I was looking back through a data log recently and the Duty Cycle shown by Datamaster never got much over 40% @ 99.6% TPS pretty much on the floor in 1st gear all the way up to 5200 rpm's. I realize that I want to stay below 90% duty cycles, but how low is too low. I am currently running rich at idle and partial throttle, but haven't pushed my spark tables yet. I have Venom injectors (approx. 69 lb/hr flow tested @ 122 cc/min) running 18+psi fuel pressure. I have some idle and off throttle issues and was wondering if I was over fueled and losing spray pattern due to very low duty cycles near idle (1.5%) Thanks, Charles

I would say over on injector size. But if idle is OK you could live with it. 1.5% I cannot relate to but I am at 1.9 mSec.

Quote: running rich at idle and partial throttle.

?? what is the A/F ratio? I am 13.3/1 at idle OL and that is Ok by me. partial throttle you cannot be rich if you are in CL and then have to be at stoich. If running all OL then could be rich. I suppose you could be over the limit of BLM in CL so then you could be rich as cannot pull more fuel.

Ronny,
Thanks for the reply, I was beginning to think I had asked something dumb. I have done some searching ,but have not found anything relevant to my question. I am currently running a BPW of 1.05 to 1.6 at idle (a few as low as .96) according to Datamaster that is milliseconds. I do not have a way to check AFR. I have not purchased a WB yet and am not sure where to weld in an extra bung for best readings. I am still running rich in closed loop in a number of cells because BLM's start sinking and engine corrects. I have several idle issues or near idle issues. The idle is not very smooth and I thought I would remove some spark to try to smooth it out; spark is 20-22 at warm idle (which seemed like alot). I also have some surging 200-300 rpms when closed throttle and rolling until the truck comes to a stop. I also have almost no engine braking because the rpm never falls to idle levels when coasting. I was just looking at duty cycle and 40-45% seemed awful low for near full throttle at 5200 rpm.
Thanks, Charles

All DC is is the % of time the injector is open compared the the time it is closed, so if you DC is 50% then you know the injector is open half the time and closed the other half. If you worried, turn down the pressure and adjust the IFR in the tune and that will raise the DC since they will have to be open longer for the same amount of fuel to be injected. There is no way your only running them at 1.5%DC. Something is wrong with that reading.

Dont forget, that is not a actual reading of anything, that is a calculated value.

I agree with your assessment about that seeming really low,(I kew it was calculated) but my millisecond readings are very low also below 1.6 and on some of my problem areas in my tune they drop to .35-.55 milliseconds. I thought I read somewhere else on Thirdgen below 1.7 they injectors get real flaky and don't pattern well. At 99.6 Kpa and 5100 rpms my BPW was 5.4 milliseconds and Duty cycle calculated to 45.9%. Static is at 100% or greater than 7.5 msec above 3800 rpm's or 4.25 msec above 6000 rpm's. At 5200 rpms 100% duty cycle is 5.77 msecs but at 85% duty cycle that would be 4.90 msecs. If my datalog is correct that would suggest that I am static or very close to it 5200 rpms. IF that is the case I could increase fuel pressure but that will make my low end fueling more difficult because idle area BPW will get even shorter; if I understand correctly larger injectors will have the same problem. How do you deal with this?

Different ECM but should still apply

http://www.thirdgen.org/techboard/tb...ing-issue.html (tuning issue)

This was from RBob:

The '747/'8063/'8746 ECMs fire an injector every time a plug fires. There are two injectors. They alternate on plugs firings.

Before I go too much further this is in synchronous mode. Sync mode is sync'd to plug firings. This is the most common mode. I have seen a few cals that are always in async mode, but never an f-body cal.

So for each revolution (V8) there are four plug firings. It takes two revolutions to fire all eight cylinders/plugs. Alternating injectors, that means each injector fires twice on each engine revolution.

At 4,000 RPM (revolutions per minute), divide by 60 for revolutions per second: 4000 / 60 = 66.67 revolutions per second. Invert this value for the time of each revolution (or frequency thereof): 1 / 66.67 = 0.015 seconds. Or, 15 milli-seconds (msec) per revolution.

With each injector firing twice each revolution we need to divide the amount of time in half: 15 msec / 2 = 7.5 msec.

This 7.5 msec is the amount of time an injector has before it is fired again (at 4,000 RPM). The higher the engine speed, the less time there is for an injector to deliver the required amount of fuel.

