Alrighty, I'm getting very close to having a supercharged Bronco now and I'm looking for advice / experience with tuning fuel delivery in the manner mentioned in the title.

I plan to run low boost (6-8 psi) to keep my Kenne Bell supercharger pulley size reasonable and to be able to use CA 91 octane without detonating. I have an Innovate LC-1 wideband kit with in cab gauge, a Vortech adjustable Fuel Management Unit, and in-cab boost and fuel pressure gauges. This is being run on a SEFI/MAF roller cam 351w with iron gt40 lightning heads (still gotta figure out how the CC volume compares to e7te's).

I want to start with the stock truck MAF (70ish mm) and 19/lb injectors. My plan is to adjust the FMU while I hit the throttle and a buddy keeps and eye on the A/F ratios and fuel pressure. How difficult is it to find a setting for the FMU that works both in neutral and under rolling load without going lean when climbing or rapidly accelerating, something that works at tip-in, part throttle and WOT??

Also what do I do about the stock regulator in the fuel rail??

I really don't wanna screw up with the money I've invested in parts.

 

FMU's are static compensators...you won't find a setting that is ideal under all conditions. Most you can hope for is good enough to be safe. My ProCharger kit cam with an adjustable 12:1 FMU and it was one of the first things I got rid of. You are going to have to run some serious pressure at the rail for those 19# injectors to keep up (which means your fuel pump is going to have a hard time supplying the volume of fuel needed, hope you have a good one or two). The FMU works on the backside of the stock regulator so that shouldn't be an issue. My suggestion is, since you already have a MAF based setup, is to get a bigger MAF, bigger injectors, and a custom tune to be absolutely safe. You'll also make more power and be happier with day-to-day driving.

 

It's not so easy for me...

I have an eec-iv truck ECU that has very little to no aftermarket tuning support. I've done a bit of looking at Tweecer and Binary Editor, but the strategy that my truck ECU uses is still being flushed out/completed and from what I'm told by users of it, the learning curve is quite steep.

I was told the stock 70mm truck MAF I have will flow like 960kg/hr before it is pegged, though I'm not sure what that means in terms of power it'll support before pegging.

I have a C&L meter with 19# 1nd 24# "calibration tubes", their meters use the stock ford MAF electronics and put it in a better flowing MAF housing, but I think they sort of 'fool the computer' with the different size orifices in their calibration tubes for different injectors.

I don't know how much safer/better an approach this is over running 19#s with a FMU or how good the driveability will be in all conditions, Kenne Bell says to avoid these kinds of MAFs "like the plague". I might just try running the C&L meter with 24#-30# injectors and matching sample tube and watching it with the wideband to see how it goes.

I have conflicting info on what originally came with this Kenne Bell kit in terms of supplementing fuel supply; their page for it (here) says that they use a Boost-A-Pump, and the manual for the kit that I bought from KB says they used a FMU.

We are probably only talking like 400-450 flywheel hp max. I don't know what to think. KB obviously stuck with the 19#s for the Gen 1 Lightning kit. Gen 1 Ls were all speed density instead of mass-air, though.

It sure would be nice to just get a chip made (maybe a switchable one for different boost levels/bad gas).

 

The FMU setup with 19's will work the best, it'll run like a stock motor when you're not into boost and should be safely rich under pressure. I was impressed with just how well this setup ran on a 5.0HO belonging to a good friend of mine, if you have an ignition box with adjustable timing you're all set. As mentioned you want to monitor fuel pressure as well to make sure the pumps can keep up under boost, you'll see upwards of 80psi on the rail if you push to 8lbs boost.
The matched MAF and larger injectors won't come anywhere close to the same effectiveness without a tune, a stock MAF computer isn't programmed for positive manifold pressure so it's going to be all messed up.

 

I figured the EEC-IV family was fairly well supported. Probably some more options out there to look at though. Maybe look at WMS's SDS standalone setups that will plug into the factory wiring harness. Have had a lot luck using those on EEC-IV Mustangs with supercharger/turbocharger combos.


I've heard it takes 1000 kg/hr to support 500hp on paper. I figure you'd need more in real life though. I know that the 80mm MAF on my truck would be hard pressed to support 450 hp which is why I went to a 90mm L-truck unit. If and when I get things rolling again on my truck I think I'll be going to an Abaco MAF.



Mr. Bell's Boost-a-Pump is as much a bandaid approach to tuning as the C&L MAF. If a fuel pump needs that much more voltage to keep up with demand then it suffices you should be looking for a bigger fuel pump or adding another. I ran a FMU with a secondary high-volume fuel pump on my truck and I would see almost 100 psi at the fuel rail with 19# injectors...I can't think that's healthy for them. With either the FMU, B-a-P, or a tricked MAF it all results in the PCM not knowing how much air of fuel the engine is really getting. Abaco MAF's are supposed to be user-programmable so you can scale the MAF properly for your setup. You can get away with the FMU/19# injector setup but I would definitely get a serious fuel pump before running with it and then only as long as it would take for me to get the tuning sorted out.

 

"As mentioned you want to monitor fuel pressure as well to make sure the pumps can keep up under boost, you'll see upwards of 80psi on the rail if you push to 8lbs boost."

I need to get some manuals/literature on the Vortech adjustable FMU. Am I wrong in assuming that at 12:1 on the FMU there will be the stock 39psi fuel pressure PLUS the 12psi for every 1lb of boost it sees? That'd be ((8*12)+39)= 135psi fuel pressure at 8lbs boost, ((6*12)+39)= 111psi for 6lbs boost ? I have a fuel pressure gauge, should it be installed hooked up to the rail or some other line in the fuel system?

I'd be looking for a drop in pressure below what I should see (111-135psi??) with the FMU getting enough volume to provide its 12:1 supplementary fuel at 6-8psi of boost?? Or instead of a drop, it never gets to the theoretical high fuel psi number needed for 6-8 psi?

"I ran a FMU with a secondary high-volume fuel pump on my truck and I would see almost 100 psi at the fuel rail with 19# injectors...I can't think that's healthy for them.:"

Probably keeps em pretty clean, wonder what it does in terms of fuel atomization, and cylinder wash... From what I've been told it's bad for injectors to be running at a high duty cycle for a long time (heats the fuel, burns out the electronic actuator), but I don't know about super high fuel pressure... My understanding is that a 19# injector running at something like 80-90% duty cycle will support around 320hp at stock fuel pressure, I'm kinda surprised that a 12:1 ratio of fuel psi/to boost psi is required for 19# injectors at lower boost; you go over double, triple stock fuel pressure pretty fast with 6-8 psi boost. Plus this supercharger is supposed to be making just about full boost at 2k-2500 rpm, so I imagine the fuel pressure ramp-up is going to be very steep/fast.

Its all very unclear to me: If I run the FMU and it's doing its job/is adjusted correctly to match the air volume, won't I be somewhere in the stock air fuel ratio range, and the 02 sensors/EEC therefore won't freak out and try to lean it out by shortening injector pulsewidth?? What will the EEC do with timing if it reads rich, lean?

"FMU's are static compensators"

What specifically do you mean by that, just that it only responds to one input?

I've been wondering; the FMU only adds fuel in proportion to boost, so is the EEC going to be adding fuel on top of it to compensate for more/less engine speed?

"I figured the EEC-IV family was fairly well supported."

It is, if you have a mustang computer. Early MAF truck EECs don't have the same strategy as mustang computers.

"I know that the 80mm MAF on my truck would be hard pressed to support 450 hp which is why I went to a 90mm L-truck unit."

I might step up to a 90mm later, but I think the 76mm C&L will support 450hp, but that's probably an overoptimistic power estimation anyway. I'm really more worried about tuning the EEC to run properly with a bigger MAF and bigger injectors.

If the FMU can be setup to safely run just 6psi with 19#s and a 70mm stock MAF on a 351w roller with g40 irons, that'd be a good enough start, but I don't feel like I know enough about their operation yet.

Man sometimes this stuff is pretty discouraging, but I'll find some way even if takes a while to figure it out.

 

From what I have seen the FMU setup works really well considering how simple it is, high pressure across the injectors isn't as big a concern as high duty cycles.. though I wonder just how much pressure the stock fuel rails can handle.

 

You'd see a steady trend of fuel pressure dropping off as rpms and boost went up...that's when the fuel pump can't keep up with demand.



It'll move very fast on your fuel pressure gauge. Almost like watching the boost gauge. High pressure or high duty cycle isn't good for fuel injectors. The effect that high fuel pressure has on them is largely the same as going static. With that much pressure the injectors won't be reacting as quickly as they should be. At that high of a pressure the spray pattern gets pretty bad too. Your math is right on what the FMU should do with that much boost. If your fuel pump isn't up to snuff then you'll know when your fuel gauge reports lower pressure at the rail.

Hopefully, by the time you are using enough throttle for boost your ECU will be in open loop and no longer responding to what the O2 sensors are telling it. It'll be the cruise/transient driving stages that will be issue.

Means it only responds to one input (manifold pressure) and is very linear in response. It's either on or off.

With the FMU route, the EEC is going to use the same fuel map it already has and will supply fuel according to the stock tune.



Welcome to the club. It only gets worse the further you go...

 

"Hopefully, by the time you are using enough throttle for boost your ECU will be in open loop and no longer responding to what the O2 sensors are telling it. It'll be the cruise/transient driving stages that will be issue."

That's something I don't know. I thought it didn't go open loop till WOT or thereabouts... This thing is supposed to make full boost at low rpm so it may operate closed loop with boost, I'm not sure.

"With that much pressure the injectors won't be reacting as quickly as they should be. At that high of a pressure the spray pattern gets pretty bad too. Your math is right on what the FMU should do with that much boost."

"I wonder just how much pressure the stock fuel rails can handle. "

I would think they'd be as quick as normal actuating (opening) but perhaps a bit slower to close against the high fuel pressure. Geez, 135psi for just 8lbs boost, that's crazy. Can you imagine how much fuel you'd have in your engine bay if you pushed out an injector o-ring at that kind of pressure, I'm getting a big fire extinguisher....

How would I go about calculating the gal/hr or L/hr of fuel required to supply 135psi of fuel pressure to the stock rails?

 

It depends on when the ECU decides there is sufficient load...generally around 60% - 70% throttle.

They'll be opening against that 135 psi so that does slow them down a little.


I guess first well have to see what 19# injectors rated at 39.15 psi delta (Ford rates their injectors a little differently than the 43.5 psi standard) would do at 135 psi and 8 psi of manifold pressure.

New Flow Rate = sqrt(New Pressure Delta/Old Pressure) * Old Injector Flow Rate

New Pressure Delta = 135 psi @ rail - 8 psi manifold pressure = 127 psi delta
Old Pressure = 39.15 psi FRPP rating without positive manifold pressure
Old Injector Flow Rate = 19 #/hr

New Flow Rate = 34.22 #/hr

Of course that is the maximum flow rate assuming 100% IDC. So 80% of that value is ~ 27.4 #/hr

Multiply x 8 cylinders and we get 219.2 lbs/hr is what the fuel pump would have to flow to support the injectors at 135 psi at the rail and at 80% IDC.

Now to see if that is enough we have to look at how much horsepower you are looking to support. Let's say 400hp. Assuming a BSFC of about 0.6 (supercharged 351W I can't say will be terribly efficient) so...

400 * 0.6 = 240 lb/hr

So it's a close run deal with that fuel pump falling short of what is needed. That 135 psi of fuel pressure is a killer for the fuel pump. I think my stock pump is a 190 lph and I have a 320 lph pump in-line with it. As it stands with those injectors, 135 psi fuel presure, and assuming 80% IDC you can support at max around 365 hp.

 

Can you explain why the boost pressure is subtracted from the post-FMU fuel rail pressure?

Anyone know what a stock 1992 bronco fuel pump is rated to flow??

Only 365hp, huh? Is that at the flywheel or rear wheels?

 

Just keep an eye on the fuel pressure and A/F ratios to find a balance. I believe you'll be over fueling with anything close to 100psi fuel pressure and 8psi on a stock long block. However, it is always better to start rich and lean it out. If you find your fuel pressure is fluctuating too much then you may need to increase the volume instead of the pressure. A 190lph or even a 250lph boost activated inline pump would be good to run anyway (if you don't have one installed already) because you'll be able to run less pressure, with more volume, which means less heat generated from compressing the fuel. Don't worry about driveability because an FMU should not affect normal driving conditions. If it does, then there is something wrong with the FMU.

On my old stock longblock combo, I was able to run 48 psi of fuel pressure running as much as 14psi and no FMU. I did have 42lb injectors and a MAF to match which does make a difference, but relaxing the fuel pressure, if possible, is a plus with a boosted application.