Fuel pressure question? AFPR, with Walbro 255 pump, TBI 60 pound injectors.

[EagleMark]

I just recalculated using one of the other sheets and it shows I need to run the 60's at 25 psi.

I was waiting for that!

This also brings on many more tuning issues as the pressure is way outside of what the injector was designed for, can be done but way to much work.

But your 80PPH injectors, at 85% Duty Cycle and .5 BSFC and 17 PSI (more realistic) will support 311 HP, now the key to get a good tune with this spreadsheet is the BPW is 102. Or at 16 PSI = 302 HP and BPW 105.5. I er to a little bigger, easy to take away fuel in tune, hard to add if it was never there... VE numbers can come down but can only go to 100 and should really never pass 95, but having WOT only need 85 is sweet....

This is really KEY to tuning a modified engine with old 160 Baud ECM.

Now you probably have a newer 16197427 and it can be fudged a little after this calculation by changing injector size, smaller = more fuel and larger = less fuel, but you still need enough fuel to feed the beast at WOT or all your idle and drive ability tuning has been a waste of time. This fudging will only work so far and you'll be able to tune in low end or high end, usually you end up short at high end and injector duty cycle goes 100% or static... so only fudge a little after calibration.

I like to calibrate a tune correctly to start. If it does not have enough fuel to support WOT High RPM, then why start?

[96lt4c4]

I was waiting for that!

This also brings on many more tuning issues as the pressure is way outside of what the injector was designed for, can be done but way to much work.

But your 80PPH injectors, at 85% Duty Cycle and .5 BSFC and 17 PSI (more realistic) will support 311 HP, now the key to get a good tune with this spreadsheet is the BPW is 102. Or at 16 PSI = 302 HP and BPW 105.5. I er to a little bigger, easy to take away fuel in tune, hard to add if it was never there... VE numbers can come down but can only go to 100 and should really never pass 95, but having WOT only need 85 is sweet....

This is really KEY to tuning a modified engine with old 160 Baud ECM.

Now you probably have a newer 16197427 and it can be fudged a little after this calculation by changing injector size, smaller = more fuel and larger = less fuel, but you still need enough fuel to feed the beast at WOT or all your idle and drive ability tuning has been a waste of time. This fudging will only work so far and you'll be able to tune in low end or high end, usually you end up short at high end and injector duty cycle goes 100% or static... so only fudge a little after calibration.

I like to calibrate a tune correctly to start. If it does not have enough fuel to support WOT High RPM, then why start?

Yes, its a 94 Truck so it has the 7427 in it. I have done what you are talking about, fudging the the injector constant while datalogging, and emulating. I watch the BLMs and fudge the consant to get it close in real time. Seems to work OK most of the time. So it looks like I may be keeping the 80;s in there and just doing some more tuning.

[EagleMark]

If you get the injector size correct then no BPW in that PCM... so it's a matter of building a smooth VE table to work with. Like 85 at WOT stuff down to little lower then stock at idle area with cam.

[pmkls1]

You need to be careful with the 255lph walbro pump. I have been running a walbro high volume AND high pressure pump ( there are 2 255lph pumps available, a standard pressure and a high pressure) on my '89 Firebird Formula for ~3 years now and have had to deal with excessive fuel pressure ever since. The car was originally running the stock 5.0l L03 TBI and 1228746 ECM, but is now running an L31 5.7 and 16197427 PCM. All the while, I have been running the stock 305 injectors and an O.E. style billet aluminum adjustable FPR with an added vacuum reference port. With the original regulator modified to be adjustable and the spring tension backed off as far as possible, the fuel pressure was still running in the mid-20's due to the flow of the pump exceeding the demands of the injectors and the capacity of the return passages in the throttle body. I then installed the billet FPR and set a base pressure of 30psi and when referenced to intake vacuum it would drop to ~21psi at idle. Even with the 350, the fuel demands aren't enough to allow for a lower fuel pressure. I know that your engine will definitely have a higher fuel demand, but it probably won't be significant at idle. I don't have any known tuning issues with that fuel pressure, but it can cause problems and most guys on here don't recommend more than 21psi on TBI injectors.

