David Lewallen

For the last month or two, since getting my MS3X, I have had a question that I cannot find an answer to. Searched the forums and either Im not using the right queries or Im completely overlooking something.

Just some quick info on the car 1999 Miata currently N/A located in Salt Lake City, soon to be running a kraken kit, with FF640 injectors.

Im struggling to find a concrete answer on how AFR table should change based on elevation or if it even should.
Everything I read says to tune for 100kpa while N/A although due to where I live, SLC, I will never see 100kpa while N/A and at most I see ~85/86kpa in the valley. When up in the canyons Ive seen as low as 71/72kpa.

Does the whole "tune for 100kpa" apply even when at elevation or should a different approach be taken?
If a different approach should be taken then should I then tune around highest kpa Ill see?

DNMakinson

iTrader: (3)

Tune for 100kPa is just saying tune up to WOT, Naturally Aspirated.

Do you plan to have a second MAP sensor? If so, set your barometric correction to 100% across the board. Then if you go to the lowlands, keep it at 100% at 86kPa, and adjust that curve up or down as required at the higher Baro.

That way, you won’t mess up your VE table that you will have created at elevation.

I have no idea how much baro correction a turbo set-might need, as no-one has posted Baro Corrections (that I’ve seen), and I have not taken mine up high around here; though I live close to Mt Mitchell, the highest place East of the Mississippi.

You will want to select and tune for either EBC or MBC before you try to tune Baro, as the operating points will be much different between the two:
MBC at 10psi boost will give you 154 kPa MAP at elevation, but 168 kPA MAP at sea level.
Closed loop EBC will try to give you the same 154 kPa at both locations.
I think Open Loop EBC will act same as MBC.

EDIT: Sorry, I did not really answer your question. Set AFR table same as typically recommended, you just won’t hit the 100kPa row while tuning N/A.

DNM

Ted75zcar

iTrader: (2)

The AFR target table is agnostic WRT ambient pressure, use a table that contains rows and columns that are appropriate for the anticipated operating points. The VE table will contain rows that you are unable to tune at your altitude, but that isn't really a big deal.

If you intend to ultimately go FI, then start with a VE table (while NA) that includes some margin above your long term target manifold pressure targets. Autotune will tune the NA cells now, and when you get FI you can tune those cells later.

Some people do some crazy stuff with baro, I will not go into that. The traditional value of baro is that it primarily provides a modifier to compensate for VE as a function of exhaust back pressure. We are talking about a few percent at most here, and is really a final touch. The method DNM suggests works well, set to 100 across the board, fully tune the VE table, then use baro for compensation at different ambient pressures.

Watch out for guidance on a topic like this that you may see on Facebook. FB has some of the most ridiculous misinformation you can get.

Bottom line is... don't do anything different due to altitude. Tune NA applications to 100kpa (some rows not tuned) and FI applications with a table that includes margin. Feel free to add a percent or 3 to the untuned/inaccessible rows (copied from the last tunable row) for safety and use EGO.

Also be aware that any tuner with a dyno will artificially inflate your numbers by a crazy amount.

David Lewallen

I appreciate the detailed responses from both of you.

Just to make sure I fully understand - I should tune without regards to my local ambient pressure and build an AFR table that is scaled from say 105kPa on the upper end to 10/20kPa on the lower end?

Does the following sound correct?
If so it looks like I need to redo my AFR table and VE table as Im hitting high 12s low 13s at 80ish kpa
1) I'm assuming load is completely dependent on amount of air in the engine so a car at sea level (101kPa) will see a higher load WOT than a car at elevation (4200ft ~85kPa)?
2) All AFR tables that I can see from a reference richen up above ~80kPa. I should do the same even though that row will likely be the max my car sees?
3) The rows that I do not hit, above 85kPa, should add 1-3% fuel to the previous row for safety?

DNMakinson

iTrader: (3)

Your VE and Spark, and AFR tables should all be scaled vertically from around 10/20 down low, to about 220 kPa at the top row. So you don't have to re-build the tables when you get the Turbo kit.

BUT, you will not be able to tune those rows until the kit is in and you can make the boost.


DNM

Malic

I am south of you, in Cedar City, about 6000ft in altitude and also used to live in Layton, so have been dealing with this stuff for a bit.

Have asked this in the past and never got a clear answer for any change in AFR target that you would have to change for being up at this altitude.

I understand that being at altitude also acts like having a higher octane of fuel, but this mostly pertains to naturally asperated engines, because of the lower cylinder pressures. Given that we force more air in this will not apply to a forced induction motor. I run the best of what is available to me, which is 91. The lower pressure outside the motor is going to change exhaust backpressure though. In my mind, lower pressure means it will flow better? But I really need to be adding more sensors to gather mode data on this.

During tuning, I ignore whatever PSI I am getting to. 14PSI at sea level is 16 PSI here at 6000ft. If I were to tune for 14 psi, I would be getting to the same MAP as someone at sea level would be turning for 12 PSI.

