94Max

Okay, I've got a newbie-type question here:

All my tuning experience is with carbureted American muscle cars and carbureted Japanese motorcycles. On any of them, increasing the air flow or exhaust flow via less restrictive components required re-jetting to maintain air-fuel ratios. (not too lean nor too rich)

If I increase the airflow thru my '94 VG via CAI and Y-pipe/hi-flow cat/B-pipe/hi-flow muffler, how is the fuel flow increased in this fuel injected engine? Does the computer automatically detect the increased air flow and inject more fuel? What is the fuel-injected equivalent of rejetting the carb?

Also, if I do something like installing higher lift, longer duration cams, does this same automatic fuel flow correction occur? Or what?

Thanks for any feedback.

Kirk

Jeff92se

iTrader: (82)

The difference between carbs and modern electronic fuel injection is the fuel mixture is not static. The ecu can determine and compensate(to a point) any differences in airflow. If there is maybe a 20-30% increase in airflow though the engine, the stock fuel injectors/fuel pressures are good enough to compensate for that. After that, they start becoming an hinderance as the injectors start going "static" or 100% open all the time to try and correct a lean condition. But this is only on NOS, Turbo or SC applications.
Search the web for "fuel injection" and there is probably a ton of info on that subject. I could write pages on this but it's better if you dig around. You will learn tons in just a few hours. Good luck

Matt93SE

iTrader: (44)

A short, simple overview of a fuel injected system is that there is some device that detects how much air is going into the engine at a given time.. In the MAxima's case, a mass air flow sensor.

The computer says "well if xxx amount of air is going in and we want a 14:1 ratio, then I need to make the injectors spit out yyy amount of fuel"

So the more air your engine sucks in, the more fuel the ECU spits out.
This is fine-tuned by the oxygen sensor and throttle position sensors, but that's the basics of it.

Yes, there is a point to this though. the ECU has the factory "specs" programmed into it, so if for some reason you're falling outside the correct range (i.e. install huge injectors on it), it can't compensate for that because it's outside of it's programmed range. it will simply think there's a bad sensor or injector in the system and go to either the max/min or some preset default "limp-home mode" value just to keep the engine running.

brubenstein

EFI systems run closed loop under most conditions. The 02 sensor determines if it's lean or rich and the computer adjusts the pulse width of the "open" signal to the injectors to maintain proper mixture. The two cases where the engine runs open loop is when it's cold and WOT. The the mixture is determined by fuel "maps" (based on reading from the MAF sensor and Throttle Position Sensor) in the ECU. The fuel maps are set to be too rich (for maximum power), but are designed to keep the pistons cool under WOT. For anything other minor modifications you'll need a custom programed ECU. You just can't just go up 2 mainjet sizes and a plug read any more.

94Max

Thanks for the info, guys.

But, let me refine my question from ANY fuel injection system, to MY '94 VG system.

If I step up to the above-mentioned "air flow enhancements" would I need to do anything to the fuel side? Or will my Max's system (ECU maps, injector sizes, MAF/TPS/O2) be able to compensate?

I'm guessing, based on the number of folks talking about intake & exhaust improvements, that upgrades like custom ECU's and larger injectors aren't necessary for just intake & exhaust bolt-ons.

Very interesting discussion. Thanks again for the info.

Kirk

Jeff92se

iTrader: (82)

Nope, basic bolts ones will NOT overwhelm the stock fuel pressures or the stock injectors. If you go turbo, nos or supercharger, then you have to start being concerned.

Nismo87SE

Unless you plan on running a big shot of nitrous or forced induction, bigger injectors aren't needed. However an ecu upgrade will be one of the better power mods. The ecu basically gives you simular results as installing a Y pipe with no noise whatsoever. IMHO I'd to the ECU right after the Y pipe and intake. The other mods will be less noticable.

94Max

I've been wondering about that.....

If the Max's ECU already will respond to the increased airflow (xxx) it sees and will provide the increased fuel flow (yyy) w/o needing more fuel pressure or bigger injectors, what does an upgraded ECU actually provide? That is, what does it do differently?

Matt93SE

iTrader: (44)

It changes the fuel maps and ignition timing mostly. it's "optimized" for the stuff you'd done to your car. i.e. when you buy the ECU, you tell them what mods you've done, and they program the ECU based on what should be best based on those mods.
Also, if you're running NOW or boost, you'll need higher fuel pressure and possibly bigger injectors, the stock ECU can handle the higher fuel pressure, but will not handle bigger injectors. again, the aftermarket ECU can easily be programmed to run different injectors.

94Max

Hmmmm, very interesting......

I can go for "optimizing" the ECU for whatever mods I've done. But then, don't I have to "re-optimize" as I go? That is, first come the air-flow improvements, then a new brain to capitalize on those gains, but then, if I go for stouter cams, head/port work or nitrous, I have to "re-brain" it.

*sigh*...starts to sound like snake oil sales after a while..... But, in the quest for more engine output, ya gotta do what ya gotta do, I guess.

Nismo87SE

Hmmmm, very interesting......

I can go for "optimizing" the ECU for whatever mods I've done. But then, don't I have to "re-optimize" as I go? That is, first come the air-flow improvements, then a new brain to capitalize on those gains, but then, if I go for stouter cams, head/port work or nitrous, I have to "re-brain" it.
It wouldn't make much of a difference if you got the JWT ECU before or after the mods. The ECU would be more than able to handle the increase in air-flow improvements without needing a reprogramming. For cams its a different story, to get the best out of cams you'd have to do a "timing sweep" on a dyno. When I say timing sweep I mean trying different base timing settings from like 13-20*BTDC. Then you can call up JWT and tell them how much advance or retard gives you the best torque curve. For example in the SR20DE with JWT S3 cams and non S3/S4 program for the ECU. With the ignition timing set at stock 15*BTDC any more advanced and the engine loses torque below 3500rpm and above 6500rpm. However with more timing retard it gains torque between 3500-6500rpm, but still loses torque above/below those points. This info was found out by SE-R owners that did timing sweeps on the dyno. Thus JWT was able to make a "S3/S4 cam program" for any SE-R that has cams. In addition to that the s3/s4 program allowed the car to idle smoother and drive alot easier at low engine speeds.
*sigh*...starts to sound like snake oil sales after a while..... But, in the quest for more engine output, ya gotta do what ya gotta do, I guess.
Its not "snake oil". Basically unless one has aftermarket cams, nitrous or forced induction all you need is the basic JWT ECU upgrade. There isn't any need to get it reprogrammed unless you add one of the previous things mentioned.