What fuel pressure?

[robomac]

I am trying to find out what the correct fuel pressure is for 4000 rpm and 12 psi is suppose to be? Figting with a miss and about to go for a test ride and then check the plugs. If plugs bad what should I use the copper ones trilogy recommends? Any help would be appreciated?
Note not use to not having spell checker fix my pathetic typing!

[Marauderjack]

Depends on you injectors??

I think I have 39# injectors and about 40 PSI is what I get at 9-10 PSI boost:cool:

Ours is a "Pulse Width" system and I think the injectors max out at the prescribed pressure while the EEC keeps them open longer for richer requirements??:beer:

I'm sure someone with more knowledge can give you a better explanation but this is what I think is going on??:confused:

Marauderjack:burnout:

[thePunisher]

what is the gap on the plugs and are you using platinum pulgs???

[robomac]

I checked the plugs and they are AGSF-22C so not platinum. They are well used and the gaps on some had opened up to at least .050. They all looled good color no damage. I down gapped them to a tight .035 using a wire gage. I am not able to buy new plugs locally that I can tell.

Recommendations for plugs is welcome.

The BAP manual says cobras should see 85 psi at 6500. I see only 40 psi at 6000. Nuthin makes it go up so maybe you are right it is all PWM. I wish someone who really knew would tell me.

Note when I down gapped the plugs it seemed that the miss went away. Plug gap and new filter done at same time.
I was planning on trouble shooting the BAP according to the instructions. Anybody done it before?

[thePunisher]

well im sure you know on a s/c car that was too big of a plug gap....at higher rpms it will blow the spark out...i bet that was your problem.....

[juno]

http://kennebell.net/techinfo/general-info/SPARKPLUGS.pdf

Very recently (like this summer) KB switched from recommending the Iridium plugs to the Denso U-gaps. They now recommend a T20EPR-U gapped at .035 for your level of boost.

IMHO the Bell's do more testing on all aspects of our motors then any other source. There is a ton of info on their site.


There are no simple answers. Do you know what your A/F ratio is at that rpm/boost? Do you know what your timing is and what octane you are running?

Are you at WOT? Is your BAP set to come on only at WOT?

Some 'safe' numbers are an AF of 11.0 to 1 and timing of 18-20 degrees. Like the punisher said, your gap could have been one of the problems.

If you consider that your spark plug gap and octane are 'fixed' values that do not change then the 'tune' will control your timing and A/F ratio.
I won't even pretend to think I know anything about the details of the actual tuning process, that is for the experts. I can tell you what I think happens at WOT and hopefully someone with real knowledge can correct me or give more details.
I do not know if these tables adjust for actual boost, I think they are set at a predetermined value and assume that your boost is x and adjusts the timing and fuel for that value.
Let's assume that the values in these tables are fixed for the maximum boost level set for your car.
When you mash the go button your car switches to the WOT tables. My impression is these tables will reference values that adjust the spark, timing, fuel pressure, and injector cycling to achieve the A/F ratio and timing to make the hp.
The keys are a/f ratio and timing.

So let's say your timing is fixed at 18 degrees. Air flow will be a function of atmospheric conditions. At a specific temperature and humidity you should get a consistant air flow to your motor. The next step is to adjust the fuel to get the AF ratio you want.
How much your injectors flow is a function of the pressure difference across them. Most people recommend not exceeding an 85% duty cycle for your injector, so say your injector cycle is set at 85%.
The higher DP you have across your injectors the more fuel will flow. If your injectors are set for an 85% max cycling/duty the flow will be determined by your fuel pressure. If your at 12 psi of boost you must have sufficient fuel pressure to overcome that boost and provide the flow to maintain an A/f ratio somewhere around 11.5 to one.
The problem is that as your fuel pressure rises, your pump is flowing less. At some point with your injectors cycle maxed and your fuel pressure maxed it is time to get higher flowing pumps. The BAP kicking in at WOT will increase the pump flow by raising the voltage up to 40-50% I believe.

I do not know what tune you have in your car. 40 psi fuel pressure at the rail seems low. Does the BAP manual indicate whether that is fuel pump pressure or fuel rail pressure? I think there is about a 10 psi drop across our fuel systems, so 85 psi at the pump will be about 75 psi at the injectors.

