fuel pressure question

[blue93steed]

hey ya'll i am new here but i won't beat around the bush with intro's. quick question: what do you boys with a similar setup to mine run your fuel pressure at, at idle without the vaccum line connected? my setup is 306 with H/C/I, 255 intank, 75 mm MAF and 24lb injectors.

i just installed the MAF meter and injectors and it is choppy down low and bucks below 2500rpms but as soon as you get into the throttle it smooths out and pulls hard all the way to 5500. i started at 39 psi and now i am down to 35 but no real difference.

thanks.

[MustangCRZY302]

hey ya'll i am new here but i won't beat around the bush with intro's. quick question: what do you boys with a similar setup to mine run your fuel pressure at, at idle without the vaccum line connected? my setup is 306 with H/C/I, 255 intank, 75 mm MAF and 24lb injectors.

i just installed the MAF meter and injectors and it is choppy down low and bucks below 2500rpms but as soon as you get into the throttle it smooths out and pulls hard all the way to 5500. i started at 39 psi and now i am down to 35 but no real difference.

thanks.


I wouldn't go any lower than 38, 39 is stock. with those mods you shouldn't have to go any higher than 45-50 psi, if you do, then you need bigger injectors. If i was building a 306, I would have gone with 30#ers IMO. We just put 30's on BDASPNY's 331, and it runs much better. When he had the 24's, it did the same thing yours is doing, and at high RPM's it would spark knock... ya might want to look into stepping up to 30#ers...

[blue93steed]

I wouldn't go any lower than 38, 39 is stock. with those mods you shouldn't have to go any higher than 45-50 psi, if you do, then you need bigger injectors. If i was building a 306, I would have gone with 30#ers IMO. We just put 30's on BDASPNY's 331, and it runs much better. When he had the 24's, it did the same thing yours is doing, and at high RPM's it would spark knock... ya might want to look into stepping up to 30#ers...

man, that's interesting cause i asked a lot of opinions before i upgraded and concensus was 24# cause 30 would be overkill... thanks for the input.... going to take it to a dyno this week and i will report back hopefully with some good numbers. i am not freaking out cause it runs like a bat outta hell once it gets over 2500.....

[Stable]

What happens if you crank the fuel pressure way up?....Not to knock on anyone setup, but 24 lb injectors @40 psi will handle 300 hp at the crank....30's 380 or so at the crank. 99% of the time it is that the Mass Air is not setup correct.....also what TB? It is possible to get the intake to large and this will cause the air speed to be too slow to keep the fuel mixed.
Check out this site for more injector sizing....http://www.rceng.com/technical.aspx

[slonomo]

i agree ,make sure that the mass air has the right calibration.i ran into that in the past.

[blue93steed]

What happens if you crank the fuel pressure way up?....Not to knock on anyone setup, but 24 lb injectors @40 psi will handle 300 hp at the crank....30's 380 or so at the crank. 99% of the time it is that the Mass Air is not setup correct.....also what TB? It is possible to get the intake to large and this will cause the air speed to be too slow to keep the fuel mixed.
Check out this site for more injector sizing....http://www.rceng.com/technical.aspx

75 MAF with only a 65mm TB..... "It is possible to get the intake to large and this will cause the air speed to be too slow to keep the fuel mixed"

please explain more.... thanks.

[Stable]

You've heard about running a too big carb? There is a idea air speed thru the motor that will keep the fuel mixed with the air. If you put heads that are set up for 8500 rpm and a lot of cube's(408) on a 306 that will dont see over 6500 rpm , it will not fall out of a tree. You can do the math, and engines are all about math, too see how much air is needed for a given hp. Granted that it is much harder to over size parts on a EFI setup, but it does happen. Just saw this a couple of weeks ago, on what I used as a example. The car made 175 hp to the ground @ 40 psi, with 30lb injectors. It was driving the A/F all over the place. We set the fuel pressure up to 70 ...the car would not idle, but keeping it running, it went up to 260 to the ground...still driving the A/F nuts.When you buy a MA setup for a certin size injector, it will be about 75-80% of what it needs to be to be right. Most of these are stoock motors with just the injector change. Anything else you do to the engine to improve air flow/speed, will change this. Just think about a engine being a water pump and how the changes you want to make will effect the output.At a fixed RPM of say 3000, a Bigger intake(MA/TB, etc) bigger exhaust(better flow mufflers/headers) will cause the speed of the water(air) to slow down.

Any of that help?

[blue93steed]

thanks for the explanation.

[nastynotchback1]

I ran a h/c/i combo last year with #24's and lightning maf.fp was 42psi.after the tune it ran so much better but i found that the bucking was from the elbow in front of the mass air.removed it and put the filter on a straight shot to the maf and it fixed the bucking idle issues i had.after the tune the car ran great but i tryed a new air filter and the dang thing would not run.it was a very touchy combo.

[blue93steed]

I ran a h/c/i combo last year with #24's and lightning maf.fp was 42psi.after the tune it ran so much better but i found that the bucking was from the elbow in front of the mass air.removed it and put the filter on a straight shot to the maf and it fixed the bucking idle issues i had.after the tune the car ran great but i tryed a new air filter and the dang thing would not run.it was a very touchy combo.


interesting... on the MAF i have it says "24 stk" which i was told means it was calibrated for the stock airbox and filiter. this person told me to go back to the stock airbox which i have not done yet. other told me that it does not matter that i am running the conical filter....

what do you think abut this theory?? kinda makes sense.

[BYEBYE2U]

i believe i'd try the stock air box.but htere is SOOOOOO much on an EFI combo that'll make it act like you'res is now.but when you go for the tune the dyno will tell the story of what it needs and don't need.

[Stable]

Run the stock air box.......On a Fox body MA it matters as to how much the MA is rotated.....These MA's do not have the sensor in the middle of the air stream, so if you rotate it, it will read different

[nastynotchback1]

Run the stock air box.......On a Fox body MA it matters as to how much the MA is rotated.....These MA's do not have the sensor in the middle of the air stream, so if you rotate it, it will read different



+1 This will greatly affect the car.

[nastynotchback1]

interesting... on the MAF i have it says "24 stk" which i was told means it was calibrated for the stock airbox and filiter. this person told me to go back to the stock airbox which i have not done yet. other told me that it does not matter that i am running the conical filter....

what do you think abut this theory?? kinda makes sense.


i do believe that could be the case.just like i said,i went from a 6in cone filter to a 9in filter and the car would not run.Put the 6 back on and it was fine.