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Got a quick question here and I think the responses should be rather interesting.
What is the highest pressure anyone has ever ran on an MPFI system with an AFPR and what were your experiences? I just sold my mega siamesed SLP runner and plenum setup and bolted a ported superram on in its place. Now, like an idiot, I never checked a very important factor before just bolting it together: The injectors.
They are 22lb Bosch D3 injectors I have been running at 60PSI (Holley regulator) for the past 4 years. However, reviewing a datalog of when the car was on the dyno with the SLP TPI, I noticed something concerning. They were pegged above 90% duty cycle for the entire pull. The car put down 314RWHP with that setup which is good for 390 at the crank give or take, factoring in parasitic loss. I'm suspecting that the increased flow from the superram may cause them to go static and take them beyond their flow capabilities. I'm left with two options. Either take the superram back apart (Nightmare) and swap injectors, or bump the fuel pressure another 5PSI to 65 and hope it gives me that extra bit of headroom. The D3 injectors have been shown to be very stable at higher pressure so that isn't a particular concern.
No problem goin higher. Most injectors can handle up to 80 psi
Thanks Jason. I'll take her out and see what the data shows. If needed, can always bump pressure and adjust BPC values accordingly.
The Holley regulator is supposed to stop at 65PSI but I wouldn't be surprised if it made it somewhere between 65 and 70. The only thing that really concerned me is I recently got into a pissing match with some guy on one of the groups claiming that the pressure I'm running would cause the oring seals on the rails to fail and set the car on fire.
Obviously not worried about that if I've been 4 years fire free but it did raise the question of exactly how much is too much?
The buna-n (nitrile rubber) o-rings on your fuel rail can easily supports thousands of psi. They aren't going to blow out unless they are dry rotted. The same o-rings are used in hydraulic fittings.
The buna-n (nitrile rubber) o-rings on your fuel rail can easily supports thousands of psi. They aren't going to blow out unless they are dry rotted. The same o-rings are used in hydraulic fittings.
GD
Last time they were replaced were 4 years ago when I dropped the engine in.
Same guy claimed to have worked with Lingenfelter and helped design the superman. Gotta love social media.
The facts presented may be sound....but I still like having the "right" sized injectors. Just my two cents...gut feeling, whatever ya wanna call it. Let's me sleep at night.
The facts presented may be sound....but I still like having the "right" sized injectors. Just my two cents...gut feeling, whatever ya wanna call it. Let's me sleep at night.
That works too!
It wouldn't be beneath me to change injectors but I'm really trying to avoid taking that damn intake back apart if I can. They look cool and work great but they are NOT fun to work on.
Tpi stuff is 43 psi, ford used 39 psi and lsx stuff was at 58 psi. There really isnt a right pressure, just use what gets you the fuel you need. Theres drag cars using 90 psi base
Tpi stuff is 43 psi, ford used 39 psi and lsx stuff was at 58 psi. There really isnt a right pressure, just use what gets you the fuel you need. Theres drag cars using 90 psi base
True. I just got done setting my regulator to 65 and adjusting BPC and VE accordingly.
I did have the regulator as high as 70 and it hadn't stopped yet. Not sure what the highest it will go is but I seriously doubt I'll need more 65-70. 65 makes those injectors 27# and 70 would be 28#.
Note that if you run them at higher pressure it will change their latency. The solenoid has to work harder against the pressure to open them and this slows down their response time which makes the injector less linear at low pulse widths. It also may not even be able to open at really short pulse widths that worked prior to the pressure change. If the pressure is significantly increased, simply changing the injector size will not likely be sufficient.
And consider that increasing the pressure may not have the desired effect of "decreasing the duty cycle".... while yes they will be open for shorter amounts of time (decreasing the duty cycle seen on your datalog), the solenoid is working very hard against that pressure and may run just as hot or hotter than it did prior and since it's open for a shorter time that is less time fuel is flowing through to cool it also.
It's not really so simple as increasing the pressure to lower the "load" on the injector. You trade one type of load for another and maybe it's better and maybe it isn't. Honestly 90% duty cycle wouldn't concern me at all. If I was at 100% and still didn't have enough fuel I might go up in pressure a little and see if I can get enough fuel but to increase the pressure when you are below 100% is probably a waste of effort.
GD
Last edited by GeneralDisorder; 02-24-2019 at 04:39 PM.
Interesting. Cant say i ever seen an issue running any injector between 40-70 psi. If there is an effect on the solenoid heat wise i dont think its enough to make a noticeable difference but i cant rule it out either. Just never experienced it yet.
I have seen injector data on off times vs pressure tho so there is a difference there
Interesting. Cant say i ever seen an issue running any injector between 40-70 psi. If there is an effect on the solenoid heat wise i dont think its enough to make a noticeable difference but i cant rule it out either. Just never experienced it yet.
I have seen injector data on off times vs pressure tho so there is a difference there
I doubt it's a problem - I'm just pointing out that if it's running fine at 90% IDC then I personally would leave it there. I don't think that changing the pressure to get it lower the 90% when it's working fine at that level is worthwhile. We see many stock applications that run them this high from the factory and the OEM's don't seem concerned about it.
I doubt it's a problem - I'm just pointing out that if it's running fine at 90% IDC then I personally would leave it there. I don't think that changing the pressure to get it lower the 90% when it's working fine at that level is worthwhile. We see many stock applications that run them this high from the factory and the OEM's don't seem concerned about it.
GD
Funny you mention that because I thought the same thing. The higher the fuel pressure, the less stable the idle seems to be even with VE and BPC values adjusted accordingly.
HOWEVER, it may also be the voltage offset table that could be causing issues. Like GD said. It takes longer for the pintle to open so the voltage compensation tables must be changed to account for it. Waiting to hear from Bob tomorrow.
Also, the highest duty cycle I saw on the dyno was 97%. So I think I need a tad bit more headroom.
Fuel pressure
I always run the lowest fuel pressure I can possibly use, because this will maximize fuel pump and component life span.
Also, high fuel pressure will reduce fuel pump flow rate.
The higher fuel pressure only increases injector flow rate- it does the opposite to the pump. So the only time you want to extend injector capacity by raising fuel pressure is if you don't mind the additional wear on fuel system components, and are sure the pump can handle the additional load without dropping too much in flow rate.
Injectors
In the past, it was a wise choice to choose injectors which were 'just the right size' so the engine would be nearing 80-85% duty cycle injector as it approaches redline. This was considered 'best' because it gave the highest resolution for fuel injection controller to deliver precise amounts of fuel using a smaller injector, both because technology in the computer (trigger and processing speed) and because technology in the injector (ability of the injector to work at low duty cycles) was not really great until around 2002 or so. So, fast forward to 2002+, now we have great computers (factory Tahoe 411 for example from 02 can use 250lb/hr injectors, flex fuel, and is $80 in scrap yards) and great injectors (newer injectors seem to be more stable, give good spray quality at low duty cycles) So the 'secret trend these days' is to go with injectors that are perhaps 150-200% too large for the amount of power you expect to run. The reasoning is that by running the injector to barely 50-60% duty cycle near redline, the injector driver is not getting hot and not drawing a bunch of current. It makes the whole injector circuit more reliable, its easier on the ecu, and gives the injectors(solenoid) more time to cool off between squirts. It also allows the owner to run lower fuel pressure which like we discussed is healthy for components.
I try to keep base fuel pressure always near 40 in any daily driver application. Some pumps are tested or rated at 60psi or more for newer fuel systems(like aeromotive stealth), so running them lower will increase they flow rates and life spans.
I got this from a google search as example how fuel flow drops with higher pressure
02-23-2019, 06:42 PM
