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69 Thunderbird | 90 Seville | 03 Corolla | 89 Marquis(scrapped) | 72 Torino(scrapped)
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Discussion Starter #1
Has anyone run more than 14 degrees of timing and got away with it?:duck:

The reason I ask is because I replaced all my injectors to 19# Bosch III's and my city mileage has dropped from avg 22mpg to 18 or so. I suspect its running a little bit rich due to the extra #/hr. I'm wondering with the extra fuel, maybe I can get away with a little more advance.:bouncy:

I could lower the fuel pressure with an adjustable regulator but damn that's expensive...
 

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'85 Fleetwood FWD
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728 Posts
I'm not sure of your engine fuel system, but the 02 sensor does help regulate the mixture, doesn't it ?? Timing is computer controlled I believe, to some degree, except maybe initial timing (?)

Just some ideas........
 

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69 Thunderbird | 90 Seville | 03 Corolla | 89 Marquis(scrapped) | 72 Torino(scrapped)
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Discussion Starter #3
Its 4.5 PFI. Right now I have the base timing at 14deg BTDC, which also means that any advance that the computer applies will be 4 degrees more than stock. I'm thinking that its just running at the richer side of the oxygen sensor's parameters. I know the system can adjust to a point but It cant be infinite... If that were the case, I could run 36# injectors and just have the ECM compensate for the higher flow by shortening the pulse width.
 

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1926 Model T street rod, 2000 Jaguar XJ8, 1999 Corvette.
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Higher flow injectors must also have more air flow to gain any power. Without more air into the engine, bigger injectors are a waste of money and time. This is a speed density injection system, it relies on a measured amount of fuel and air to run correctly. On Mustangs for example, changing the injectors requires a larger throttle body to increase air flow to get more power as well as a different mass air flow meter. This car has no mass air flow meter so a larger throttle body is the only mod that would be beneficial.
 

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69 Thunderbird | 90 Seville | 03 Corolla | 89 Marquis(scrapped) | 72 Torino(scrapped)
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Discussion Starter #5
Higher flow injectors must also have more air flow to gain any power. Without more air into the engine, bigger injectors are a waste of money and time. This is a speed density injection system, it relies on a measured amount of fuel and air to run correctly.
Right, I understand that. I'm not trying to gain any power by adding more fuel. The system was designed for 18# injectors and I'm running 19# so I suspect that the engine is getting slightly more fuel than it should seeing how I've lost some mileage after the upgrade. I'm assuming that the ECM is seeing this and compensating to some degree but I still think its running richer than before.

I'm wondering if the suspected extra fuel might give me a margin of error to bump the timing to maybe 16 degrees to bring the mileage back up into the 20's again.

Now that I think about it, maybe it was just running lean on the old injectors? I didnt have any issues with them at 14d BTDC and the mileage was outstanding. Hmm....:hmm:
 

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1985 Sedan DeVille
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1,883 Posts
You might try resetting the computer by touching the positive wire to ground (disconnect from battery first obviously) so as to force it to relearn.
 

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69 Thunderbird | 90 Seville | 03 Corolla | 89 Marquis(scrapped) | 72 Torino(scrapped)
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Discussion Starter #7
You might try resetting the computer by touching the positive wire to ground (disconnect from battery first obviously) so as to force it to relearn.
Been there done that and got the t-shirt. Did the reset and idle relearn, cleaned the TB and EGR, and recalibrated the TPS and ISC. Gonna bump the timing up to 15 as soon as i get a chance....

:stirpot:
 

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1926 Model T street rod, 2000 Jaguar XJ8, 1999 Corvette.
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6,738 Posts
If I remember correctly, this engine has no knock sensor. So extreme advance might cause pinging? I'm still unclear on the reason for the change? You said yourself the car was running well and mileage was in line. Why change it? Years ago I replace the throttle body on my 84 Eldo 4.1 with a larger unit from an 81 Eldo 5.8. My thought was bigger air intake, more power. What I got though was bigger injectors as well which caused an overheated Cat Converter, it glowed cherry red!! Swapped my original 4.1 injectors into the larger t/body and everything was fine plus the throttle response was crisper because for the same throttle imput I gave it, the throttle blade was larger at that same angle. Top speed remained the same because the intake runners and valves were the same size. Around town it was much more responsive but pulling a steep grade it was still a 4.1 liter with valves to match.
 

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69 Thunderbird | 90 Seville | 03 Corolla | 89 Marquis(scrapped) | 72 Torino(scrapped)
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2,026 Posts
Discussion Starter #9 (Edited)
-4.5mpfi 67k mi @ 14deg BTDC, no cat, flowmaster muffler, Accel super coil, MSD wires
-93 Shell always
-no codes
-no maintenance part has more than 10k miles
- mileage was 22 around town
-#6 injector dies (1.2ohms)
-Swapped injectors from 18# to 19# Bosch III(5% increase)
-Mileage dropped from 22 to 18city avg
-I suspect its running richer than before
-Wondering if the suspected richer mix will give me room to advance the timing some more without pinging


I understand that to properly make use of more fuel requires more air but more air is not possible in this case.

I only have a basic understanding of how the engine control system works and most of that is speculative.



On one hand, my thought is: The ECM's tables are written for a given flow rate at a given fuel pressure resulting in a specific volume of fuel sprayed for a pre determined pulse width. So say 43 psi into 18# injector equals X volume for Y miliseconds. So if you increase the injectors flow rate to 19#/hr 'X' increases by 5%. Conversely, if you keep the 18# unit and raise fuel pressure 'X' will also increase accordingly. So the computer is running the engine as if nothing has changed resulting in a 5% richer mix....

BUT.... My other thought is: The ECM has no control over 1. Rail pressure, and 2. Injector flow rating so the only way it can affect the resulting fuel volume at the given psi is to reduce the pulse width. The only way it knows how much to change the p/w would be by the values from the oxygen sensors. So the oxy sensor says " Hey assclown, were running a little rich down here!'' ECM says "OK I'll shorten the pulse width..."

So if im correct, then the system is compensating for the extra #, the fuel ratio is normal, mileage should be the same, and I've just been hitting the sauce a little too hard.:cookoo:
 

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1985 Sedan DeVille
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The primary input is the MAP sensor, which tells the computer how much air is flowing into the combustion chambers, which is then used to determine how much fuel should be mixed in.
 
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