I just did some calculations on injector size and the flow rate of the walbro 255hp fuel pump. my results are cause for alarm. i hope i'm wrong.
it seems to be common practice to get a 12:1 fmu and raise fuel pressure as high as 110 psi in hopes of increasing flow through the stock injectors. this would work fine if the fuel pump had the same flow rate at all pressures, but it doesn't. as you increase the fuel pressure, the pump flows less GPH (gallons per hour).
the walbro 255hp is capable of the following flow rates:
15 GPH converts to 946 cc/min. that breaks down to only 236 cc/min for each injector!
theoretically, your stock 234cc/min injectors should flow 374cc/min at 110 psi. this number seems to be severly limited by the fuel pumps ability to keep up the flow at that pressure.
this data suggests that we should not expect the walbro 255hp to produce sufficient flow at incredible pressures. if you do, you will run lean and we all know what that means. i would think 90 psi is a reasonable MAXIMUM fuel pressure. that pressure would be capable of feeding 4 injectors at 568 cc/min each. this is enough to support about 285-290 hp. lower fuel pressures would also reduce the risk of injector lock-up. 70-80 psi would be an even better pressure range to operate in.
the trick is choosing the right size injector to flow enough without needing extremely high fuel pressures, yet be small enough to either sneak by the stock ECU or be managable by a S-AFC. I know injectors are a little pricey, but after reading this, do you still want to run your stock injectors at 100 psi or higher?
#1230, "RE: BEWARE: fuel pump flow rates ..." In response to Reply # 0
Why does it seem that we are all worried about our stock injectors and how much they can flow with these turbo kits???
here is the deal..they do not flow enough to allow us to run more than about 3 lbs. of boost...they are pushing way too much pressure through a tiny little opening!!!! That causes problems like injector lock up, not too mention the fact that pushing that kind of pressure doesn't allow them to atomize the fuel, all it does is dump fuel into the motor! Yes, it works, but you are cheating yourself if you think it works efficiently...
#1232, "RE: BEWARE: fuel pump flow rates ..." In response to Reply # 2
LAST EDITED ON 28-Jan-01 AT 07:47 AM (PST)
LAST EDITED ON 28-Jan-01 AT 07:16 AM (PST)
Unfortunately there are basically 2 types of injectors, low impedance/resistance (peak and hold) and high impedance/resistance (saturated). We have the low impedance (correct me if I'm wrong) while the GST/X's have the other. The only way to use the high impedance is with the use of a fuel computer (like the Super AFC or Haltech).
BTW, I believe the first post about the Walbro pump only supplying 234 cc/min of fuel is wrong, I don't have formulas or figures to argue with, but if all the GST/X owners upgrade to this same pump, or a variation of (the Nippon Denso pump flows at 255 gph also) as well as this same pump is good for a Mustang GT (2 times the numer of cylinders as us) then this should be good enough for our turbo charge needs... (once again, correct me if I'm wrong)...
DoN
'99 GS Sports Edition Star Stage 2, CFDF Clutch Greddy Manual Boost Controller Greddy BOV and EVO Exhaust Greddy TT and shift knob B&M Edge Short Shifter Eibach Sportlines with Tokiko Blues
'99 Eclipse GS Sports Edition Star Stage 2 Greddy EVO Exhuast AEM Big Brakes Greddy Type S BOV Pioneer DEH-P7200 Bazooka EL1500 running 2 MTX 10" Subs Fosgate Separates throughout
#1233, "RE: BEWARE: fuel pump flow rates ..." In response to Reply # 3
Good post - rigtht idea, just mixed up injectors - we have saturated, GS-T have peak and hold - we can use the peak and hold if using a proper ecu like HRC's Accell
#1234, "RE: BEWARE: fuel pump flow rates ..." In response to Reply # 4
Thanks for clearing that up...
Don
'99 GS Sports Edition Star Stage 2, CFDF Clutch Greddy Manual Boost Controller Greddy BOV and EVO Exhaust Greddy TT and shift knob B&M Edge Short Shifter Eibach Sportlines with Tokiko Blues
'99 Eclipse GS Sports Edition Star Stage 2 Greddy EVO Exhuast AEM Big Brakes Greddy Type S BOV Pioneer DEH-P7200 Bazooka EL1500 running 2 MTX 10" Subs Fosgate Separates throughout
#1235, "RE: BEWARE: fuel pump flow rates ..." In response to Reply # 5
jason, i'm NOT worried about trying to use the stock injectors. i thought i made it clear that i think it's a bad idea. if you need that much more fuel, it's time to call RC Engineering and order some new squirters.
don, i'm saying that if you try to run the walbro pump at 110 psi, it will only flow 946 cc/min. (236 x 4) if the GST guys are upgrading to the walbro pump and running it at these insane pressures, then they are cheating themselves out of fuel too.
there is NOTHING WRONG with the walbro pump. it is plenty adequate for supplying fuel to a turbo'ed 420A. the problem is that asking the pump for 110 psi is expecting too much.
run the walbro at 80 psi and you'll have enough flow for 4 injectors at 678cc/min each. that should be enough, eh?
moral of the story: get new injectors and a S-AFC if you're going to run more than a few pounds of boost.
#1236, "RE: BEWARE: fuel pump flow rates ..." In response to Reply # 6
This thread elaborates upon the inadequate fuel flow aspects of the Walbro pumps that I brought up recently.
A correct conclusion: use a pump that can keep up with the demand at hi pressure. The stock injectors have shown themselves to work well in our HRC turbosystem installations with a correct fuel supply such as our ACCEL pump.
The Walbro pump, as I mentioned earlier, is adequate for factory turbo applications (GS-T, GS-X) which never see more than 65-70 PSI pressure. It is NOT adequate for VROG equipped installations such as our 420A turbo cars, as it was never intended for such high pressure delivery.
Please DO NOT try to run at a lower fuel pressure under boost as mentioned in this thread. If HRC customers have further questions about these issues (although they have nothing to worry about, since their systems come with a properly engineered fuel setup), they can contact HRC Tech Support daily at 630-801-9065.
Bill Hahn Jr. 98 RS Turbo: 10.87 @ 136 MPH HRC Stage V with N20 Nation's Fastest Street FWD 2G DSM www.turbosystem.com
#1237, "RE: BEWARE: fuel pump flow rates ..." In response to Reply # 7
please re-read my posts carefully. i did NOT recommend simply turning down the fuel pressure and leaving everything else the same. that would certainly kill your engine.
i suggested getting larger injectors and a S-AFC so you wouldn't have to run such ridiculously high fuel pressures.
440cc/min injectors wouldn't need 110 psi to flow enough fuel, correct?
sure you can get the accel pump if it can supply enough flow at those pressures. i don't have the specs on that pump, but if you say it can supply enough fuel, i'll take your word for it. i was simply stating that it's a bad idea to run the walbro at 110 psi with stock injectors and think everything is fine. it's not.
#1239, "RE: BEWARE: fuel pump flow rates ..." In response to Reply # 9
Not trying to knock STAR - just making a point here - this is what happens when you just throw a kit together without real development and testing ....
