Somebody check my IFR math - won't start now!

[Mr. P.]

I replaced the injectors on my LQ9 with some aftermarket ones, they are flow-rated at 640cc @ 43.5-psi. Just to try it, I started the truck and it ran (!) and after I shut it off I did the following math:

1. rated flow = 640 cc/min
2. 640 cc/min = 62.75 lb/hr (rated flow)
3. rated psi = 43.5 psi rated
4. operating psi = 58 psi (measured)
5. SQRT(58/43.5)*62.75 = 72.45 lb/hr -or- 9.1288 g/sec actual flow

OK SO I put "9.1288" in every cell of the IFR table B4001, the truck has a return-style system with FPR, and now the truck refuses to start. The original value (3.xxx) from the factory tune is doggy but WILL start the truck. Has anyone seen this behavior before? Better question, anyone got the right IFR values for the Delphi Mini-60lb injectors sold by Five-O Motorsports??

Mr. P. :)

[SSpdDmon]

I get 640cc/min = 60.95lbs/hr. Using the old IFR spreadsheet I have, that equals 8.8677537grams/sec or 70.3802lbs/hr for 58psi.

Now my car runs a vac. ref'd FPR with the Motron 60's at about ~42psi @ 0kPa manvac IIRC and I have 64.485lbs/hr in my IFR using a stock MAF curve. The quick and dirty way I found what my car liked was to change B0120 to 400rpms instead of 4,000rpms. Then using the stock MAF curve, I made adjustments to the IFR to bring my AFR in line. Then based on that IFR, I would tune the VE if I was going to return B0120 to stock. I never did though. :)

This isn't the accepted method of tuning. But, it got me close enough to be happy.

[5.7ute]

As has been discussed a lot lately on this forum, just changing IFR is only part of the process.
With your larger injectors needing a very small pulsewidth at idle, small pulse adjust, minimum pulsewidth, default pulsewidth etc will all come into play. Plus the injector offsets will play havoc as they will make up a larger proportion of the final pulsewidth.
The few people that have this info are not giving it out for nothing, but a search on this & a few other forums will give you enough info to get started.(or frustrate you even more)
One trick I learned while playing with the 60lb injectors is to always log IBPW. This & a calc pid for actual IPW for fuelling will give you an insight into what is happening.
Check out this http://forum.efilive.com/showthread.php?t=10179&highlight=ibpw
Work on this has stopped since I am not using 60lbers on my new combo. Once this is sorted I will be getting back to it, but it will be a little while yet.

[Mr. P.]

OK how about this:

I decided to take another attack on this whole IFR thing, the factory IFR is 3.1797 and the factory injectors on an LQ9 are rated at 28.8 lbs/hr. So, the new injectors are 72.5 lbs/hr, which is exactly 2.5-times more flow; I figured since the new injectors were 2.5-times larger to just multiply the factory value 3.1797 by 2.5 = 7.9493 - I plugged this value into the IFR table and the truck started and drove. It ran doggy but the truck was very driveable (enough to easily get you home) and sure beats the non-starting behavior of the 9.1288 value I attempted to use from the INJECTOR.XLS spreadsheet.

As for the other injector-related tables, these injectors are modified Delphi 8.1 units (originally 44-lb/hr) so what if I could copy the injector values from an application which uses those Delphi injectors to get the offset and pulse width tables close, then last step put in the larger IFR value???

Comments????

Mr. P.

[5.7ute]

Obviously something doesnt add up. Either the standard injectors are 25lbhr not 28.8 or the spreadsheet is calculated incorrectly. I would think that GM has understated the injector to allow some tolerance for low fuel pressure, different stoich ratio fuels etc.
Once the injector has been modified all those other parameters will have also changed. But they should be close enough if you arent trying to get too OC with your tune.