TunerPro shows my W.O.T. injector DC% as being over 50%, I'm assuming that's for one of the two injectors?
With the stock 55 lb/hr LO5 injector should I look at increasing the fuel pressure and redoing my VE table to compensate?
In the attached screen capture my injector base pulse width looks like it's decreasing as RPM rises, is that correct?
The duty cycle is the same for both injectors, it is one value. You should not worry about DC until it is over 90%. you are fine with the DC.
Square body stepsides forever!!!
Your duty cycle calculation is wrong!!! At 7 msec at 4,000 rom you are very close to 100% duty cycle. With two injections per crank rotation at 4,000 rpm you have 133.33 injections per second. 1000 ms per second gives you 7.5 ms between injections. It looks like they are not doubling the value for two injections per rotation. 7.06/7.5×100 = 94% DCOriginally Posted by Kitch
Last edited by Fast355; 08-23-2015 at 05:31 PM.
Hi Fast,
From what I had previously read it sounded like I should expect the stock TBI LO5 injectors to be getting maxed out.
I was noticing a trend while using the spreadsheet to correct my VE table, the corrections were working well except at the 90-100kpa end of the table where it kept staying lean. The fuel pressure gauge would read nice and constant and if tweaking the VE table was making no difference then I was wondering if that was all the injectors had to offer!
From the two attached images what variable do yo think needs to change, something in the DC% equation or is there a problem in the actual base pulse width calculation?
Incorrectly displayed DC% aside, from a tuning perspective is increasing the fuel pressure the next step?
Duty cycle equation is wrong. For an easy fix multiply by 200 rather than 100. That equation is assuming one injection per rotation and TBI has two meaning half the time available or two times the duty cycle.
At close to stock fuel pressure I would expect a TBI engine especially with headers to be running out of pulsewidth at higher RPMs. Next step is to increase pressure or injector size. I prefer increasing pressure for mild changes.
Your stock injector is 61# @ 12 psi. At .5 bsfc and 85% duty cycle they are good for ~210 hp. The 55# in the 305 are good for about 185 hp worth of fuel. The 46# 4.3 injectors are good for 165 hp. GM closely sized the injectors and fuel pressure for the HP of the stock engine and did not leave much room for added airflow and fuel requirements at the stock fuel pressure or injector size. I have had a L03 F-car that headers and a catback required me to toss 350 injectors into it to keep it from running lean and static as early as 3,500 rpm.
Last edited by Fast355; 08-24-2015 at 08:15 AM.
That was quick!
I'd previously tried the multiplier change to 200 and it it worked pushing my DC% off the graph above 100% but I thought that was too easy and changed it back
I take it that upping the fuel pressure means a VE table redo?
Do you think my base pulse width mV's falling off as RPM rose was because the injectors were working so hard or would there be another reason?
Base Pulse Msec follow the VE table. If you have 99% @ 3,000 and drop to 85% at 5,000 your base pulse will drop but the duty cycle can still increase because RPM has increased and time between firings has decreased.That was quick!
I'd previously tried the multiplier change to 200 and it it worked pushing my DC% off the graph above 100% but I thought that was too easy and changed it back
I take it that upping the fuel pressure means a VE table redo?
Do you think my base pulse width mV's falling off as RPM rose was because the injectors were working so hard or would there be another reason?
If you are talking O2 mv pushing the injectors over 85-90% DC will make them go static and really do some weird things to your air/fuel ratio.
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