Replacement Fuel Injectors for 3.2's - Why's it so hard?

[Peter M]

On 20/01/2022 at 22:01, Mike D'Silva said:

hi Peter

did you change injectors when going up to 3.4 litres?

mike

Yes, to another set that I had on hand at the time.  They are actually higher flow than ideal (max actual duty cycle around 60% according to my data logging) so wouldn't normally recommend these larger ones for a naturally aspirated 3.4 engine at all. 

Currently I'm running a set of 0280 155 868's (261g/min @ 3 Bar N-Heptane).  In hindsight the 0280 155 931's (187g/min) recommended in this thread  would have coped fine with the bigger engine  capacity as 60% of 261g/min is only 156g/min. (Interesting! I've never done this calculation before today! According to Greg Banish's recommendations I should aim for about a 20% overcapacity in flow rate or 187g/min which matches the 931's perfectly!)

After saying all this the Motec manages the oversize 868's fine so the practice doesn't always reflect the theory fortunately.  The engine actually runs so well that I've never been tempted to change the injectors even though it only involves an easy couple minute update of characterisation data in the tune software.

Irrespective of my sloppy injector sizing practices, I still recommend these modern multi-hole injectors over the old tech ones any day.  Plus they are relatively inexpensive and very available as the two models referenced here for example are widely used in Holden engines.

For a turbo'ed street version of my 3.4, you could proportionally increase my calculated  injector flow rate to get a pretty sound indicative size based on my requirement of 187g/min being required for a MAP of 84kPa.  ie For a 0.5 Bar increase in intake manifold pressure (ie.to 134kPa absolute), the injector would need to increase to around 134/84 x 187 = 299g/min.

(Note my WOT MAP of 84kPa is lower than most because my data logging has been done at around 1100m elevation but this reduction is safely covered by the 20% buffer in the injector sizing calcs.)    

[autojack]

I've had this thread bookmarked for awhile... I have an '88 with Steve Wong chip as well, though my exhaust is all stock. As far as I know the injectors have never been serviced, and the idle has never really been as smooth as I'd like, especially when cold. I'm now debating whether to just send my injectors out for cleaning ($240) or make the jump to the newer injectors Peter used ($450). New injectors are tempting, I'm just a little wary of messing around with fueling when I don't really know what I'm doing.

[Peter M]

Jack,

The only extra messing around might be a couple clicks on the Fuel Quality switch on the ecu under the passenger's seat.   However, I would still recommend a dyno run to check AFR's are satisfactory.  That's probably not a bad idea as it will confirm the rest of the fuel system is operating satisfactorily as well, particularly the Air Flow Meter that is subject to aging.   So "yes" some extra messing about, I agree.

Before I changed my injectors, I had a very unstable cold start where the engine revs would isolate wildly for the first 10 -15 seconds before settling down.

Both solutions have their merits. 

[autojack]

Thanks Peter! My idle is not that bad, but just doesn't feel as smooth as it seems like it should. A bit rumbly and uncertain, with a bit of vibration. I think I'm going to go the cleaning route first and see how I go. If they are mechanically fine for now and that buys me a couple more years, I think I'll be satisfied with that. If the cleaning guys report bigger problems, maybe I'll go down the route of the replacements you used.

[sus911]

I ended up having mine cleaned. The guy said it did take a bit of cleaning to get them flowing back to spec. but the car continues to run really well.

[tomo]

All that tech stuff beyond my standard knowledge,  But in saying that, Have had injectors cleaned before on other cars, Worked OK for 6 months, But if old I think new is best. 2 c worth

[autojack]

FWIW, I emailed Steve Wong to see what he thinks about this. I was surprised that he isn't offering an updated chip matched to modern injectors, if there is a need for that. He responded that he doesn't think there is anything inherently wrong with the stock injectors, as long as they are clean and working correctly. He definitely thinks having them cleaned and serviced is worthwhile, but otherwise doesn't see a need to replace them just because they are an older design. Just figured I would share that here. I responded and told him about Peter's research and the newer Bosch injector he is using, in case that's ever of any use to Steve.

[tomo]

On 14/04/2023 at 10:57, autojack said:

FWIW, I emailed Steve Wong to see what he thinks about this. I was surprised that he isn't offering an updated chip matched to modern injectors, if there is a need for that. He responded that he doesn't think there is anything inherently wrong with the stock injectors, as long as they are clean and working correctly. He definitely thinks having them cleaned and serviced is worthwhile, but otherwise doesn't see a need to replace them just because they are an older design. Just figured I would share that here. I responded and told him about Peter's research and the newer Bosch injector he is using, in case that's ever of any use to Steve.

He might have got it Wong