Injector Flow vs Pump Flow
 

I posted this in the fuel section, but everyone thinking about upgrading their fuel system needs to read this:



So I was bored doing homework today and needed something to distract myself. I was thinking about my track outing last weekend and was pondering over why I was so out of fuel with huge injectors, and then the realization set it that pump flow decreased much faster than injector flow increased with pressure. My suspicion was raising the base fuel pressure actually hurt me in terms of total fuel system flow.

I made this spreadsheet comparing the flow rates of the most common fuel pumps in single and dual configurations; walbro 255, walbro 400, walbro 450, aeromotive 340, bosch 044, and denso 1020 (my current pumps). I found all of these flow rates on the internet and interpolated for values I didn’t have to have for adequate chart resolution. I attached the spreadsheet to this post, but for the lazy ones among us I am posting a graph for discussion. I used my current 127lb/hr @4bar injectors as comparison.

To use this spreadsheet on your own setup, simply download it and change the green box to whatever your injectors are rated at 4bar. The pump flows will not change. The spreadsheet provides a nice compilation of these as well.

Assumptions
This analysis makes a few assumptions. Namely, the fuel lines, and rails are of adequate size to not cause an appreciable flow restriction (and therefor pressure drop). The voltage to the pumps is constant at 13.5v. In dual pump configurations, there is a doubling of flow. 1 US gallon=3.7854 liters, and 1lb of gasoline weighs 6.073 pounds. The injectors operate at a maximum of 95% duty cycle. Gasoline is the only fuel being used (although an e85 analysis is similar, not all pumps are e85 rated).

Attachment 125852

The thing to keep in mind is that the fuel system upper bound is determined by the least flowing component (pumps or injectors). Basically the injectors can’t flow more fuel than they are given by the pumps and the pumps don’t need to pump more fuel to the injectors than can be used.

Notice the black line, which is the flow rate of my injectors with respect to rail pressure. At some point it crosses all of the pump curves. It is at this intersection (to a given pump) where the injectors are using the exact amount of fuel the pumps are providing. There will be nothing returned to the tank no pressure drop at the rail at this point. To the left of this intersection there is more pump flow than injector flow, so some fuel will be returned. Similarly to the right of this point there is a lack of pump flow so rail pressure drops (which moves the pumps back to the left so they flow more).

To clarify, it is impossible to operate to the right of the intersection point since the injectors cannot flow more than the pumps. If it were on the right, the pumps would see less pressure and move to the left on their curves, where they flow more, and rebalance with the injectors. When your desired flow rate is to the right or above the intersection, the air/fuel ratio will be lean. The further away, the leaner you will be.

With larger injectors, the black line will shift upward (and obviously smaller injectors force it down). The important thing here is the intersection point. The intersection point is your maximum fuel system flow capacity. With more powerful pumps or higher voltage (boost-a-pump) the pump curves are shifted upward. The higher this intersection on the vertical axis the more power your fuel system can support.

Boost and Base Fuel Pressure
The second topic I want to talk about is boost. If you are boosted you need to be using a boost reference regulator, this will increase fuel rail pressure to compensate for boost. Notice how the pump curves decline with pressure, this is why FI needs a lot of fuel pump. With a boost referenced regulator, pressure is maintained at a constant at the fuel injector, so the injector line would be flat while the pump slopes down. It is the same conceptually, where these lines cross is your maximum fuel flow.

Reading the Graph
To use my spreadsheet, we can look at the above graph. Let’s say you have dual walbro 255s and 127lb injectors and want to run a max of 20psi. The question is what is the best base pressure to use? You want this to be high enough so the injectors flow enough at idle to have good control, but low enough so you don’t max out the pumps before the injectors. How you would do this is a little tricky. You want the curves of your injectors and pumps to be separated by your max boost pressure. Because injector rail pressure is held constant, it will be a flat horizontal line instead of the sloped one on the chart. So going back to my example of dual 255s, if you draw a line straight up from 58psi to the injector curve, and move 20psi to the right, where is the pump curve? It is far below the injector line, meaning our system is very pump limited. If you draw a line straight up from 35psi to the injector curve and move 20psi to the right it will hit dead even to the walbro pump curve. This means the ideal base pressure for the combination of dual walbro 255s and 127lb injectors is a 35psi base pressure. If we repeat the same process for dual walbro 400s, we find 55psi to be the ideal base pressure.

