How low can I safely set my fuel pressure?

[GTLee]

Hey Guys,

Thanks for all the wisefull replies in the "Is 209 rwhp Ok for my mods" thread. By reading the replies it seems as though I am seriously over injected.

So I went to the Fuel System Analyzer in Mustangworks.com and plugged in 250 fwhp (~ 209 rwhp) with 24# injectors and it told me my injectors only need 25 psi to supply the necessary fuel to the engine.

Also note that my mods in my signature made 200rwhp initially at the stock 40psi. The guy at the dyno shop lowered the fuel pressure to 35psi and I got an extra 8rwhp and 11ft/lb of torque. It was late so he couldn't mess with the fuel pressure again and left it at 35 psi.

Can I safely lower my fuel pressure to 30psi. By "safely", I mean no detonation. I am running 93 octane fuel and 14 degrees of timing.

Thanks


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90 GT -
-Engine: K&N, 75mm MAF with 24# Injectors, 65mm TB, Trick Flow Street Intake, 1.7 Roller Rockers, Adj FPR (set at 35psi), 190lph FP
-Exhaust: MAC 1 5/8" Headers, Catco 2.25" h-pipe, MagnaFlow mufflers
-Drivetrain: World Class T-5Z, Aluminum Driveshaft, 3.55 gears
-Suspension: Subframe Connectors and K-Brace, KYB Shocks and Struts, BBK Upper and Lower Control Arms

[jimberg]

You have a computer controlled engine. Adjusting your fuel pressure is a waste of time other than it will probably idle a little leaner. Since your MAF is calibrated for 24# injectors your computer knows how long to open the injectors to supply the correct amount of fuel.

Our computers adapt to changes in fuel pressure. If you noticed an improvement at the dyno, it's likely that your O2 sensors are old and need replacing, or it's possible that your engine runs better a little leaner. Any changes that were made have been adapted to by now, however, and are most likely gone. See the Fuel Pressure thread for more details.

A fuel pressure change will only change your air/fuel curve temporarily until the computer adapts. The only way to adjust your air/fuel curve permanently is with a chip or a computer like the SpeedBrain.



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351W 89 Mustang GT Convertible

[jimberg]

I read the other thread and Unit 5302's comment about the spray pattern being bad is correct, especially if you try to make an adjustment through fuel pressure. Optimal spray pattern will be at normal fuel pressure. The biggest problem you'll have is at the low end since the computer will not be able to open and close the injectors fast enough to keep the air/fuel curve normal. Going back to 19# injectors for your mods would give you better fuel control. If you start having fuel starvation issues then you can bump up your fuel pressure to compensate.

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351W 89 Mustang GT Convertible

[andy669]

I'll second that. 19#ers would be a better match for your combination. However, you already have them and you also have a new MAF calibrated for them so changing back to 19's wouldnt be cost effective. Your money would be better spent on a custom chip (not a one size fits all "box chip") that can take out fuel. You can then put the pressure at 39psi where the injectors have the best spray pattern. You didnt mention what your A/F ratio was on the dyno. He does have a wideband A/F sensor, doesnt he? of course thats just my opinion, I could be wrong.
andy

[jimberg]

Before changing air/fuel ratio, it is important to make sure that all of your feedback sensors are working properly. Getting a chip to adjust your a/f ratio would be an expensive way to compensate for bad O2 sensors or ACT sensor.

I think Andy is probably correct about cost effectiveness. My assumption was that he still has the 19#ers sitting in a box somewhere along with the old MAF. If that's not the case, then just keep your fuel pressure at 39psi.

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351W 89 Mustang GT Convertible

[This message has been edited by jimberg (edited 06-04-2001).]

[GTLee]

I replaced the 02 sensors a year ago so they should still be ok. What are ACT sensors?

I did have the old 19# and MAF in the garage until my parents decided to "clean up". So at this point, I guess all I can do is live with what I have until I get heads.

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-Engine: K&N, 75mm MAF with 24# Injectors, 65mm TB, Trick Flow Street Intake, 1.7 Roller Rockers, Adj FPR (set at 35psi), 190lph FP
-Exhaust: MAC 1 5/8" Headers, Catco 2.25" h-pipe, MagnaFlow mufflers
-Drivetrain: World Class T-5Z, Aluminum Driveshaft, 3.55 gears
-Suspension: Subframe Connectors and K-Brace, KYB Shocks and Struts, BBK Upper and Lower Control Arms
-Dyno: 208.7rwhp, 262.9ft/lb torque

[andy669]

Youre right Jimberg, I assumed those things were good. My bad. And the reason I didnt mention the stock MAF, was because youre limited by that too. When I was "chippin'" (i'm PMS ing) now) I picked up .5 seconds and 3 mph in the quarter. That was with an over the phone custom chip in my heavily modified n/a 94. Try to tell me my car wasnt out of tune!!...lol... I look back at how foolish I was thinking that all the mods I used to do to an old carbureted car, would have the same effect on a new fuel injected car. For my money, the chip is the last piece of the puzzle!!
of course thats just my opinion, I could be wrong.
andy

[GTLee]

I agree with you andy. I want to install the chip after I'm done with my car. Not every time I install a power adder (heads, blower).

