Modify stock FPR?

[decathect]

I know that in the Honda world this has been done...

I know most of you will suggest that I get an aftermarket one. However, for my own purposes, and for the sake of argument, I am wondering if it would be possible to maybe drill a small relief hole in the inner piston of the FPR to relieve the excess pressure from a walbro 255? Right now I think my pump is overrunning my stock FPR and the pressure is too high.

Would this at all be possible? Has this been done before? VFAQ out there about it? (ha, I looked but didn't find one).

THanks!

 

[decathect]

Gotta keep my car stock, or at least stock looking for rallycross and autocross purposes, can't use an aftermarket AFPR... I think I said that earlier. I'm gonna see how much of a pain this becomes, but its not too bad at this point. Also want to see how much my mileage improves.

And yea, implementing a 2 step voltage source would probably be over my head.

After keeping a close eye on my fuel pressure, when the car is really hot after being driven a while, it sits closer to 30 psi, so I think I might just leave the hole at that size. I think its just at 34 when the car is cold... when the fuel pressure solenoid is taking effect.

 

[steve]

That 2 step voltage trigger would be kinda hard to make.

The hard part is the trigger. The rest is a power resistor and a relay to bypass it under boost. You can snatch those from a 3000GT VR4, Stealth TT, or an EVO since they come setup with the two speed fuel pumps. Since this goes in the power feed to the fuel pump and doesn't do anything but reduce the flow it's as legal as changing the fuel pump was in the first place.

 

[donniekak]

I'm thinking this could be done pretty cheap. I may try this myself. The switch could be a cheap hobbs switch set to lift a ground to a resistor on the power feed to the pump. I would think setting the crossover at 10 psi would take care of any overrun problem. A walbro 255 sould be able to feed a 4 cylinder at < 10 psi with 8 volts or so, then have the grounded resistor lifted at 10 psi to get full battery voltage.

 

[jrohner]

The 1G FPR has a bigger outlet, and it's easy to put on a 2G -- all you have to do is drill the holes offset a little since they are spaced differently. I had a 1G regulator on my 2G at one time.

 

[steve]

On the other cars the resistor is in series with the pump and the relay bypasses the resistor. So the hobbs switch either supplies 12v to the relay coil with the other side grounded or the reverse the relay coil is always hot and the switch grounds the other side to complete the circuit.

 

[donniekak]

On the other cars the resistor is in series with the pump and the relay bypasses the resistor. So the hobbs switch either supplies 12v to the relay coil with the other side grounded or the reverse the relay coil is always hot and the switch grounds the other side to complete the circuit.

I've had some turbo 3s's before. They don't use a hobbs switch, they use a figure based off of engine load to bypass the resistor. The circuit i was talking about would be a voltage divider circuit. I would measure the resistance of the pump and use that figure to find out what resistor would need to ground on the feed to get about 5/8 of line voltage going to the pump and 3/8 through a ground. The hobbs would power a normally closed relay on the resistor ground so that when it activates the ground will be lifted and full battery voltage will be seen by the pump.

 

[decathect]

hrm, not a bad idea, i'll have to think about this as a secondary option. Thanks steve and everyone!

 

[steve]

I've had some turbo 3s's before. They don't use a hobbs switch, they use a figure based off of engine load to bypass the resistor. The circuit i was talking about would be a voltage divider circuit. I would measure the resistance of the pump and use that figure to find out what resistor would need to ground on the feed to get about 5/8 of line voltage going to the pump and 3/8 through a ground. The hobbs would power a normally closed relay on the resistor ground so that when it activates the ground will be lifted and full battery voltage will be seen by the pump.

Perhaps I could have explained better.

I wasn't implying the 3S used a hobbes switch, they use an ECU output to activate the relay based on engine load like you said, you had mentioned using one in your suggestion. I was pointing out that the typical setup doesn't use a voltage divider with one leg grounded, they use a series resistor that is bypassed with a N.O. relay. It's still a voltage divider when the relay isn't activated.

Either way would work.

 

[donniekak]

I think the hardest part would be getting the right value for the crossover. Some 3s's develop an issue where they run lean under light boost because the relay has not activated and the car is using more fuel than the pump is delivering.

 

[decathect]

yeah, how exactly would I link it to boost level? Is there a specific switch out there somewhere that can be used for this purpose?

 

[donniekak]

http://cgi.ebay.com/NEW-HOBBS-PRESS...in_0?hash=item2a004a1754&_trksid=p3286.c0.m14

 

[Fabian]

doesnt B&M make a fuel pressure regulator MODIFIER? I think you cut off the top half of your stock FPR and they supply a bigger top half that you join to the bottom of the fpr and then you have a home made adjustable fpr. But that was when I had my civic si and to be frank, I dont know shit about dsm fpr.

 

[viprez586]

Correct me if I'm wrong.
Aren't 1G N/T FPR run higher fuel pressure?
That way you can keep the stock appearance, while getting the most from your fuel system.

 

[steve]

doesnt B&M make a fuel pressure regulator MODIFIER? I think you cut off the top half of your stock FPR and they supply a bigger top half that you join to the bottom of the fpr and then you have a home made adjustable fpr.

The B&M reuses the part that is causing the problem. It's the orifice that can't flow enough fuel back to the tank to regulate the pressure. Also when used with a fuel pump that doesn't overrun the stock FPR the B&M can only raise the pressure not lower it.

Correct me if I'm wrong.
Aren't 1G N/T FPR run higher fuel pressure?
That way you can keep the stock appearance, while getting the most from your fuel system.

Again, the issue is fuel pressure overrun. Raising the base pressure doesn't fix the problem if the orifice that can't flow enough fuel back to the tank for the FPR to regulate the pressure.
The other factory FPR's are all pretty much the same design with different springs in them to get different base pressures.

The simplest solution to decathect's problem would be put a smaller fuel pump in the car. He doesn't show bigger injectors as a mod so it's not clear why he's running a 255 lph pump to begin with.

 

[decathect]

yeah, I don't really need to be. I just suspected that my stock pump was going bad (I don't believe it actually was). I could just put the stocker back in, but I was intrigued at how to solve this problem. I think its a relevant problem to solve. I mean... why do people really need expensive aftermarket FPRs? You shouldnt really need to turn up fuel pressure a whole lot unless your car is pretty serious, or you're maxing out your injectors. I think this solution is more industrious and economical.

 

[landrocket]

what size resistor would be used to supply the pump with 8 volts?

 

[steve]

what size resistor would be used to supply the pump with 8 volts?

Don't know.

On the 3S the resistor spec is 0.6 to 0.9 ohms. I expect it's a 25W or larger resistor.

 

[decathect]

i'm going to bring this back from the dead, cause I am about to attempt to do this. I think I understand how to do this, but could steve or donniekak draw up a circuit diagram on how you envisioned this to work? Thanks!

I assume I would use a relay/hobbs in conjunction with a voltage divider circuit such as:

You must be logged in to view this image or video.

Could I use a potentiometer in place of actual resistors so I could play around with the voltage that the pump sees under no/low boost?

Any idea what PSI I should have the hobbs switch activate at? I'm thinking something like ~5 psi.

Edit: Here's a diagram I drew up. Am I missing anything? Do I need to incorporate diodes into this somehow?

You must be logged in to view this image or video.

Updated with my plans to rewire the pump. Might as well while I do this, right?

You must be logged in to view this image or video.

 

[350war]

If you wanted you could probably eliminate one relay.The potentiometer would control voltage until the hobbs switch pressure is reached. Your plan looks good there is no reason it wouldnt work that I can see.

 

[jeffo]

This is how I would probably wire it up:

You must be logged in to view this image or video.

One of the problems though with this setup is that the bigger the fuelpump you install, the more current it draws, which makes the resistor value you need smaller and harder to select. You can't use a regular potentiometer to test it, they can handle <1 watt generally and you will smoke it immediately. You need a low resistance 25 watt reostat.
I would probably set the Hobb's switch for a very low crossover point, like 0psi if possible. I forget how low they will go.

 

[steve]

Mitsubishi does it like this. Note that they use a relay with two contacts and the resistor is by default out of the circuit so that if the control function fails the fuel pump sees full voltage. That can save your engine.

The resistor they use with the factory Denso pump is 0.6 to 0.9 ohms and likely a 25W or larger resistor. If you use a different pump the resistance might change.

The relay is connected to the existing fuel pump power feed between the MPI relay and the fuel pump (so after a rewire if you've done one) and the control switch grounds the control pin D when it wants the pump voltage reduced.

 

[decathect]

This is how I would probably wire it up:

You must be logged in to view this image or video.

One of the problems though with this setup is that the bigger the fuelpump you install, the more current it draws, which makes the resistor value you need smaller and harder to select. You can't use a regular potentiometer to test it, they can handle <1 watt generally and you will smoke it immediately. You need a low resistance 25 watt reostat.
I would probably set the Hobb's switch for a very low crossover point, like 0psi if possible. I forget how low they will go.

ah, yeah i was wondering i had the Hobbs setup right.

This is how I would probably wire it up:

You must be logged in to view this image or video.

One of the problems though with this setup is that the bigger the fuelpump you install, the more current it draws, which makes the resistor value you need smaller and harder to select. You can't use a regular potentiometer to test it, they can handle <1 watt generally and you will smoke it immediately. You need a low resistance 25 watt reostat.
I would probably set the Hobb's switch for a very low crossover point, like 0psi if possible. I forget how low they will go.

why so low? 0psi?

 

[steve]

why so low? 0psi?

Because at atmospheric pressure (WOT on a NA engine) the engine should be consuming enough fuel to keep the FPR from overrunning and give the relay time to switch before you need more fuel as boost kicks in.

 

[decathect]

ok, so i've been working on a revision that I think is perfect. First, I found this device on ebay. It is like a hobbs switch, but has both normally closed and normally open contacts. So I can wire it up to ground out the relay UNTIL my engine hits a certain boost level, then it will activate the normally open contact. Adjustable from 2-24 psi. Hopefully 2 is low enough?

Boost Pressure Switch - For Turbo Water Injection hobbs:eBay Motors (item 330388975649 end time Dec-28-09 11:13:54 PST)

Here is my new diagram. Any input is encouraged.

You must be logged in to view this image or video.

 

[steve]

What's the point of two relays?