FAST XFI tuning question

[GregWeld]

I hope Jody will add to this -- but VE's are the efficiency of the engine at a particular MAP point... once the VE is "on" -- then you should leave that alone and just change the A/F ratio that you want to run at that spot. Check your O2 sensor readings -- and if the motor is 'putting out' the A/F ratio that you've set and it's not adding/subtracting more than say 5% -- then I'd just leave the VE where it is.

I have Accel -- so it's hard for me to tell you what your screens are... but the Accel has a 'screen' that is similar (I think) to the Fast system - where you can see a scale of what the O2 sensor is doing.... and if the VE is on the money - the O2 sensor is doing 'little'. That would mean the VE would be right on... If the O2 sensor is adding and subtracting 10 or more % - then that should be corrected first. Once the VE is on - then you should be able to change the A/F ratio and the ECU will just simply figure that ratio out and that's what the engine will get.

Does that make sense? Ideally -- you'd want the VE table so well set - that the O2 sensor is just along for the ride. That is not always possible of course... but if the VE tables were perfect - you wouldn't even need an O2 sensor.

[camcojb]

Originally Posted by GregWeld

I hope Jody will add to this -- but VE's are the efficiency of the engine at a particular MAP point... once the VE is "on" -- then you should leave that alone and just change the A/F ratio that you want to run at that spot. Check your O2 sensor readings -- and if the motor is 'putting out' the A/F ratio that you've set and it's not adding/subtracting more than say 5% -- then I'd just leave the VE where it is.

I have Accel -- so it's hard for me to tell you what your screens are... but the Accel has a 'screen' that is similar (I think) to the Fast system - where you can see a scale of what the O2 sensor is doing.... and if the VE is on the money - the O2 sensor is doing 'little'. That would mean the VE would be right on... If the O2 sensor is adding and subtracting 10 or more % - then that should be corrected first. Once the VE is on - then you should be able to change the A/F ratio and the ECU will just simply figure that ratio out and that's what the engine will get.

Does that make sense? Ideally -- you'd want the VE table so well set - that the O2 sensor is just along for the ride. That is not always possible of course... but if the VE tables were perfect - you wouldn't even need an O2 sensor.

getting the VE's right with the O2's working as little as possible is the goal. But if you change the a/f desired at a particular range the VE will also change to keep it ideal. I treat the VE table like jet sizes, larger numbers are richer and smaller are leaner, so if you change your a/f table you'll want to dial in the VE table as well.

In closed loop the computer will compensate by adding or subracting fuel to reach you desired a/f. You can limit how much it adds or subtracts in the tuning software, usually most aftermarket systems have a max limit of 25% +/-. That means if your VE table is within 25% of the ideal number to reach your desired a/f number, then the computer will automatically add/subtract to get you there. Your job as the tuner is to get you there without a lot of computer intervention, so it still has room to add/subtract fuel due to varying conditions like temps, baro changes, etc.

Jody

[GregWeld]

Jody --

I was trying to get him to understand the VE --- get that close --- then leave that alone and fatten up the A/F... figuring that going from 14.3 to say 13.8 wouldn't "disrupt" the VE much.

I personally like to not make too many changes in too many tables - when trying to sort out a problem. Since he's happy with the tune - except just this one area he described... I was kind of thinking that if his VE is okay - then he could just change the one table (A/F) and see if that's the issue (a lean miss?).
Rather than messing up the VE table AND the A/F table too... if you get my reasoning?

I agree - that if - lets say he fattened the A/F to some point - and the miss went away... then he could go back and make sure his VE was accurate?

I might be wrong here - so I'm glad you chimed in.

I actually like working in my A/F tables and VE tables with the O2 in open loop... and when I'm seeing A/F's that match my desired A/F's with no correction - then I switch it back to closed loop... but I didn't want to complicate matter for him either.

[camcojb]

Originally Posted by GregWeld

Jody --

I was trying to get him to understand the VE --- get that close --- then leave that alone and fatten up the A/F... figuring that going from 14.3 to say 13.8 wouldn't "disrupt" the VE much.

I personally like to not make too many changes in too many tables - when trying to sort out a problem. Since he's happy with the tune - except just this one area he described... I was kind of thinking that if his VE is okay - then he could just change the one table (A/F) and see if that's the issue (a lean miss?).
Rather than messing up the VE table AND the A/F table too... if you get my reasoning?

I agree - that if - lets say he fattened the A/F to some point - and the miss went away... then he could go back and make sure his VE was accurate?

I might be wrong here - so I'm glad you chimed in.

I actually like working in my A/F tables and VE tables with the O2 in open loop... and when I'm seeing A/F's that match my desired A/F's with no correction - then I switch it back to closed loop... but I didn't want to complicate matter for him either.

you're right, assuming he has enough correction set up in the software he could simply adjust the a/f and let the competer adjust the ve to get there and see if it solves his issue.

Jody

[67ragtp]

I appreciate all the input from you guys, I ordered the fuel pressure sender kit from FAST today and will hook it up and head to the highway this weekend. This way I can data log whats happening with A/F ratio vs fuel pressure and look at all the paremeters, once I have this data I will let you guys know.

The closed loop correction limits at the area(52to 58kpa-2.0k-2.4k rpm, light cruise) that IM having problems is -25% and +22%

Thanks for all the help, more to come- Rich

[67ragtp]

Well I installed the fuel pressure sender and hooked it up to the ecu. Seems accurate, the regulator is vacuum referenced so my idle pressure is right around 40psi. If I pull the hose off the regulator it goes up to 44-45psi.

I had the opportunity to get it out on the highway for a solid 1 hour of highway time and data logged two 30 min sessions. Its at least 15 to 20 degrees(70F) warmer out than it was when the stumbles were occuring during last weeks criuse(52F). I let it idle in my driveway for 10 to 15 min hood closed to get it really warmed up before we hit the highway. Were cruising at 2200 rpm and at around 22 minutes into the first cruise we feel a very slight stumble and thats it the second 30min never felt a thing. So I look at the data around that time and I dont see anything major. The average fuel pressure is between 36 and 40 psi the actual A/F is around 14.5. I noticed the fuel pressure tracks the KPA graph very closely, I imagine this due to the vacuum reference. When I roll out of the throttle and rpm comes down getting off the highway the kpa goes down significantly(18kpa- 36psi) as does the fuel pressure. I really thought the stumbles would happen more frequently since it was warmer out but it didnt. Does the colder air make it much leaner? What do you guys think?

Thanks Rich

[awr68]

FWIW, I really enjoy reading this type of thread...I always learn something if not lots!!

Rich, you and the boys will figure it out!!