afc to control boost

Post by **tony/ka24de** » Thu Mar 20, 2003 5:01 pm

Can i use an afc instead of a fmu to control my stock injectors under boost?(for a ka24de) If so, whats the difference in price between the two? The reason I am asking is because I want to start off with low boost(4-5 psi) , and for this I need a fmu. But if I want to try and boost it up later(7psi), I will probably need a afc and injectors. If I ran low boost and then wanted to boost it up would the fmu be usless for the new inJectors? How much boost can the stock injectors handle and the fuel pump with no problems? If I go with a fmu what ratio would be best?

Post by **huguetpj** » Thu Mar 20, 2003 5:01 pm

This has been asked before you if you do a search a lot more information will be available... but since I'm not doing anything...

The amount of fuel that is injected is directly related to mostly 3 things:

-> Injector size-> Fuel press-> Injector duty cycle

To modify this values you can use:

-> Injector size: change injectors-> Fuel pressure: FPR (FMU)-> Injector duty cycle: modified ECU or AFC

Theoretically stock injectors @ stock fuel press max out (80% duty cycle) at about little less than 4PSI, but I'm gonna try and see myself so I can be sure using a A/F gauge.

To run more boost you need to add an FMU, so idle fuel press stays stock while it rises under boost. Stock injectors with and FPR shouldn't be used at more than 7PSI. Look at the nsport kit for example. But more fuel press can bring other issues, leaks for example.

The other way is to change to bigger injectors, but you need to control their duty cycle at idle (and other rpm points) cause the ECU won't close them enough and your engine will die if the injectors are too big. So you'll need and AFC or ECU reprogram. AFCs usually allow to change duty cycle by +-50%.

Price wise the FPR is usually cheaper than AFC.

If you wanna use boths, you can use the FMU to add fuel under boost and the AFC to control idle and fine tune other rpm points.

Post by **tony/ka24de** » Thu Mar 20, 2003 5:01 pm

thanks huguetpj

Post by **Jeff240sx** » Thu Mar 20, 2003 5:01 pm

huguetpj wrote:Theoretically stock injectors @ stock fuel press max out (80% duty cycle) at about little less than 4PSI, but I'm gonna try and see myself so I can be sure using a A/F gauge.

Which A/F gauge are you gonna use? The nifty autometer pretty blinky light gauge? Anyway... I ran my car at 4.35 (.3bar) for 10,000 miles. This was a ton of street races, many drag passes, and fairly hard driving. It works w/o problems. Just run premium, and you can get by.-Jeff

Post by **huguetpj** » Thu Mar 20, 2003 5:01 pm

Yep the pretty blinky light gauge... just to make sure I'm not running lean. I'm also gonna be monitoring injector duty cycle and fuel press.

.3 bar, huh? Sounds like you were using the same Tial WG spring as I'm gonna. I wanna run .3bar at first to make sure I ain't got no leaks or othre bugs and then change injector, retune ECU and up the boost

I had my doubts whether I could do it on stock fuel system (+Walbro fuel pump to be safe)

So you ran .3bar with stock fuel injectors and stock FPR? Cool, I see if I can get by without premium gas... there's none available here, it's either 92 octane (give or take - mainly take - a few octanes) or race gas and race gas ain't cheap.

Post by **cnichols** » Thu Mar 20, 2003 5:01 pm

There's really no point in wasting your money on the Autometer gauge. I assume you're not using a Wideband O2 sensor, so there is no way that thing is going to read accurately. Also, remember that an O2 sensor works using voltage...even if you splice the gauge in perfectly, I would be afraid of messing up the signal.

If you want a real measure of Air/Fuel Ratio to tune by, FJO Racing makes a pretty nice kit for around $600. It's pricey, yes, but worth the trouble and money of replacing your engine. Which, you most likely will have to do if you rely on a 3 wire O2 and a crappy Autometer gauge.

Post by **huguetpj** » Thu Mar 20, 2003 5:01 pm

Yes I know that the stock O2 sensor isn't very accurate and add to that the A/F gauge accuracy and I wouldn't tune by that and you can rest assure I won't. By the way I already bought the gauge and it's installed in my car. Actually I'm very happy with it's functioning and I believe that the gauge has a very high impedance A/D converter input so the voltage signal of the O2 sensor isn't affected at all.

Don't worry, when it comes to that, tuning will be done on a dyno or at least with a lambda (Wideband?) sensor. The A/F gauge is just to make sure nothing is wrong during daily driving. But thanks a lot for the advice, all advice is always welcomed cause I'm still learning my stuff as I go along :pface

Now if only I had installed an oil press gauge before I blew a piston... :rolleyes

Post by **Matthew** » Thu Mar 20, 2003 5:01 pm

Lots of people are running turbo'd hondas off of an AFC. They reduce the voltage input from the MAP sensor (or MAF for nissans) and run 450cc injectors. It's also nice because you can adjust your fuel by throttle points, and lean it out at idle (and before the turbo spools) in order for your car to idle stock.While this can be quite reliable for low boost, you have to take into account that the s-afc cannot directly retard your timing under boost. So, while you are upping the boost, you could quite possibly be upping the timing advance as well.I personally would take the larger injectors + afc over a FMU and high fuel pressure numbers anyday for low boost applications.A company called SMC also sells an unit that is much like the first s-afc's (which were analog). You use dials to set the MAF voltage reduction in the desired rpm range.http://userwww.sfsu.edu/~ecg/smc.htmFor only $120, it's a pretty good deal, i know some people that have had good luck with it.