Standard air pressure here tends to be about 88 kPa when I turn on the key and is also what second baro sensor sees too.
One thing I did change due to airpressure is change my fan activation temps, as when you are burping the system, water boils at 201F, and stock Miata fans are programmed to turn on at 208F. I lowered mine to come on at 195F to have that buffer. Putting on the pressure cap just lets it go to whatever PSI above what you started, not magickly raise it to seal level atmo + cap, so the temp it will boil at will be lower. If you have any leaks, will make it more apparent.
https://en.wikipedia.org/wiki/High-altitude_cooking

Driving an hour south gets me to St George, and it is about 2000ft in altitude, so I use it to gather data for altitude changes during my trips there.

DNMakinson

iTrader: (3)

@Malic And what did you find different at 2000' vs 6000'? Share the knowledge.

David Lewallen

The reason I have started this thread is mainly due to differences in AFRs due to elevation when out driving. Ive tuned the car in the valley which is ~4200ft (85kPa) but often drive up into the mountains/canyons which can reach elevations of 9000ft + (70-72kPa).

Due to the change in elevation my MAP decreases and Ive seen near WOT attempting to hit cruising AFRs because of the drop in ambient air pressure.

How do I handle differences in elevations regarding AFR or is that not something that I need to worry about? In another way is 70kPA floored in the mountains the same amount of load on an engine as 70kPa at sea level?

Here is an my AFR table

DNMakinson

iTrader: (3)

What ECU? Post a tune. Post two logs, ON SAME TUNE, but at the different altitudes.

Do you have a second MAP sensor to detect Barometric Pressure? Or are you using KOEO readings to set it.

You can use EGO to maintain your target AFR's. The fact that you different actual means there may be an issue with that.

Yes, almost exactly (besides the backpressure mentioned above) 73 kPa is 73 kPa. kPa is the commanding "load" for most all tables.

To the extent that it is different is 2-fold.... EGO is not correcting your fueling to the target / you need Baro Correction. To the extent that EGO does need to make correction (which it sounds like it is not), you need to bring it back to 100% EGO with the Bare Correction.

DNM

David Lewallen

Im running a MS3X - currently away from my computer right now so I dont have access to tune/logs but can upload later.

I do not have a second MAP sensor but will be adding one soon. Yes Im using KOEO to reset (although baro correction is zero'd out across the board)

EGO is currently turned off but will look to turn it back on although Im not sure that is the problem Im running into. Im almost certain this is a knowledge issue around engine load based on MAP that im not understanding

If this is true then that would explain what Im seeing with AFRs when driving up into the mountains.
WOT in the valley (85kpa) in between the 89/81 rows on my AFR table which should produce high 13s low 14s.
When Im WOT in the mountains (70ish kPa) Im between the 73/65 kPa rows in my AFR table which should produce low 15s as thats my cruising cells when in the valley.
As long as that is correct and is "safe" then I think my questions are answered and I appreciate the help with this.

Ted75zcar

iTrader: (2)

I encourage you to research the science of combustion. I personally find it very entertaining. Real research too, not the opinion and speculation you find in forums or social media. There are a bunch of edited books, as well as numerous published technical papers.

At a high level there are 2 things that may lead you to adding fuel where one may not typically see fuel added. The first is that for gasoline, you can get the most power at ~12.6 AFR. This due to science, see recommendation above. The second is that more fuel increases detonation resistance and may allow you to run MBT timing. Now for FI, this can be a big deal. Assuming you are NA and running premium fuel, it probably isn't as important because you can get close to if not run MBT without the added detonation resistance. Same thing with running at 12.6 AFR, you may not gain too much power with a NA. If you lived at the higher elevation I would say go for it. Only visiting high elevation... meh

David Lewallen

Yeah I definitely think doing research on the topic will help with my understanding. Do you happen to have any recommendations that you would suggest for reading up on combustion?

If you were looking to get the most potential out of your engine (N/A) at elevation and at sea level then you would likely have multiple tunes?
One that will achieve ~12.6 AFR WOT at sea level (101kPa) and one that will achieve ~12.6 AFR WOT at elevation (< 101kPa)
I guess you could have one that is ~12.6 AFR across the whole table which should do the same but would definitely hurt fuel efficiency when cruising

Ted75zcar

iTrader: (2)

The obsession has long past, and I don't remember any specific publications, but I also read pretty much every one I could find and there are a lot. I do remember that ethanol content vs flame front speed had good stuff, obviously optimal ignition timing for gasoline is fruitful, other good keywords would be swirl, tumble, static vs dynamic compression, intake manifold tuning, optimal cam timing... seriously there is a ton of stuff out there. Don't forget that a big piece of the AFR puzzle relates to optimal emissions, especially with a catalytic converter, and that for fuel efficiency less fuel doesn't always mean more MPG.

I did do a tune for a guy one time who insisted on running at 12.6 everywhere, so it has been done before. My suggested approach would be to take some extended logs of mixed driving, identify the areas for true cruise, acceleration, deceleration, idle and the transitions between these areas using MLV and the table generator (filters can help with this). Then configure the breakpoints in your AFR target table to bound these areas cleanly, and use the appropriate AFR for each area to suit its purpose. Keep in mind that with an NA, the most effective way to increase acceleration is with the transmission, and the AFR table has two axis

Edit - one tune, all objectives can usually be accomplished with one tune.