AFR and air density altitude?

[tgeers]

Hi,
I don't recall having seen any posts concerning the relationship between AFR and air density altitude. Air density certainlly has an effect on a car's performance, but what is the best way to make corrections? I'm thinking small changes in the PE table but others might have a better solution.

Also is there any rule of thumb to use when making changes according to changes in the ADA?

Thanks
Tony

[SSpdDmon]

I was under the impression that if the MAF is completely dialed in, it should adjust the fueling for you as your altitude changes. The car will also reference different points of the VE table as the density/altitude changes. However, the MAF tends to be more consistent from everyone I talk to.

The other thing the car does to account for altitude and pressure changes is reset its barometer every time you start the car or go WOT. This helps keep the injectors in line while atmospheric conditions change.

[TAQuickness]

MAP sensor measure manifold absolute pressure, while MAF measures the incoming airmas. If you tune is dialed in, theoretically, it wouldn't make a difference if you were in Denver (85-95 kPa) or Houston (95-105 kPa).

[tgeers]

That's very interesting. Does that mean that we are better off NOT running in OP with the MAF disconnected as in a SD tuning mode ?

Thanks
Tony

[TAQuickness]

Op?

[Tordne]

Must mean Open Loop (l key is close to p) :)

[SSpdDmon]

Open loop simply means there's no feedback from the O2's during stoich cruise (14.7AFR). If you can, you should run closed loop incase changes from day to day decide to alter your tune. As far as running speed density (no MAF), that's a constant debate. I'm pro MAF and have my tune set to run awesome with the MAF. Others have a different opinion sometimes and then there are the few running so much boost that they can't use the MAF. I figure if you can use it....use it. SD should be left for those running forced induction that would normally max out the stock MAF.

[tgeers]

Recently I went to one of the classes at EFILive University. One of the things the instructor told us was that we should tune our cars in SD (OL) and leave it there. I questioned him and he affirmed that that was what he meant. What you are saying though makes more sense.

Tony

[joecar]

hi tgeers :wave:,

Some comments about what you said...

SD = speed density (uses RPM and density calculation) = MAF-less
OL = open loop (ignoring O2 sensors)

You can have OL and SD together.... OLSD :cheers:

Using my GM tune I run OLSD all the time, this is how I dialed in my VE table;
or some others run OLSD using a custom OS (COS3, COS5);
others dialed in their VE in OLSD, then dialed in their MAF, and now run with MAF (can have OLMAF or CLMAF);
can run either way, pros/cons to each, they're all good, neither is wrong.

VE table has units g.K/kPa... it takes temp. and pressure into account (as long as the VE is dialed in and the IFR table is correct). :cheers:

[tgeers]

Hi JoeCar,
I always appreciate your comments. Thanks

Here is what I have done to tune my Corvette.
I started with the OLSD. I set my command fuel in OL (B3605) high enough that my PE would always be governed by PE modifier based on RPM (b3618). This was easier for me. I began with PE set at 1.0 and turned my attention to the VE table. My goal was to get my base Efficiency 1-LM1 map to average as close to 1.0 as I could get it. That took a lot of doing but my numbers range from 1.0 to 1.08. Next I worked with my PE table. I wanted to idle at 14.6 (1000 rpm) then run a bit lean in the low rpm/map numbers (1500-2500 rpm/30-55map) for fuel economy. But when I accelerate I tried to get an AFR reading of 13.8 (2500-3500 rpm)(Map over 55). 13.8 AFR is a good acceleration number for my car. Then at wot I've been trying to maintain a 12.5-12.8 AFR throughout the 3000-6800 rpm range (map 95-100). Right now I am trying to leave the VE table alone and make my fine adjustment with the PE table.

I'd appreciate your comments on this approach.

[dc_justin]

Recently I went to one of the classes at EFILive University. One of the things the instructor told us was that we should tune our cars in SD (OL) and leave it there. I questioned him and he affirmed that that was what he meant. What you are saying though makes more sense.

Tony

Was Ben Strader the instructor of that session?

I'll probably run into him a few times over labor day weekend and can verify that for you.:)

[caver]

THe PE table only comes into play at over 60% or so TPS depending on what you have set it to.
If you are in open loop SD you need to be playing with b3605

[superls1]

Recently I went to one of the classes at EFILive University. One of the things the instructor told us was that we should tune our cars in SD (OL) and leave it there. I questioned him and he affirmed that that was what he meant. What you are saying though makes more sense.

Tony

I am fairly certain that EFI Univeristy classes primarily focus on aftermarket EFI systems. I don't know of anybody running a Motec and a MAF. :) The issue of getting GM PCM's to reliably operate in Speed Density mode appears to be challenging at times. This is probably more a result of GM's VE table being more an "air mass" table vs. a Volumetric Efficiency table.