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Fuel injector guru's - Relation of flow to duty cycle/impedance/ boost

In the process of tuning safely at higher boost levels, and well into the 400's (Got to be able to hang with Reap's ground-based SR-71 come December). The boost knob works perfectly, holding great up to 1.1bar. TECGT is fantastic, light years from the TEC1

http://uk.youtube.com/watch?v=D55DJasN-sU

However, my tuner was having concerns that the injectors were operating at the peak (possibly over?) of their duty cycle, as was still seemed to be running overly rich at boost, despite fuel changes. Sure enough, we lost an injector on a run soon after the vid was shot. The guy that built the engine may have known what he was doing from the build standpoint, but when it came time to tune the car for high boost, left that dept for others. Pulled the present injectors (some sort of daimler injector) that were probably crying for mercy when we started to ramp stuff up. Gonna install 62lb injectors next week. Anyway, am trying to wrap my mind around the concepts of fuel injector flow and the relationship of duty cycle to boost and what is required of a fuel injector as boost increases. Also, what exactly does high vs low impedence mean in terms of injectors for a boosted engine. GJF has enlightened me immensely on these topics, but if Goran or other can add their knowledge, would be great.
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Old 11-09-2008, 11:40 AM
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Fuel flow is a function of fuel pressure in the system. I have 60lb Mototron injectors and if I need more fuel delivery, I bump up the pressure. These Mototron injectors deliver 60 lbs at 45 psi and something like 72 lbs at 60 psi.

Duty cycle is a function of work over time. Your ECU should be telling you the duty cycle of your injectors. If the ECU indicates 85% duty cycle at wide open throttle, 6500 rpm and low AFR's, you're good. If it tells you 100%, you've reached the capacity of the injector and subject to failure. You have to have some wiggle room.

The mototron injectors I use are high impedance because that's what my ECU calls for (12-16 ohms), not boost. Most all ECU's I know of do not supply power to the injectors - they supply the ground source. For example, my injectors are wired directly to a dedicated 12v fused source. The ECU provides the ground when the injector is needed. At idle, the ECU completes the circuit by grounding the injectors for about 1.1ms at 45 psi.

I adjust the injector timing to whatever the AFR meter calls for. If I need more fuel, I increase the fuel delivery in tenths of a millisecond through the fuel table in the software. Boost has very little to do with injector timing as long as I watch AFR's. I think timing and boost are more related
Old 11-09-2008, 01:36 PM
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Hello!

There is nothing magic that happens with injector when boost is raised. FPR makes sure that fuel pressure is raised when boost is raised, so that pressure differential across the injector is constant (usually 3 bar). You usually choose injector size so that they use around 80% duty cycle when engine/mass flow is maxed.

An injector cannot operate "over peak duty cycle". Duty cycle is just a ratio between time injector is closed and open. It cannot go over 100%...as it means that injector is constantly open. Regarding high- vs- low-impedance injectors, you usually choose low impedance injectors over certain injector size as they open quicker. It's quite hard to find hi-Z injectors once you step over 50lb/s or so. Low impedance injectors are opened with full voltage (roughy 4 amps) and then kept open with lower voltage (resulting in 1 amp hold current). If your EFI has possibility to control low-impedance injectors (= peak & hold) then its only a matter of fitting new injectors.

So to wrap it up: there is nothing special that happens once you raise the boost. You just make sure AFR's are kept in check and injectors are kept between 70% and 90% at max mass flow and you're square. Also, fuel pump must support whatever the flow is at system pressure (usually 3 bar) + boost pressure (whatever it is).

Adjustable FPR is a messy and crude way of adjusting fuel delivery and should be avoided. Injector capacity does not scale linearly with pressure, spray pattern might change with "odd" pressure and fuel pumps will be taxed. It's best to use standard 3-bar FPR.

I don't quite understand how tuner "has concerns" about injectors operating at their peak (and are opening more than 100% ?)? They are operating on whatever duty ratio EFI tells them to. And if they are doing the mapping, it's should be just a matter of looking into fueling maps? If AFR's can be kept on safe margin without running injectors over 90% then it's OK. If they cannot, you install bigger injectors and redo the maps.

Regarding you failed injector: If you operate injector at high duty cycle for prolonged time it might overheat. Also, if you run a low-impedance injector without peak & hold circuitry it will fail (overheat).


