FAST fuel parameters and VE table

[mosk]

Here's an odd question: Which tables are affected by the injector size parameter? I assume the VE table uses this parameter, but do other tables use it as well?

Here's why I ask: I got my FAST system retuned today by a local tuner, and it runs great -- very smooth, no pinging, etc. He did a great job, and I have no complaints. (BTW, this was a retune after I made some changes to the motor.) Anyway, after I got home and I was looking over the old and new tuning files and noticed that I had mis-entered the fuel injector size parameter, and we did the tune with this incorrect parameter in place. This was my fault, as I was trying to make some table-wide increases in fuel before his re-tune and I forgot about it. I have 550 cc/min injectors (52.3 lbs/hr) and had entered 47 lbs/hr, a value that is ~11% undersized.

So, what are the effects of this mismatch? The system runs well as it is, so obviously the tuner was able to compensate OK. If the VE table is optimized for all conditions, and is running with minimal correction, does this mismatch matter? We had to pull a lot of timing out of and add a lot of fuel to the WOT cells in the table to eliminate pinging, so I want to make sure I didn't inadvertently limit our tuning potential.

Thanks for whatever info anyone can share.

-Jeff

 

[bruce]

Originally posted by mosk
Here's why I ask: I got my FAST system retuned today by a local tuner, and it runs great -- very smooth, no pinging, etc. He did a great job, and I have no complaints.

That's all that REALLY matters.

But, be sure to store a backup copy of the file somewhere, and note the error. The problem would be in trying to duplicate things in case of a problem down the road.

I'd probably save that file, then make a new one with the injector size being corrected, and try changing the VE entries the 11%, then verify the tune was in fact right. Datalogging the first file and then the second tune, and comparing things should tell you where you are.

 

[berrelauto]

Actually it makes a difference in that your VE, duty cycle, & lbs/hr will not be accurate. The system is true speed density and it uses the injector flow rate, cubic inch displacement, and various other inputs to calculate the above mentioned numbers. Your vehicle might be running great, just don't rely on the above values as they won't be right.

 

[bruce]

Originally posted by berrelauto
Actually it makes a difference in that your VE, duty cycle, & lbs/hr will not be accurate. The system is true speed density and it uses the injector flow rate, cubic inch displacement, and various other inputs to calculate the above mentioned numbers. Your vehicle might be running great, just don't rely on the above values as they won't be right.

The Duty cycle will remain the same. With whatever injector size they will have to be open x amount of time to deliver the fuel needed.

While VE is based on % of cylinder filling, if the injector size is off then it just skews the entry for that cell, and if all the cells have been done with the same error, what is the difference?.

IMO, it's the same as having the base timing off 2d, and then accouting for it in the table. If you know it exists, and account for it, it's handled, IMO.

 

[mosk]

Thanks for the info. I've already duplicated the tune we did yesterday and made changes to the duplicate VE table, increasing all the values by 11%. I'll do some driving and datalogging and see how it runs.

It seems like this error would only be a problem if I were to change injectors. If things were tuned properly using the correct injector size value, I could change injectors and just change the value in the fuel parameters. Since I won't be changing injectors, this won't be a problem for me (although I feel pretty foolish for forgetting about this).

One problem we did have was with pinging. To eliminate pinging at WOT (and in a few other circumstances) we had to limit advance to 9° and had to max out the a/f table in the affected cells. The tuner seemed to feel this was probably because of imperfect squish area in the piston design, and suggested either higher octane fuel or water/methanol injection, which I may or may not followup on (this was at 10-14psi and on 91 octane pump gas on a street vehicle, and I'm not sure I want to get away from a stright pump gas setup). We were able to tune this out, but I wonder if correct fuel injector size values would hazve made any difference in the amount of timing we were able to run?

Thanks again for the help!

Jeff

 

[bruce]

Originally posted by mosk
One problem we did have was with pinging. To eliminate pinging at WOT (and in a few other circumstances) we had to limit advance to 9° and had to max out the a/f table in the affected cells. The tuner seemed to feel this was probably because of imperfect squish area in the piston design, and suggested either higher octane fuel or water/methanol injection, which I may or may not followup on (this was at 10-14psi and on 91 octane pump gas on a street vehicle, and I'm not sure I want to get away from a stright pump gas setup). We were able to tune this out, but I wonder if correct fuel injector size values would hazve made any difference in the amount of timing we were able to run?

Sounds like you just might be out of fuel.

The trick to timing is running the LEAST amount while retaining max performance. If the car is running it's best at 9d and the AFR is correct then, don't get hung up on numbers.