[red82gt]

About the O2 sensors and the computer adapting I'm going to maintain that the computer has no concept of what peak power is and fuel pressure and WOT A/F ratios are controlled mechanically and without more mods to make more power your computer will not be able to compensate at 39psi to keep the fuel mixture lean enough to make decent power.
Try going down to 30psi, the spray pattern won't be great but it'll get the A/F more inline. It's been proven to work too many times to deny it.

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GT-40 heads (ported, polished, + milled), B303 cam, 1.7rr's, JE pistons, Offy intake, Carter AFB 625cfm carb, Flowtech 1 5/8" shortys, Flowtech X-pipe, MAC Flowpath exhaust, MAC pulleys, 373's, subframes, Eibachs+Tokicos, B&M ripper, FMS Clutch, Zoom Quadrant+cable, 17" CSA Ultra rims, 235/45ZR17 Yokohama AVS S4's, MSD 6A ignition+coil, FMS 9mm wires, Carbed, Naturally Aspirated, and Nasty!

[This message has been edited by red82gt (edited 06-04-2001).]

[andy669]

I agree with you GT, but it should only cost you $50 to get a chip reburned for whatever mods you add later. If you spend the $250 (or whatever they are)on a chip and harness now, and you decide to add a blower later, they should just reburn your existing chip. You shouldnt have to spend $250 for a new chip everytime you change something. Man, that would suck! If they wont do that, find a different tuner! of course thats just my opinion, i could be wrong
andy

[GTLee]

That's good to know andy. It WOULD suck if I had to pay $250 every time.

[This message has been edited by GTLee (edited 06-04-2001).]

[jimberg]

If air/fuel ratio were strictly mechanical at WOT, what's the point of having a mass air flow sensor? Clearly you must accept that the computer is metering fuel based on what the MAF is reporting. The ACT (air charge temperature) as well as ECT (engine coolant temperature) sensor is also active at WOT. Do you think the computer would care what temp the air or coolant was unless it was compensating for it?

You're right, the computer has no concept of peak horsepower or what the fuel pressure is. EVER. It doesn't need to. The only thing it is capable of doing is controlling the length of time for which an injector stays open. It can also control air to the degree that the idle air bypass can, but that's unimportant for this discussion.

Let's say that the MAF reports a certain voltage based on the amount air moving through it. The computer looks up a value based on that voltage and the values reported by other sensors to determine how long to open the injectors at a given RPM. These values are predetermined by a test engine that is in perfect condition. The computer then checks the oxygen content of the exhaust to see how much oxygen is present to decide whether it's too lean or to rich a mixture. The computer then starts making corrections with the injector pulse duration until it achieves the desired O2 content in the exhaust. If it has to make this correction over time, the computer stores this correction in an adjustment table in KAM (Keep alive memory). What I just described is Closed loop control. It learns about the engine and works to maintain the best possible fuel economy and emissions output.

At WOT, or open loop control, the O2 sensor is no longer used to fine tune the air/fuel ratio. Instead, the computer simply looks up values in its WOT lookup table and adjusts them based on the adjustment tables that were updated during close loop control. The computer still measures air and dispenses fuel accordingly, but it won't do it as precisely as it does during closed loop control. The computer will compenstate for any injector throughput you may have added to, or removed from, the fuel injectors by changing the fuel pressure.

Saying it's been proven doesn't make it so any more than just proclaiming that you're right. I'm open to the fact that I may be wrong, but I know that my experience and what I have read support each other.

This is one of those things that being wrong could be fatal to your engine. That's why I'll jump on it every chance I get. If the computer has adapted to 24# injectors and then all of a sudden the fuel pressure was dropped to only make them put out fuel at the same rate as 19#ers and the driver then just started running it at WOT before it built new adjustment tables, it's possible that it would run seriously lean and cause serious damage. I'm not clear on how much the computer can do to prevent this, either. Maybe it does a lot or maybe it will do nothing.