This is why it is important to keep an eye on the injector PW. Once the PW value exceeeds that given amount of time, no more fuel can be added. The darn injector is on ALL of the time.

At this point the only thing that can be done is to increase the delivery of fuel. Bigger injectors and/or higher fuel pressure is required.

As another data point at 6,000 RPM a TBI injector is static once it hits 5.0 msec of on time. For best results TBI injectors shouldn't be open for more then 85% duty cycle. At 6K RPM this is 4.25 msec's.

I believe the thing to do is get a vacume adjustable fuel pressure regulator.

Interesting that you brought that up, I have been investigating that all day today. It just seems like the resolution with large injectors is not accurate enough to get the idle dialed in, but I am running out of fuel from the looks of some recent datalogs of the BPW above 5200rpm. Anybody else have thoughts on vacuum referenced fuel regulator? I will obviously need to adjust my tuning and need a WB also to dial it in. Thoughts? Also I am running Venom injectors that appear to be stock GM 61# remanufactured to flow more like 68#'s I am also currently running about 20 psi of fuel pressure. I have also considered getting BBC injectors to rule out problems with the venoms but a larger injector is only going to make my low rpm tuning harder.
Thanks, Charles

I am running a 350SBC (360 cu in) 10:1 with aluminum heads, comp cams (Summit part# CCA-08-304-8) Duration at 050 inch Lift: 210 int./220 exh.
Advertised Duration: 266 int./276 exh.
Valve Lift with Factory Rocker Arm Ratio: 0.500 int./0.510 exh. lift
Lobe Separation (degrees): 114
50mm TBI
What are some other OD users running for injectors / fuel pressure/ vacuum FPR or not and BPW at idle

To help improve idle quality when using large injectors at higher fuel pressure one can do several things
VAFPR. I am uncertain if all are same quality. aeromotive is prob the best.
Run open loop with mixture richer than stoich(<14.0/1).
Run synch fueling(disable asynch). I believe that allows consistent pulse width.
Not sure if this is recommended but I idle better at 16-18d SA.

I think GM sets some parameters to help with emishions. I am non emishions.

Ronny,
Which Aeromotive, where did you get it? Part number?
Thanks, Charles

I think this is what you're looking for.

Definately too much fuel at idle. That small a pw and inj control is real sketchy, compounded by too much pressure at idle and you have real sluggish injectors. Response time decreases with fuel psi increase. You might tweak the low pw offset table in addition to a vafpr.

It is the one with two springs. One is 1-20 lbs and the other is 19-60 lbs for FP. I find the weaker spring set at 19 lbs for me allows the VAFPR to pull the pressure down further at idle. I will guess it ends in 133**01.

patcon: what on Earth prompted you to install 69 #/hr injectors for a 350????? No wonder you have tuning issues. And you are running at 18psi fuel pressure!!! Save yourself alot of tuning pain and install Ford Racing 24 #/hr injectors. Heck, I am running Ford Racing 30 #/hr injectors in my 395 cu.in. stroker.

Actually, the idea is to install an injector size that will put you around 80 to 90% Duty Cycle while running at WOT. Also, its best to run as high of a fuel pressure as possible. With a properly sized injector, you will have a nice idle as far as fuel tuning is concerning. If I remember correctly, the shortest injector pulse width should be about 0.8 msec.

FWIW: Before I went with the 30s, I ran 24 #/hr injectors at 52 psi fuel pressure in the 395 cu.in. stroker and turned a 12.12 sec 1/4 mile.

DOC. We are assuming he is TBI. I however did not check his personal profile.

You are correct Ronny, I am running TBI. I am also currently running about 20 psi of fuel pressure ( works out to about 106# when adjusted for pressure; if I did that right). I had BBC injectors early on from Fuelman I believe ,but if I remember right they didn't pattern well and dripped (18 mos ago); any way I removed them for the Venoms (they were recommended by my chip programmer) and upped fuel pressure. I am just trying to figure out what will give the best results. I could try to find some 75# injectors and run VAFPR 20psi or so at WOT, I could run 80#ers and VAPR (I found some flow matched ones, or I could up my fuel pressure to 22-23# could also add VAPFR to try to help with low BPW all three options should feed about 400 HP. I am just trying to figure out was is going to present the least amount of problems.
Thanks for you help,
Charles