VE value calculation when outside table range?

shauer · Post by **shauer** » Fri May 26, 2006 8:36 am

I've been driving my car daily now for the past several weeks and have the basic tuning set the way I want it and even have AE pretty close. I'm now looking at the finer details of tuning and have a question.

My ITB setup idles with a MAP of about 50 so I set the lowest MAP row on my VE table to 40. This works great except for times when the MAP is lower due to closing the throttles during decel or shifting gears. My logs show the MAP going as low as 20 in these cases and the mixture goes very rich. It looks like the injector PW is not changing significantly from a MAP of 40 to 20. This causes some hesitation during gear shifts as I get a rich stumble for a split second. I go from an AFR of 12.8:1 at 5500RPM to an AFR of about 8:1 while the thottles are closed during a gear shift. This over rich condition sets up a bad start condition for AE when I get back on the throttle.

My question is how does the code arrive at a VE value that is outside the range of the VE table on the low side? I know the code interpolates between values on the VE table but how about off scale low? Does the code interpolate between the lowest entered MAP value and "0"?

Should I add an entry to my VE table for these low MAP transients?

Bernard Fife · Post by **Bernard Fife** » Fri May 26, 2006 10:35 am

Steve,

When outside the table range, MegaSquirt uses the last value (I.e., the lowest MAP value at the current rpm in your example). It does not interpolate between the last value and zero.

If the current rpm is between rpm bins, the rpm is still interpolated, so you would see a value between the two lowest MAP bins that bracket the current rpm.

Lance.

shauer · Post by **shauer** » Fri May 26, 2006 2:18 pm

Thanks lance, that information helps a lot. I think I can remove a MAP bin in a linear portion of my VE table and add a MAP bin down around 20 or so. There seems to be enough variation with my setup in that range to warrant an entry on the VE table to keep the mixture under control.

Jedrik · Post by **Jedrik** » Fri May 26, 2006 5:43 pm

Some decel in the AE settings might help you. start with 85-90%
Though this is only active WHILE the throtle is moving.

I also have a set of bins at 20 kPa set very lean for this reason. Still goes ritch but not bad.

PSIG · Post by **PSIG** » Sat May 27, 2006 5:27 am

Jedrik wrote:...I also have a set of bins at 20 kPa set very lean for this reason. Still goes ritch but not bad.

Why is that?

shauer · Post by **shauer** » Sat May 27, 2006 6:22 am

Jedrik,

I do have decel set at 85% but it does not solve this specific problem since as you said it only cuts in while the throttles are moving. Decel cuts the fuel for the duration of the throttles moving from WOT to closed on a gear shift but then the mixture goes out of control like I described above.

I have a MAP bin at 70kpa that I can remove since my VE table is very linear between 60 and 80kpa. I'll remove that row and add one at 20 this weekend and I'm sure that will help.

Based on how non-linear my VE table is for my ITBs it does not surprise me that a single MAP bin at 20 will not be perfect but I'm sure it will help a lot compared to having no VE definition below 40kpa like I currently have.

Thanks for the information. I have been able to get my ITB setup running smoothly very quickly with the information I have found here.