Ok, some of the veterans will need to help me out here. My 24 pound injectors just won't cut it anymore and I got a good deal on some larger ones. They are actually much larger, on the order of 42's. Now I know this is a lot of overkill, 30's are what I really need but I couldn't pass up these 42's. I installed them this morning and programmed my injector constant.

They are SVO injectors so I used the following formula

Effective Flow Rating = advertised rating * sqrt(running pressure/rated pressure)

This gave me about 44 for my injector constant. I programmed a chip with this value and sure enough the car will run. The car idles and seems to run fine but it just has 2 small problems.

1. Sudden hard throttle or "blips" of the throttle cause a slight stumble

2. INST MPG values are way too high.

Now before you all go jump on me and say these injectors are too large try to think about it and help me out. Consider that a supercharged car running these same injectors is still going to idle and run for the most part still under vacuum. These driveability issues can be tuned out of those cars so it can't be impossible.

Where should I start? I'm guessing the injectors are giving too much fuel with the tip in throttle response, hence the stumble. I'm also guessing that for cruise the pulse width is so small that it thinks the injectors are actually closed, hence the high INST MPG readings? For the record my INST MPG didn't behave very strangely before when I had borrowed a set of 36# injectors. In fact the car drove VERY well without changing anything but the injector constant.

Another thought I just had, I'm thinking when I programmed the 36's I programmed them as 39's because I thought SVO injectors were rated at 36 PSI. I found that SVO injectors are supposedly rated at 39 PSI and that's what I used for these injectors. Assuming 36 is correct I should rate these new injectors closer to 46#'s.

Any thoughts are welcome? I would really like to use these injectors if at all possible. I can't know for sure what the future hold for this car and I might need them later. If there is any way to tune them out I'd love to do it, I'm just not sure exactly where to get started.

 

I know you said don't but In my limited experience.. if you run too big an injector even with tuning you never run near the Optimum operating range of the injector thus not getting the proper or optimum spray.. atomization or whatever you fancy.. You are sort of trying to use a 4" fire hose on a garden faucet for lack of a better analogy.. just my 2 cents and I may be wrong.. I tried to use the svo 30 lb injectors on a stock lt1 and never got the tuning perfect.. and this was no where near the size difference you are dealing with.. I switched back.. The only reason i did this was due to the price and I needed new injectors at the time.. ended up costing me more when i had to buy a new set of stock replacements as well as the SVO's. I know your engine is no where near a stock but the 30's may fare better for you ... Let me know how it comes out for you..


[Modified by Ericthewolf, 10:43 PM 5/16/2003]

 

Have you checked your fuel pressure? I am running 42lb/hr Lucas injectors in my supercharged 396 LT4 and I don't have any problems with stumble or hesitations. I am running a static 50 PSI with the vacuum hose disconnected at the regulator and btween 43 and 44 with the hose connected.

SVO injectors are rated at 39 PSI as far as I know so your first calculation was correct at 44lbs.

 

Fuel pressure is fine, it's in the low 40's at cruise, going up to around 50 at WOT. Of course under high vacuum it goes way down, around 36, like if I just let off from 6000 rpm's.

 

OK if the fuel pressure is fine, then if you have a scan tool be it Datamaster of Freescan or something else. Check to see what your fuel trims are doing at cruise and at idle. Perhaps you have one injector that is out of wack or leaking that can be causing a stumble. With a stock or near stock engine you should be seeing something close to 128/128 or there abouts. If you see one at 128 and another at 108 it means its running rich on one side and its correcting for it. Or it can go the other way around as well one side can be real lean (>128) and it would cause the same problem.

 

42# is too big man .. gotta be spraying or blown to need that big - course you'd want like 50# then, lol.

 

It seems like everybody is still ignoring the fact that forced induction motors still operate under vacuum a large precentage of the time, especially in the case of a centrifugal supercharger such as a vortech.


In any case I don't believe any of the injectors are leaking because it holds it fuel pressure after you shut it off. Unfortunately I let a friend borrow my cable so I can't data log right now. I should get that back today and I'll have more time to play with it later in the week.

 

Nathan,

The injectors are too big for your needs. Forced air increases your injector performance. So your rich! Find a buddy with a wideband and determine what your a/f is and tune from there....this is the only sure way to see what the problem is.......

Frank :flag

 

You don't need a wideband ... if you're over 1000mv, you're way rich anyway. The stock shortband can read to 1000mv (what i'm told), and will do fine since you want 870-890mv.

 

The injector my not be leaking but it could be spraying a lot more or less fuel than what its suposed to be. Get your scan tool back and check it out. I used lots of 42 and 50lb injectors on blown engines that are running not radical cams but cams producing a relatively smooth idle. I never had this problem with any of them. I may have seen some rich indicating fuel trims but the PCM was still in control within its range of control. Adjusting the injector offsets moved the trims back to within the proper LTFTs.

And you are correct under most conditions a blown engine is not under boost and does not need more fuel. Having the larger capacity 42lb or larger gives me the extra fuel when I do need it. But even then the added boost enrichment needs to be written into the PCM or ECM prom image for that enrichment. It not going to just happend by installing them.

 

Yup, really need some scan data first.....