fuel pressure adjustment 19 or 30lb injectors.

mostsmooth

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Hi all,
95 gt.
gt40 intake and heads, some type of moderately aggressive cam. 65mm throttle body.

a long time ago, the car had a vortech supercharger that was installed by somebody i assume knew what they were doing.

the car has been sitting a long time and now im getting it back on the road. the charger is gone.

car runs really well on the low end, but wehn the speed hits 85-90ish, it seems to struggle. in my head im thinking its like theres too much fuel, if that makes any sense.

it has 30lb injectors which i think are oversized now that the charger is gone.
i have the stock 19lb injectors in a bin on a shelf.

i dont know what the hp is.

im thinking the 19lb injectors would be fine, but its a hassle to swap out injectors.
so, would it be stupid to simply lower the fuel pressure? (assuming im getting too much fuel at the moment) i was thinking somewhere around 26psi would work out to equivalent to 24lb injectors. somebody can correct that if i am way off on the calc.

this isnt a daily driver, i can play around with it as much as i want.

one other tid-bit, i dont know if the mass air is for 19 or 30 lb injectors. (thats important, right?)

thanks
 
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jozsefsz

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If it's running very rich at the top-end, but is doing ok at the bottom-end, I'd suspect your car was tuned for boost and has a piggy-back chip on it. This might also include an upgraded MAF so just bringing down the pressure might cause you to run very lean at lower RPM's.

You're correct that dropping the fuel pressure can make the injectors behave as-smaller. Here's a good calculator: https://www.deatschwerks.com/fuel-calculators/flow-vs-pressure-calculator - 30's designed for 39psi running at 26psi will in fact get you to about a 24. It's probably unnecessary though - 30's should run just fine on a stock engine, they're not that huge.

If you do that, you'll need to replace the MAF with stock and remove the chip. Or, just have it re-tuned with no hardware changes. My 2c.
 

mostsmooth

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thanks all

jozsefsz, you just reminded me i forgot to mention that my computer is from a 93 i think. it was swapped in when the supercharger went on to allow for more tuning capability if i recall correctly. so, this chip idea sounds plausible. how do i find this chip? does it plug in to the box like a thumbdrive or am i going to have to take the box apart or ?
i am a little confused by your last line. seems youre saying remove the chip only if i lower the fuel pressure? or does it seem like i should remove the chip regardless. i can play around with it if its not too much of a hassle getting it out and back in if need be.

same with the fuel pressure, i have an adjustable regulator and a pressure gauge.
 

jozsefsz

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On our generation a chip connects to a port on the side of the computer, but sits within the metal casing overlapping the circuit-board. Here's a decent vid: View: https://www.youtube.com/watch?v=JNyFF2jhwSg


I'll try and clarify a bit. The amount of fuel you get is controlled by injector-size, fuel pressure, the MAF, and the tune.

If you reduce fuel pressure without removing the chip (tune) then you'll probably be lean everywhere you weren't rich before.

If you remove the chip (tune) then you can reduce the fuel pressure, but you'll also want to swap the MAF back to stock because the tune is probably set to use a bigger MAF.

This all assumes you have a chip, and that the MAF is non-stock. With all these variables, it may just be easier / cheaper to have the car re-tuned than to swap out hardware or try and make pressure adjustments for a specific load+rpm range because those will affect that car at all ranges where it's currently running good.
 

mostsmooth

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On our generation a chip connects to a port on the side of the computer, but sits within the metal casing overlapping the circuit-board. Here's a decent vid: View: https://www.youtube.com/watch?v=JNyFF2jhwSg


I'll try and clarify a bit. The amount of fuel you get is controlled by injector-size, fuel pressure, the MAF, and the tune.

If you reduce fuel pressure without removing the chip (tune) then you'll probably be lean everywhere you weren't rich before.

If you remove the chip (tune) then you can reduce the fuel pressure, but you'll also want to swap the MAF back to stock because the tune is probably set to use a bigger MAF.