Now, you do have options that would allow for use of this pump with your current configuration and also have lots of room for any future upgrades. You can always use a block off plate in place of the stock regulator and use an external regulator that will allow for the use of just about any injection & fuel pump combo. I would have done this myself a long time ago, but there's always something more important I need to buy. I'm a little surprised that you haven't had any issues with a vortec application fuel pump, but I don't know the flow rating of them either. Anyhow, I just wanted to give you a heads up before you install the walbro. There's nothing wrong with them, they just put out a little more than a typical TBI system can handle without some accompanying mods/hardware.

[96lt4c4]

You need to be careful with the 255lph walbro pump. I have been running a walbro high volume AND high pressure pump ( there are 2 255lph pumps available, a standard pressure and a high pressure) on my '89 Firebird Formula for ~3 years now and have had to deal with excessive fuel pressure ever since. The car was originally running the stock 5.0l L03 TBI and 1228746 ECM, but is now running an L31 5.7 and 16197427 PCM. All the while, I have been running the stock 305 injectors and an O.E. style billet aluminum adjustable FPR with an added vacuum reference port. With the original regulator modified to be adjustable and the spring tension backed off as far as possible, the fuel pressure was still running in the mid-20's due to the flow of the pump exceeding the demands of the injectors and the capacity of the return passages in the throttle body. I then installed the billet FPR and set a base pressure of 30psi and when referenced to intake vacuum it would drop to ~21psi at idle. Even with the 350, the fuel demands aren't enough to allow for a lower fuel pressure. I know that your engine will definitely have a higher fuel demand, but it probably won't be significant at idle. I don't have any known tuning issues with that fuel pressure, but it can cause problems and most guys on here don't recommend more than 21psi on TBI injectors.

Now, you do have options that would allow for use of this pump with your current configuration and also have lots of room for any future upgrades. You can always use a block off plate in place of the stock regulator and use an external regulator that will allow for the use of just about any injection & fuel pump combo. I would have done this myself a long time ago, but there's always something more important I need to buy. I'm a little surprised that you haven't had any issues with a vortec application fuel pump, but I don't know the flow rating of them either. Anyhow, I just wanted to give you a heads up before you install the walbro. There's nothing wrong with them, they just put out a little more than a typical TBI system can handle without some accompanying mods/hardware.

Well that stinks, I just got the pump too. I guess I will stick it in and see why happens. I have AN style lines going from the filter under the truck to the TBI and back so if I have to add a regulator its no big deal.

[96lt4c4]

I was just looking throught the stock tune and tell me if am wrong but is the stock BPW 397.6? So it needs to go down almost 200 usec?

[EagleMark]

No! I think I led you to BPW being all to important calibration factor before tuning, which is the case on other ECM. Just change the injector size in your ECM/PCM, if running higher pressure use the spreadsheet to get injector flow.

[gregs78cam]

See, this is where using the wrong term over time gets people confused. BPW is the time in milliseconds that the injector is commanded to open. You are thinking of Base Pulse Constant - the conversion factor used by the old ECMs to figure out the required PW or BPW.

Those parameters that you have open, are simply the minimum time in micro-seconds that the PCM is allowed to open the injectors. There is a point as you go shorter in time at which the amount of fuel dispensed by the injector is not consistant, and idle will get erratic.

You don't really need to worry about those.

