totallimmortal

OK so i'll be buying injectors soon, I have a na with an safc, i plan on a supercharger or turbo intalled on this engine, not a swap, so i want my injectors to be able to supply the fuel i need when i do get forced induction. What size should I go with? should i only do the secondaries? And what is the largest injector recomended for use with a safc? or will any size still be tunable?

J-Rat

iTrader: (2)

The SAFC is better at turning down injectors then turning them up. You need to have a HP goal, then picking your injector size may be a little easier. In my opinion though, go bigger then you need, and then turn them down.

Digi7ech

iTrader: (1)

I might guess 680's or just going 550 primaries then 680/720 secondaries.

Not really sure since it would be a high compression engine taking forced induction.

Aaron Cake might know since he slapped a turbo on his ride.

NZConvertible

Electronically the S-AFC is just as capable of turning them up as turning them down (+/-50%). The limit (as always) is the size of the injector, since if you increase the setting up to the point where injector duty cycle hits 100% they won't flow any more fuel no matter what you set it at.

totallimmortal

well i'm not really sure on a hp goal but what kind of hp would 550 primaries and 720 secondaries support? I'd be fine with running 1600s if i thought that the safc could turn them down enough. i guess we can go with 300 flywheel hp as a goal what would be needed then? I know there are clalculators on the net to find out but i don't know how to figure in the fact that we have secondaries and not just 4 injectors as a 4cyl would

J-Rat

iTrader: (2)

Not to mention the fact that the ECU wont let the injector see anything close to 100% duty cycle. Hence my point.

At 2 550s and 2 720s, the comuter I use gives a max HP of around 270- 320 given the BSFC.


All the SAFC does is trick the ecu into thinking the AFM signal is bigger (or smaller) then it really is. Its still limited by the stock parameters of the ECU itself.

Which leads me to another question, what AFM percentages (open) do you see on your SAFC at WOT? I only see about 74%.

NZConvertible

There is no duty cycle limit built into the ECU. That is a myth. The ECU calculates injector opening durations (pulsewidths), not duty cycle. If you increase the airflow signal enough (whether electronically, with actual airflow or both) and rev high enough then the pulsewidth can exceed the engine cycle duration, i.e. 100% duty cycle.

J-Rat

iTrader: (2)

Okay, now I am confused. Pulse width and duty cycle should be interrelated.

Puse width being the amout of time that the injector stays upen (usually measured in some sort of time), and duty cycle being a percentage of an ENTIRE cycle (open and closed) that the injector stays open.

Now, Can the ECU drive circuts actually go to 100%? If so, thats news to me. I am not doubting the veracity of your statement, I just havent known anyone to have measured that. I would assume that if you max out the value of the AFM it should be a max value of 100%?

Also, what AFM percentages are you seeing at WOT?

totallimmortal

i'm not even seeing about 70 i don't think, i';ll have to check tho

Icemark

actually I have two different very well known injector sources now saying that the 86-87.5 US model ECUs (N326 and N332) were set to a max of 65% duty cycle with the saturation type injectors, rather than adjustable for up to the maximum duty cycle as found with the peak and hold injectors found in the 87.5 and later US models.

If this really is the case, then it would explain the rumor of a duty cycle that seems to keep popping up from time to time.

J-Rat

iTrader: (2)

What about the 87.5 and up?

Icemark

since the 87.5 and later are peak and hold, it wouldn't matter

J-Rat

iTrader: (2)

Can you please explain that? Far as I know, the terms saturated and peak and hold only refer to the resistance of the injector.

NZConvertible

They are, but the ECU only cares about pulsewidth.
Right, so the same pulsewidth results in higher and higher duty cycle as rpm increase.
Why wouldn't they? The ECU doesn't calculate duty cycle, only pulsewidths.
Not unless the factory deliberated calibrated the ECU and AFM so the two parameters matched, but I doubt they would bother, as there's no point.
Me? I haven't got an S-AFC.

But a duty cycle that low corresponds to the peak duty cycle of a totally stock engine. If that were really the limit, modifications would be impossible. As soon as airflow started to increase above stock levels, dangerously lean mixtures would result. Yet we know from experience that that doesn't happen.

neptuneRX

too hell with it, I was gonna run into this problem when I got my supercharger, I'm just gonna go ahead and fork the cash for a aftermark ECU. Have it tuned with new 550cc/720cc injectors before and after the supercharger upgrade.

Roro

What if the stock levels were say.. Around 40%? Wouldnt that leave room for error (read, increase to 60%)?

Using 4x550 and a BSFC of .65, the 550s should be able to handle around 257 hps.

Your assuming that the stock levels represent 100% using this statement.

NZConvertible

They aren't.

J-Rat

iTrader: (2)

And how do you know this?

HAILERS

I don't understand most of what is written here, but......using a Fluke 88, set on Percent Duty Cycle, and the lead connected to a Primary injector, on a very slightly messed with 87turboII, the duty cycle WILL hit in the low 90percent area just prior to the secondary injectors coming online, whereupon the Primary and secondary drop to about 40 percent and then both start to rise as long as the foot is planted on the floorboard. Sorry, but I don't know exactly how high the duty cycle will go after that. Never held it down long enough. Unlike J-Rats car, mine is extremly fast. and won't stall out like his after 130mph.

As I mentioned a while back in another post, I noticed at the Radio Shack they have a meter for approx sixty bucks that measures duty cycle and probably the duration also most likely.

NZConvertible

Because 880cc/min (four 550cc/min injectors at 40%) is not enough fuel to support 200hp (S5 TII). 55-60% would be more realistic.

Icemark

Another (from Injector.com) Marren Fuel Injectors:

NZConvertible

Again I ask, if that really is the case, how are any engine mods possible on those cars? It just makes no sense.

totallimmortal

OK so now that every one has argued points not regarding my question i'll make it simpler. What is the largest injector recommended for a stock port NA? And not the largest reasonable i'm i'd be fine with running th safc at -50% untill i got forced induction if i have to i just want injectors that will not have to work hard at all noww, and still be not maxed out with forced induction. I'm thinkin 550/720 that sound good? i shouldn't ever need to support any more than 300 hp

Icemark

stock port, without forced induction, the max size I would use would be 460cc on primary and secondarys.

The stock 460cc with stock computer run crazy rich from 5000 up (with up to about 200 FWHP).

Once you go with the forced induction then go with the larger injectors. But before that, is just a waste of money and time.

And remember, even using a AFC, you don't want to lean the upper RPM fuel curve down less than 18/20 or you run into lack of rotor lubrication issues at high RPM. The fuel and oil, act as lubricants in the system at upper RPM.

NZConvertible

Stock.
Why bother replacing the injectors before you need them? There's no point. You're going to be pulling everything apart to add forced induction anyway, so install them then.
That would be fine for 300hp.