robertbartsch

How does one determine if the injectors need to be larger:

If the duty cycle is 80% or less does this mean the injectors are OK?

Thanks...

slow

iTrader: (7)

80% is the general rule of thumb, if you stay under that, your probaly fine.

if you continually go over that, at wot, and high RPM's, its time to start looking into larger injectors.

Ryan

gillbot

iTrader: (27)

I just ran into this problem myself. Looks like I may need to look into 30#'s after adding just a cam and headers.

https://ls1tech.com/threads/showflat...=7&fpart=1

robertbartsch

I've heard a lot of conflicting stuff on this topic; some say stock injectors are good for 500HP...others say upgrade if you have a H/C car....

Colonel

I've learned to trust what's happening in the REAL world rather than in the world of not proven (and oftentimes disproven) calculation and theory.

For street and drag racing purposes, stock 26.4 lb injectors are safe at 450 RWHP with an A4. (I made 480 with an A4 on slicks.)

For stock 28.6 lb injectors, make it 490 RWHP with an A4.

In M6 cars they'll go even higher due to less drivetrain losses.

In my '02 I have stock 28 lb injectors. I make 423 RWHP NA and I wouldn't be at all scared to run a 75 FWHP dry shot of nitrous on top of this.

***ALL FIGURES ARE ON A DYNOJET CHASSIS DYNO***

gillbot

iTrader: (27)

Well, according to my O2's (I know they aren't the most accurate), My AFR is falling off at about 6100 RPM and has a slight dip right before I shift at 6400 where my injectors hit 95.5% duty cycle. I'd rather be safe and get bigger ones, but that's just me.

Colonel

Gillbot, if you are truly out of fuel system capacity, then something is wrong. You're nowhere near what that system should be able to pull. If you adjust your WOT mixture richer from 6100+ with LS1Edit, what happens? If nothing happens then yes, you're maxed out. If it does in fact get richer, you obviously are not.

And, are we talking about a major change (like going from 12.8:1 to 13.8:1)? If you're using your car's O2 sensors for fuel/air then you really don't know the answer to this question. In fact, you could be running rich and not know it.

Is your fuel rail pressure still at least at 55 PSI at all times?
If not...
Pump bad?
Filter restriction?

It's not always the injector's fault when mixtures go lean. The pump in my '00 ran out of capacity before my injectors.

BTW, I do agree that it's always better to be safe than sorry. But you do have to ask yourself if what you are doing to correct a problem (and if you really have a problem at all) is really the right thing to do...as in going to bigger injectors when in actuality the problem is that the fuel pressure is dropping to 45 PSI at WOT, for example. In that case, the bigger injectors would be a bandaid at best.

Colonel

Oh, and you are correct, your injectors are a VERY poor indicator of what your WOT mixture is actually doing. You might be very suprised to see what your mixture is actually doing if you get it on a wideband...I know I was!

gillbot

iTrader: (27)

I'm well aware that there are many other factors and I'm NOT saying that I am in fact out of "injector" since my car has yet to see a dyno pull or a wideband sensor. I was actually suprised that (with my minimal mods) I was seeing over 85% duty cycle, let alone 95%.

I have yet to get it on a dyno and truly tune in the top end to stabilize the AFR. I'm sure that there is plenty of injector left for me to add on top of but the fact remains that after I add any further mods I'm teetering on the edge. THIS is why I've decided to just go ahead and go larger now, since I plan on adding some items this winter, if budget (My Wife) allows it. I can't honestly answer how large the swings were since I don't have a wideband handy to tune with. I'm sure with wideband tweaking, the duty cycle may come under control but I'd rather have too much injector and run a little fat rather than go lean.

Colonel