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351W 89 Mustang GT Convertible

[red82gt]

The computer can only adapt a small amount, which is why a combo like his doesn't have a chance of running a proper A/F at WOT, I do agree that the computer has the ability to adapt, but it is only a small amount and with him picking up 8hp by dropping 4psi it would indicate that the computer is a ways out of its window. I think he still has a large window before he leans out.

GTLEE, if you can get some 19lb injectors and a stock MAF your car will probably pick up hp.

[jimberg]

If the MAF is calibrated for 24# injectors and he has 24# injectors, the computer shouldn't have to adapt at all. His air/fuel ratio should be correct. The difference will occur at idle. I don't think it will, though, since the injectors probably can open and close fast enough. You'd start running into problems with 30#er or more. Just because the injectors are rated for 24#/hour, doesn't meant they will be putting out that much.

There's no basis for you to say that the computer can only adapt a little. The example in the book was that they set the fuel flow to the engine to run 20% rich and the computer adapted within an hour of normal driving. To put that into perspective, GTLee would have to lower his fuel pressure to 25psi to be 20% out of spec.

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351W 89 Mustang GT Convertible

[red82gt]

Okay, I talked to a Ford Technician today, an acquantance of mine. Here Goes:
The fuel tables for open loop are locked. Adaptive strategy is a survival feature. Adaptive Strategy or KAM (Keep Alive Memory) is a correction factor added to the table that the computer uses to compensate for the engine aging and degradation of the fuel system components. It allows the computer to add to the pulse width of the injectors in case the injectors are partially clogged or if the motor needs more fuel. If the injectors are flushed it will adjust and go back to the base settings in the table that is set from the factory, the computer can only add extra pulse width, not take it away. (after all if it did then there would be no sense in upping your fuel pressure because the pulse width would shorten)This he showed me from the ford technician manual.
He went on to say that that is why we are able to calculate ideal fuel pressures when we know how much power we are making and get a close match, further tuning should be done on a wideband sensor. Also he said that the factory lists the safe operating pressure range as 30-40psi.

[jimberg]

It couldn't possibly be true that the injector pulse cannot be shortened. If that were the case, the example in the book about running 20% rich and being able to adapt would be false. To do so it would have to decrease the length of the injector pulse. Other than that everything else that he said doesn't contradict what I've been saying.

You still don't seem to understand what I am saying about why you would raise fuel pressure.

(after all if it did then there would be no sense in upping your fuel pressure because the pulse width would shorten)

It makes perfect sense since you don't raise fuel pressure to increase the amount of fuel in relation to air. You WANT the pulse to shorten. I've said all along that fuel pressure has nothing to do with air/fuel ratio across the rpm range. The reason you raise it is if you are pulling in more air for which your injectors can supply fuel. Leaving all of the adjustments aside, let's just say that 19# injectors can support 304 HP at 39psi. You've made modifications to your car that makes your car capable of 336 HP. You start running lean at WOT, though, since your injectors can only handle 304 HP. You need 21# injectors, but there's no such thing. You could always get 24# injectors to support up to 384 HP, or you can bump up the fuel pressure to increase the flow rate of your 19# injectors to be the equivalent of 21# injectors.

Now let's take the flip side of that. You're at warm cruise going steady at 70 mph. The computer is in closed loop mode ensuring that you are getting the best possible gas mileage and producing the least amount of emissions. It has a target air/fuel ratio that it needs to keep. Whether you have 19# or 24# injectors, the amount of fuel it uses to maintain the a/f ratio is the same. The difference is the pulse width of the injector. It's shorter for the 24# injector. If you raised your fuel pressure, the computer would simply adapt by lowering the injector pulse length.

I suppose a lot of guys on here with superchargers would be disappointed to find out that they can only be safe at 40psi.


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351W 89 Mustang GT Convertible

[jimberg]

Hmm, this is funny. I just did a search on Google about this subject and found this interesting article.

EEC-IV ADAPTIVE CONTROL

It would seem that I have independently come to the same conclusion as Mike Wesley. I suppose you should tell Dan to pull this article since it's wrong.

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351W 89 Mustang GT Convertible

[red82gt]

jimberg, what you've basically said to me is that if you lower your fuel pressure then you'll lean out and wreck the motor. I disagree, the adaptive strategy will keep it alive and the fuel pressure will keep the motor from dumping in too much fuel at WOT.
I do understand that in practice you can bump up the fuel pressure to make a 19psi injector act like a 21psi injector (and I alway knew that this works), all I've been trying to explain is that you can make a 24psi injector act like a 21psi injector by going the other way. I don't see why you can't accept that.

