 

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Hi Dan,

Can you do a log?  I don't have access to the old software today so I'll take a look at your map in a bit more detail tomorrow.

The spike in your fuel map is certainly pretty odd and not typical.  Does the engine go lean in the same RPM regardless of if you free rev it with no load Vs driving with significant load?

The example smooth map that you have posted uses MAP as load whereas yours uses TPS so that is not particularly relevant.  In general a TPS fuel table is usually pretty flat above 50% throttle so if we imagine yours without the spike it has roughly correct shape.  As for the super smooth example from the other mapper - in my experience you wont often get a fuel map as smooth as that if the engine has been mapped properly steady state - there is always intake and exhaust resonance that will create small peaks and valleys relative to RPM so that one looks a little fudged to me.  You certainly shouldn't see the Rocky mountains like yours but some ups and downs are normal.

 

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Thanks for the input, as for the lean spot, it would do this free reving also. It was so lean that it would splutter and struggle to rev past it. Im just thinking perhaps the TPS has a worn spot at that particular rpm. Have ordered a new one anyhow as the one on there is no doubt 20+ years old. 

 

If you could have a look over the map when possible that would be great. Would be nice to just have someone else look it over and see if theres anything else that looks out of place

 

many thanks 

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Log 28-08-16 10;27;29 am.llg

sorry theres no afr log as I haven't got the wideband connected up yet as been using tailpipe mount. just had a look at log I noticed when the hesitation occurs at 2000rpm approx. the map is reaching 100kpa so just going into positive pressure. could it be anything connected with that?

just to confirm it does run lean when this occurs. doesn't seem to run lean at 2000rpm when free reving like I before,  however the map sensor is still reading in vacuum at that point which is making me wonder whats related to it doing it when the map sensor reading goes into positive pressure..?

something to do with the 4d maps, where its switching from the tps fuel table to the MAP fuel table?

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Dan,

Normally with a "4D" mapped ITB turbo engine like yours you would find the bulk of the "tune" comes from the main TPS/RPM fuel table (fuel table 1) and the 4D map usually only needs to make very small corrections to the main table as things like back pressure and turbo efficiency start to move outside "normal".  So in my experience the 4D table (Fuel table 2 in your case) would normally be just about all zeros with only small numbers in the extremes such as high boost/RPM regions.  Yours is completely different to what I would expect with corrections applied to just about the full table and up to 73% added!

Your problem area seems to be mostly where the Fuel table two starts to apply corrections - you kind of have two tables fighting each other.  So although we can possibly improve the situation by tweaking this "fuel table 2",  really its been done wrong and its never going to work great like this. 

As for how the mapper has got it wrong I am not sure, perhaps he never got Table 1 right before adding Table 2? or perhaps its something like your MAP sensor calibration or fuel pressure that was having an influence during tuning?  Personally I would be starting the fuel maps from scratch but if you are happy to stick with the odd setup and its really only the flat spot that you are worried about then I can possibly help you tweak it.

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Another thing in your base-map. You have the load of Fuel Table 1 as TP(Main), which is quite typical on an RB26. However the 4D table (Fuel Table 2 for you) would normally have the load axis set to MGP, whereas yours is MAP.

The other thing is that your setting for Fuel Equation is set to Load=MAP. This should be set to Load=BAP/MAP Xover. See attached section of PCLink help file.

Scott

 

Fuel equation.pdf

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Just another thing i noticed the 4d fuel table box is ticked off, should this be on?

In your base-map Fuel Table 2 is being used as an 'overlay table' instead of the 4D table. This is not a problem, in fact fuel table 2 has more resolution, so may be a better choice.

Scott

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Thanks that makes sense. The mapper is actually one of the top guys at syvecs so abit surprised hes not done this in the way you would expect. Im going to re do it all now as it should be and go from there. Many thanks for your help guys

Dan,

If you haven't already seen it, you might learn something by joining HP Academy for a month (~$20).  They have a webinar showing tuning the G4+ on an RB26, using 4D strategy so it should be quite relevant.  I haven't seen any of their webinars myself but I know Andre and his logic and technique is generally very good:  https://www.hpacademy.com/previous-webinars/4d-tuning-link-g4-plus/

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