one last time...Fuel Cut...

[WJM]

So...what voltage do we know is the Fuel cut triggered at?

 

Its time to build a fuel cut defender.

 

the issue with this is...

 

at that voltage...is the ECU already dumping max fuel, or what?

 

or lemme rephrase...

 

at 9 psi...is the engine using the smae amount of fuel as at 13 psi?

 

would adding the FCD in mess up the fuel delivery, as in lean it out and upper RPM's/higher than 9psi boost?

[Vanislru]

From what I remember at 9 psi the injectors are at the max of their duty cycle so i don't think you'll chang much flow wise by putting in a zener diode and limiting the signal to the computer. I remember someone on the board having success with a FCD way back. An EGT meter might be a good idea while you're looking for the FCD.

[WJM]

hm.

 

more fuel pressure.

 

where's my 450's at?????

[WJM]

 

or

 

 

and yes I know we do not have map sensors.

[garner]

I know you don't want to hear this but I have the sollution, it's called....

 

MEGASQUIRT

 

I really don't think you can get the stock ecu to deal with the higher boost and not fuel cut....You may find some help from a RRFPR at like a 1:1 ratio but you have got to get the ECU to ignore the boost level

 

garner

[JWX]

but the stock ECU doesn't see boost, only the volume of incoming air

and why WJM no megasquirt?

[Do It Sidewayz]

the ECU's dont' see pressure...just the volume of air.

 

and a RRFPR....at 1:1 ratio..how the heck is that going to help you??? the STOCK FPR is a 1:1 ratio RRFPR!!!!!!

[Myxalplyx]

I'm sorry guys if I seem to sound like a prick or anything but I'm going to ask this again.

 

Where is there documented information that states fuel cut is determined by the ECU seeing volume of air over a determined limit (On our EA82Ts)?

 

I've been searching and reading and re-reading and can not find it. There has got to be something somewhere that says this is the case in the later modelled Subarus.

 

I could've overlooked someone posting this info and I apologize if this has been the case. Up to now though, this just seems like it's assumed that it works this way.

[JWX]

how else would it work? the ECU doesn't see anything else

[Do It Sidewayz]

you need to remember that back then the ECU's were fairly basic. the only sensor that can measure pressure of any sort is a MAP sensor (manifold pressure sensor) The EA82T does no have one...and has no actuall idea of pressure...just air flow.

 

 

The FSM will actually tell you what sensors the computer takes into account to give you fuel.

 

 

You need to know how the MAF works. It's a Hotwire type, and the computer gives the MAF like 5 volts, and the hotwire is a resistor. the more air that flows over it, the less resistance (close to 5 volts output). the less air that flows over it...more resistance (anywhere from 1-5 volts output).

 

The ECU will ALWAYS try to keep you just on the edge of fuel cut. It will basically ramp up to 4.7 volts (i don't know the voltages exactly) and start to open the wastegate, by energising the Boost Soleniod. Subaru has picked a number (prolly like 4.7 Volts), which tells them that when the MAF is reading 4.7 volts, it is flowing the mount of air to make "X" amount boost (Fuel Cut).

 

From what i understand the Boost Soleniod is directly controlled by the ECU, in relation to the MAF sensor output. It will basically remain closed until it get's to a certain Air Flow number (4.5 volts or so).

 

This is why everyones boost which these cars varies greatly. Even on your own car, boost from season to season...or even day to day will change greatly.

 

On cold days you might only run 4-5 PSI because the cold and dense air will cool the MAF better and produce the "waste-gate open" voltage sooner.

 

On hot days you might make 12-13 PSI because the hotter and less dense air will cool the MAF worse and produce the "waste-gate open" voltage later.

 

In effect...on a hot day running 12-13 PSI, and a Cold day running 4-5 PSI you will experience the same amount of performance. You still use the same fuel for 4.7 volts......but it just takes more or less PSI to get the same air density.

 

Now....the computer i believe needs to see this "over boost signal" for more than 5 Seconds to trigger the Fuel cut. The ECU opens the wastegate solenoid.....because it assumes the wastegate is stuck closed....but if you are running an MBC, the soleniod can open all it wants...but if you don't have enough pressure there...because you have bled it off the wastegate still won't open. After the computer has tried the open the wastegate, it figure it is stuck closed... and will cut the fuel.. to prevent engine damage....however it dosn't prevent damage...it actually accellerates it....going instant SUPER LEAN at high PSI is no good.

 

It's a hard subject to explain to someone......so sorry if you didn't understand what i wrote....

 

WJM....to answer your question......you will need more fuel over 12 PSI..... And if you are smart. you can build a FCD with little potentiometers...so you can adjust the cut-off point to the very last point of where the fuel cut hits. However you would be better off with an FCD, than hitting it 12 times a day...Hitting fuel cut definately isn't good for it!

[kingbobdole]

Thanks for that info,

My old 87.5 XT turbo would sometimes overboost to 15 psi... wonder if it was the maf.....

Good read,

[rallyruss]

well I doubt its the last time but here we go.

 

ok yes the MAF is the determining factor for fuel cut as far as I can tell. the boost switch is just for the stupid light. the waste gate control by MAF? hmm not to shure about that one. I will double check my facts before I call it though.

now I have to say that with a properly matched turbo (TD04) I have a verry stable max boost level. the stock waste gate just is too small for a high flow intake and exhaust and that causes over boost. espically when cold or under a hard load. I no longer have this issue with a slightly larger turbo with a bigger waste gate.

I am now gonig to experiment with boost control above stock. my Saab APC system is almost complete. I will be able to report back with results of that test soon.

now to dispell the myth that fuel cut causes super lean mixtures at high RPM.

there is only a slight amount of time that the engine May run lean due to fuel cut. then during fuel cut it pumps air only. this is a COOLING method on many newer cars(and some old cadillacs) due to the fact that its MPFI the injector is right next to the cylinder. when fuel stops the motor stops.