Note too that a Focus SVT pump has an internal bypass feature that will start diverting flow at 85 psi, so getting over 85 psi pump pressure with those is improbable if you have them.

Other factors that will retard your timing to protect the motor are the knock sensors and the tranny modulation.

If your knock sensors are turned on (1 for 03's and two for 04's) they will retard timing if they sense knock to protect your engine. This could be a function of poor gas, high ambient/inlet temps, etc.

I believe the tranny modulation also reduces timing to give you a 'softer' shift. That can be adjusted in your tune also.

So the tuner controls the dance between all those values. There are a lot of variables, like what the MAF signal does. Does that kick everything off by using predetermined tables for whatever voltage it is producing based on the air flow? Is that variable at WOT or does it reference a set value?
Someone with real tuning experiience can tell you what really happens. :D

[robomac]

I have a fairly strong technical background and you went where I can not currently follow. I know that the flow rate is a function of the pressure diferrential (NA is 40 psi rail vs 0 psig inlet). So when on boost it is now 40 psi rail vs 12 psi inlet = 28 psi. All factors beig equal (and they never are) says that at the same commanded duty factor (85%) the flow rate is down 12/40 or 30%. So in my ignorance I assume the BAP, which is set to come on at boost (need to check installation instructions) cranks up the voltage to get more flow (lpm) then the FPMU cranks up the pressure to get more fuel for the same duty factor. This is where my ignorance shows up I just don't know. It may be possible that the high flow injectors make up for it but I am betting that the flow rating is based on 40 psi differential.


So what do I run for plugs the U gaps? If so where do I get then and how much?

I think I need to call Lidio and get a read on fuel pressure. My understanding is that I have a custon tune from him (400 rwhp) done in June 2005. If he can recover it then I will be so much smarter.

Thanksfor all the info

[thePunisher]

the pcm will adjust the fuel pressure accordingly by duty cycling the pump longer......this is how a returnless system works....

[Breadfan]

Have you called Trilogy yet?

Plugs should be gapped to 0.032", AWSF-22C are optional plugs, they are one heat-range colder and recommended. I'm running them with no issues at all. Be sure to use anti-sieze. As the parts counter to cross ref. the part #, I seem to recall my dealer finding them listed under a new model # - or something odd, they had them in stock under that part # above but something about them not showing up in the computer right away...

Honestly if you haven't called Trilogy yet I recommend that.

Was King Fubars car a stock trilogy or did it have a different pulley and tune? That too is important info.

Did your car come with the install manual? That can be helpful too.

[robomac]

OK when I regapped them to a tight .035 I did use anti-seize. I have been working on aluminum motors for over 30 years so I know all about steel black oxide plugs and antiseize.

King Fubar had a custom tune from Lidio in June 2005 with a 3.2 pulley JLT CAI

I have not called Trilogy yet good idea I will do it tomororw

Ford said the part number I had in the car was the cross for the Trilogy book plug. They said it was a good number but they did not have it didn't want to order it etc etc.

So I am looking to figure out where to get plugs. Lidio or Jerry are proably both good canidates plus they can tell me the latest 411 on what to use

Yes I have the insatll manual it is a work of art.


Yes punisher I understand the basics of retrunless but I also know that th einjectors are designed to flow the rated fuel flow at a certain delta P. That dleta P is not there when you are on boost. The BAP manual syas 85 psi for a Cobra at 6500 rpm. Is a Cobra returnless or something else.

Good advice I will do it

[thePunisher]

hmmm...depends on what year cobra...but i believe the 03-04 is returnless......

[Dennis Reinhart]

I am trying to find out what the correct fuel pressure is for 4000 rpm and 12 psi is suppose to be? Figting with a miss and about to go for a test ride and then check the plugs. If plugs bad what should I use the copper ones trilogy recommends? Any help would be appreciated?Note not use to not having spell checker fix my pathetic typing!

nothing under 38 PSI, use a Xcall 2 to data log this

[Breadfan]

You might want to PM a user named "TheDealer" aka Ray, he's a parts guy at a L/M dealer and may be able to send you the plugs you need if he has them in stock.

I'm also curious if there is an Autolite # for them...aren't they just about the same as Motorcrafts?

[Dennis Reinhart]

I have them in stock.