[carcaper]

In my opinion the size of the injector flow (lbs per hr) is what you should calculate first for the demand of fuel needed to match the engines consumption. This figure should be at no more than 80 % duty cycle. Larger Injector sizing sometimes is taken to large which also can cause poor performance regardless of pulse width. Injector sizing is no different than carburetor jet sizing, you can be to small or too large, there is still a sweet spot. As far as the pressure setting, this would only be fine tuning of the injector sizing and you don't want to beat up your injectors with excessive pressure, the higher the pressure the harder the injector pintle slams closed which will cause excessive, unneeded wear and erratic operation. I have never seen a gm throttle body application higher than 16 psi that was not erratic. Its important that you have enough reserve fuel flow from your fuel pump but don't overkill the flow rate, there is a percentage thats engineered into every vehicle, they do this for a reason. All systems can very per application.

[907 Chevy]

I was waiting for that!

This also brings on many more tuning issues as the pressure is way outside of what the injector was designed for, can be done but way to much work.

But your 80PPH injectors, at 85% Duty Cycle and .5 BSFC and 17 PSI (more realistic) will support 311 HP, now the key to get a good tune with this spreadsheet is the BPW is 102. Or at 16 PSI = 302 HP and BPW 105.5. I er to a little bigger, easy to take away fuel in tune, hard to add if it was never there... VE numbers can come down but can only go to 100 and should really never pass 95, but having WOT only need 85 is sweet....

This is really KEY to tuning a modified engine with old 160 Baud ECM.

Now you probably have a newer 16197427 and it can be fudged a little after this calculation by changing injector size, smaller = more fuel and larger = less fuel, but you still need enough fuel to feed the beast at WOT or all your idle and drive ability tuning has been a waste of time. This fudging will only work so far and you'll be able to tune in low end or high end, usually you end up short at high end and injector duty cycle goes 100% or static... so only fudge a little after calibration.

I like to calibrate a tune correctly to start. If it does not have enough fuel to support WOT High RPM, then why start?

So instead of starting a new thread I'm digging this one up because this is an issue I'm faced with at the moment. I have two sets of injectors (61&80pph) and need to decide which to use. Either set I'll need to run at higher than stock pressure and I will be sending the injectors off to be cleaned and flow rated as well as having witch hunter measure latency.
My engine setup should hopefully be close to 400 hp and assuming 0.45 BSFC and at 85% DC I'd need to run the 61pph and 80pph injectors at 35 psi and 21 psi respectively to give the engine the required fuel. So larger injectors/lower pressure or smaller injectors/higher pressure? What are the pros and cons of each?

I'm running a vortec EP281 pump so higher fuel pressure is not an issue with respect to fuel supply.

Whatcha guys think?

[EagleMark]

Bigger injectors at lower pressure! Pro is they work fine at that pressure, Con at higher pressure is they open slow and casue issues, no need for latency on TBI injectors, just a service.

[907 Chevy]

Bigger injectors at lower pressure! Pro is they work fine at that pressure, Con at higher pressure is they open slow and casue issues, no need for latency on TBI injectors, just a service.

Sounds good, thanks for your input. You don't think it would be beneficial to have latency checked at the pressure the injectors would be operating at though?

[gregs78cam]

Sounds good, thanks for your input. You don't think it would be beneficial to have latency checked at the pressure the injectors would be operating at though?

If it doesn't cost any more it wouldn't hurt to check the numbers against what is in the PCM.

[EagleMark]

It's be cool to see how they stack up to factory specs too!

But I don't think it's called latency is it? Voltage offset?
In $42 it's "INJ - Injector Correction Multiplier"
In $0D it's "Injector Offset vs. Battery Voltage"

Or did I miss the boat?

[907 Chevy]

It's be cool to see how they stack up to factory specs too!

But I don't think it's called latency is it? Voltage offset?
In $42 it's "INJ - Injector Correction Multiplier"
In $0D it's "Injector Offset vs. Battery Voltage"

Or did I miss the boat?

No, you got it right. I'd really like to have some solid offset numbers at say 22psi on 80pph injectors. It'd def take the guess work out of adjusting injector offset for increased fuel pressure. Witch hunter charges $25 per injector to measure latency or lag time, whatever you want to call it.