EFI fuel pressure ?
 

I am planning on using one of Ricks new efi tanks that is set up for 58psi regulator. Brings up a good question. Why do aftermarket EFI system suggests 42-45psi vs some other pressure? What could or would be the downside of running more or less?
I will be running accel Gen 7 but that's just the ECM . I am not running their regulator or fuel pump, etc.
Is there a difference with how the injectors perform with pressure difference? I can imagine there is but....?

most of the later model LS stuff is all 58 psi. I have heard the reason they did that was because they also went to returnless systems and the higher pressure helped avoid any vapor lock issues. (actually they still return, but the regulator is built into the fuel filter and bypasses there instead of going to the fuel rails and then bypassing to the tank.

With that said I don't think there's any issue running the higher psi as long as the tune is set up for it; if you convert from a 43 psi tune to 58 psi the higher pressure will make it richer and require re-tuning.

Jody

Good points. Makes sense that if one has an ECM with preset tuning then changing pressure would be an issue. With many aftermarket ECMs being tunable one could adjust as needed. thx

That's not entirely true. On most aftermarket systems once the O2 turns on it'll correct to the programmed air/fuel ratio. The only time it fattens it is in cold start or any other condition that the O2 isn't operating in. Most injectors are rated at 3 bar or 43.5 psi, that's why most regulators are set at that. Some older style injectors get slower as you increase pressure. Doesn't mean much on a programmable Sequential ECM like the Gen 7 but on others it can effect performance, especially at idle. Increased pressure can enhance the spray pattern and atomization though on some injectors, just depends on the type.

OEM stuff works differently. Most of it is programmed to run at stoich throughout much of the rpm range. However they will run richer in power mode with more pressure cuz the O2 turns off and goes to as premapped fuel curve.

If you need help on your Gen 7 let me know. I'm the guy on their website.

Mark

that's not entirely true............ :) Do you tune a car just enough to get it so closed loop can add or subtract and keep the desired a/f? Or do you tune it so the add/subtract from the O2's is minimal when in closed loop? Bottom line, bumping pressure from 43.5 to 58 psi is going to richen the a/f, and should be accompanied with a tuning change. My humble opinion anyway.......... ;)

Jody

I tune it for minimal correction so if you bump the pressure and the O2 is correcting it'll correct right back to the preprogrammed air/fuel, period.

If it's tuned to where the O2 has a big correction to begin with then yes it will fatten it, only because the O2 runs out of room so to speak.

my point was that if it's tuned for minimal correct and then you bump the pressure that much, it will no longer be tuned for minimal correction.... You are correct, should have enough room for the O2's in closed loop to correct it, but my point was that it will change the O2 correction and a tuning adjustment will be needed to get it back as close as it was. Not saying it won't run. Just saying I would tweak the tune to get it back to optimal.

Jody

O.K but you said it would run richer, it won't, not if the O2 has enough room to correct.

The opposite is true when someone has tuned it for minimal correction and then you use fuel with more ethanol in it, guess what, it'll have to add fuel. But as long as it was done correctly there shouldn't be an issue, no more so than an older EFI'd car that was calibrated for 100% petroleum based fuel and now runs on todays 90/10.

well, it will run richer during warm up, cold start, and any open loop areas. That's also a big "if" on if there's enough room for the O2 to correct. Should be I agree.

We're not disagreeing, just playing semantics. My point was raising the fuel pressure will affect the a/f and I would re-tune to compensate to get my trims back in line and to get my cold start and open loop areas back where they need to be. But you are correct in that there should be enough correction available in the O2's to at least keep the closed loop areas in line, but again that assumes the tuning is right in the first place. If the O2's are already maxed out or close to it, and are pulling a bunch of fuel in closed loop to maintain the desired a/f then raising the fuel pressure will cause it to run richer then desired all the time........... :cheers:

I do appreciate your info and feedback.

Jody

Thanks for all the input guys:thumbsup: Didn't mean to start something...lol Supreme once I get my project to that point I just my hire you to help some. I do have a person that came highly recommended and well versed in Gen 7 stuff lined up locally. After reading some posts here even expert (?) people can struggle with this Gen 7 stuff.

pressure vs flow
 

Most older OEM fixed fuel pressure regulators were set at 3 bar (43.5 psi), therefore the injectors were rated at that pressure. If you change the fuel pressure to 58 psi (the OP's question) the injectors do flow more, search the web for the formula. For the Gen 7, simply plug the new injector flow rate into the config screen and all is good without a re-tune.


Peace,

Rob

Not true, it won't change the VE number automatically.
The Gen 7 uses 7 parameters in a mathematical formula, one of which is the VE, to calculate how long to pulse the injector to achieve the desired air/fuel. If you change the fuel pressure after doing the tune and change the fuel pressure value in the config as mentioned, then you'll need to change all the VE numbers to a lower value. However this can be done easily by clicking and dragging across the entire table, pushing the X key and multiplying everything by .90 or whatever. In theory you would have the same amount of excess in all areas.

I guess I will throw my 2 cents in here. I put most of this out there as you only mentioned the tank and ECU, but not what engine, what injectors and whether or not it is naturally aspirated or a boost application. The basic question you asaked has been answered here. I am just adding more info that will hopefully help you out as you move along. I apologize ahead of time as I may bounce around a bit as my thoughts come through.

I do a few LS engines here and there :D and we are always in the 60 PSI fuel pressure area as that is what the factory is based on. You can run 45 PSI which is, as mentioned, a very common EFI pressure since the L98 days.

There are exceptions and one is fuel injector choice. Most of the common Bosch, Siemens and GM replacement injectors work great at 60 PSI and even more. If you use the RP or Rochester Products injectors from GM like those in an L98, Or the Holley RP injectors, I have seen them begin to lock up and begin to not work over 48 PSI. I found this out on John Parson II Much Nova when trying to go from 45 PSI to 60 PSI.

I personally like the higher pressures for better fuel atomization and, in most of my cases, what the injectors were designed for.

The factory ECM's as well as FAST XFI and Accel DFI, when equipped with an oxygen sensor, preferably a wideband, will adjust A/F to a specified AF ratio. The factory ECM's can adjust as much as 50% up or down and the aftermarkets usually 25% up or down. It is always best to tune any system to as little as much correction as possible for the best transitions on accel and decel throughout the RPM band. Since you mentioned you had an Accel Gen 7, hopefully you have the additional wideband for it also. Big help for sure.

Changing fuel pressure will definitely change a tune and though making the proper changes to the base calibration for the fuel pressure or injectors, it should never be assumed that that change will m ake it 100% correct. Always verify with proper equipment for sure. If it was correct before the pressure change, it should be close after the pressure and calibration change. If it very far off then something does not like the pressure change.

Now, most aftermarket systems with an oxygen sensor, again preferably a wideband, will adjust fueling on the fly up or down based on the allowed amounts set in the program, but it will not learn and constantly be changing everytime it goes though any certain RPM and load range that is not 100% correct for what is commanded.

Factory ECM's have a learning feature called long term fuel trims which over time the ECM learns these compensations over time and will adjust automatically as the comes into a learned are of the RPM and load map. This drastically helps the transitions, but still not 100% perfect.

Now there are aftermarket systems such as the new Holley packages have a very fast and effective self learning system. Accel does also, but not as effective and still takes good tuning.

It was not brought up here, and not an issue with aftermarket speed density systems, but with a factory EFI such as a late model LS engine using a MAF sensor, air intake design and tube diameter as well as MAF placement in the intake tract can drastically alter a tunes A/F base and compensation.

Mike Norris