RB26 multi throttle fuel mapping procedure

[Alex B]

We have 2 GTRs that will be converted to run Motec ECUs shortly and question that worries me is fuel mapping. Both cars have stock intake manifolds with multiple throttles. I know this was discussed in tuning section before, however I do have practical question which is what is the correct way of mapping the fueling on such engines?

The most logical seems to me to have 4D fuel with throttle used as Z axis. So do I make a standard 3D fuel map on max possible th positions under given MAP rows and then copy it over to each Z table and make corrections from there? Or there is some other way?

Also what interval is recommended for the Z values? 10%, 20% or some other.

Would the chassis dyno be enough for this task?

Regards
Alex

[MarkMc]

It depends on if you are measuring the Manifold Pressure from the engine side of the throttle bodies or the plenum side. If you are measuring it from the engine side then it would be easier to tune it as a MAP vs RPM engine.

For idle quality you should measue the manifold pressure on the engine side of the butterflies.....to make things easier for your self.

What will the car be used for? Street, race only? 4D mapping can be done a number of different ways and can get pretty complicated.

[Alex B]

Hi Mark,

Both cars are for street use, so we need good idle and part throttle mapping.

I was planning on using MAP from the engine side, connecting it to the same source as the stock fuel pressure regulator.
As you probably know RB26 has a sort of vacuum tank in a form of rail with all the cylinders connected to it as well as idle control valve.

On my previous tests with MAP on the engine side the MAP x RPM table does not work correctly since for the same MAP there can be different throttle positions (eg. different gears) so airflow will be different. Also I got the feeling that MAP readings were lagging the actual engine condition. Not sure why, perhaps the stock vacuum tank was causing it or idle valve was skewing the picture.

Do you think using 2 MAPs (1 on engine side, one on plenum side) may help solve part throttle fueling? So that second sensor can be used for comp?

Alex

P.S. I've seen one ECU that is using 2 MAPs connected to cyl 3 and 4 respectively and relies just on MAP x RPM table for the fueling.

[Martin]

For what its worth, ive had this same problem on multiple butterfly turbo engines.

What I did was use MAP for efficiency and then afterwards a secondary efficiency table (2nd load) that was throttle based. It was a track car so it doesnt see as much part throttle as a road car, but it worked pretty well. From below 90% throttle I had to trim more and more fuel out. The maximum trim was at about 30% percent throttle from where the trim got less again. The throttle trim was not much below 2500rpm where there was not much airspeed through the butterflies. The more boost it had the bigger the influence of the butterfly.

If you intend to run multiple boost levels things can get quite interesting i think. For that I think 4D will be the answer.

[MarkMc]

All engines will have same manifold pressure readings at different throttle positions, turbo or naturally aspirated. The manifold pressure is telling you that there is more or less restriction from the butterflies if you are measuring it on the engine side of the butterflies.

The trick with your setup is that you probably want idle control which really means you need manifold reference at idle RPM this kind of cuts out doing it by TPS and then MAP on boost. I think Martins way is probably the best suggestion at this point but there are a number of ways to do it.

[Alex B]

You are correct that all engines have same manifold pressure at different throttle positions, however single throttle setups are less affected by this fact and single fuel RPMxMAP map is usually suffice since all cylinders share manifold volume without throttle plate restriction.

What I have in mind about 4D mapping is using TH as Z axis with 10% step rate and then run the engine at this fixed TH position at different gears to correct the fuel. The only question that is I am not so sure is what to do with the remaining parts of the Z tables i.e. from which table to copy them.

What do you think?

If we disregard the idle control which strategy would you recommend? TH position as main table with MAP as secondary table?

Best regards
Alex

[Hanzie M48]

Thanks for this link Alex!

I Have read this topic a few times to understand.

So, with the differents th.positions the MAPreading can be the same. Only the airflow gets higher when the throttle opens more.

So I have to figure out when the MAP reading is the highest together the smallest th.opening at that time. When I open the throttle more at this point the mixture gets leaner then I think. The mixture must be tuned then by the 2nd load table when the throttle opens more. I'm correct?

What do you mean whith copy the table? I use a M48 and how i can see the 2nd load is a % of the maintable.

on the web I also found this http://77e21.info/mstuning.htm

Hans

[Alex B]

Hi Hans,

Thats a very good link. A lot of useful information there. The NA engine is pretty straight forward to map, however we have turbocharged multi throttle engines which adds complexity.

By copy I meant saving map and then importing it in one of the Z tables.

The Mx00 series ECUs have capability of 4D fuel mapping where you can assign one of the channels as Z axis of your MAP and have a individual 3D map (say RPM x MAP) for each value of Z.

Best regards
Alex