There are few things you should ask your tuner:

1. Are AFR's OK troughout the range?
2. What are duty ratios for current injectors?
3. Are current injectors low or high-impedance?
4. Can your EFI trigger low-impedance injectors correctly (peak & hold)?


Regards,
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Last edited by beepbeep; 11-09-2008 at 01:50 PM..
Old 11-09-2008, 01:47 PM
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Ah, i see. Thanks Don. So with higher boost, the injector timing is the constant 'K', with variables (Delta's) being pulse duration and fuel pressure. Have been reading some in Corky Bell's book, but topic is spotty.

I may be getting into apples and oranges, but GJ stated that at with higher boost, you want to go with lower impedance (sequential) injectors, which fire at each ignition event, as instructed by a cam sensor. Why do you need this at higher HP applications?
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Old 11-09-2008, 01:55 PM
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Thanks Goran, just missed your post while answering Don. I understand better what's going on and can ask the proper questions of my tuning guy. What are your thought about when to go to 'low impedance' injectors?
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Old 11-09-2008, 01:59 PM
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Quote:
Originally Posted by juicersr View Post
Thanks Goran, just missed your post while answering Don. I understand better what's going on and can ask the proper questions of my tuning guy. What are your thought about when to go to 'low impedance' injectors?
Well, as soon as you step up to 60+ lbs injectors, choice will be made for you automatically as you simply won't be able to find high-impedance injectors of that size any more.

Reason for almost all big injectors being low-Z is that the biger they get, the shorter time they have to stay open in order to maintain the idle. Hi-Z injectors open slowly, but that's usually not concern as long as opening times aren't too short. But when injector flows a lot, you are forced to very small opening times at idle. And if you try that with hi-Z injectors,they start to open unevenly at their margins and mess upp idle, emissions and driveability.

For those reasons, most big injectors are more or less forced to use low-Z coils which "yank" the spindle fast enough to provide precise metering at very short pulse durations.

It works like this: you open the injector with full 12V in the beginning (resulting in 6A of current on 2 ohm injector, then go down to 3V after certain period of time just to keep them open but not overheating them. (figures vary from injector to injector).

Fitting low-Z injector on EFI that cannot trigger them properly (peak & hold) will result in overheated/burnt output stage of EFI and/or overheated injectors.

16 ohm injector is typically triggered with 0.75A. 2 ohm injectors is triggered with 6A. That's a lot more current/power that goes trough injector, and that the reason why it opens so quickly. But it also demands more from EFI.

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Old 11-09-2008, 02:16 PM
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Justin the reason you fire big injectors (over 55-60 lbs) in sequential mode is to be able to acheive decent idle AFR's. If you try to fire the injectors in batch (all fire simultaniously) you will not be able to acheive a desired AFR at idle. It will be too rich because the injectors can't be fired low enough not to over fuel the engine in batch. So by setting up the EFI to fire each injector for every spark event (sequential) you will be able to acheive a respectable idle AFR. This is from Electromotive's web site from the TEC GT manual.
High vs. Low Impedance Injectors
The TECgt is equipped with 6 fuel injector channels. These channels are pull-to-ground outputs, meaning that they provide a ground connection for the injectors when they fire.
The injector channels can be configured to run in one of two modes – either 2 amps peak with 0.5 amps hold for one low impedance injector per channel or 4 amps peak with 1 amp hold for a pair of low impedance injectors per channel.
To differentiate between high and low impedance injectors, simply measure the resistance across the two terminals on the injector. If the resistance is around 2.5 ohms, it is a low impedance injector. If the resistance is around 1.2 ohms, it is a throttle body-style injector. If the resistance is above 12 ohms, it is a high impedance injector.
Low resistance injectors are used when the injector is large and finer control of the low opening time is required. Most OEM injectors are smaller and are of the high resistance type.
When selecting your output from the 4/1 or 2/0.5 type you should match the output to your injector configuration. (see Figure D.1. 1 for parallel wiring instructions).
As a rule, always use the 4 amp setting for running injectors in pairs.
Use the 2 amp setting to run injectors individually one per driver.
The one exception to this rule would be the 1.2ohm injectors found in throttle body injection (TBI) setups. These injectors should be wired with only one injector per driver in the 4amp mode.
High impedance injectors can be used in place of low impedance injectors at any time, as a rule, use the same logic to adjust your outputs.
When the injectors are wired such that the circuit will flow more than 5 amps, the TECgt fires them in peak-and-hold mode. When this is happening, the injector current ramps up to approximately 4 amps very quickly, then drops to 1 amp for the remainder of the pulse width. See Figure D.1. 2 for details on peak-and-hold mode.
When an injector circuit is wired so that the circuit will flow less than 2amps, the injectors will be fired in saturation mode. When this happens, the injectors will ramp up to the maximum amperage determined by the circuit (should be less than 2amps), and keep the current at this level for the remainder of the pulse width.
The bottom line here is that the injector channels should never be required to remain turned on (in “hold-mode”) at a level greater than 2 amps. As long as the circuit is capable of flowing more than ~ 5 amps, the TECgt will enter peak-and-hold mode, thus limiting the current output. Care must be taken when using injector circuits that flow less than 5amps, since they will not engage the peak-and-hold mode. As a result, the injector channels will remain on full current for the entire pulse width. Also, if a low impedance injector resistance measures less than 2.2ohms when cold, its resistance may increase when warm, so it should not be used on a one-injector-per-driver basis.
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Old 11-09-2008, 03:56 PM
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Thanks Don (again). Think i need to go back and read my manual a little more... read over most of it but i kinda glazed over the injector part.

