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Old 02-05-2001, 04:54 AM   #1
Talon
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Question capabilties of stock fuel system

Does anyone know what power (and/or boost if 'charged) the stock fuel system (ie pump, pressure contoller, injectors, computer??) on the '98 spec. 2.5L are capable of handling safely?
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Old 02-05-2001, 05:00 AM   #2
SteveS
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Stock setup probably won't handle much more than 180hp. If you tack on something like the S-AFC, then you should probably be able to handle up to about 230hp (about 5 psi of boost). Not sure how long the stock fuel pump will last under these conditions though. The stock fuel injectors are only 320cc, IIRC.

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Old 02-05-2001, 05:10 AM   #3
hacman
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oddly enough... I just caught up on Project Impreza from SCC Mag.

They were running 5psi, intercooled, from Minnam turbo, on the stock EFI system with no audible knocking. They got it up to 7psi, with a little fine tuning of the J&S Safeguard and A'PEX AFC... putting 183 HP to the wheels on a 4 wheel dyno.

I'm up to the suspension part, and all they've replaced in the fuel system is the Fuel Pump.
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Old 02-05-2001, 06:31 AM   #4
Nick
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Hey Steve, actually the stock injectors were benched at 280CCs.

hacman, if you read further in that series about the setup that the SCC car had they were running horribly lean and the only reason that it worked was because they took advantage of the mechanical inefficiencies of the injectors running them over 80%. Basically they were flooding the engine all the time with gas and it kept it going. I believe that the stock injectors went over 80% somewhere about 5psi.

-Nick
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Old 02-05-2001, 01:34 PM   #5
Diz
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Actually, the stock injectors reach 80% duty cycle at less than 1psi. But higher boost will make them go wide open, dumping the remaining 20% all at once.

Shiv -

How did you verify the the stock injectors did this? I was working through this today and I figured that our cars will hold up to 7psi with the stock injectors. My calculations show that the injectors go full duty cycle at 6500 RPM. Assumptions:

280cc injectors
2.5L displacement
12:1 Air/Fuel ratio

BTW - Horsepower at redline was around 220. These numbers match up pretty well with my observations with my car. I was running a Bell RRFPR (20007), AFC and Holley FP.

-Paul
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Old 02-05-2001, 02:54 PM   #6
Tensecgn
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We ran a 92 legacy with a turbo @7psi and the egt were in the 1400`s.All we did was raise the base fuel pressure when we get to the track.The pump should be upgraded and we use mustang 190l pump.We also installed a hot wire kit for the fuel pump.Now the pump gets 14.2 volts instaed of 12volts.It helps spin the pump faster for more pressure and volume.
Tony
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Old 02-05-2001, 03:23 PM   #7
tonytiger
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I'm off the subject, but can the fuel pressure be altered higher manually? What would be the consequence of doing that on normally aspirated? Is that what the so-called superchips mainly do like the ones in here http://www.superchips.co.uk/ . Am I completely wrong? Those chips promises quite big power and torque increases...
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Old 02-05-2001, 04:10 PM   #8
shiv
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Diz-- I verified stock injector duty cycles by running the TEC on an NA MY99 RS (Imprezer's car). Running stock air/fuel ratios, peak duty cycle approached 80%. Running any amount of boost would force those injectors to go static. This isn't unusual as almost all OEM cars run injectors sized to operate at 70-80% duty cylce at peak hp.

They all seem to subscribe to RC Engineering's injector sizing guidelines.

165hp= 4 injectors x .8 peak duty cycle x (Y injector size in lbs/hour)/BSFC or 0.5 for NA car

165hp = 4 x 0.8 x Y/.5

82.5=3.2Y

Y= 26lbs/hour
= 273cc/min

Of course, this is all assuming standard 3bar (43psi) of static rail pressure. Using rising-rate or adjustable fuel pressure regulators will change the amount of hp the stock injectors can safely support.

Cheers,
Shiv
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Old 02-05-2001, 10:03 PM   #9
Eric SS
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I'm at 215hp (on the bottle) and it is handling the power fine...

