incendui

Can I just wire the fuel pump to the battery through a 12v relay which is connected to a switched power source (ie one that turns on when the car is turned on)?

I'd rather bypass all the stock ecu wiring, and I'll be dyno tuning this weekend anyway, so I don't care if it being a constant 12v throws off the mixture. But would it cause any problems if the fuel pump is on when the car is on but not running?

NZConvertible

It will if you crash and break a fuel line...

If you just want to bypass the dropping resistor then use the fuel pump's wires to switch the relay. That way the pump will still only run when the engine's running.

Why do you want to bypass the dropping resistor anyway? There are no gains, only losses.

incendui

I'd rather deal with a consistant voltage at the fuel pump, trying to work around the fuel line pressure randomly changing seems more trouble than its worth.

What does the ecu use to step up the voltage though, is it when it sees boost, a tps reading, or something else?

Scott 89t2

if anything the constant power would create bigger presure changes then stock....

the pressure should never change. it is set by the FPR not the pump. but the pump can over power the FPR causing higher pressures. which it might with 12v at idle.

NZConvertible

But why? What's the big deal? Most FC's do this and it's not a problem. Fuel pressure is constantly changing anyway, as the FPR keeps it a constant amount higher than manifold pressure.
In my testing it was at ~3psi boost, but it is most likely at a certain airflow reading that just happens to correspond to ~3psi on my car.

ska3260

iTrader: (1)

it costs $130 for the stock relay and $20 for a genral parts, somtimes it is just more cost efficet to go and rewire it

incendui

I'm assuming the fuel pump changing voltages will change the afr (even assuming a constant injector duty cycle), and I don't know precisely when it will happen. The stock ecu is flakey enough as it is, I'd like to take out every variable I can. Admittedly, it probably wont make a big difference, but since I'll be tuning it anyway, I dont see a downside to having it at a constant 12V.

Though using the fuel pump's wires to switch the relay is a good call, burning alive seems like a bad way to go.

NZConvertible

But we're not talking about replacing broken parts. The relays rarely fail anyway and you can find a used replacement far cheaper than that.

Injector duty cycle is never constant, it constantly varies according to load and rpm. The ECU's fuel maps will already take account of the change in fuel pump voltage, so there won't be a sudden change in mixture when it occurs.
Flakey? It may not be very powerful or sophisticated, but it's not unreliable. Doing this isn't going to make any difference to reliability.
Tuning it with an interceptor? They can only alter fuel based on rpm and (if you have an S5) throttle position. Since the fuel pump voltage changes according to load, you're not going to be able to tune out the ECU's allowance for the voltage change. You're actually adding a variable, not removing one.

I think you'd making this into a bigger deal than it is. This system rarely causes any problems, and rewiring the fuel pump while keeping the dropping resistor working is easy to do.

Ranzo

It seems everyone on this forum is so adamant about using the dropping resistor. This discussion has came up so many times I can't even remember the number.

If you are still using the stock pump and ECU and so forth there is really no reason at this point to do any rewire unless you are not currently getting the proper voltages at the pump.
One problem on FC's is that the changeover does not happen in time or at all in some cases. Even when it does happen some cars will still not see a full 12volts at the pump. This is the reason people started doing rewires in the first place.

On my car I have a Nismo GT-R pump and it is better for this pump to see 12volts all the time and it did not perform properly with the stock wiring. I rewired it using a relay being turned on by the stock wiring pulling 12volts from the battery. I have been using that same pump since 1998. If you are installing a larger pump then rewire it to ensure it gets the power needed so its performance will not fall off. It is your choice to use the dropping resistor or not but I personally don't see the point. However if you are running a pretty much stock engine then there will probably be little to gain from rewiring it.

NZConvertible

Any modded engine should have the pump rewired, even if still using the stock pump. It ensures the voltage does not drop off when there are a lot of other electrical devices being used. It's cheap insurance.
Judging by the number of threads on this forum, this is a very rare problem. Even when it does it's usually the relay which is cheap and simple to fix. Remember the system is fail-safe, so if any part doesn't work it simply feeds full voltage all the time. The relay would have to fuse closed (highly unlikely) for the pump to never see full voltage.
Well since the common rewire keeps the dropping resistor functioning normally, this is obviously not why people started doing it. If it didn't work before the rewire it won't work after. Rewiring the fuel pump via a relay to minimise voltage is a mod as old as the hills. It is complicated a bit on FC's (and other modern cars) by the dropping resistor, but the reasons for doing it and the effect of doing it are still the same.
What does "not perform properly" mean exactly? A pump is a pump; they all work pretty much the same. There's no reason why the system should work the same with a larger pump.
At high load the pump is fed full voltage anyway, so how is its performance goin to fall off?

Ranzo

You seem so knowledgeable about the fuel pump NZ.... I think you make alot of assumptions though. I have owned and worked on more than one FC that never had more that 10volts at the pump.......no matter what the load was. Also with electric motors you have to deal with current and load. The wires in some of these cars cannot support a larger pump which pulls more current.
I have even witnessed a FD that was running screaming EGT's from going lean and putting a guage on it showed fuel pressure falling off at high rpm. Rewiring the pump resolved this problem. This is a similar problem to what I experienced with the Nissan pump it would not run proper fuel pressure through the entire RPM and boost range.

Again I have seen a car set up with normal wiring and the changeover in voltage was not happening until about 5500RPM. This was causing the motor to konck and it bucked violentley at that point as it took of down the road. This was a wierd problem that took a while for me to figure out. I rewired the pump and it solved the problem. As far as the dropping resistor I don't see the point.....some people said it is for emissions or gas mileage but I don't care about either one. The pump is designed to get 12volts and I ensure that it does.
Both methods are fine for ensuring that the pump gets the power it needs but I kind of look at it like a amp in a stereo system.....nobody reccomends using a skinny brittle wire to feed your amp and you definatley would not drop the voltage going to it at a low volume setting.

Everyone has different experiences and you obviously believe in the way you do it and I believe in mine....but it would be interesting to hear some of the losses you mentioned earlier about applying a constant voltage to the pump.

xfeastonarsex

iTrader: (5)

would a walbro overpower the stock fpr? would it cause any damage if it did?

incendui

In NZconvertible's defense, I think he was recommending rewiring the fuel pump. He was just saying to do it in a way that kept the dropping resistor.

I'm going to give it 12v though and see how it works out. If it gives me any problems, I can always rewire the dropping resistor back in.

NZConvertible

Thanks, that's exactly what I was saying. I thought that was pretty clear, but I guess not...

My fuel pump has been rewired and now has practically no voltage drop between the battery and pump, other than from the dropping resistor at low load. At high load (when it actually needs it) the pump gets full battery voltage.

If you operate the pump at full voltage when the ECU thinks it's not you're going to get richer than normal mixtures. Note that you spend far more time driving at light load than full load. These engines aren't that economical as it is, so I see no good reason to make it even worse.

Ranzo

Cool I am glad we got that straight. I am not trying to discredit anyone or be argumentative...even though it might appear so. I just think eliminating things that don't aid driveability or performance is a big step forward to having a powerful rotary that lasts. What NZ is saying about the rewire is correct.

I am just curious as to how much changing voltage on the pump affects economy and performance and so on. For example the stock ECU is connected to the O2 sensor and fuel adjustments are made to run the proper AFR at cruise so would it not be safe to say that even if the pump was seeing 12volts all the time that the ECU would compensate via the O2 sensor?
Personally I think being a little richer would improve the cars performance and possibly give longevity. As for me I have a Power FC and I adjust the maps to run the AFR I want at cruise and WOT as well as every other situation so having a higher than stock fuel pressure which does not change somewhere in mid acceleration is a plus. Has anybody ever monitored fuel consumption or Fuel pressure changes by voltage at the pump. With my pump at 12volts idle I get 3.5kilo of pressure static (49.7psi) with the vacuum connected I see about 3.2 kilo (45.5psi)

NZConvertible

Closed-loop is used a lot less than you might think. You have to be travelling at a constant speed and light throttle for a certain amount of time before the ECU starts looking at the O2 sensor. That doesn't happen all that often.
Making the mixtures richer always results less power. The extra fuel lowers combustion temps and hence the amount of energy extracted.
Ah, you never mentioned that! Obviously my arguments were for those with stock ECU's, since they can't compensate for the fact that the ECU is expecting slightly lower fuel pressures. Having control over the mixtures changes the situation, but there's still no gains to be had from higher fuel pump voltage at low load. It's just a bit simpler.
Hailers measured the AFR at idle with the resistor in and out of the circuit. There was a small but definite change. I can't remember the exact number but it proved that is does make a difference.

HAILERS

I am just curious as to how much changing voltage on the pump affects economy and performance and so on. For example the stock ECU is connected to the O2 sensor and fuel adjustments are made to run the proper AFR at cruise so would it not be safe to say that even if the pump was seeing 12volts all the time that the ECU would compensate via the O2 sensor?

Let's put it this way....... I recently bought a SAFC. I leaned out the Low and High throttle positions a good deal. I noticed that I only saw results under high throttle. OOOps. So I disconnected the 02 sensor. When I did that the afr during low throttle was wat lean, in the 16-17 afr range whereas when the 02 sensor was connected it was showing in the 14.5-14.7 range.

I'm saying that the stock ECU was working its butt off to maintain a 14.7 afr after I had leaned the mixture out with the SAFC. I only knew how much I had leaned it out by the removal of the 02 sensor from the circuit.

Under normal driving I'd venture most of the driving is within the series four tps range. The secondary throttles begin to open about the very time the tps reaches full travel. I can maintain 70mph in fifth with the secondarys (read tps not at full travel) not cracked yet.

And yes, I watched the afr using a wideband from oz, could see the afr rise/fall when the resistor in the fuel pump circuit was engaged/disengaged. I was a doubting Thomas until I saw that myself. It's an easy project to do. Just depin 3D on the ECU and idle the car. WAtch your afr. Now reinstall the pin 3D. The afr WILL change when doing this. Its approx 9v when pinned and 13.5 when depinned. The afr is written on a thread from within the last month or so.