Auxiliary Fuel Injection controller VS an FMU

Used a couple quicky injector calculators and both said that at 90% duty cycle my injectors need to be around 200cc, minimum. The stockers are 140cc are almost maxed out as is.

Keith Tanner wrote: I'd rather see you run an FIC than a single extra injector. The fuel distribution of a single injector is bad.

Depends on the manifold and injector placement. Millions of vehicles have been built with ALL of their fuel provided by a single injector.

The single injector would be much simpler to install and quite a bit cheaper. Not sure what to do here. There are a couple AEM FICs on eBay for under $300 right now.

Want to buy a Greddy Rebic IV?

Did you get the info I sent?

Zomby Woof wrote:

Keith Tanner wrote: I'd rather see you run an FIC than a single extra injector. The fuel distribution of a single injector is bad.

Depends on the manifold and injector placement. Millions of vehicles have been built with ALL of their fuel provided by a single injector.

Not for a long time, they haven't. And for good reason. The old GM throttle body injection saw about a 10% variance from cylinder to cylinder.

The AEM FIC is very powerful. I have one for a different turbo project i've also yet to assemble.

One thing to keep in mind is that while an FIC allows you to scale for different injectors, you're somewhat limited on how big of injector you can run due to the ridiculously small pulsewidths that would need to be accurately controlled during idle and lean burn on this particular engine.

I think i'd WAY rather see someone using an RRR than trying to shoot an extra injector worth of fuel through a 'dry flow' manifold.

In reply to Zomby Woof:

Yep, got it. Thanks!

So on the AEM FIC, I would probably have to increase the injectors by 70% (140cc to 240cc) am a little worried about idle with that large of an increase. Obviously, the actual numerical increase isn't THAT big. The 240s run a D16 which overall combustion size isn't much bigger (330cc vs 400cc per cylinder).

I don't know the FIC at all, but modern injectors are really fast. We can idle well with 1200s on 1.8 engines.

Question: I seem rising rate things/FMUs mentioned on here for very cheap builds often. I don't understand how cars don't blow up with them. Do they only work on MAF cars (not MAP?).

I've never owned a boosted car. I've ready about them frequently on Saturn forums, and my understanding there is that the a RRR, FMU or even EIC = guaranteed engine death.

Car builds boost @ part throttle (closed loop). EIC/RRR/FMU/whatever detects this and adds fuel. O2 sensor on stock ECU detects too much fuel and cuts fuel to get it back to 14.7:1 ish. Motor assplodes. Apparently they work fine if you only do use WOT under boost.

What am I missing? How do other cars get around this?

In reply to Keith Tanner:

Good to know. Thanks. Chatting with zomby a bit off line but the fic seems a good option.

ProDarwin wrote: Question: I seem rising rate things/FMUs mentioned on here for very cheap builds often. I don't understand how cars don't blow up with them. Do they only work on MAF cars (not MAP?). I've never owned a boosted car. I've ready about them frequently on Saturn forums, and my understanding there is that the a RRR, FMU or even EIC = guaranteed engine death. Car builds boost @ part throttle (closed loop). EIC/RRR/FMU/whatever detects this and adds fuel. O2 sensor on stock ECU detects too much fuel and cuts fuel to get it back to 14.7:1 ish. Motor assplodes. Apparently they work fine if you only do use WOT under boost. What am I missing? How do other cars get around this?

I don't know why they wouldn't work on MAP cars.

You've found one of the problems - closed loop. Stock ECUs only have so much authority to trim fuel, so the technique is to simply overwhelm them until they get into open loop. Then you've got way too much fuel all of a sudden. The usual result is lean tip-in. The car won't "assplode" necessarily if you run 14.7 with some boost, although you'll want to make sure the timing is on your side. The Mazdaspeed Miata actually has the tendency to run at stoich under boost right from the factory, and I know that some of the modern OE setups run AFRs that garage tuners can't imagine.

