No wonder I'm confused. It isn't easy shooting at a moving target.



The injectors duty cycle is determined by the ECM in PE mode. How does the duty cycle change with the fuel supply? Do you have a wide band O2 and a computer programmed to maintain a 12 to 12.5 A/F ratio in PE mode?

It would seem to me that a good way to test the fuel system delivery would to go for a ride with a fuel pressure gauge taped to the windshield. Not as fancy as a dyno runs, data logging, etc, but it works.



[QUOTE=Demonic85 It sounds like the TPI pump cant handle the upgraded 80 lb injectors if all is flowing well otherwise.[/QUOTE]I don't know why it wouldn't. Two 80# injectors put out 160# of fuel at a 100% duty cycle at rated pressure. That is the equivalent to supplying eight, 20# per hour, [80 X 2 = 160]; [160 ÷ 8 = 20] injectors for a TPI. Even at 20 psi, less than half the pumps normal working pressure, assuming they will deliver the claimed 115#/hr. the pump only needs to supply, [115#/hr X 2 = 230#/hr]; [230#/hr ÷ 8 = 28.75#/hr], less than the equivalent of a TPI with 30# injectors. Duck soup!

RACE ON!!!

 

As the fuel pressure drops the ECM will try to correct by adding more fuel by incresing the duty cycle. Duty cycle is variable and will change over time based on fueling needs. If you run the injectors at 100% duty cycle they will burn up eventually. The common taget is a duty cycle of 85% to guard against them going static.

FWIW - A stock stock set of injectors (i kept them the same even though they have a different rating for simplicity) will support 233hp based on a 85% duty cycle. This is based on flowing them at 1 Bar (14.7 psi)

Yes I have a WB02 and it is setup properly. The tuning setup is 8746, with a HAM adapter, EBL and Ostrich. I use a combination of driving and dyno time. The dyno time has been no cost thus far so I use it when it is available.

 

My question, boiled down, is how does the ECM know the fuel pressure has dropped?



I only mentioned duty cycle in comparing the demands of the two injectors for the Crossfire to the eight for the TPI for Demonic85. I understand about 85% and 100% duty cycle. I shouldn't have even mentioned duty cycle in that post.



That power on that fuel has to be based on an "assumed" BSFC. That is interesting. The calculators suggest that I have 230+ rear wheel HP. However, I require less fuel to make that power, because I'm not operating at 1 bar.



I am not familiar with all that computer stuff, but you are fortunate to have the dyno access you have.

RACE ON!!!

 

The ECM would not know the fuel pressure has dropped per se. It would however know it is running lean (pressure drop) by the O2 sensor and then attempt to compensate by adjusting the pulse width hence increasing the duty cycle for more fuel. It will also reduce the duty cycle for less fuel. If you were to look at the code for the 7747 or 8746 you could easily see that is how it works. Luckily for me I wrote software for embedded systems many moons ago. Needless to say I can follow the code and have gained an understanding on how it works.

The calculations do not suggest you have a 230hp motor it shows the injectors can support 233 hp at that flow rate. You are correct on the assumed BFSC.

 

ElK: explain how in PE the ECU knows that the fueling is insuffcient(inadequate FP)? I thought when the ECU involkes PE it will look back at the last seen BLM in that cell and if high BLM(140-adding fuel ) it will use the high BLM in the calc for PE. If low BLM(114-removing fuel) it will use 128. Iin PE I dont believe the mv of the 02 sensor is used nor has any meaning in PE.

Are you using the WB02 to provide feedback to ECU in OL? If so then that is different.

 

I did not mention anything about PE but in general the O2 sensor is what is used for feedback. The O2 is the only feedback mechanism available.

 

You didn't mention PE, but since you didn't, *I* ***-U-MEd it because I couldn't picture the mixture going lean in full closed hoop at "4000rpm and at 5500rpm the pressure was at 15psi" at less than WOT. Admittedly, you don't say so, but also, you don't say not. The fact that the O2 isn't effective at WOT A/F ratios, and the ECM ignores O2 input in PE mode, was the reason for my question, "how does the ECM know the fuel pressure has dropped?". Possibly your silence is promoting unnecessary and incorrect assumptions. It appears Rohn was making the same assumptions as I.



