Z28 Boy

ok, now i am trying to figure these things out.

is the duty cycle basically a percentage of the rated output an injector is working at? i guess what i mean is lets say an injector is rate at 100 lb/hr at a given pressure. does this mean that if the pressure is lowered enough to only allow the injector to operate at 80 lb/hr is is at 80% of its duty cycle? or if say the pressure was increased so that the injector was injecting at 120 lb/hr, the duty cycle would be 120%?

if so, all of the equations i see concerning fuel injector sizing only say to use an injector at no higher than 80% of it's duty cycle.

supposedly the GM injectors are rated at like 13 psi. yet, people run them a lot higher. if they are rated at 13 psi and all of these charts say to go no higher than 80% of that, the highest allowable fuel pressure is only 10.4 psi.

i basically understand the BSFC numbers..but th duty cycle has me thrown off a little.

-brian

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1991 Camaro RS
GMPP 350 HO crate engine - Holley TB, MSD ignition, SLP exhaust, everything is new
700R4 - 2,400 stall converter, vette servo, shift kit
My car's webpage - z28boy.cz28.com
President - Central New York F-Body Association
Email - z28boy@twcny.rr.com
AIM - "Z28 Boy"

fast_broker

iTrader: (1)

I'm not a master but my understanding of 100% DUTY CYCLE is the fastest rate that an injector can PROPERLY fire at and not fail over time. Highest rated "firing" rate, and it is more or less independant of fuel pressure. 100% duty cycle is the fastest the injectors guts can "move" back and forth, if you will. If the ECU tells the injector to fire faster than 100%, weird things happen as the injector can not keep up, causing misfires, etc, and improper fuel delivery.

Increasing pressure may lower the 100% number as the injector now has to overcome more pressure to move its parts. Likewise, a drop in FP would allow a slight increase in the number of possible firings. I don't know how much the FP impacts the actual duty cycle/firing parameters, just know that it can effect it, but I believe the effect is rather small???

On the opposite end, the injectors can only fire so "slow", too, before they just don't fire at all, ie, too big of an injector and/orr too much FP, mainly at idle.

This is why proper injector choice is so critical. I am sure Grumpy will give you the better/correct response to this question, in properly worded format to boot.

[This message has been edited by fast_broker (edited December 11, 2001).]

Z28 Boy

hmmm...anyone else?

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1991 Camaro RS
GMPP 350 HO crate engine w/TBI
Homepage z28boy@twcny.rr.com AIM - "Z28 Boy"

Brent

iTrader: (1)

Duty cyle is the amount of time an injector is commanded on by the ECM, divided by the amount of time availible (time it takes to complete one crank revolution). It's not related to pressure. It's a time ratio.

At 6000 rpms the crank takes 10msec to complete one revolution. Since TBI fires each injector twice per revolution the max time we have is 5msec per injection event. 80% (duty cycle) of 5 is 4msec.

100% DC means the injector is on constantly. The 80-85% number comes from mechincal limitations of the injector to meter fuel predictably. There are other considerations like FB mentions.

Z28 Boy

oooooooooooohhhhhhhhhhhhhhhhh

that makes a LOT of sense.

thanks a lot brent.

by the way, do you know f any good sites (or even books) that have a lot of this info? i looked at all the equation pages, but i am looking more for a general info type page.

thanks again man.

-brian

Brent

iTrader: (1)

www.diy-efi.org

Searchable Archives (very large source)
www.thirdgen.org/gmecm/

www.thirdgen.org/diyefi/



[This message has been edited by Brent (edited December 11, 2001).]