FMU/RRFPR with Megasquirt? And what fuel pressure?

[boardkid280z]

(as posted on zcar.com) - I was wondering what fuel pressure you guys are running with your Megasquirt setups. Both N/A and Turbo, but specifically turbo since that is what I am doing.

Do most of you keep the stock FPR and program the MS to work with that psi? Or do you have an aftermarket FPR that runs constant psi? Or do you have an after FMU that raises fuel pressure with boost? (and from what psi to what psi?)

Thanks guys.

I am thinking of running an FPR off of a Turbo Dodge van at 55psi with turbo ZX injectors and just program the MS for that constant psi instead of messing with rising rate. Seems like MS will work better like that.

[blake culp]

I am running the stock one, according to my guage it runs at 30psi at idle i believe it is 32 at 0 manifold pressure. Im not sure of the technical name for the stock regulator, but the stock regulators are referenced to the manifold. The stock regulator will increase pressure evenly with the boost at a 1 to 1 ratio. For example, at 12psi of boost the fuel pressure on the rail will be 44, 32psi base plus 12psi to overcome the 12psi you are boosting. make sense? Im sure the turbo dodge regulator does the same, just with a higher 55psi base setting so at 12psi of boost your fuel rail would be at 67. I am running dsm 450cc injectors so i dont need the higher base pressure to get more flow from them, kind of overkill to begin with. However if you are planning on running smaller injectors you might want to use the 55psi regulator to allow them to flow a bit more fuel, although im sure there is a limit to how much pressure you can put on them. I always heard that FMU's were a "band aid" fix, im sure they do the job good enough, but if you are running megasquirt you are actually fixing the problem.

[BLOZ UP]

Rising rate regulators are nearly impossible to tune. They are a band-aid fix and are not healthy for your injectors. If you have megasquirt and bigger injectors you shouldn't "need" a rising rate.

 

I used a AEM adjustable FPR on my Z31. The only thing you could change was the base pressure, which I set to 45psi (about the standard rate that injetor sizes are measured at). I used DSM 450cc (440?) injectors. Then you just tune away.

[Thumper]

47psi base. 40 under vacuum. Weapon R FRP 1:1 440cc injectors

[z-ya]

Stock Z regulator and fuel pressure. MSD pump.

[boardkid280z]

On ZCar.com http://www.zcar.com/forums/read.php?f=1&i=651451&t=651451 there have been two responses, basically saying:

1) the flow rate out of the injectors is constant at constant psi (no matter what the air pressure is, so it will be much easier to tune MS with a constant psi.

2) at boost the intake manifold pressure pushes against the fuel pressure and reduces flow from the injectors which is why the stock FPR increases fuel pressure as the boost comes on, which is what you need.

 

Is one of these wrong and one right?

[cygnusx1]

Just an FYI. I run the stock ZX Turbo injectors from stock pressure up to 65psi fuel pressure during boost. A BEGI FMU controls the fuel pressure during boost and the stock regulator controls pressure during vacuum. I've been running like that beautifully for almost two years with no trouble at all.

 

However, I am switching to a standalone ECU with larger injectors (HP is addictive). I will be selling the FMU because I want a constant fuel pressure to program a map to. Too many variables is bad for my brain.

[z-ya]

Yes, a stock FPR will keep a constant pressure at the injector nozzle. The pressure in the fuel rail will increase as the throttle opens. The regulator keeps the pressure at the nozzle constant by allowing the pressure in the fuel rail to increase as manifold pressure increases.

 

A rising rate FPR will increase the presure a preset amount for every pound of pressure increase above atmosphereic pressure.

 

Personally, I don't think either one is "easier" to tune. My turbo car has a rising rate FPR, and it was no harder to tune than ones I've done with non rising rate FPRs. The rising rate FPR raises pressure in a consistant linear fashion, so tuning I not more difficult.

 

Bottom line, if you have a stock one in good working condidition use it. If you are purchasing a new one, and you have injectors with plenty of capacity for the application, get a non rising rate regulator. If you have stock turbo injectors, and don't plan on replacing them, you might want to go with the rising rate regulator.

[mobythevan]

the flow rate out of the injectors is constant at constant psi (no matter what the air pressure is, so it will be much easier to tune MS with a constant psi.

 

A consistent setup makes tuning easier. So I don't believe it matters if the FPR is static or raises pressure as long as it does its function the same way every time, then it will be easy to tune and AFRs will stay the same, disregarding every other variable in an engine.

[zguy36]

On ZCar.com http://www.zcar.com/forums/read.php?...51451&t=651451 there have been two responses, basically saying:

1) the flow rate out of the injectors is constant at constant psi (no matter what the air pressure is, so it will be much easier to tune MS with a constant psi.

2) at boost the intake manifold pressure pushes against the fuel pressure and reduces flow from the injectors which is why the stock FPR increases fuel pressure as the boost comes on, which is what you need.

 

Is one of these wrong and one right?

 

Both are correct. Think of flowrate out of the injectors as an equation of the difference in pressure between the inlet of the injector and the outlet. A rising rate regulator increases fuel pressure as manifold pressure increases at a one to one rate, keeping the differential pressure exactly the same. This makes for a good consistency in the flow characteristics of your injectors.

[Pyro]

Another thing to consider is efi pumps don't like making 55 psi all the time. A pump must work much harder to make 55psi as compared to 30psi.

 

A 1:1 boost referenced regulator will keep the pressure consistant across the injector and will be easier on your fuel pump during off boost driving. Areomotive makes a nice one (summit racing, 135.00, AEI-13109).

