One problem when installing some bigger injectors : they are difficult to tune when the engine is not fully loaded (mid RPM's) but are ok at WOT where the engine needs the extra fuel.

Since the LSJ fuel pressure (PSI) is constant at 60-62 PSI under every conditions, it throws waaaaaaaaaaay too much fuel at midrange RPM's with the bigger injectors so it's difficult to tune them (with a Mini AFC or a MAF translator) to achieve a flat air fuel ratio curve.

Here it is : Along with some just nicely calculated bigger injectors than stock (maybe 42's, 48's or 50 lb/hr ones, depending on your mods), how about controlling the fuel pressure (less than 60 PSI at low load, then more pressure at midrange and then the max pressure for WOT) ?

It could be done with this :

SUPER FUEL MANAGEMENT UNIT (SFMU) (a kind of ADJUSTABLE fuel pressure regulator)

http://www.vortechsuperchargers.com/.../fmu_sfmu.html (the second model; the "Super" one).

(Don't get me wrong; I know it will not add extra fuel with the stock injectors neither rising the stock 60-62 PSI fuel pressure at the fuel rail. And it's not a pump booster too, since the stock fuel pump is flowing enough).

I continue ... The formula for injector duty cycle include the differential pressure passing through the injectors. So the problem with running a small supercharger pulley is that the differential pressure (passing through the injectors) at WOT is too small ...

Here is an example :

Let's say I run a 2.6 pulley and boost 20 PSI in the manifold, so the differential pressure passing through the injectors is :

62 PSI (stock fuel rail pressure) minus 20 PSI (manifold pressure) = 42 PSI only ! If I calculate my injectors needs for (as a pure example) 350 BHP (engine power), it woukd need some 47 lb/hr injectors (rated at 43,5 PSI).

Now I could install the SFMU unit, install some 48 lb/hr (or 50's one), then (as a pure example) tune the pressure down to 30 PSI for idle, 35 PSI for 3000 RPM, 40 PSI for 4000 RPM, and 62 PSI for 5500+ RPMs (those numbers are some quick non-calculated examples).

What do you guys think ?

 

So you would need to go to a return style fuel system?

I think it's a good idea if you ask me.

I have thought about that from time to time but never as in depth as you have.

 

I think that the fuel pressure sensor will couse the PCM go nuts my self.
Try it and see.

Later

 

This does sound like a good idea!! Have you try it yet?

 

When I had fuel injection on my Mustang I had a SFMU. I had the idle set at 30 PSI and by the time the 14 PSI hit I had 85 PSI of fuel pressure. It does what it is supposed to do.

 

Please hurry up and test your therory soon, so we can know if this works. Then I can have this set up for the return of my engine which will need some extra fuel some of the time.

 

can't you just change the injector's constant to what ever lb you are using? at least that's how we did it on thirdgen camaro's...you'd change the rating form 23 to 30 lb and it would adjust the hole curve...plus at part throtle the ecm is adjusting to 14.7 anyway, so if your injectors contant is set to the right lb, it shouldn't dump fuel....I ran 24.5's on my TPI350 without touching the ecm, it took care of my lean condition on top but never dumped any extra fuel in the midrange...dunno haven't done tuning in a very long time maybe the cobalt is diff.

 

sounds god to me , but couldn't you use HP Tunners fix fix this situation?
wouldn't it be better to throw a little more money in and by HP Tuners and be able to modify every aspect of the car instead of screwing around with addition equipment?

 

I think what he's trying to acheive here is a quick fix for track days, while on the road, ect. I think it's a great idea, personally.