One way to solve the FMU lean tip-in problem is to spoof the O2 signal. Remember my comment earlier about a Hobbs switch? That's a spot you can use it. When the car goes into boost, change the O2 signal to show a lean condition. Stock ECU responds by adding fuel, you just paved over the tip-in problem. Yes, it's a bit crude but it does work. We have a slightly more sophisticated version in our Voodoo Box, and used to use a voltage clamp/Hobbs switch combo on our Protege setups years ago.

The more sophisticated and fast-acting your stock ECU is, the more difficult it is to make an FMU setup work. The reason you usually associate FMU setups with engine death is because they're a half-assed way to do things these days, so they're often combined with other half-assedness.

Keith Tanner wrote: I don't know why they wouldn't work on MAP cars. You've found one of the problems - closed loop. Stock ECUs only have so much authority to trim fuel, so the technique is to simply overwhelm them until they get into open loop.

I was speculating about MAP cars because many (most?) MAP cars the ECU cannot read positive pressure. So in addition to the O2 reading suggesting too much fuel, the MAP X RPM X TPS also suggests too much fuel. Do MAF cars have the same limitation?

Hmm. Not sure if the Saturn ECU is overwhelmed into open loop or not. Or maybe not very easily. Also, no way of controlling ignition timing, so maybe that is a factor as well.

The idea of an FMU (or other device to add more fuel, electronic or mechanical) is to take over when you go into positive pressure. You don't expect the stock ECU to be able to handle it, so you take care of it.

There are other ways to handle timing, it's easy to retard the spark. You can do it either by misadjusting the base timing (that can be as simple as twisting a distributor) or you can do it electronically with a programmable box that will pull timing based on factors such as MAP and engine speed.

I don't know anything about Saturns other than the fact that they haven't built cars for a while. Older cars spend more time in open loop than newer ones do. It's not necessarily a matter of overwhelming the car into that condition, generally speaking the stock ECU is more likely to run in open loop at higher loads.

Alright gang, thanks for all the input. I think that I am going to go with the AEM FIC (Sorry Zomby Woof for flip flopping). I have been researching all morning and think that even though the install is a bunch more work, the end result is far superior. This is definitely one place not to cut corners.

Little late to the party, but I've got an ms1 megasquirt you could have for a little over a Benjamin. Setting it up for a fuel only extra injector should be pretty easy.

In reply to bluej:

I talked that route over with a friend and the AEM is just such a clean way to handle what I want. Thanks for the offer though. I might remember that if I need an MS1 for one of my other crazy projects (Like a Turbo Ford Galaxie).

Keith is right on with the FMU, it gets a bad rap because of the KIND of things it goes on (cheap/lazy). It's another tool in the toolbox for getting fueling right, and it's only a good answer some of the time. I think it'd be a good one in your case, but I'd like to hear how the FIC works out. I haven't really had luck with that unit in the past (though all on German cars, which are notoriously grumpy).

Oh, I don't think FMUs are that bad. In the Saturn world they get no love, but I've seen them work, and work well, on lots of cars (Dave Coleman's Lemons Miata comes to mind). I was mostly curious on how some cars they seem to work well (Miatas) and other cars they are pretty big failures.

Depends on who you ask in the Miata community They were very common, but have fallen out of favor with almost everyone so deals with them professionally. 20 years ago, they were an okay option but time has moved on.

I was mostly curious on how some cars they seem to work well (Miatas) and other cars they are pretty big failures.

Tuning an adjustable base pressure and an adjustable rate of gain, while not horribly complicated, is not the easiest thing in the world, especially if you're among the probably ~90% of RRR owners who don't really even understand how it works and are only surviving based on the fact that copying someone else's advice/settings got them 'close enough for low boost'. How many RRR-equipped cars don't even have an in-cab fuel pressure gauge? You'd be surprised how many people, even with 'better' equipment, chase their tails trying to 'tune out' a fuel pressure problem because they.. wait for it.. don't know what the fuel pressure is!

They can be made to work by smart people who are not horrible cheapasses. How great of a success rate is that?

Keith Tanner wrote: Depends on who you ask in the Miata community

Incredibly cheap people. Like those running LeMons cars

Yeah, if it only has to last a race or two and mostly runs wot, I can see that. I tend to look at years of street use.