man, I hate making the "hey, I'm an idiot" MS threads, but here's another...REQ_FUEL.

[740ATL]

Ok... in the MS manual, under the tuning section where it says "get the engine started and idling (http://www.megasquirt.info/v22manual/mtune.htm#howto)"

it says:

Once you have it started and running, let it warm-up fully, and note the injector pulse width at idle. Then you can reset the REQ_FUEL value back to its original number. It is very easy to change without having to re-enter the VE numbers. In MegaTune, go to 'Tables/VE Table 1/Tools/VE Specific/Reset ReqFuel' and enter your original and new REQ_FUEL values, make sure the 'Reset Req_Fuel in Constants' box is checked and click on OK. This adjusts the VE table and resets the Req_Fuel.

Then go to Settings->Constants and change the REQ_FUEL from the value you used to start the engine to the value calculated in MegaTune. The injector pulse widths will be the same, but the VE numbers will more accurately reflect your actual volumetric efficiencies.

what exactly is the purpose of this? Is it simply saying "hey, your VE table was initially way too rich, so we're going to scale the whole thing down for you, and you'll get to keep your original REQ_FUEL number" ?

2nd question... does having REQ_FUEL right on really matter? The reason I say this is that because in this thread (click me (http://forums.turbobricks.com/showthread.php?t=39540&highlight=req_fuel)), there were several people saying that REQ_FUEL was too high and to lower it...

I'm a bit confused because it seems like the REQ_FUEL should be exactly where the REQ_FUEL calculator says it should be...

please be kind to the idiot and help the poor boy out.

many thanks.

Mike

[fryea]

REQ_FUEL is all relative to the VE map you have. remember that the equation for pulse width is :

PW = REQ_FUEL * VE * MAP * E + accel + Injector_open_time

Ideally, you want a REQ_FUEL that will give you 100VE, at 14.7afr, at peak map level (highest boost on a turbo, 100kpa on a NA) at peak HP RPM (according to the megamanual somewhere...)

but with all that jib jab aside, you can scale the VE map and REQ_FUEL accordingly and within proportionality and still come out a winner. As a simple example, i could have 14 for req fuel, at say a VE bin of 5, which when multiplied together is 70. Now along that same principle, i could have a Req_fuel of 10 and the same VE bin of 7 and still come out to 70. Its all just a math equation really. Now dont ever try scaling it that way, because you will end up way the **** lean or way the **** rich, so you would have to take in all the other variables in the PW equation to get the CORRECT value, but that is just a very basic example i showed.

Hope this helps clear up some values of VE

The way i have mine set up is 13.6 REQ_FUEL, 2 squirts per cycle, with a VE table that has 20-30's on the bottom kpa row (20 methinks) and high 90's to low 100's on the top kpa row (205)

if this is completely wrong, someone make a call me out

[740ATL]

REQ_FUEL is all relative to the VE map you have.

works for me.

Thanks dude.

[The Aspirator]

Here's my question, and Matt tried to explain it to me a long time ago but I guess I'm just thick or something. If req fuel doesn't have to be exact and is relative to your VE map, how is duty cycle calculated? Cause if req fuel isn't accurately showing the injectors' size, how does MS know the difference between 30 lb/hr and 50 lb/hr injectors?

[Matt Dupuis]

MS doesn't give a shiz what size your injectors are, it only cares about how much fuel to squirt. (Reqfuel X load scaler X VE entry X corrections) + injector open time = total time injectors are open.

If your reqfuel is too high, your VE entrys will have to be low to compensate. It's that simple. Lower your reqfuel, you've got to increase your VE entries to keep the current AFR. Reqfuel is a quick & dirty way of leaning or enriching the entire map an equal amount, in theory.

The importance of having the reqfuel "dead on" is only so that if you decide to play with it, you can quickly return to the beginning by re-running the reqfuel calculator, and you don't have to remember some goofy number days or weeks later.

Having said all that, if you're running MSII you might want to shoot for as low a reqfuel value as you can get away with, so that your VE map numbers are really high. This gives you maximum table resolution to take advantage of the high calculation resolution. 150 is a very small change from 149, while 50 from 49 is three times the jump. Some would say that it doesn't matter, but if you're idling in the 20% VE range and you want to be SLIGHTLY richer or leaner, you're going to be hooped 'cause one step either way is roughly a 5% difference - enough to get you from 14.0 AFR to 14.7 or 13.3 in one step.

[fryea]

isnt duty cycle just the amount of 12v going to the injectors to give it x amount of pulsewidths? thats what i always thought, so when im trying to tune for max. MPG, i shoot for the lowest DC at cruise possible, because thats when the injectors are being used the least

[Paul_VR6]

Duty cycle is the % on time of the injector during one injector cycle. And yes lower PW per a given engine and road speed is better mpg.

[740ATL]

thanks guys. This has helped wonders. For whatever reason, that didn't sink in from studying the megamanual.

I shall go forth and multiply.