21lb Injector max PSI? - Mustang Evolution

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Old 11-09-2012, 04:19 PM   #1
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21lb Injector max PSI?

What is the maximum PSI a pink 21lb. Bosh injector can SAFELY take with the vacuum line on? I've got mine at 36. STFT is -10 in park...

I'm trying to compensate a bit for not having 24lb'ers in my 4.6l DOHC for now.
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Old 11-09-2012, 07:40 PM   #2
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The 21 lb/h rating is Ford's spec at 40 psi differential across the injector (fuel rail pressure + -[manifold pressure in psi]). I.e. at -9 psi manifold pressure (19 inHg vacuum at idle) the rail pressure would be 31 psi--31 + -(-9) = 40 psi differential.

At 43.5 psi differential (the standard pressure for rating injectors) they are spec'd as 22 lb/h. Here is the formula for recalculating injector capacity as differential pressure changes:



So for the Bosch 22lb/h rating at 43.5 psi the GT pink injectors flow:

(40/43.5)^0.5 * 22 = 21.09 lb/h

At 36 psi with the vacuum line off (I assume at idle) the pressure "across" the injectors would be around 45 psi (36 psi in the rail, + -(-9 psi vacuum)). Meaning that under those conditions the 21 lb/h injectors (at 40 psi) have become;

(45/40)^0.5 * 21 = 22.2 lb/h injectors

Which is why your short term trim is at -10%. The PCM thinks they are 21 lb/h injectors (19 lb/h if your are running the stock tune, which I suspect you are), but they are behaving, due to the increased fuel pressure, as 22.3 lb/h units injecting too much fuel. The PCM has compensated for this by injecting -10% less fuel--just about the difference between 19 lb/h and 22 lb/h.

Are you running a custom tune? If so do your know what the low and high injector slopes are set to? IF it is the stock tune do you know the box code/catch word--it will be a 4 character code like XSH2, or MDE3 and should be on a label on the PCM.

If you can get the box code I can look up the tune (I have almost 6000 in my library) and see what the injector setting are. What year is your car, as you have the pink injectors I assume late 2002 to 2004.

Back to your original question, Bosch rates them to 100 psi, however that is mostly because that is the maximum working pressure for SAE 30R9 rated hose--the most commonly used OEM fuel line hose. I have run other Bosch injectors up to 135 psi.

Tell me more about your setup...
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Old 11-09-2012, 08:30 PM   #3
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96 GT auto with a 96 DOHC swap

I have it at 36psi with vacuum ON

DDX3 or DDX2 computer

It does have a tune... bought it off my dad so I don't really know what the slopes are... it has a 250 450 .500 cam

He got tired of the way it drives and didn't want to get to a dyno for them to tell him he has to spend more money... so he bought a Z28 :/

The tune is a can tune.. something tech.. duratec or ... idk.
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Old 11-09-2012, 08:53 PM   #4
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I know all that I'm doing is shortening the pulse widths... which to me is a good thing
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Old 11-10-2012, 03:31 AM   #5
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That would be a CDAN4 strategy tune.

What is the fuel pressure with the vacuum line disconnected?

You said "I know all that I'm doing is shortening the pulse widths", are you referring to the -10 short term fuel trim? The problem with that is that the long term fuel trims used in open-loop mode are learned from the short term trims and may be removing fuel at higher loads and WOT when it is needed. Disconnecting the battery for 5-7 minutes would clear the KAM (Keep Alive Memory) and reset the LTFT table; it would be an interesting exercise to do that and see if the engine runs differently.

Do you know what the AFR looks like at idle/low loads (it should be 14;7:1 as the STFT is not maxed out), and at higher loads and WOT? That is essential to sort out drivability issues.

"something tech.. duratec": Could it be the Hypertech Power Programmer? If so do you have the handheld unit, if not how was the tune implemented? Is it flashed to the PCM or a "chip" attached to the J3 diagnostic port?

What does the engine do (or not do) that made your father tire of the "way it drives"?

Sorry for all the questions, however they are the same ones I'd ask if I had your car in the shop for a tune...
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Steeda UDPs, Ralco flywheel, RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.

New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq
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Old 11-16-2012, 03:42 PM   #6
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With the vacuum line disconnected I think it drops about 10lbs or so, Haven't been able to check it just yet.

Over the coarse of me having the car I've played with the butterfly/tps voltage @.99v, using a rubber spacer various hole sizes on the IAC

Yeah thats it!, Hypertech... It must have been flashed, don't see a chip in there.

The car doesn't want to down shift unless you dangerously mash the gas. The idle hangs around 1200k until you completely come to a stop after a few seconds. The shifts are really sloppy unless under high load... it has 3:55's without a speedo gear but I believe he changed that in the hypertech...

The engine does run differently when I clear the KAM, until it learns itself, then its back to runny like garbage. I stopped clearing the KAM cause I was tired of burning extra fuel.

My fuel pump won't kick on until I crank the engine over, turn the key off, then back on, and I hear the pump (or repeat those steps another time). Other than that I think I hear the relays every time... or the pump-pumping but no gas. the pump has about 50k on it. But has done it ever since the
swap.

BTW, I've got pink top bosh's with a 96 MAF... Should I get a 02-03 off of Ebay?

---------- Post added at 04:42 PM ---------- Previous post was at 04:41 PM ----------

The car throws a KAM code in a KOEO. But not in the trouble codes stored in the pcm.
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Old 11-16-2012, 04:12 PM   #7
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Just double checked my STFT's now that my computer has had time to learn the adjusted fuel trim.. its now at +3.

On a hunch, I synchronized or... calibrated the MAF and the TPS. MAF was .91 TPS jumped up to 1.01 from .99.... MAF is now .98-1.0v; TPS .99v... Leaving for work in a bit... I'll see how it drives... but its not going to fix the problem, only band-aid.
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Old 11-18-2012, 10:12 AM   #8
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For 21lb pink injectors can I use the stock 96 MAF? Not getting any codes... I'm thinking I would need an 2002 MAF right?
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Old 11-19-2012, 09:55 PM   #9
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Quote:
Originally Posted by Smokum281 View Post
For 21lb pink injectors can I use the stock 96 MAF? Not getting any codes... I'm thinking I would need an 2002 MAF right?
MAFs are not calibrated to any particular injector size. Those that are "calibrated" MAFs are calibrated to allow larger than stock injectors to work with a stock tune; they are a kludge left over from the old days when tunes could not be easily or at all changed--more about that here.

So the MAF will be fine. However the fuel injector low and high slope values in the tune will need to be changed. If you are running an adjustable fuel pressure regulator and the pressure across the injectors rise above 40 psi then you will have to also rework the MAF Transfer Function, and the Stabilized Open Loop Fuel table.
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SOLD! - 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum, Delta Force tuned,
Steeda UDPs, Ralco flywheel, RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.

New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq
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