Choosing Injectors, for different engine sizes Options

[McMark]

A question about which injectors to use on a 1911 engine with stock fuel injection made me dig in deeper. So I collected the following info for stock injectors. The critical piece when comparing injectors is the testing fuel pressure. I was able to look up the flow-report some other members had posted, which show the after-cleaning flow rates. A set of 2.0 injectors flow tested at 42 lb/hr, confirming the 380cc @ 2bar rating that's been passed around. So using that, I built the following table using the typically reported flow rates for the other stock injectors.
(IMG:http://www.914world.com/bbs2/uploads/post-419-1526397753.png)

I was looking at these injectors at FiveOMotorsports. The FiveO injectors don't explicitly state their testing pressure, but industry standard is 3-bar. So using the chart above you can find the best fit injectors for stock replacement. BUT YOU MUST CONSIDER WHAT FUEL PRESSURE YOU WANT TO RUN. Considering a stock 2.0, you either need 475cc@3bar injectors at 28psi, OR 380cc@3bar injectors at 43psi. And higher fuel pressure improves atomization, so I'd upgrade to 3bar fuel pressure.

and now to get to the actual question, "What injectors to use with a 1911?" you also have to consider the collection of components your using. I'm going to answer this based on the assumption that you're using a complete and correct setup, meaning matching ECU, MPS, etc.
(also note: cam choice can make a difference here too because different cams will have different VE results)

Okay, so here we go:
(IMG:http://www.914world.com/bbs2/uploads/post-419-1526399085.png)

So a 1911 is 14% bigger than a stock 1.7. So assuming you had a perfect, complete 1.7 fuel injection system (which implies 'perfect' tuning) that you were installing on a 1911, you would use 380cc@3bar injectors, run at 28psi to compensate for the increase in displacement.

The 1.8 injection would work best with 250cc@3bar running at 28psi and 2.0 injection would work bets with 450cc@3bar injectors run at 28psi.

My thinking, as I type this, is that the tuning of the stock fuel injection is tied to the ECU, MPS and Injectors (mostly). So which collection of components you're using would change which 'upgraded' injectors you would need for a 1911.


Okay, I'm even confusing myself with some of this info. Ask questions...

[Mblizzard]

Just to be clear, McMark was speaking of the options related to “modern” low impedance injectors available for the 914. What I am presenting below is for old style stock injectors that you would purchase as a “stock” replacement.
OK I am just going on memory here so I might have this wrong. This is a lot of my thought process I went through when trying to decide to use stock injectors on my Micro squirt 2056 build.

I think the relative simplicity of the early FI systems required the use of higher flow rate injectors because the ECU was not capable of making all of the changes across all areas of performance. For the D-jet systems, it is important to note that the pulse width of the injector is not a calculated cycle value. Its maximum length is defined because the trigger points mechanically define the time the injectors can fire. You can only fire one pair for the length of time the contact is closed based on the trigger points. I am not sure exactly how the D-Jet varies the pulse width if it is a duration that is less than amount of time the points are closed but the maximum pulse width is limited to exactly 178 degrees of distributor cycle. One cycle has to end when the other contact point closes. Certainly, as RPMs increase this length of time decreases.
The actual capacity of the D-Jet injectors used with stock systems are closer to only providing a maximum of 60% of their maximum rated flow. This is why by modern standards the 914 injects seem huge! Ultimately when increasing displacement you reach limits that stock injectors with stock ECUs just can’t overcome.

Now with that said, some people try to overcome limitations of the stock injectors by increasing fuel pressure. There is a linear relationship between volume and pressure. Obviously until you hit the built in flow limitations of an injector the higher pressure you put in the more flow you get out. The information I have always worked on was that the stock injectors (because they were low impedance) simply did not have the power to operate correctly at pressure much higher than 35 psi. So you lose that linear increase as pressures go past 35. So building a 2.3 and trying to get enough fuel to by operating at 43 PSI leaves you lacking. This is exactly what I experienced even on my 2056.

So these are just a few of the things to consider in relation to use of stock injectors and ECUs. In the end it became very apparent if you are going to modify your engine past a certain point you were truly limited in being able to take full advantage of your modifications if you stick with the stock systems.