EFI injectors-too big?

[hitz]

 
  Why not use an injector that is bigger than you need if you can adjust the minimum time to idle OK.

  Harv

[dieselgeek]

We run "too big" on a lot of cars.  If it's your first time installing/tuning EFI, it's a good idea to try to size them appropriately, but if you're familiar with techniques to make big injectors give good drivability, then it's not that big of a concern.  I'm sure some over-tech guy will suggest potential atomization issues, but I've never run into problems.

GH's car has 160 lb/hr injectors and we're only using about 60% duty cycle on them.   The car starts and runs no different than a 2009 Chevy EFI engine, cold/hot/whatever, fires right up and idles happy.

[SPARKY]

Fuel dropplet size?

[McRat]

Probably as long as fuel economy and engine life aren't a great concern, too big can't hurt too much.  We put "too big" injectors in our 2002 Vette, and it runs fine other than makes your eyes water if you stand near the exhaust idling, and 10% drop in range.

Liquids don't burn, only gases do.  The smaller the droplets, the faster it changes over to gas state, and the less likely it is to wet the walls. 

There are a number of tuners of late model gas engines who always recommend putting in bigger injectors no matter what.  I think it's the "one less thing to go wrong" philosophy; if the engine goes lean, it wasn't because the injectors couldn't keep up.

[Rex Schimmer]

Fuel dropplet size is probably more a function of the fuel presssure, and electronic injection is typically fairly constant fuel pressure. I know on my street roadster I have larger than required injectors and way to big of butterflys and I have done some fooling around with changing fuel pressure to lean or richen the mixture and I find that my motor runs best with 45 psi fuel pressure and setting the injector max open time, this is in milli seconds, to set the basic fuel setting, then tuning with the throttle opening rate to enrichen at throttle opening and then modify the main max opening time using the mapping feature to try to get the fuel curve to fit the engine's needs. All of which, I am sure would be easier on a dyno but I am slowly creeping up on it.

I would certainly think that for an engine running WFO an injector that was operating at some place in its 50-60% flow range would be best as almost anything doesn't perform as well in the "mud" around 0-10% and also at the top end close to 100%.

Rex

[hitz]

Thank you gentlemen,

  Rex, I also am running ITB's (46mm) that are too big for the current state of tune of my stock 1.9L Saturn engine. It idles great and has 26# injectors but has not been run under load. I have brakes now and the drive line is being made. I'll be able to load the engine soon. I've installed a wide band fuel ratio gauge and a have a EGT gauge so maybe I can dial the basic fuel curve close enough.

  I don't know if the TEC II will read the wide band sensor. It reads a narrow band on the data recorder that is also 5 volts but that is only accurate at stoic. When I first ran the lakester it was really rich and the data read 10 to 1. I am looking forward to running it again soon. I have the pulse width set at 6.5 ms. I should be able to work the formula back through and get a rough estimate of the HP. I have a WAG of 160 to 170 MPH. I am the eternal optimist.

Thanks to all for your input.

Harvey
 

[robfrey]

One of the biggest problem that we experienced with running very large injectors is that there is a minimum turn on time. For most injectors, it is about 1.2ms. When you are pulling really great vacuum as we do with our turbo engine the turn on time was about 1.25ms. As you can imagine, .05 of actual time was pretty dependent on battery voltage would determine if the injector would even turn on enought to flow anything. This was much more an issue with the Electromotive when we were trying to run in the semi sequential mode as we were injecting on every stroke.
Anyhow, with Electromotive stuff, it's okay to go a little big but don't get carried away.

[dieselgeek]

One of the biggest problem that we experienced with running very large injectors is that there is a minimum turn on time. For most injectors, it is about 1.2ms. When you are pulling really great vacuum as we do with our turbo engine the turn on time was about 1.25ms. As you can imagine, .05 of actual time was pretty dependent on battery voltage would determine if the injector would even turn on enought to flow anything. This was much more an issue with the Electromotive when we were trying to run in the semi sequential mode as we were injecting on every stroke.
Anyhow, with Electromotive stuff, it's okay to go a little big but don't get carried away.

Rob, if you step up to a system that lets you tailor the opening time - tuning oversized injectors becomes much easier.

Motec, Megasquirt are a couple examples that will let you tune PWM time threshold and opening time versus voltage.  FAST, Electromotive - not so much.  I tune a 1000hp, 170 cubic inch SVO Ford engine that's street driven, running 160 lb/hr injectors and it idles at mid 14s on the wideband, at 750rpm.  With his previous bigstuff3, it would idle 10:1 AFRs unless you idled it at 2000rpm.