The calculations I was referring to, weren't yours. This time, *I* wasn't clear. The available formulae, if you believe them, says my 1/4 performance indicates my engine is putting out the max power you say is possible with the stock injectors, but with less air. An interesting coincidence.

RACE ON!!!

 

To clarify the rpm... It was the range from 4000 to 5500 not 4000 and 5500 only. I thought that was clear but apparently not. As far as PE mode is concerend I need to look at the code again to put forth and informed response. No silence here just busy at the moment.

 

Well here it goes........

While in PE mode the ECM does not ignore the O2 sensor - according to the code anyway. O2 sensor readings are part of one of the realtime service routines. It is used to calculate the AFR at that moment of time when the interrupt is happens. It is then used as part of the routine that calculates the pulse width at a specific RPM. PE or not it is there. While in PE will it set the pulse width to a point where the injectors will go static? yes it may especially if the fuel pressure is dropping off it will get to a point where the injectors will go static (100% duty cycle). The ECM also calculates the theoretical fuel need based on gm's/sec (based on injector size) per given rpm and map reading or load on the motor. That number is also used in the calculation for pulse width. There are other influencing factors for pulse width, the physical characteristics of the injectors, injector bias, AE, PE etc. But I am not writting a paper on TBI injectors nor do I care to. It has already been done and available. If you wish I can post the assembly code and you can have a look for yourself. Heck, it would be good to have someone else look at it so I could validate my thinking. If I am wrong in my understand of what is going on I will be the first to admit it.

Getting back to my original issue... Fuel pressure dropping off from 4000 to 5500 rpm. The way to correct this is by parallel plumbing the TBs to correct fuel pressure drop off and higher rpm's. Supply both injectors a the same time (parallel) versus one at a time (series or stock). A stock motor will not experience this because the fuel demand is well within what the stock setup can supply.

Are there still tweaks to the tune.... yes of course there are. Is the saga complete no not really.

 

sorry for not posting sooner. The results are in and parallel plumbing does work. My duty cycles have been significantly reduced. Without changing fuel pressure they have gone from the 90's down to the 70's and no fuel pressure drop. This means I can now provide more fuel for hopefully more power.

 

Crazy. I would have never thought that would have made a difference. Congrats on the improvement though!

Let us know when you get more dyno numbers.

 

help IF you have the time ?? I have a 82 ported intake, heads, 355cid, mildish cam, all good parts on this re-build 355cid. health and $$$ an issues now. OK I have a T/Buick SFI and after many, many hrs on it I got the right inj/f/p etc. best time to date 10.56@128,[complete dif. animal] Ok now I have my Crossfire back together I KNEW that I would need at least 80lb/hr inj's to MAXIMIZE this combo, however engine was done EXCEPT inj's/Tb-51mm bore, I had the same prob. at or above 5000rpm's leaning out again I EXPECTED that, w/stock inj's, however put it on dyno anyway. [had 2 free hrs] I'm no newbe however I don't have the knowledge w/crossfire [like I do w/SFI] don't NOW have the $$$ to trial and error it. I need ta get it right the 1st time[or as close as possible?] so maybe you could answer a few ?'s I have? Since the tbi's are staggered can I JUST use stock set-up using the rear f/p reg to adjust f/p OR will I need to parallel plump them? Then in parallel will I only need 1 f/p reg and 1 f/p gauge? set-up between the pressure side/f/line to the tbi's? Again $$$ are a SEVERE problem now, plus time is running out for me......using stock ECM and yes I'm aware of its limitations using a PCD performance chip. ANY and ALL help would be GREATLY appreciated! Respecfully: Lance yes race on, life is short!

 

Produced 368hp@388ftlbs at the crank I KNOW theres more there w/ the right set-up on inj's/f/p However I won't be able to put it BACK on the engine dyno. They are friends of mine however they ARE runninng a business not charity. Its possible? MAYBE? that I MIGHT? get a lil time on the chassis dyno,,,,yet to be seen??? Again Thank you!