This all assumes you have a chip, and that the MAF is non-stock. With all these variables, it may just be easier / cheaper to have the car re-tuned than to swap out hardware or try and make pressure adjustments for a specific load+rpm range because those will affect that car at all ranges where it's currently running good.
thanks
the problem would be trying to find a tuner i think.
would a tuner need to know what the MAF is designed for, or they dont care?
 

jozsefsz

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If it's a non-stock MAF, the tuner would want the "flow data sheet" that came with the MAF (or minimally the part-number so they can try and find the specs). If you went with a canned-tune (something like Bama tuning from American Muscle) they'd want your injector size information and probably the MAF information as well. Or you could just pick up a stock MAF and some 19's, remove whatever chip you might have, and call it a day. Those can all be had used or for-cheap.
 
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mostsmooth

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so, I took a look a the computer. doesn't seem to be any chip.
Note: the computer is from a 93, I seem to recall that that was because it allowed for more tuning than the 95 computer. the 93 computer was installed when the supercharger was installed. would a 93 computer be programmable itself, or would there still need a chip?

also, the computer is under my passenger seat, and I found that the MAP sensor is under the seat as well. Seems like a weird place for the map sensor? is the map sensor supposed to have a vacuum line connected to it?

Thanks
 

General karthief

wonder how much it would cost to ship you a pair
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I'm confused about the use of a 93 computer in a 94-5 car, I think there is significant wiring/computer differences, also the 94-5 uses a ccrm that works with the computer to control thingr like the cooling fan, that the 93 computer does not support(this is my understanding), I have never heard of this modification and I could be wrong.
I'm also confused about the computer being under the seat.
 

mostsmooth

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I'm confused about the use of a 93 computer in a 94-5 car, I think there is significant wiring/computer differences, also the 94-5 uses a ccrm that works with the computer to control thingr like the cooling fan, that the 93 computer does not support(this is my understanding), I have never heard of this modification and I could be wrong.
I'm also confused about the computer being under the seat.
ok thanks.
yes, so there is something called a PIH installed on this car as well which i didnt know was relavant when i typed th. Initially, i thought it only had to do with the fans (there is a module with dials to set on/off temps), but when i saw your post, i went and looked at the manual a bit and it says right off the bat that the purpose of the PIH is to allow for the conversion from a 94-95 computer to a 93 computer.

The PIH is from 'Best Products' and is called 'Next Generation Performance Improvement Harness'. Research seems to be telling me they were fairly popular back then.

I am going to read this manual more closely soon, but i just noticed it says a crane MSD high performance ignition is required to allow the ignition system to function properly. this may explain one of the things i noticed (car runs kinda crappy at higher speeds?). i took it off because i thought it was just for the supercharger which was removed.

the manual talks about connecting a 'BAP' sensor to the harness. then it talks about locating 'barometric pressure sensor' (which i assume is the BAP sensor they talked about) in the dash or wherever.
when i looked under the seat the other day, the part number on one of the things was apparently a ford MAP sensor. I dont know if the MAP and BAP are teh same thing though.
 

revhead347

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93 doesn't use a map it uses a bap but you don't need it

the 93 ecu is a downgrade from the 94/95 ecu. A lot of people did that down grade years ago because they had sub par tuners.

No point in going to 19s or swapping mafs if all works now. Best bet is to dial in what you actually have using a moates quarterhorse.



They didn't have tuners. The 93' computer let you get away with more.

Kurt
 
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revhead347

Apparently my ex-husband made that mistake.
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hey Kurt, how much does the 93 let you get away with? Could a mild 347 with say 400 ponies be doable?
You mean like 93 Octane, or a 93' Computer? No one is going to recommend not getting a tune these days.

Kurt
 

nickyb

WAIT,you now have a pair?
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93 a9l,yeah a grand or more on a stand alone is more wood to chop.
 

revhead347

Apparently my ex-husband made that mistake.
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93 a9l,yeah a grand or more on a stand alone is more wood to chop.

Steeda #18 cam with Edelbrock Performer RPM heads which is about 300hp. That was the last bolt in combo that was designed to work with an A9L. Anything other than that, and there are no guarantees. Might work, might not. You don't need a stand alone computer, you just need a piggy back chip to edit the tune on the factory SN computer. I have a 347 engine on the factory SN computer in the 425-450hp range with a mail order piggy back chip. Depends on where you live. Here I just take the car down to Moe's, and the guy tunes the factory computer to work with whatever you have for $500.

Kurt