I may have been misinformed with what I was told about it not correcting to shorten pulse width, (I still haven't seen any evidence to support that it can shorten the pulse width from the stock tables in open loop) but what I said would happen in theory, you only proved me to be correct by pointing out that article. I claimed:
(after all if it did then there would be no sense in upping your fuel pressure because the pulse width would shorten)

Now in the article:

Now if you were to increase the fuel pressure, the amount of fuel delivered for a given pulsewidth would go up since more fuel will be forced through the injector. As soon as the EEC goes into Closed Loop control, it will 'see' this increased fuel pressure. The reason for this is for any given pulsewidth, the A/F ratio as measured by the oxygen sensor will be richer than what the EEC wanted it to be since now there is more fuel delivered with the same pulsewidth. The EEC will calculate the difference from what it wanted and what it got and update the Adaptive table with a 'correction factor' and use this correction factor to reduce the injector pulsewidth the next time the injector fires. Eventually what happens is the EEC is able to 'dial out' the extra fuel that was added by increasing the fuel pressure. Now you can probably see why raising the fuel pressure is only a temporary 'fix' for a lean problem. Soon you will be right back where you started from. The EEC is continuously updating the Adaptive table anytime it is in Closed Loop.

Straight from the article, If the EEC is going back to where it started from then you setting your fuel pressure to 47 psi is only a 'temporary fix'.
quote:
I suppose a lot of guys on here with superchargers would be disappointed to find out that they can only be safe at 40psi.
/quote
What Ford calls safe and from what works in practice is different, we both know that! I'm just using it to illustrate that at 30psi GTLEE is not going to lean out and wreck his motor.

[red82gt]

Oh yeah, GTLEE got his original fuel pressure estimate from the Stang Analyser, maybe you should tell Dan to remove it because you feel that it's wrong!

[jimberg]

I think we're almost there, red82gt.

First, I'm not disputing what you said about lowering pressure to make a 24#er act like a 21#er. My point is that there is no benefit from doing that. In fact, by lowering pressure too much you could cause the injector to start dribbling instead of spraying. At what point that would occur, I have no idea. Since the MAF sensor is calibrated for the injector size, the pulse width will be correct without much adaptation.

Your quote of the article supports what I've been saying. The computer has no clue what the fuel pressure is and definitely has no way of controlling it. The temporary fix they are referring to is the air/fuel ratio throughout the rpm range. If you raise fuel pressure you will richen the mixture but then after a while the computer will adapt back to what its tables say it should be at. Where it matters and is a permanent fix is at the top end.

Let's say that an injector (24# @ 39psi) can open for a maximum of 10 msecs. That would mean approximately 0.00007 #s of fuel for that pulse length. The computer wants to have the air/fuel ratio at 13:1. That would mean that only as much as 0.00091 lbs of air could be in the cylinder in order for the computer to keep that a/f ratio. If we get 20% more air than that (0.001092#s which means .000084 #s of fuel is needed), we're going to be much leaner than what the computer wants because we passed the physical limits of the injectors at 39psi. We'll also be losing a considerable amount of HP. By increasing fuel pressure, we can increase the amount of fuel able to be delivered for that 10 msec pulse. To compensate, we can bump it up to the point that it can deliver 0.00009 #s of fuel. Now the computer can keep it at 13:1.

The other option is to use bigger injectors, 36#ers for instance. They can deliver 0.00010 #s in 10 msec. We only still need .000084#s though. The computer just opens the injector for (You should like this since it may explain part of what you've been claiming to some degree) 8.4 msecs. But, if the computer can only control the injector width in 1 msec increments (probably not that coarse a timer, but should have some resolution limits), it will either choose 8 or 9 msecs. 8 delivering a little less fuel than needed and 9 delivering a little more. In closed loop operation the computer would alternate between the two times since it's going to base its decisions on what the O2 sensors are reporting. In open loop it will probably just pick one or the other. This is were it's possible that small tweaks could make a difference since you could reduce or increase fuel flow to fine tune the margin of error caused by timer resolution. This would all depend on the resolution of the timer, of course. If it can control at a resolution of 10 thousandths of a second than it probably wouldn't help much.

Dan's calculator simply tells you what size injector you need and what fuel pressure would give you that throughput. I don't think it's there for recommendation of fuel pressures. Maybe he should add a link to the EEC article on the results page, though. We can suggest that.

You're right about the computer being able to protect from being overly lean at WOT. Code 43 covers that. This means that the O2 sensors are monitoring at WOT, just not for the purpose of adjusting a/f ratio.



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351W 89 Mustang GT Convertible