IMO, if your trying to run good numbers with less then a F/M, alky injection is a good thing. Running sans Intercooler, I dropped my actual MAT temps, over 100dF, with alky.

 

[mosk]

Originally posted by bruce
Sounds like you just might be out of fuel.

The trick to timing is running the LEAST amount while retaining max performance. If the car is running it's best at 9d and the AFR is correct then, don't get hung up on numbers.

IMO, if your trying to run good numbers with less then a F/M, alky injection is a good thing. Running sans Intercooler, I dropped my actual MAT temps, over 100dF, with alky.

Thanks, Bruce. I don't think I'm out of fuel, but I'll verify that I have adequate pressure and fuel at WOT.

By the way, what's "F/M"? I'm wracking my brain but not coming up with a definition.

Also, these are screenshots of the a/f, VE, and spark tables. These are the current tables, tuned with the incorrect injector value:

http://www.well.com/user/mosk/images/fast_tables/VEtable47lbs.gif

http://www.well.com/user/mosk/images/fast_tables/air_fuel_table.gif

http://www.well.com/user/mosk/images/fast_tables/spark_table.gif

Thanks for your response,

Jeff

 

[bruce]

Originally posted by mosk
Thanks, Bruce. I don't think I'm out of fuel, but I'll verify that I have adequate pressure and fuel at WOT.

By the way, what's "F/M"? I'm wracking my brain but not coming up with a definition.

Also, these are screenshots of the a/f, VE, and spark tables. These are the current tables, tuned with the incorrect injector value:

F/M Front Mount Intercooler.

Make a pass and look at your Duty Cycle at WOT, if it's over 90% your out of fuel/injector. Also what the WB is reporting.

A 100% DC, and the WB showing going lean is an absolute that your short on fuel.

The 9d timing looks to pretty well match the rich commanded AFRs.

 

[ablackFormula]

As said before the duty cycle will not change. For a given engine condition, the injectors need a certain pulsewidth. Whether the computer knows the correct injector flow rate and correct VE, or a lower flow rate and therefore higher VE, the same pulsewidth is calculated in the end--or the engine simply wouldn't run right. And since the duty cycle is defined as the pulsewidth divided by the amount of time for an intake stroke (depending upon engine RPM,) changing injector size does not have an effect on dutycycle (as entered into the computer.) Obviously, with larger injectors, a smaller pulsewidth is needed to deliver the same amount of fuel (or if you were to raise fuel pressure, also,) and that would lower the duty cycle.

My question is this:
How does the FAST unit account for fuel pressure? I see where you enter in the injector size and VE tables, but if you keep the injectors the same size, and don't make any changes to the engine, but raise fuel pressure, the pulsewidth needed would drop. Is there any way to account for this besides then changing the VE tables? Is there a special formula to change for this in the injector size?

 

[ijames]

I would guess that you have to change the injector size. Fuel flow is proportional to the square root of the pressure drop, so effective injector flow = rated flow * sqrt(new pressure / pressure at rated flow). Most injectors are rated at 3 bars or 43.5 psi, so for example using 72's at 40 psi makes them look like 72*sqrt(40/43.5) = 69 lb/hr.

 

[mosk]

Originally posted by ablackFormula
As said before the duty cycle will not change. For a given engine condition, the injectors need a certain pulsewidth. Whether the computer knows the correct injector flow rate and correct VE, or a lower flow rate and therefore higher VE, the same pulsewidth is calculated in the end--or the engine simply wouldn't run right. And since the duty cycle is defined as the pulsewidth divided by the amount of time for an intake stroke (depending upon engine RPM,) changing injector size does not have an effect on dutycycle (as entered into the computer.) Obviously, with larger injectors, a smaller pulsewidth is needed to deliver the same amount of fuel (or if you were to raise fuel pressure, also,) and that would lower the duty cycle.

That makes sense, and makes me realize I had things backwards. If I understated the fuel injector size by 11%, it sounds like I would want to input the correct size and then remove 11% across the entire VE table. Is that right?

-Jeff

 

[ablackFormula]

Jeff, sounds like you got it right. And since duty cycle doesn't change, that should be a good way to judge if you're doing it right--datalog it as is, get duty cycle #'s for all kinds off different map/rpm situations, make your changes, and do what you gotta do to the VE tables to get those same duty cycle #'s back (although I think the datalog also lets you log pulsewidth directly, which would be pracitcally the same thing...)

 

[berrelauto]

You are correct, the duty cycle will be the same regardless of what is entered into the fuel injector size parameters but the amount of fuel it correlates to will be innaccurate. That is what i was trying to say earlier. The bottom line is that the system requires you to enter a number, it mine as well be as close to correct as possible so you get the most accurate information and performance from the system.