Post by **Jeff240sx** » Thu Mar 20, 2003 5:01 pm

PJ - Yeah. I was running the Tial spring. If you can't find one, or need one, I don't need mine anymore.Yes, I did run with stock injectors, fpr, pump, ecu, absolutly no fuel control. No problems at all.The problem is that a O2 sensor works in the range of 0-3V, and the blinky light gauge runs on 0-1V. A wideband O2 runs 0-5V, as do the expensive A/F gauges. One caveat to going WB O2 is that they are meant for tuning, and you'll be buying a new ($150+) WB O2 sensor just about every year.So, my final though... save the $60 on the blinky-light gauge, and buy an EGT, or pyrometer. It will tell you the same thing. Hotter than usual = lean, cooler than usual = rich. They are more accurate, also.Also, I was running 93 octane. 92 is good too, and in Cali, 91 will work for stock fuel system vs. 4.35psi.-Jeff

Post by **huguetpj** » Thu Mar 20, 2003 5:01 pm

Already got the Tial with .3 bar spring, but thanks for the offer. I also bought the A/F, although I did buy a EGT also (and fuel press to be safe).

91-92 that's good, I'm pretty the crappy gas we get here at least has that much octane.

Post by **tony/ka24de** » Thu Mar 20, 2003 5:01 pm

what does egt stand for?

Post by **Jeff240sx** » Thu Mar 20, 2003 5:01 pm

tony/ka24de wrote:what does egt stand for?

Exhaust Gas Temperature.-Jeff

Post by **WDRacing** » Thu Mar 20, 2003 5:01 pm

Not really on topic, but for safety purposes I would highly recommend a MSD knock sensor. I tuned my Skyline all the way up to 28 psi using that and alcohol. Since the AF gauge will react slow at best, the knock sensor will detect detonation at a very minimal level before it becomes catostrofic. So you can up your boost while having the reasurance of not detonating.

Just somthin to think about...

WD

Post by **encasemyheart** » Thu Mar 20, 2003 5:01 pm

So the AFC alone would only really be able to do anything up to 4psi, at which point the injectors are maxed out. So you would definately need injectors above that.

So could the SAFC compensate for 50#+ injectors at a lower psi such as 6-7? I ask because I don't want to rely on a FPR and want to get everything early so when I am able to move up to the 350 or so whp range my injectors will be there. I know the AFC can only tune + or - 50%, can it go low enough without running too rich at a low psi with such big injectors?

Post by **Jeff240sx** » Thu Mar 20, 2003 5:01 pm

270cc injectors come stock. 50# = 525cc injectors. 50% of 525cc = 262.5cc. That means yes.-Jeff

Post by **huguetpj** » Thu Mar 20, 2003 5:01 pm

WD: doesn't the KA have a stock knock sensor to retard timing? Haven't got my FSM with me. Wouldn't that be of use? I was actually planning to connect one of the Blitz SBCs optional inputs to this to and monitor it's output.

Anyway, if I'm talking out my as. and find out that it doesn't, then I'm definitely interested in this aftermarket sensor. Right now I'm pretty low on cash since I'm rebuilding the motor, but will look into when time comes.

Quote »So the AFC alone would only really be able to do anything up to 4psi, at which point the injectors are maxed out. So you would definately need injectors above that.[/quote]

I beleive so, since at 4PSi you're maxing out the injector duty cycle so the AFC can't make them open any longer. Now 50lb injectors, wouldn't know.

Post by **cnichols** » Thu Mar 20, 2003 5:01 pm

The KA does have a stock Knock sensor.

You could also change the fuel pressure, which I assume would allow you to use the stock injectors above 4 psi and still make use of the AFC.

Post by **WDRacing** » Thu Mar 20, 2003 5:01 pm

Yeah, the KA has a knock sensor that sends a signal to the ECU. But the ECU pulls out a ton of timing if any knock is detected. Alot of times, with forced induction, the ECU is far to late when pulling out the timing. Your engine detonates and explodes.

With the MSD, it simply tells you the severity of the knock with a visual led bar as well as an audio alarm. So you can monitor the knock. I simply added alcohol then added boost. Monitor knock for a few high rpm pulls, then add more alcohol and add even more boost. If I ever got a knock indication, I would add more alcohol.

This method will work for you as well. Accept you'll be increasing injector pulse and or fuel pressure with boost.

WD

Post by **encasemyheart** » Thu Mar 20, 2003 5:01 pm

cnichols wrote:The KA does have a stock Knock sensor.

You could also change the fuel pressure, which I assume would allow you to use the stock injectors above 4 psi and still make use of the AFC.

Yeha I hadn't thought of this. You are safe to 7-8psi on an FMU, which just raises pressure, so prehaps you could raise base pressure to handle 2-3 more psi? I assume the stock KA ECU can recognize positive manifold pressure?

Post by **huguetpj** » Thu Mar 20, 2003 5:01 pm

Quote »KA ECU can recognize positive manifold pressure[/quote]

Nope it can't. It doesn't have a MAP sensor.

Quote »the ECU is far to late when pulling out the timing.[/quote]

Yep, I thought so.

Quote »With the MSD, it simply tells you the severity of the knock with a visual led bar as well as an audio alarm. So you can monitor the knock.[/quote]

That's what I'm planning on doing by conecting the stock knock sensor to my Blitz SBC optional input display. This way I can monitor the knock sensor voltage by way of a digital bar reading.

afc to control boost

afc to control boost

Nissan Forums

Infiniti Forums

Datsun Forums

Technical Forums

Classifieds / Marketplace

Nissan Motorsports

General Forums