420A 420a turbo fuel help

[Oneslow2g]

So I’ve been putting together my 96 the past couple of months and over the past few weeks have been trying to get my fuel set up right cause it keeps leaning out around 5k or if I go to wot. At idle it reads between 10 and 12 but doesn’t stay that low in boost. My build is simple and will upgrade parts in future but I have:
Stock ecu
Stock injectors
12:1 fmu
Stock fpr
Evap delete
Fcd

 

[Canadian_CD9A]

It shouldn't be that rich at idle with stock ECU/injectors/fuel pressure, something is wrong there, but I see lots of issues. Are you on the stock fuel pump? If you're at 8 psi of boost, multiplied by 12:1 FMU ratio plus your base fuel pressure = approximately 140 psi fuel pressure at full boost, which your fuel pump can't keep up with. It's delivering a pitifully low amount of fuel at that pressure. For example, the typical Walbro 255 delivers around 240 lph at 40 psi, but flow collapses to 50 lph at 105 psi. Imagine what's happening at 140 psi.

Ideally (and I use that term very loosely with an FMU), you'll want bigger fuel injectors and a way to drop your base fuel pressure to keep the ECU happy at idle and cruising, and use a smaller FMU gain ratio, around 8:1, to give you a fuel pressure that keeps your pump happy. This will give you a safer AFR in boost, but it still won't be ideal.

 

[Oneslow2g]

Sorry I should’ve added that I do have a Wally 255 pump so your thinking I should ditch the the fmu and get a sfmu?

And 5psi boost with 6psi spring in wastegate

 

[Canadian_CD9A]

Sorry I should’ve added that I do have a Wally 255 pump so your thinking I should ditch the the fmu and get a sfmu?

You can either run the FMU plus a usual aftermarket fuel pressure regulator (my old turbo Neon had that when I bought it), or the SFMU, yes.
5 psi x 12:1 gain = 60 psi + 48ish psi base fuel pressure = 108 psi, just for 5 psi of boost. A Walbro GSS342 can support roughly 140-150 hp of fuel total at that fuel pressure, which is what the stock engine makes without a turbocharger. That's why you're running lean. If your FMU is adjustable, drop the rate of gain to see how it reacts. Less pressure gain will allow more fuel pump flow, which will actually richen up the top end, but I'm not sure it will provide a rich enough AFR throughout the boost curve. Worth a try, anyway. You can't add any more fuel pressure and you can't modify the injector duty cycle without a standalone, so your only option is to increase the injector size. If you increase the fuel injector size enough without changing the fuel pressure, you'll run overly rich at idle and the ECU can't compensate for that within its existing parameters, so you need all of it. This is why I don't recommend that people go with FMUs - the costs can add up to what a MegaSquirt system costs, and the result is really bad.

 

[Oneslow2g]

Alright with that being said could I get a set of pink top evo injectors and a sfmu and more than likely not have fuel cut? I will eventually get a stand-alone I just wanted to get a feel for low boost for a little bit

 

[Canadian_CD9A]

It's not so simple. Those Evo injectors are 550ish cc at 43.5 psi and stock is around 220cc at 43.5 psi. Remember - the ECU doesn't know what size injectors it has, it simply has a defined open/closed strategy given its known parameters. It will dump more than double the fuel at all times with those new injectors (it can cut 25% of total as a fuel trim but that's it), and likely not run at all due to being so rich at stock fuel pressure. I'm guessing you'll need an injector in the 275-300cc range to make it work properly. My 2.4L neon had 36 lb/min injectors, which is 378cc, and needed 22 psi base fuel pressure to run them. It wasn't ideal since fuel injectors do not atomize well at low fuel pressure. Adjusted for a 2.0L, the absolute maximum injector you should have is 315cc to be in range.

 

[Oneslow2g]

Alright just so that I’ve got exactly what I’ll need I got in my cart currently an adjustable fmu that you can set base pressure so my other question, if people have run the same setup on other cars with stock injectors why isn’t it working in my case?

 

[Canadian_CD9A]

Alright just so that I’ve got exactly what I’ll need I got in my cart currently an adjustable fmu that you can set base pressure so my other question, if people have run the same setup on other cars with stock injectors why isn’t it working in my case?

Like I said, try a different FMU gain rate first, the 12:1 runs you out of fuel pump - you still have enough fuel injector. If you drop 10-20 psi of fuel pressure, you’ll have enough fuel flow to support your current boost level, but watch your AFRs very carefully as you adjust.

 

[Oneslow2g]

What should I set the base to?

 

[Canadian_CD9A]

What should I set the base to?

Keep it at stock base pressure for now and just change the gain to see what happens. Don’t spend money if you can get it running okay on the 5 psi.

 

[Oneslow2g]

That’s what I’m thinking it’s on stock internals and for the low boost I really want the stock ecu to stay for now. I’m waiting on more people to use the speeduino ecu so there’s more out there for support.