Eric
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Old 02-05-2001, 11:39 PM   #10
ImprezaRS dot com
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Well, I'm moving closer to SHIV's camp - I have found a used Simple Digital Fuel Injector kit (SDFI). It adds two 440cc injectors (similar to what HKS did with their Impreza prototype turbo kit) and has it's own map sensor.

I find the RRFPR does not seem to raise fuel pressure linearly, and it seems to rise faster early on, and flatten out above 5 psi. This is just a guess from 3 months of tuning without a fuel pressure gauge and only EGT's. I am using a 255l fuel pump, so that is not limiting anything. If I tune fuel pressure for 7 lbs boost, then it is too rich for 5 lbs boost when I use my electronic boost controller to change the boost. If I tune fuel pressure for 5 lbs boost then it is too lean above that.

If I just leave it at one boost level and tune for that, the RRFPR with stock injectors works great. I had basically planned to just tune it for 7 lbs of boost and leave it that way until I found a good price on the SD FI system. Also, remember that 7 psi at 6,000 feet altitude is the same cylinder filing pressure as 5.5 psi at sea-level for my 10:1 pistons. So, I am not putting a lot of demands on the fuel system yet. I am not even boosting enough to need a J&S or ITC yet (I can get detonation at 9+ psi right now, and the car is very sensitive to changes in the gas I use).

So, the point is that the fuel system with a high flow pump and a RRFPR will work well with stock injectors to about 7-8 psi at 6,000 feet altitude. But, rather than continuing to use a mechanical RRFPR to boost fuel pressure and force more fuel from the stock 280cc injectors when under boost(makes them act like 330cc injectors at over 80 psi fuel), I will be able to electronically tune the fuel delivery by RPM and BOOST with thew SD accessory ECU. For instance, at 3,000 rpm and 7 lbs boost the new FI system will supply less fuel than at 5,000 rpm at the same boost - right now, with the RRFPR and full boost the injectors are static, and at 7 lbs I get the same fuel delivery at any rpm that is seeing boost.

And, with the SD FI system and my 255l fuel pump, I can ditch the RRFPR, and be able to fine tune the fuel delivery from within the car (by the EGT's) - I will removing the EGT probe from the turbo up-pipe and am trading for a header with the EGT already at the #4 exhaust port, and that will help me tune it better. I know a lot of people say to do the EGT at #3 (at the end of the fuel rail), but I hear more falures are in #4 anyway.

The costs for my fuel management setup will total only $250 Torque Chip II, $300 for used FI system ($600 new), $200 for gauges, and the fuel pump was part of the turbo kit. As for timing control, I will likely add water/alcohol injection before I do a timing controller, since that will actually lower EGT's and help with the hot intake charge on my small turbo. I understand retarding timing too much will result in higher EGT's...

Larry www.ImprezaRS.com

PS: rambling on it is too late at night. Where am I...
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Old 02-06-2001, 12:10 AM   #11
shiv
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Actually, the stock injectors reach 80% duty cycle at less than 1psi. But higher boost will make them go wide open, dumping the remaining 20% all at once. Not ideal for performance but suffient for a few psi of boost. But running 7psi with just an AFC and J&S knock sensor yielded over-retarded ignition timing and uncomfortably high EGTs.

Cheers,
Shiv

[This message has been edited by shiv (edited February 05, 2001).]
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Old 02-06-2001, 07:24 AM   #12
Diz
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Talking

Shiv -

Thanks for the numbas...

Larry -

I have been thinking about this quite a bit and have made the decision to upgrade to 440cc injectors and a Haltech if I go back to boost. We can meet up and go over the details.

Cheers -

Paul
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Old 02-06-2001, 10:37 AM   #13
yebokmj
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So the MY99 injectors are shown to be 280cc, what about MY98 DOHC? Any one have some firm numbers on them? Is there a chance that the orgionaly belived 320cc number to be that of years before the engine chage?
Joshua
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