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Discussion Starter · #1 · (Edited)
I've read some article somewhere stating something like a 60-70% percent duty cycle being ideal for longevity and atomization.. I can achieve this by cranking up my psi and tuning for the next size up fuel injector.

The only thing I could think of is the fuel enters at a higher psi than an actual 24lb injector and wouldn't fog (atomize) as efficiently...

So I could set my 19lb (21) to 55psi and tune for 24lbers and achieve a 60% duty cycle at the same time?

Anyone? Let it rip!

P.s. I ran out of dough, green backs, skins... and have a big n/a cam and bolt ons to supply it and a factory sct tune... otherwise I'd upgrade the injectors and dump a dyno tune it..my car goes silent under heavy throttle... gap is set at .32 step hotter, msd Edis and 8mm wires 96. Gt aode with a cobra dohc swapped in... imrc removed but slides are there.. full exhaust, 80mm maf cai, fuel reg, 250lph fp.

The more I crank up my fuel psi, the earlier my car goes silent in rpm range under heavy throttle
 

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Discussion Starter · #6 · (Edited)
Well, I have no imrc plates. Just left the slides in... had no way to control it anyway, it'll help raise cylinder psi up top or down low if I go s/c... planning on gettin a 3.90 or 4.10 on a set of 18s anyways....I wouldn't need a bigger tb or maf when I move up to 24lb... i have the red tops out of my for gt now
 

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Discussion Starter · #7 · (Edited)
I can't do much about how rich it runs, the pcm won't let me by... it runs nice and rich until I've used the car a few times.


That's with my car tuned with the handheld for 19# (21#) at 53 psi (new flow rate would be around 24#) it just cuts out, but it even cuts out down at 32-40psi just with more throttle and a thousand rpm or so higher... on two different fp's, a walbro 190 & aeromotive 250 or 255 ... I want to tune it for 24s... return style... there like a 2 or 3% difference in fuel trims on each bank
 

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Discussion Starter · #8 ·
I know the injectors would run cooler at a much lower duty cycle, most of the line in our return cars are aluminum, heavy duty hose at the tank, new hardcore hose in the tank lol... and even up at my aftermarket rails with brass fittings.. so it's not a question of bursting...what about the injector slides at the nozzle?
 

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I've read some article somewhere stating something like a 60-70% percent duty cycle being ideal for longevity and atomization.. I can achieve this by cranking up my psi and tuning for the next size up fuel injector.

The only thing I could think of is the fuel enters at a higher psi than an actual 24lb injector and wouldn't fog (atomize) as efficiently...

So I could set my 19lb (21) to 55psi and tune for 24lbers and achieve a 60% duty cycle at the same time?
Running the orange 19 lb/h injectors at 55 psi makes them effectively 22.3 lb/h injectors, operating the pink 21 lb/h injectors at 55 psi would make them 24.6 lb/h units.

Here is the formula for recalculating capacity as fuel pressure is changed:



(55/40)^0.5 = 1.173

19 * 1.173 = 22.3

Anyone? Let it rip!

P.s. I ran out of dough, green backs, skins... and have a big n/a cam and bolt ons to supply it and a factory sct tune... otherwise I'd upgrade the injectors and dump a dyno tune it..my car goes silent under heavy throttle... gap is set at .32 step hotter, msd Edis and 8mm wires 96. Gt aode with a cobra dohc swapped in... imrc removed but slides are there.. full exhaust, 80mm maf cai, fuel reg, 250lph fp.

The more I crank up my fuel psi, the earlier my car goes silent in rpm range under heavy throttle
You need to get a wideband O² system on there and monitor the AFR when the engine bogs.

What tune are you running (the stock GT tune or did you install the Cobra PCM as well)?

What MAF are you running?
 

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Discussion Starter · #10 · (Edited)
Nah, it's the Gt pcm ddx3, 80mm maf, 24lb sensor, but the sct is telling my car I have 19# inj, out of the box sct 93 octane tune for a gt... i can't remember if this is sequential or batch fire... cause it sounds like it's batch firing at idle no codes except for th evap and egr is detecting high voltage. Evap is just deleted. Egr is shut off. Everything egr related is not there. Sensors, tuBing, diaphragm. I don't want hot air or soot in my engine.
 

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Nah, it's the Gt pcm ddx3, 80mm maf, 24lb sensor, but the sct is telling my car I have 19# inj, out of the box sct 93 octane tune for a gt... i can't remember if this is sequential or batch fire... cause it sounds like it's batch firing at idle no codes except for th evap and egr is detecting high voltage. Evap is just deleted. Egr is shut off. Everything egr related is not there. Sensors, tuBing, diaphragm. I don't want hot air or soot in my engine.
Actually the EGR system reduces combustion chamber temperatures (already burned fuel cannot burn again). This reduces NOx emissions and allows ignition timing to be advanced a bit for improved performance and fuel economy. However it is only active at low loads and lower engine speeds so it is not a performance related system whether its working or not.

The EECV PCM fires the injectors sequentially.

Your engine is likely running very rich in open loop mode as the stock tune is set for 19 lb/h injectors and your are effectively (at 55 psi) running 22.3 lb/h units. The PCM will calculate an injector pulse width based upon the target AFR from the Stabilised Open Loop Fuel table, the airflow reported by the MAF, and the fuel injector capacity which it believes to be 19 lb/h.

However the resulting fuel delivery will be 17.4% (22.3/19) more then required--I.e. the engine will be running quite rich, and is likely bogging from that. For example if the desired AFR is 12.4:1 however 17.4% more fuel that required is being injected then the resulting AFR would be 12.4/1.174 = 10.5:1.

The short term fuel trims will be messed up as well (pulling fuel to meet the 14.7:1 closed-loop target AFR.

The short story is you cannot raise the fuel pressure without altering the injector slopes in the tune, and expect the engine to run properly.

[edit]

Oh, one other thing I have run into before.

The CDAN4.DDX3 stock tune (actually the whole CDAN4 series) has a wacked out MAF transfer function that is only accurate to 4.0V output. This is roughly 450 cfm, which is what would be just about the limit for a non-PI 2V. The maximum amount of air a 281 in³ engine could possibly draw at 6000 rpm in 490 cfm. At a VE (Volumetric Efficiency) of 85% (perhaps too high for the NPI engine) that would be just 420 cfm.

So making the MAF function only accurate to 4.0 V (450 cfm) was all that was needed...

[/edit]
 

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Discussion Starter · #13 ·
Awesome info! Should I ditch my pcm and look for a mark viii one to hook up with the 4r07w? And put my fuel psi back to 41 vacuum off? I have an LLX2(if I'm correct) but it needs a reflash, don't have the file.
 

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Awesome info! Should I ditch my pcm and look for a mark viii one to hook up with the 4r07w? And put my fuel psi back to 41 vacuum off? I have an LLX2(if I'm correct) but it needs a reflash, don't have the file.
I am confused, do you have a Cobra engine or a Mark VIII engine?

What injectors are you running, the stock GT orange ones?

Returning the fuel pressure to 40 psi would likely help, although the 19 lb/h injectors are too small for the DOHC engine--you need at least 24's, and a tune setup up to operate them properly...
 

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Discussion Starter · #17 ·
I have a 96 Gt 4r07w with a 96-98 cobra dohc and red top bosch injectors running on my stock Gt ddx3 pcm... I have A llx1 pcm sitting here for a 4v auto mustang but its corrupted and needs a reflash..
 

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Those may be 30 lb/h injectors, do you know the Bosch part number. If they are there is no way they will work properly with the stock GT tune even at 40 psi--at 55 psi they were the equivalent of 35 lb/h injectors.
 
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