I’m going to start buying fuel system parts this week but am having a little trouble resolving a few questions. Any help, based on experience or defined theory rather than baseless speculation, will be greatly appreciated.

The goal: supply enough fuel for the operating envelop of the stock twins with a street port while also leaving a good margin of headroom.

The Plan:
-four-850 injectors
-max duty 88%
-fuel pressure 43.5
-oem supra pump 255-270L/hr (depending on who you ask).

The Questions:
1. At 45psi of fuel pressure will I have trouble getting good drivability at low rpms and a stock-like A/F ratio at idle?
Obviously the 850s are bigger than the stock 550s. I’m concerned that this change coupled with additional fuel pressure will take the injectors too far out of their sweet spot for idle/low rpm part throttle use.

2. Does the Stock fuel system vary fuel pressure at idle/low rpm?
I’ve seen a couple people post that the stock ecu varies power to the pump at idle and low rpm. This seems like an easy way to resolve the idle issue. In theory I suppose I could also solve the problem by using a RRFPR but I’m trying to avoid it if possible because I think a 1:1 will be less of a headache to tune.

3. Are there any other limitations to upping the fuel pressure?
Is it safe to go to 45psi with the stock lines? At the very least I believe I will have to change out the FPR if I want to adjust it. If that is incorrect someone please let me know so I don’t waste $130. Obviously the pump is another factor but I’m upgrading that regardless.

 

1. I'm using (4) 850's with a base pressure of 38 psi. It doesn't sound like you know what base pressure is so I'll explain and answer question 2 in the mean time. Flow from injectors increases with pressure, the higher fuel pressure may cause problems running rich with 850's. I'm assuming you have a PowerFC or other aftermarket computer that allows you to change injecter sizes.

2. Yes, fuel pressure varies based on manifold pressure/vacuum. Base pressure is the fuel pressure when manifold pressure is zero on your boost gauge. The stock fuel pressure regulator is a 1:1 regulator (& most aftermarket ones) so for each pound of boost fuel pressure increases 1 psi and under vacumm it decreases fuel pressure. So with a base pressure of 38 psi, I read about 28-30psi at idle with 13"hg vacuum and under full boost it goes up to around 50psi (only running 11 psi right now).

3. Shouldn't be a problem for the fuel lines. An adjustable/aftermarket regulator would be best for fine tuning especially when running the larger primaries.

How much boost do you want to run, ported motor? Due to a weak primary I spool slow and can only run 0.8kg/cm2(11.3psi) to redline with the secondary. This equated to 310rwhp with all the bolt-ons and large street port. After racing a WRX the other night I noticed peak boost of only 0.78 and max duty cycle of 84.1% so (4) 850's won't make it too far once I can straighten out the turbos and go back for more boost. If starting over, I'd skip the 850 primaries and get 1600 secondaries.

Some of your questions are kind of elemental for fuel systems. Check these sites out for more fuel system info. and let us know what new questions come up.
http://www.wvinter.net/~flanham/wlanham/
http://www.maxcooper.com/rx7/how-to/...tem/calcs.html

Jack

 

Jack:

Thanks for the reply. I understood all the above before posting. I also spent a lot of time at both of those sites. Special thanks to both authors for great work.

All my fuel pressure numbers are relative to the mani/fuel rail gradient. So for example when I say 45psi of fuel pressure, I'm assuming the mani is at zero vac. A FPR compensates for changes in the air pressure/vac in the manifold by upping fuel pressure. As you said if you desire to have a gradient of 45psi and you have positive boost pressure of 10psi you need fuel rail side pressure of 55 (50-10=gradient of 45psi). The stock system runs at a gradient of around 40 from what I understand.

If I start there and do the fuel calculations for 4 850s at a max duty of 85% with a BSFC of .62 I get a max of 362rwhp. I believe I can reach that capacity limit with my mods, especially during the fall (see sig).

Now if I up the fuel gradient pressure to 45psi my max rwhp goes from 362 to 385. The reason behind this, as you said, is simple; the higher the pressure the more fuel that comes out per duty cycle. However, if this is true even at idle, in vac ect, it seems like it would be problematic since the 850s are already 300ccs larger than the stock primary injectors.

If the FPR is the only device that regulates this area it is concievable that I could get the best of both worlds by using a RRFPR. However, as previously mentioned I'm trying to avoid that if possible. If on the other hand the ecu is able to restrict pump function at idle ect, fuel pressure could be reduced without the intervention of the FPR. I've seen people post in the past that the power going to the stock pump does vary. As you pointed out the variance in power may be done just to maintain a consistent fuel pressure gradient and not to lower the gradient.

So I guess what I'm saying is it seems like it would be ideal to have electronically variable fuel pressure that could run at say 37.5 at idle and under light load, giving you stock injector like flow, but then increase to a gradient of 45 as you get on it, providing you with extra capacity.

I've thought about going with 1600s but I have yet to speak to someone that has a seamless transition at part throttle. A local FD owner (actually now owns the blue ASP car of kevin w.) has 4 850s and is putting down around 400hp with them. That's more than I expect to get out of the car in the foreseeable future. 850s are cheap to acquire too. If I need to rebuy down the road when my needs change it won't be much of a loss.

