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Normy's Avatar
 
Join Date: Mar 2007
Location: Ft.Lauderdale, FLORIDA
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22 psi fuel pressure at idle-?

Does this sound correct to you?

I just installed a rail-end fuel pressure gauge on my '85 S2, which has the LH-jetronic injection from the '85 and '86 32 valve cars. I was surprised to see how low the fuel pressure actually is. At idle, it reads around 22 psi, and when I use a long screwdriver to lift the throttle suddenly, it spikes up to 30 psi or so.

I have a rising-rate fuel pressure regulator from 928 Specialists in North Carolina, and this device has TRANSFORMED the car! I suspect that this device alone, with careful tuning...has added at least 30 hp. The device is a "rising rate" sort, according to www.928gt.com's website. I mounted an air-fuel ratio meter in my glove box about 4 years ago, and this was one of the best mods I've ever made! I used this gauge to set the fuel pressure regulator. I think I have it right: I put a set of Falken RT-615's on the car on September 19, 2008. They are now TOAST no more than 3000 miles later! These are soft compound tires that just barely meet the requirements to be "street" tires, and their treadwear number of 200 kind of bears this out. They are soft, and I drive like a bat out of hell. I guess I have nobody else to blame other than Scott Rothstein for my problems. Lord knows it can't have ANYTHING to do with the way that I throw my 928 around almost any intersection-

[Yes, you read that right. My '85 has 151k miles now, and I'm going through tires every 3k~]

What sounds strange is this fuel pressure reading. I have owned this car for 11 years now, but only now do I attach a fuel pressure gauge. For the past 15 months since I FINALLY got the fuel pressure regulator from 928 SPECIALISTS ONLINE STORE - 30,000+ 928 Parts - 928 SUPERCHARGER KITS! to work right......I have owned a hooligan. I'm sure it is making upwards of 350 hp, the way this thing just explodes when it hits the powerband, and literally "hurls" the 3300 pound car forward tells me of this.

Yet....this fuel pressure gauge? It says 22 psi at idle. I goosed the throttle a few times, and it tries to climb toward 30 psi, but it always returns to 22.

From the WSM's:

"Fuel pressure should stabilze around 2.0 bar, and during transitions, it should be between 2.3 and 2.7 bar."

My fuel pressure sits constantly at around 20-22 psi. At idle, I consistently see 15.0-16.0 air fuel ratio. At wide open throttle, I see a consistent 12.0 /1 ratio. Thoughts? These numbers don't make sense but the car runs like a raped-ape-

N!

Last edited by Normy; 05-20-2010 at 04:26 PM..
Old 05-20-2010, 04:16 PM
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JK McDonald's Avatar
 
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Every one should install that Gizmo -

Hey Normy, To me - it sounds like your variable fuel pressure regulator is working as advertise and is providing the needs of the engine according to demand. If 22 psi at idle and 30 psi when pushed is generating a sweet running 350 HP vehicle, we should all install that gizmo.

Michael

Last edited by JK McDonald; 05-20-2010 at 05:21 PM..
Old 05-20-2010, 05:18 PM
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Hi Normy !
The book idle pressure is 29psi. So you're close.
Old 05-21-2010, 06:52 AM
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Well, I'm not going to mess with what works right! The fuel pressure sounded low is all.

N!
Old 05-21-2010, 03:15 PM
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Sweet Spot -

Hey Normy, The 22 PSI that the variable fuel pressure regulator produces at idle is probably still within the oxygen sensor/fuel computer operating range. The fuel injectors are simply being hit with a little longer pulse width to keep the engine purring at low rpm. As the engine load is increased and the fuel pressure ramps up toward 30 psi – the fuel computer recalc’s the HP demand vs the required injector pulse width. You may have found a nice setup that provides a very desirable (air/fuel mixture) sweet spot with reasonable fuel mileage (a little lean at lower speeds) and an instant response when you put your foot into it….

Michael
Old 05-21-2010, 03:53 PM
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Quote:
Originally Posted by JK McDonald View Post
Hey Normy, The 22 PSI that the variable fuel pressure regulator produces at idle is probably still within the oxygen sensor/fuel computer operating range. The fuel injectors are simply being hit with a little longer pulse width to keep the engine purring at low rpm. As the engine load is increased and the fuel pressure ramps up toward 30 psi – the fuel computer recalc’s the HP demand vs the required injector pulse width. You may have found a nice setup that provides a very desirable (air/fuel mixture) sweet spot with reasonable fuel mileage (a little lean at lower speeds) and an instant response when you put your foot into it….

Michael
My car, an S2, doesn't have an O2 sensor. This LH system runs "open loop" full time. The computer doesn't even have an input for the O2 sensor, according to John Speake. This car has NO pollution controls whatsoever. It is literally a 1960's muscle car in the year 2010, yet it could probably pass California emissions testing today. Why? The LH isn't very good at making horsepower in stock form, but it CAN pas almost any emissions test! That is why Bosch built it: emissions, not horsepower.

N!

Last edited by Normy; 05-21-2010 at 06:31 PM..
Old 05-21-2010, 06:22 PM
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Full Time Open Loop

Hey Normy, Sorry for the assumptions – I’ve worked on a number of cars equipped with various configurations of the Bosch LH-Jetronic fuel injection system but not an 85 928. The difference between the early L and LH systems that I’ve crossed paths with - primarily had increased the sensitivity to the computer inputs and changed the method of metering the throttle air. Perhaps Porsche caught the edge of the V8 emission standards before a regulation change. I did not know that there was a full time open loop system that could pass U.S. emissions but I guess if anyone could do it - Porsche could.

Michael

Last edited by JK McDonald; 05-22-2010 at 08:30 AM..
Old 05-22-2010, 08:17 AM
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Old 05-22-2010, 08:17 AM
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