BTW, couldn't find the distributor gear-driven cam sensor on Clewett's website. Is it a relatively new item?
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Old 11-09-2008, 06:52 PM
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I just looked and is isn't listed anymore. I've used them in the past we just used one to set up a DTA system on a 3.8. But these have been around for a while. I suspect the new cam tower mount is the way everyone is going. One could be made using an old dizzy.
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Old 11-09-2008, 07:22 PM
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If you mount a dizzy, you'll likely have to re-engineer the crank sensor mount. Asuuming, it's using the dizzy mount stud for the sensor mount as they usually do, be it a clewett unit or one fabricated.

The cleweet trigger could get tricky as well because on one cam you've got a scavenge pump, while the other is blocked by the engine mounted oil cooler., assuming you still have a scavenge pump and engine oil cooler of course...
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Old 11-09-2008, 07:52 PM
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I have mounted a cam trigger on the driver's side cam while using the scavenge pump, it was crudely fabricated but it works.

I'm trying to remember exactly what I did, but I know I used a flat round piece of steel, and cut out all but a notch in the right spot, and used another VR sensor like one on the crank sensor to trigger it. I mounted the sensor to the original housing that held the sprocket for the smog pump, and replaced the sprocket with a trigger wheel. But, sometimes the scavenge pumps are mounted directly to the cam carrier and don't have a housing for the smog pump drive so not sure what you should do at this point.

Typically cam synch sensors are more reliable when using a hall-effect sensor, though, as the lower speed from the cam generates a weaker amplitude of the signal, where as a hall effect sensor is self powered and simply uses the presence of ferrous material to trigger the sensor internally. With the VR sensor, you are actually generating the voltage used for the signal...
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You can set up the tec unit to use either hall of VR type for the cam...
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Old 11-09-2008, 07:57 PM
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jesus, I can't type tonight...
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Old 11-09-2008, 08:04 PM
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Mike, the clewett cam trigger sensor looks like a pretty easy install (pelican on the Technical board did one on his ITB EFI), and will fit on the passenger side cam site if the engine oil cooler is removed and replaced with an in-line flow filter unit. This is not a problem for cooling if a front mounted cooler is in place, which is necessary anyway for these cars.

Looks like a sequential injector system. cam sensor and all the fixin's will be in the future as i push the envelope with this motor. Until i get completely familiar with GT system, gonna have some fun with lower boost and non-seq set-up for a while tho.

Thanks for the explanations guys. Really digging learning about tuning in boosted applications.
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Old 11-09-2008, 09:08 PM
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Keep us posted on your idle quality running batch fire with the 62lb injectors. Working on a similar setup right now and wondering if I should go ahead with the cam sensor and sequential injection setup right out of the gate.

Thanks, Dan
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Old 11-10-2008, 11:59 PM
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