To restate the main question: Will it idle like a pig (rich A/F) and lurch around town if I upgrade the primaries to 850s AND up the fuel pressure to 45psi? Alternately does the stock ecu (and/or the pfc) regulate the fuel pump in any way to create a pseudo rising rate system?

 

Well, I can't give you an exact abswer to your question on the pig rich idle with an A/F ratio. Don't have a wide band, yet. When playing around with SX FPR adjustment the "eyes watering" factor is definitely affected by fuel pressure. From this, I suspect 45 psi would result in a very rich idle. With 38 psi base pressure and mid pipe, its smells pretty damn rich but not overly for a car with a mid pipe - thats all relative.

I'm comfortable with the way the car runs but there have been a couple of occasions where the car idles for a while then gets rough and acts like its loading up and a blip of the throttle will clear it up with a nice puff of smoke from the tail pipe. I belive this is due to poor atmoziation from having a short injector body in the primary rail. I've had the primary rail out to observe for leaks and pulsed the injectors to make sure they weren't sticking - the 850's fuel stream sprays in a narrow cone pattern but some droplets still cling to the bottom of the rail. This concerned me more than the size of the injector (tuning should be able to compensate for that) and that is why I milled down the primary rail to bring the injector tips close to the outlet from the rail.

I've seen quite a few people post that they have problems with rich idle using 850 pri while others have no big issue. This lead me to belive that tuning plays a big factor here. I noticed many of the folks with no issues had their car tuned by RP, I went to them for advice because of this and I also knew I would be bring my car there for tuning once I finished the engine. Chris told me to mill down the primary rail 1/8", use 38 psi base pressure and gave me a base map for my PFC (main changes on the map were in the idle fuel cells & timing for the street port)

As far as I know the ECU has nothing to do with fuel pressure. The low to high speed jump will definitely result in a higher rail pressure but only momentarily until the fuel pressure regulator can compensate.

Never thought of it but I do notice my fuel pressure will overshoot its highest setting based on peak hold from my SPA gauge that has recorded a peak up to 10psi above target. Meaning that I was running 11 psi but will peak at 55-59psi.

As far as I know, no one has defined all the parameters that cause the fuel pump relay to switch from low to high, ie. rpm, boost, etc. Workshop manual only relates it to low and high engine load. I have a feeling the switch over point changes similar to the pri/sec injector transition. I believe the PFC sw/sensor screen shows when the high speed relay is engaged. Unfortunately, I've got datalogit with no laptop to datalog that change over yet. Might try to get a passenger to watch the Commdander.

I know people that run with full battery power to the fuel pump and have no issues. This makes sense because the FPR will keep rail pressure under check, it just has to bypass a bigger volume of fuel at idle.

I started a project in my car (wires and relay installed) to retain the low speed but provide direct battery power to the pump upon high speed change over. I had problems with relays backfeeding power when the engine was off and never completed this project due to problems with a well-respected engine builder that sent me three piece of crap engines before I went to RP for a motor. So tired in the end, I just wannna drive it for a while after the 19 month ordeal. I plan to pursue completion of this project after I can get some credit cards paid off. It should only be a matter of using some diodes to eliminate the backfeeds.

I don't understand how you plan to increase your base pressure without installing an aftermarket, adjustable regulator.

To fully answer the 45psi base pressure question, I'd try asking the tuners. Its hard to reach most of them since they're running a bsuy show but you sometimes get lukcy and catch one near the phone, might try: Chris/RP, Ralph Friend/Excessive Motorsports, Steve Kan/Gotham Racing.

Jack

PS: noticed your sig, I've also got a 94Mbm/tan - here are some pictures
http://ourworld.cs.com/_ht_a/talvendi/id19.htm

 

Doesn't the ECU attempt to correct FP +-15% ? I may be wrong, but I swore I heard that somewhere.

 

For the ECU to adjust fuel pressure it would need to have fuel pressure as an input. Never seen anything to indicate that the ECU uses fuel pressure or has a sensor to even read it.

Jack

 

Trout2 is right, the ecu does not read or adjust fuel pressure and there is no way to ajust fuel pressure with the stock FPR.

The stock ecu runs a lower voltage to the fuel pump under low load conditions to increase the life span of the fuel pump. From what I understand, the PFC does NOT do this.

In any case, Ccarlisi, you should be able to just take fuel out of the low load and rpm cells with the PFC to adjust for the 850 injectors and higher fuel pressure.

 

then I guess you have been talking to guys without a fuel pressure gauge.

I got news for ya.........it does.

Below 5psi of boost, fuel pressure rises to about 35-40psi or so. At 5psi, fuel pressure shots up to about 50psi and rising about 1psi of fuel per 1psi of boost. It's kinda hard to look at booth gauges while doing a WOT run

 

So the PFC DOES have two settings for the fuel pump, just like the stock ecu? Interesting....thanks for the info flybye.