 

If you don't use a 1:1 pressure regulator, the injector will not flow as much during boost. For example if the fuel pressure is 40 psi and the boost is 40 psi there will be no flow at all from the injector. Of course this would never happen but you get the idea.

 

Go to RC Engineering. There are some formulas to figure injector flow at different pressures.

 

Here is one formula: (((new pressure/old pressure)**1/2) x old flow rate).

So if an injector flows 550cc at 40 psi then it would only flow 435cc at 25 psi (40 psi fuel pressure-15 psi of boost).

[cygnusx1]

This is an older thread but since I am now jumping into the world of MS-II, I have a question, related to this.

 

How much boost will the stock 280ZX Turbo regulator maintian 1:1 pressure rise to? Does it have a useable boost pressure limit. I can test it with my compressor eventually but my car is still winterized in the shed for a few more weeks.

[z-ya]

I know people that have run upwards of 18psi with the stock regulator. You should be OK under 15psi.

 

Are you going MS-II, or MS-I with extra code?

[Clifton]

Mines seen 25 psi, checking it. I'm sure it will go higher but I haven't checked it higher. There shouldn't be a limit on the boost you can run and have it compensate for, just the volume it will flow.

[Tony D]

The way the regulator works, the initial pressure is arrived at by a spring pressure against a diaphragm, pushing a needle valve against a seat. Fuel pressure pushes against the seat, and when it overcomes the spring pressure present, the valve cracks, bleeding off excess pressure down the return line.

 

Now, put a vacuum line on the regulator, and manifold vacuum works on the same side of the diaphragm as the spring. This works with a net effect as lessening the spring's effective pressure against the seat, making it bleed off at a lower pressure. Conversely, you have (under boost) boost pressure+ spring pressure raising the fuel pressure.

 

The stock FPR therefore will give confusing reading to people who don't understand how it works. They don't understand why with 'key on' but not running the thing makes 40psi, but when idling it only makes 36psi. Knowing now how it works, it becomes obvious that the pressure adding to or subtracting from the preset spring pressure is what is making the different pressures. In the case of 'key on' being higher than idle pressure, in effect what you are simulating is "0" manifold pressure at that point, the same pressure present at WOT in a N/A engine, and that is at least 18" of Hg higher pressure (roughly 9psi) than at idle.

 

Anyway, the only limit would be burst pressure of the diaphragm. I have a spare....now I'm curious as well.... Mechanical failure will probably not be an issue. What will be more apt is what Clifton alludes to: if you have a fuel pump capable of supplying the flow for a 45psi Turbo Engine, the small orifices of the stock return lines on the stock FPR will most likely be more of a problem than mechanical integrity.

[cygnusx1]

Z-YA I am going into semi-unchartered territories of MS-II. I plan to run 18-20psi if the knock doesn't come knocking.

 

I figured the diaphram was the limit but I wondered if anybody has found the limit...also I am curious if it can maitain it's 1:1 linearity at higher boost levels and the required fuel flow. If it can't, you are in dangerous waters! Although it would probably vary high pressure rather than low which is safer.

[Tony D]

Yes, the limitation on bypass will the the orifice size in the regulating needle.jet combination---as flow increases beyond bypass capability pressure rises.

 

This is how RRFPR's work, incidentally, along with some pneumatic amplyfication relay functions, which I don't EVEN want to get into!

 

But yeah, at the limit it would "go safe" by raising fuel pressure. With larger fuel injectors, you may run into pulsation issues, check out JeffP's installation of an aftermarket pulsation dampner to smooth out pressure fluctuations and make the regulator action more smooth.

[z-ya]

If you keep you timing and mixture right, I wouldn't worry so much about knocking (you have a stock L28ET with 7.4:1 CR, right?). I'd worry more about what the stock head gasket might do at 18psi (unless you have a metal one). What turbo are you running?

 

Sometime over the next few months I will be tuning Chris Holmes NA 280Z with MS. Let me know if you want to bring your car out at the same time for some dyno tuning on a Dynapack dynometer.

[cygnusx1]

Z-YA, thanks for the offer to come out to NH. Unfortunately, with our new son Nicholas, my schedule is going to be erratic. I am not sure how long it's going to take me to get the MSII installed and running. If I get it done soon enough, I could make a weekend out of it. That would be great.

 

The engine is a bone stock L28ET with custom exhaust, large I/C, 240SX TB. The turbo is a T3/TO4B "Super-V" trim. The headgasket may blow, it has 120K miles on it but I already make 265rwhp and 300ft-lbs with it at 15psi with the stock ECCS and an FMU.

 

The FMU will go ebay, stock injectors will be replaced with 42#, the distributor and coil will stay, stock CHT sensor, stock knock sensor, 240SX TPS, Mapdaddy MAP/Bar sensor, GM IAT sensor, LC1-wideband O2, and I will use the Megaview in the dash, and a laptop for tuning.

 

I loaded all the software onto the laptop last night and started playing with it. I should have the MSII within a few days.

 

My plan is to get it all working and debugged, drive it for a season or two and then do the big engine rebuild in 2008 or so.

[z-ya]

Seems like your turbo is the weak link if you are only making 265HP at 15psi. You should be closer to 300. I don't think that the AFM is the source of restriction, but I might be. Might be your turbine. I know because I have the turbo on my turbo Z and it makes 250HP and 310ft-lbs at 12psi. I'm running standalone speed density system on that car. The last L28ET I tuned made 325HP and 310ft-lbs at 13psi, but it had a T04E/T3 with stage 3 turbine.

 

Also, why not just use the MAP sensor built into the MS-II?

 

Why did you choose MS-II vs. MS-I and extra code?

 

Do you have a dyno shop near by?

 

BTW, we met at the Syracuse convention. I was helping out with the dyno tuning.