With larger injectors or smaller pumps the ideal base point is to the left, and with smaller injectors or larger pumps the ideal base point is further right. If base pressure is too high with large injectors, they will have a difficult time idling due to their minimum pulse width.

Case In Point
Lets take the case of my truck last weekend; I was running dual denso 1020s at a 70psi base and 20psi boost. If you see the chart, the means my injectors could potentially flow 650l/hr but I was pump limited to closer to 270l/hr. Which perfectly explains why adding fuel in the tune did absolutely nothing! Remember your fuel system is defined by the lower limit. If I had run a 50psi base, my injectors would only flow about 560l/hr but my pumps would flow 525l/hr. That is nearly double my total fuel system capacity at 70psi base! If I had done this analysis before last weekend I would have gotten my 9 second pass almost certainly.

Conclusion
I have not seen a discussion on this before regarding pump flow and injector flow, but the analysis is surprising simple and extremely relevant. I encourage people to download the spreadsheet and play with the injector sizing to see what size injectors you will need to support the pumps you plan on going with to support the power you want to make. Fuel system flow can be related to power output by knowing your BSFC.
The main thing to take away here is running a higher base pressure does not mean you will make more power, and I was actually surprised how quickly pump flow falls off after doing this analysis.

Additionally pumps (some of the big magnafuels for example) can be easily added and examined in the same way.

Hope this helps everyone :D

Log looks good. Lmao!

Wait. Did he say The logs look good??? Hahaha


But yea i always wonder why guys turned their fuel pressure up when pumps flow falls off fast with higher pressure. But always just wrote it off.

But your science lesson above explains it perfect. Thank!! Mr Rocket Scientist.

I want to know the story behind this 'logs look good' phrase lol

Its funnier to hear Chris tell the story, but me and him were driving Friday night trying to get my truck to cooperate and it would spit and sputter and sound awful (but still haul ass) when I gave it throttle. So we are on the interstate and I open it up and its sounding terrible but im watching the narrowband o2 voltages and tell chris the log looks good because its not misfiring, even though it sounded like shit. The log looked good so everything must be fine :lol:

Lulz, I might need to make a video of me tellin the story..... You can't describe in in words..... Yet Richard could most likely after his space rocket explanation in the first post! Lol, but yea we started around 50mph and he flat footed the accelerator, tps was off so at actual 100% in the truck only showed 79% on the log. We had initially been fighting spark issues and plugs and wires were checked and changed. We hit the interstate to "test" and from 50- beyond pegged on the dash, he was busy watching the log to check for misfires all while the truck is popping (similar to blowing out spark on a wide gap spark plug). And that's how we came up with the logs look good! :)

I cant find the flow numbers for a flex fuel pump to put into this single pump chart. I wanna see where my stock injectors and pump compare. also if this pump and 60lbers are enough for my 500 goal.

Also how do I find the variable of "injectors at 4bar" for stock and other injectors? also im still running the stock map sensor, which will not go up to 4 bar right?

Great write up!!! Lets get some flow numbers from ff pumps please

4 bar is 58psi.. which is our fuel pressure..

majority of the injectors out now are rated at 4 bar.. where they use to be rated at 3 bar (43.5psi)

Can u explain that to me because My truck idles and drives at 43 not 58. Should I still use 4 bar? It only increases Fp when in boost?

What would my variable be w stock injectors. I'm still unclear how to calculate it for my trucks injectors and ones I'm looking at

who runs a base of 70 psi!!!! :eek2: