The Top DSM Community on the Web

For 1990-1999 Mitsubishi Eclipse, Eagle Talon, Plymouth Laser, and Galant VR-4 Owners. Log in to remove most ads.

Please Support Rix Racing
Please Support STM Tuned

Fuel pump re-wire and AFPR overrun fix resistor

This site may earn a commission from merchant
affiliate links, including eBay, Amazon, and others.

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

tkelly27

15+ Year Contributor
1,712
27
Mar 11, 2007
Gresham, Oregon
I've got a neat little B&M AFPR cap, but it overruns now that I have a 255 pump. Many cars drop the fuel pump voltage at lower RPM (you can find it in Mitsubishi, GM, Acura, and Nissan, among others) so I thought I'd give it a shot.

RRE has a good write up about the EVO system

RRE's EVO Fuel Pump Info

I found that the EVO 6 uses a .55ohm, and EVO 7 and up use a .75ohm. These are big resistors to handle a fair bit of power. If you've ever seen one, they have heat sinks. I'll be able to have a better idea of how much power I need when I figure out the drop across it.

Not having a potentiometer that can handle big wattage, I :aha: used some spare spools of speaker wire. With the stock wiring the whiney sob is tamed somewhere between .8ohms and 1.8ohms. I will wait until I run a big wire to try and figure out what the final value should be.

I'm torn between triggering the 2nd relay with an ECU output (reliable, adjustable, repeatable) or a hobbs switch set at whatever vacuum level seems to fit best (leaves reliable, adjustable, repeatable output from ECU free).

All in all, I think it will be somewhere around $20, plus the cost of the re-wire kit. If you already have relays and wires, the final resistor should be in the $5 range. Seems cheap and easy to me compared to an Aeromotive. Plus, no braided stainless. I can't stand braided stainless.

I'll run the big wire tomorrow. You'll know more when I know more.
 
Yes. You could also do this for a stock FPR, the B&M just happened to come with the car.
 
Sure. This seems like a cheaper route then getting a AFPR since, like you said, it can be used with the stock FPR. My question though, how do you plan on regulating the overrun going into boost when fuel pressure needs to rise 1 to 1 with boost pressure? Seems like you'd go EXTREMELY rich after going beyond the set RPM that would limit voltage to the regulator for idle.

I'm curious because I like looking stock under the hood and if you can control this using a resistor rather than a big ass AFPR/gauge I'd use it.

:dsm:
 
FPR overrun only really happens at low RPM, low pressure (vacuum) conditions.

Lets assume you can calm down a walbro such that it was like stock pump at idle. There would be no problem and you could easily boost up to 10-15psi without any issues.

All pumps have a curve, such that they flow the most without any pressure and flow nothing at all when they are dead headed (capped off). At higher pressures the pump will have less ability to flow. Higher pressures are anything that is not your idle, so part throttle or full throttle before boost.

Another factor helping you out is that at any rpm above idle and any manifold pressure above idle your injectors are flowing more. This dumps more of the fuel that would have to pass through the regulator into the manifold.

As soon as the amount the pump is able to flow at the current pressure minus the amount the injectors are flowing is less than or equal to what a pump like a 190 out of an evo (that doesn't over run) can flow at idle pressures you can switch it without any problems. The FPR can flow the same amount at idle as it can at full boost, it just doesn't need to when the injectors are flowing a ton and the pump isn't flowing as much.

Have faith. Evo's do it, 300zx's do it, integra's and NSX's do too. : D

I didn't do a damn thing to it today, got caught up in putting a stereo into my DD. Spilled juice alllll over it, smells like kindergarten and smoking electronics. Still seems to work though.
 
So, just clarifying, with this mod you won't need an afpr with an aftermarket pump?
 
Basically, yeah.

My talon came with a wal-mart amplifier supply wire, but no amp, so I made it the fuel pump re-wire wire. I'll probably wind up with a pair of pumps in the future, the big wire should handle it OK. I need a distribution block, the super ghetto electrical tape is not safe.

With the car running there is 13.8V at the end of that wire while hooked up to the pump. I found I had to drop the voltage to about 7V at the pump to get it to calm down enough to work properly. This required a 1.5ohm resistor (feet and feet of speaker wire). Doing some math, it means it needs to handle 35w.

I'll need to pick up a 20A SPDT relay, and I think it will be easist to get 3 .5ohm resistors that can handle 15W each. I'll have to call the electronic surplus place tomorrow.

For now, runs good, doesn't seem to be over running. If you're a perfectionist, you can probably tweak it with smaller high resistance resistors. Right now it's not spitting out black smoke, and the fuel pressure looks good, so I'm happy with it.
 
I'll need to pick up a 20A SPDT relay, and I think it will be easist to get 3 .5ohm resistors that can handle 15W each. I'll have to call the electronic surplus place tomorrow.

If your wiring those resistors in parallel, you'll need 3 Five ohm resistors. This will give you 1.66 ohms total resistance.

What is the current draw of the fuel pump at 7 volts?
 
Why buy resisters when you can just tune the car properly and have no problems.

No matter how good you tune, your stock fpr will stick get overrun by a 255 walbro.

Tkelly, please do a write up if you get this working properly, much less noticeable and cheaper than an afpr
 
Three .5ohms in series will give me 1.5ohms with each resistor dropping the voltage 2.3V, and each resistor will have about 10W of power going through it. It looks like there is 4.6A going through the system. I picked the 20A relay because I don't think the pump will be able to draw above that

http://www.jayracing.com/images/pro...lbro GSS342 (255HP) vs Deatschwerks DW300.jpg

I have DSMap for tuning. As far as I know you can't tune out FPR over run. No one has that I've seen anyway. When you drop the IDC's to compensate for the fuel pressure being higher, the fuel pressure goes higher. This all happens non-linearly in a spot in the table where small changes in fuel make large changes in AFR. As I said earlier, OEM's do this in quite a few cars so I don't think it is necessarily improper, just different.

Actually, in looking through this I found a modified box for Nissans

AshSpec Dual Fuel Pump Controller

An enterprising individual could build something like that for DSMs. If it also had a built in voltage booster to extend the usefulness of a 255, or make one of these new mega pumps even more BA I'd get one if I had the money. A jay racing @ 18V when you need it with a stock regulator when you don't, hell yeah. : D
 
Last edited by a moderator:
Three .5ohms in series will give me 1.5ohms with each resistor dropping the voltage 2.3V, and each resistor will have about 10W of power going through it. It looks like there is 4.6A going through the system. I picked the 20A relay because I don't think the pump will be able to draw above that

Gotcha! Have you priced 10 watt resistors? They might be pricey... Also, if you are in fact drawing 4.6 amps, then each resistor will be dissipating over 10 watts each. 10.6 watts by my calculation. Might be a good idea to step up to 15 watt resistors.

This mod is a good idea, i'm interested in how it works out. :cool

Lol, nevermind. Just found this: Power Resistors | Alltronics

25watt, 0.5 ohm resistors for $2.65 each :p
 
:thumb:

Hoping to be able to walk into the electronic surplus shop with $10 and walk out with everything I need.
 
No matter how good you tune, your stock fpr will stick get overrun by a 255 walbro.

Tkelly, please do a write up if you get this working properly, much less noticeable and cheaper than an afpr

volume is what matters, pressure is meaningless (only one of several controllable factors).

Before anyone even tries to argue - I don't expect anyone who's ONLY ever tuned a DSM to fully understand this, but it's how it is. If you want to understand why this is the answer, I recommend learning about returnless fuel systems, how they work, and how they are tuned. And then you'll understand why "overrun" is a non-issue (in most cases - there are times when it matters, when close to certain limits, but 99% of cars are never in those situations)

Most returnless fuel systems do actually use a variable voltage pump supply, but in the case of a DSM it's not needed for most applications. You could probably do this with a stock reg with a pump like a DW300 or a Bosch and it'd work pretty well. Good idea actually, since those will over-run the stock reg to the extreme. I'm not saying you never will over-run it so bad that it's not a problem, because you certainly do, once you go to a double pumper or a bosch size pump - but my original explanation is in reference to the OP's mentioning of only running a 255 pump.
 
volume is what matters, pressure is meaningless (only one of several controllable factors).

Before anyone even tries to argue - I don't expect anyone who's ONLY ever tuned a DSM to fully understand this, but it's how it is. If you want to understand why this is the answer, I recommend learning about returnless fuel systems, how they work, and how they are tuned. And then you'll understand why "overrun" is a non-issue (in most cases - there are times when it matters, when close to certain limits, but 99% of cars are never in those situations)

Most returnless fuel systems do actually use a variable voltage pump supply, but in the case of a DSM it's not needed for most applications. You could probably do this with a stock reg with a pump like a DW300 or a Bosch and it'd work pretty well. Good idea actually, since those will over-run the stock reg to the extreme. I'm not saying you never will over-run it so bad that it's not a problem, because you certainly do, once you go to a double pumper or a bosch size pump - but my original explanation is in reference to the OP's mentioning of only running a 255 pump.


So your saying a 255 won't overrun the stock fpr because it doesn't flow enough fuel? Or i guess your saying you can tune out a 255, because i know for a fact that people have had the fpr ovverrun by 255s.

Either way, I'm subscribing to this thread. Please keep up the good work, its always good to have innovation on the dsm front :thumb:
 
So your saying a 255 won't overrun the stock fpr because it doesn't flow enough fuel? Or i guess your saying you can tune out a 255, because i know for a fact that people have had the fpr ovverrun by 255s.

Either way, I'm subscribing to this thread. Please keep up the good work, its always good to have innovation on the dsm front :thumb:

A 255 will overrun a stock reg, much moreso on a 2G than a 1G, but most of the time it's well within the limits of tuning to get the fuel straight. Fuel volume is a product of pressure and pulsewidth. If pressure increases you can reduce pulse to compensate. If the car is tuned properly vs load then it works perfect as it would with a static fuel pressure. This works as long as you aren't running injectors that are super huge. When you get over 1200cc it can get a little finicky, but that's more due to inadequate ability of the ECU to properly drive the injectors. The new style high Z injectors may do better in this respect.

Most new cars run a returnless type system that works this way. They hold pressure at the rail constant (Mopar for example runs a static 59 PSI) and fuel is removed via compensation based on vacuum reading. Pulsewidth is reduced and pressure held high.
 
A returnless system that holds the rail at a constant pressure is much different than FPR over run. If it were just nonlinear it wouldn't be too terrible to tune out. The "base" fuel pressure changes with the vacuum and based on the pump demand until you reach the point where the injectors are flowing enough to allow the regulator to regulate. I'm not saying it's impossible, and if you tuned your car by yourself to get to that time slip you likely know what you are talking about, but that kind of tuning is beyond my ability and I assume beyond what most people are willing to attempt. Well, maybe not beyond their ability, but with the amount of people willing to shell out $150 for an aeromotive it's not something people are willing to dedicate time to.

The electronics store had .86ohm 22W resistors for $1 each. Hopefully I got lucky on the +-10% and it works out to be just right.

10 PACK 12V DC 20A/30A Bosch Style Relay & Socket SPDT | eBay

If a trip to the junkyard doesn't get me what I need I may try the 10 pack. Either way that would put the whole setup right at $5.
 
Last edited by a moderator:
The problem with tuning for FPR overrun is that it happens dynamically based on engine load and can happen any time you lift off the throttle or when the road dips changing the engine load. If it were a static problem tuning for it would be easy.

On a car used for drag racing you can pretty much ignore it since most of the time your at WOT and using enough fuel that the regulator won't overrun but on a daily or a road racer it can happen over a good part of your part throttle fuel map. You can't tune it out because if you try to you wind up lean when it doesn't happen.

Easiest solution is to fix it and changing the fuel pump voltage is a tried and true method car companies use to address the problem, as is using an aftermarket regulator that doesn't overrun.

Thanks for figuring out the voltage a walbro 255 needs and the resistance. As you've noted the other stock pumps from different cars need different voltage drops and the resistors to accomplish it.

At least one other person solved it by using two factory resistor packs wired in series to get the higher value. Sounds like a pair of EVO VII resistors would do the trick and have plenty of power rating.
 
Just a couple of notes:

1. Most conventional returnless setups use a regulator located in the tank. It's just a long deadhead line to the rail and fuel pressure is held at a static pressure with no manifold reference.

Some of the newer systems (Ford and GM come to mind) use a controller that reads a manifold-referenced fuel pressure sensor and control the pump via pulse width modulation. Variable voltage using resistance wastes a lot of power as heat, and every amp OEMs can reduce from the alternator is good for fuel economy. Therefore, they run the pump at full voltage, and vary the duty cycle. These types of systems DO still have a 1:1 relationship between manifold and fuel pressure. I see this as the direction fuel systems will go in the future, actually. Possibly with brushless DC fuel pumps.

2. If you design a variable voltage setup (and this *should* go without saying), you absolutely have to design it so that in case of failure, the pump defaults to FULL power. The last thing you want is a failure to switch your pump to 8v when you're on full boost.

Easiest way to do that would be a SPDT relay and use the NC contact for full power, with the NO contact for reduced power. That way the control signal has to actively switch to the reduced power circuit. That way if it fails, the pump will have full power and the worst that will happen will be a bit rich at idle/low load.
 
A returnless system that holds the rail at a constant pressure is much different than FPR over run. If it were just nonlinear it wouldn't be too terrible to tune out. The "base" fuel pressure changes with the vacuum and based on the pump demand until you reach the point where the injectors are flowing enough to allow the regulator to regulate. I'm not saying it's impossible, and if you tuned your car by yourself to get to that time slip you likely know what you are talking about, but that kind of tuning is beyond my ability and I assume beyond what most people are willing to attempt. Well, maybe not beyond their ability, but with the amount of people willing to shell out $150 for an aeromotive it's not something people are willing to dedicate time to.

The electronics store had .86ohm 22W resistors for $1 each. Hopefully I got lucky on the +-10% and it works out to be just right.

10 PACK 12V DC 20A/30A Bosch Style Relay & Socket SPDT | eBay

If a trip to the junkyard doesn't get me what I need I may try the 10 pack. Either way that would put the whole setup right at $5.

You're not understanding how fuel pressure system works. It is damn close to linear. That is how returnless systems are tuned. I tune returnless systems every day, It's a simple concept. You just have to stop thinking about things as needing to be straight lines and look pretty when graphed and just make the car run how it's supposed to.

Correct me if I'm wrong, but returnless fuel systems modulate fuel pressure by adjusting voltage to the fuel pump right?

Yes they do. There actually is are tables in the ECMs on returnless cars for correction via voltage regulation. But, there is also compensation for baro/map via inj. pulse adjustment.
 
The problem with tuning for FPR overrun is that it happens dynamically based on engine load and can happen any time you lift off the throttle or when the road dips changing the engine load. If it were a static problem tuning for it would be easy.

On a car used for drag racing you can pretty much ignore it since most of the time your at WOT and using enough fuel that the regulator won't overrun but on a daily or a road racer it can happen over a good part of your part throttle fuel map. You can't tune it out because if you try to you wind up lean when it doesn't happen.

Easiest solution is to fix it and changing the fuel pump voltage is a tried and true method car companies use to address the problem, as is using an aftermarket regulator that doesn't overrun.

Thanks for figuring out the voltage a walbro 255 needs and the resistance. As you've noted the other stock pumps from different cars need different voltage drops and the resistors to accomplish it.

At least one other person solved it by using two factory resistor packs wired in series to get the higher value. Sounds like a pair of EVO VII resistors would do the trick and have plenty of power rating.

That's why we tune by load vs RPM and not just by RPM alone. On a properly tuned vehicle, over-run is meaningless and transparent.

When the reg is over-ran, it will have a lower limit it can't go below. Below this, it is basically running like a returnless system. Tune it as such. Above that number, it's rising rate, return type. Tune it as such.

Unless you're still on a 5-knober, it's simple to do. These days there's so much out there that is so cheap there's really no reason anyone should be tuning these cars with a system that is incapable of tackling a non-linear fuel pressure curve

I made 625whp on a boosted (small kenne bell roots blower) hemi last week with the stock returnless system and a boost-a-pump on top of that. Truck drives like stock and lights up all 4 tires second gear roll (SRT8 Jeep). Seems like non-linear fuel pressure works just fine to me.

Some cars are pulse width modulated at the pump to control fuel pressure, but others actually have the regulator right after the pump in the fuel tank.


Pretty sure most use both. The regulator at the tank is more of a "relief valve" type thing
 
It still doesn't seem that you understand the problem. Tuning with a static fuel pressure is not hard. Tuning with a fuel pressure that varies with manifold pressure is even easier. To tune with speed density with a static fuel pressure you'd simply take
sqrt((static pressure - absolute manifold pressure)/(fuel pressure the injector is rated at)) * soft VE at said absolute manifold pressure. Tweak it a bit and it is done.

What is not easy is when the fuel pressure changes based on injector flow, and can change in two ways based on manifold pressure. It is not running like a returnless system. If you have the injectors open less to make up for the higher pressure, flow needing to go through the regulator goes up and the unregulated pressure goes up. As soon as you crack the throttle the engine absolute pressure goes up so the flow delivery of the pump goes down, changing the pressure again. Eventually you might be able to get it, but so far no one really has. If you are comparing FPR over run to a returnless system then you haven't really dealt with it.

With gas at $4/gallon I'm comfortable spending $5-10 to fix it instead of tune it.

Some progress, and some bad news. Trying out these resistors it seems that my multimeter has some precision, but the accuracy is way off. I had it going, but it seems like it may be more in the 1ohm range than 1.5. I think the temperature dependance of resistivity may have come into play
Electrical resistance and conductance - Wikipedia, the free encyclopedia
the resistors were getting H.O.T. Like sizzle the oil on your skin hot. Give you nice fat burn lines on those fingers if you've had too many beers and touched it again hot.

I think the voltage values are much more reliable. I have a job now, so I have no idea when I'll work on it again.
 
I understand the problem. I have dealt with it, I know what it is, why it is, and how to fix it. For 10 years. I've explained all the concepts required to do so. If you guys don't want to believe it, because you haven't figured out how to do it yourself, so be it.
 
Support Vendors who Support the DSM Community
Boosted Fabrication ECM Tuning ExtremePSI Fuel Injector Clinic Innovation Products Jacks Transmissions JNZ Tuning Kiggly Racing Morrison Fabrications MyMitsubishiStore.com RixRacing RockAuto RTM Racing STM Tuned

Latest posts

Build Thread Updates

Vendor Updates

Latest Classifieds

Back
Top