Stock Fuel Cut

james_gt · Post by **james_gt** » Thu Mar 25, 2004 9:37 pm

What psi is the stock fuel cut, I just received a manual boost controller and i barely turned it up at all and i am hitting fuel cut i think, ill be going along and the car will fall flat on its face even if i have the pedal to the floor. Thanks

james

ciper · Post by **ciper** » Thu Mar 25, 2004 10:28 pm

Search.

legacy92ej22t · Post by **legacy92ej22t** » Thu Mar 25, 2004 10:44 pm

FC is at 13 psi.

Are you running a boost gauge? If not I recommened that you don't use the MBC until you do.

ciper · Post by **ciper** » Thu Mar 25, 2004 10:48 pm

Its not 13psi.

The first dang thread in this forum which is also a sticky has infinite information on this exact subject.

The pressure sensor reads upto about 27.5PSI total pressure before cutting fuel (if I remember right).

legacy92ej22t · Post by **legacy92ej22t** » Thu Mar 25, 2004 11:11 pm

Umm...it is 13 psi ciper. I'm absolutely positive about it too.

What are you doing, adding in atmospheric pressure just to be difficult?

You will hit factory fuel cut at 13 psi manifold pressure garaunteed. If we could run 27.5 psi then there would be no need for FCD's then would there?

evolutionmovement · Post by **evolutionmovement** » Thu Mar 25, 2004 11:27 pm

I think he's combining atmospheric and boost, so total pressure. So the fuel cut would vary depending on barometric pressure, which wouldn't always be at 13psi, but probably pretty close on average.

Steve

ciper · Post by **ciper** » Thu Mar 25, 2004 11:47 pm

legacy92ej22t: If you read some of the other threads you would see people have hit fuel cut early while others hit it later. Saying exactly 13 isnt right.
What if he tunes his MBC for just under 13 and still hits fuel cut?
I will try to post a couple links. For one I remember someone running 11 psi on stock boost control while another was able to hit constant 13 without fuel cut.

boostjunkie · Post by **boostjunkie** » Thu Mar 25, 2004 11:57 pm

Wouldn't it also depend on the condition of your MAP sensor? Those do tend to wear over time.

I know when vrg3 hooked up his scan tool to my car my boost gauge would read a little higher than the MAP sensor reading on the ecu. From what I've heard (and experienced), the fuel cut is initiated when 13psi (27psi absolute) is reached for about 2-3 seconds.

Now whether or not this was due to a worn out MAP sensor and my being at, or close to sea level, well there's a bunch of different variables.

Just rambling here.

LegacyT · Post by **LegacyT** » Thu Mar 25, 2004 11:57 pm

Ok lets say 13 psi @ sea level for a period over 3 seconds. I'm not sure of the exact time but I know for a fact that you can run above 13 psi for a brief amount of time. Hell you can run like 20 psi for only a few seconds

, but then the fuel cut spoils the fun

Mark,

boostjunkie · Post by **boostjunkie** » Thu Mar 25, 2004 11:58 pm

LegacyT wrote:Ok lets say 13 psi @ sea level for a period over 3 seconds. I'm not sure of the exact time but I know for a fact that you can run above 13 psi for a brief amount of time. Hell you can run like 20 psi for only a few seconds , but then the fuel cut spoils the fun

Mark,

But then you just get the vrg3/ciper fix and be done with it!

evolutionmovement · Post by **evolutionmovement** » Fri Mar 26, 2004 2:12 am

Of course there's error in every system that accounts for different results in otherwise identical situations, but we could then say that 27 absolute is the what the system was designed for. Actual results will vary depending on the tolerances of the combined parts so it's more likely than not that you won't get exactly 27 psi. Then too entropy increases with time, so accuracy will suffer as the systems age.

Steve

ciper · Post by **ciper** » Fri Mar 26, 2004 2:41 am

We should probably figure out what james_gt boost gauge read when he had the problem.

vrg3 · Post by **vrg3** » Fri Mar 26, 2004 3:21 am

You could also be experiencing misfire. As boost increases, the voltage required to create a spark increases too. If any part of the ignition system is marginal it might fail to ignite the charge at high boost, which would feel a lot like fuel cut.

Does it happen as soon as you hit high boost, or does it happen after you sustain high boost for a few seconds?

ciper · Post by **ciper** » Fri Mar 26, 2004 3:23 am

How about timing retard because of knock? What does that feel like in a turbo?

vrg3 · Post by **vrg3** » Fri Mar 26, 2004 3:26 am

I don't know for sure whether or not I've experienced it, but I'd have a very hard time believing it feels nearly as severe as complete failure to combust.

rsstiboy · Post by **rsstiboy** » Fri Mar 26, 2004 7:35 am

ciper wrote:How about timing retard because of knock? What does that feel like in a turbo?

timing retard just feels like you put the foot down and are going nowhere, the reason you can go over fuel cut for a few seconds is because of the size of the hoses going to the map sensor, the pressure takes a little time to get into the map sensor, your boost guage will always be a tad off as well.

ciper · Post by **ciper** » Fri Mar 26, 2004 7:44 am

Since you can run over the fuel cut threshold for a few seconds, he should have noticed the boost gauge stabilize before the problem happened.

BAC5.2 · Post by **BAC5.2** » Fri Mar 26, 2004 9:05 am

But we still don't know because he hasn't replied at all, lol.

vrg3 · Post by **vrg3** » Fri Mar 26, 2004 6:21 pm

Delay in the signal is not the reason you can overboost for short bursts. The signal is almost instantaneous, and the delay is certainly nowhere near a couple seconds. The hose is 3.5mm, which is plenty big, and contains no restrictors.

On a naturally aspirated car, someone once connected a MAP sensor with 30 feet of 1/8" (~3mm) hose to a manifold and compared signals between the MAP sensor and the TPS, and found the offset to be less than a millisecond. This test was done to show the suitability of remote MAP sensors.

boostjunkie · Post by **boostjunkie** » Fri Mar 26, 2004 7:14 pm

How about my statement about the MAP sensor's sensitivity? Valid or not? Just curious.

vrg3 · Post by **vrg3** » Fri Mar 26, 2004 7:33 pm

Well, certainly the tolerances in the sensor's precision will affect the pressure at which fuel cut occurs. In your particular setup, though, boostjunkie, you weren't feeding the sensor the proper reference voltage, since you were using a 7805 instead of the ECU's own 5-volt rail. The difference of a few hundredths of a volt probably accounted for the difference between the ECU's pressure reading and the actual pressure.

NemesisEJ22t · Post by **NemesisEJ22t** » Tue Apr 13, 2004 4:36 am

I would have to agree for the most part that it probably has something to do with the condition of the sensors rather than altitude. I doubt that the engine is going to take into account atmospheric pressure when calculating when to stop the fun (ie. fuel cut). Take me for example, i live at about 1000 feet above sea level on average. On an average weather (and pressure) day I could expect my Patm to be about 14.1-14.2 psia. Now, assuming my boost gauge is correct, i usually hit fuel cut at about at about 11 psig. That gives me a Pabs of 25.1-25.2, lower than the theoretical 27 psi absolute pressure. I am pretty sure that my gauge is accurate as it reads the same as the old one did under the same conditions. So, most likely either the 27 psi pressure (27.696 psi if it was done at sea level mean pressure) is not true, or the condition of the sensors is the main deciding factor in fuel cut. As for me, i plan to remove the fuel cut as soon as i get a new clutch when i can take advantage of the added boost.

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Stock Fuel Cut

Stock Fuel Cut