lorenolson888

Question... At the point where injectors cannot deliver enough are they at 100% duty cycle... i.e constantly squirting...

MarkRobinson

pretty much. you can still get quite a bit more out of them by increasing fuel pressure, though, safer to stay under 90% or so if it can be done. (I think 90% is the percentage)

Imo000

If the engine is leanign out and the injectors are at 100% then YES, they have reached there capacity. At 100% the injectors are always open. Anything over 80% is over stressing the injectors.

Tony

Ok, this is where a misunderstanfing comes up for me regarding duty cycle.
When you are WOT in this car and the RPMS pass appx 5100 rpm, the injectors will go 100% duty no matter what size you run.
I wish some one with a stock S4 would log this so i can see for my self if this is true.
19lbs...24 lbs..30lbs..42lbs...they will all go to 100% duty at appx 5100rpm under the right amount of load..this case WOT.

100% means they are open all the time. When this occurs the amount of fuel you can get into the motor and the result AFR is determined by
1) the size of the injector
2) the pressure behind the fuel.
3) the pump capacity

Everyone says thats you shouldnt run injectors at 100% duty for this or that reason..or for this or that duration...but you cant get around it in this car with the stock maps at WOT beyond appx 5100rpm...

..Is this true?

So in a stock S4 for example, on the dyno, putting out 267ish RWHP, the 19lb injectors dont go to 100% duty?

Am I missing some basic concept here. Its always puzzled me, espcially after lookign at my data logging. I just wish i had it when the cars was NA and stock.

MarkRobinson

You've datalogged all those injector sizes on your car on boost? JC..

blau928

Tony,

It is my understanding as well, that the 100% duty cycle means that the FI is open all the time.. That's why I opted for a secondary fueler to control the high RPM secondaries a bit more accurately and keep the LH..

I think the 24# is within the bandwidth of the LH's ability to be controlling the pulse width of the FI accurately. I think that 30# may be too rich at the bottom end...

HTH,

Tony

No i havent, i got my logger after the SC install. I wish i would have logged the 19lbs. But i will bet a beer or two that the 30lbs will do the same thing at 5100 rpm...as will the 42lbs...60lbs etc etc
After 100% it will be my fuel pressure that will determine the AFR not the pulse width of the injector...there is none so to speak..they are open all time at 100%..just like an open end garden hose..
Really is a grey area for me as far as understanding..
What makes it(the LH) go to 100% when we all say they should be ran there?

Are the stock maps only made to take the stock 19lb injectors to 80%..if so, i have never seen that becasue the LH has no clue what injectors i have in the car, never does.

BC

Yes, the LH shouldn't know anything about your injectors. Under closed loop (which seems to be a rare thing in the LH system) it just is seeing the Burned fuel on the 02, and adjusts from there. We all know that.

Why is the LH going to 100% at 5100 when there is 1400 more RPM for which there are varying levels of fuel needed? Its off closed loop at 5100rpm, at least at WOT, so nothing is comming in from the 02 to tell it to max out.

So that would leave the only thing telling them to open 100% - the Map in the LH.

Did you say that you thought this happens on stock cars?

At a base thought, its scary that when there is this boost level you have to tune the ARF with fuel pressure.

I guess I don't have much help - just more questions.

The SHark Tuner should allow you to run proper sized injectors at this level, such as about 60lbs, and reduce the ms times at lower rpms.

Jack '84 928s

Ive seen 100 pound injectors run at 65% duty cycle but they were in a wrx, stock injectors with the ends cut off.

tv

That is a great result! Now we need some tales of what you leave behind in the dessert dust.

all4woody

Tony,
I am running the stock S4 TB. I am looking at buying an APEXI Super AFC to piggyback the MAF, this will allow me to control the fuel per rpm. I am still doing research to see if it will work, check it out and see what you think. http://www.prostreetonline.com/buy/a...er%5Fafc%5Fii/
Later, Woody.

FlyingDog

1 Why is your datalogging rpm scale off by so much?

2 It looks like you're hitting 100% dc before 5100 if the dyno tach is correct. Looks like around 4900.

3 The LH going to 100% dc for 5100+ may be a combination of anti-detonation safety and extra power for running richer than stoich. They may have tried to make it run 12.5:1 (or close to it) at 5100 full load 100% dc. As you go from 5100 to rev-limiter you'll slowly lean out reducing power and making people want to shift before the fuel system runs out of steam.

Tony

Im not sure why the RPM is off....Im still getting the hang of this thing(LM-3 and logworks) . What i can do on it is basic compared to its capabilities.
Im going to try a simple inductive clamp next. right now the tachsignal is coming right off the LH and i dont know why it is off?.

I think Louie Ott mentioned some thing peculiar about the injectors at the 5100rpm point..cant recall. Somehting along the line of firing twice per revolution instead of once??? dunno


Anyway...
I put the 30Lbs in last night in about 2hrs...leak checked and pressure tested again to 90psi. Good to go!
Inital start up is fine and the idle is WAY better than it was when i had the 30lbs in originally . I used to have NUMEROUS airleaks ...the idle would hunt up and down.
Now with it all completely air tight and plenty of miles on it, the 30lbs seem to idle fine, just a little high on the RPM.
Since i had airleaks originally, the richness at idle was being masked a bit i think.
The AFR is now rich at idle and at 2000...3000..4000...and 5000rpm.As a quick test I simply placed the car in park...reved to each rpm, held it for some time as the logger recorded. The neighbor hood loves me this sunday

This is where the SHARKTUNER will be an obsoulte MUST! right now I assume the look up maps in idle dont have a pulse width short enough to control the 30Lbs injectors at idle. My fuel pressure is set as low as im taking it and can actually. The AFR does not hunt on my ARM 1 or LM-1. Sits at appx 12-13:1 and the last 2 blues lights on the ARM1.
Ill post the log here shortly and try another "top end pull" soon enough .

Time to seriously read up on the Sharktuner.

Jim_H

Correct.

Ed Scherer

Tony, do you happen to have a copy of handy? There's all sorts of info in there that might help answer these kinds of questions. I just flipped through my copy (I'm not sure I've ever cracked it, to be honest), and it looks like its filled with lots of detailed info.