I understand the fear of too much air meaning lean. It makes sense if there is still some fuel to burn. but under fuel cut there is no fuel. no burn. no lean.

 

ok how to defeat it. my solution. push right to the edge of the fuel cut limit and be happy there with timing and boost both under control in case you run run lean for some reason both timing AND boost can be pulled back.

 

the better solution? ask garner. programable fuel and spark control can be the answer. this is assuming that things are programed correctly. I highly respect those who dare to venture down this path. that is the best solution for you boost junkies. the rising rate fuel pressure regulators and other methods are mere bandaids in my opinion. they cannot control fuel/air/boost/timing like a true programed system for a specific motor.

Will I support your desire to go fast. do what you need to get that extra boost.

now I am interested how fuel cut effects a higher compresion turbo motor?

Caleb? any thoughts?

[Myxalplyx]

We are still talking about what we believe is the case and no documented info. The info provided is really informative though. Thanks guys!

 

I think I may have a lead on some interesting info that you guys would want to read. I'll report when I get home from work in about 2 more hours. It may or may not be one step further to what EXACTLY is going on here. I'll report once I get home if my red eyes don't burn out of my head. I've been looking all night for some fuel cut info on the internet and came across something(s).

 

 

Edit: Arrrgh! I'm to sleepy to look much further. Check this patent out by

"Abstract not available for JP60222537

Abstract of corresponding document: GB2157853

The cylinders of the engine are divided into two notional groups, and the control system is provided with an engine speed sensor, a throttle closed sensor, and a fuel cut off circuit responsive to a signal of the throttle sensor provided engine speed N is greater than n1 for producing a fuel cut off signal. A cylinder group selecting circuit responds to the fuel cut off signal by disabling fuel injectors in one of the groups of cylinders, and then disabling fuel injectors in the other group after a delay. When engine speed N is less than n2 fuel supply is resumed in the same two stage way.

 

Patent number:JP60222537Publication date:1985-11-07

 

From here: http://v3.espacenet.com

 

Lots of information about Subaru Fuel Cut device and lots of diagrams. I'm beat after working 12hrs and can't search through this when I get home. See if you can find any further information on Funi Heavy Industries fuel cut patent and exactly how they go about. Again, lots of useful diagrams. Would be more useful if you could read Japanese.

 

More info:

 

"Abstract not available for JP1032040

Abstract of corresponding document: US4846127

An idle switch is provided to respond to release of an accelerator pedal of a motor vehicle, for producing an idle signal. A large increment of the engine speed after the production of the idle signal is detected and an engine speed increase signal is produced. The increment is compared with a predetermined value and an immediate cutoff signal is produced when the increment is larger than the predetermined value. In response to the immediate cutoff signal fuel supply is immediately cut."

Patent number:JP1032040Publication date:1989-02-02

[Do It Sidewayz]

We are still talking about what we believe is the case and no documented info. The info provided is really informative though. Thanks guys!

all you need is the FSM for this vintage of car.

 

it will tell how to diagnose the MAF, what voltages it produces....blah blah blah.

 

it will tell you the factors for fuel maps and all that.

 

You need to remember that these computers don't have much "on the run control" over things. They basically switch to a pre determined map, which is adjusted slightly by a few factors.....the Coolant Temp Sensor, the MAF, the 02 sensor under open loop operation. This is really all that effects the Fueling potion. Timing is controlled by the knock sensor...however i believe the computer can only make adjustments like +- 5 Degrees, i think they also play with timing when it is cold...but not a great deal.

 

 

This is why tuning these computers is soo easy....you just go in.....find the fuel maps, and adjust them for more less injector duty cycle (more or less fuel).

 

I know for example....for a easy fuel adjustment on my Justy i threw a potentiometer in the Coolant Temp Sensor output line....by cranking the knob and applying like 1k ohms resistance, i can force the computer into giving the injectors about 4-5% more duty cycle... This was shown by my emanage....i just pulled up the real time display, cranked the knob, and noticed a 4-5 % increase in injector duty cycle.

 

 

and for the Wastegate control....how else would it be controlled without relation to the MAF sensor???? There is no pressure switch....modern cars like the WRX are related to the MAP sensor which gives the computer an exact idea of manifold pressure.

 

The wastegate control, and therefore boost is depentant on air density/flow.

[JWX]

ok so I have the parts book here and I see that the fuel pump is listed as 85-87 and 87-90, which me thinks would indicate that 87 would be the cross over year and some might have the 85-86 pump and some would have the 87-90 pump. there might be something there.

[garner]

the ECU's dont' see pressure...just the volume of air.

 

and a RRFPR....at 1:1 ratio..how the heck is that going to help you??? the STOCK FPR is a 1:1 ratio RRFPR!!!!!!

 

I respectfully disagree, read below clipped from a post by ALL_TALK, an explanation of the "pressure switch(s)" Page 80 in volume 2 of the 1989 FSM. This clearly explains that the "pressure switch(s)" not only send a heavy load signal to the ECU cancelling the AFR feedback control and the other sends an over boost signal which initiates fuel cut. Further, the waste gate valve control you speak of is really an alltitude compensator not a boost controller. It simply allows the turbo to develop stock or near stock boost at high elevation/ low oxygen environments. See page 82 of the same FSM.

As for the RRFPR!!!!! WHAT THE HECK I meant an adjustable where base pressure can be turned up so your base starts at 40psi or so. (and I, like RallyRuss believe this to be a band aid)

 

All I'm saying is the stock ecu is not prepared for 14psi boost however it "sees" it. If you could "hide" this level of boost (like when you unplug the over boost switch) from the ecu AND give the injectors more fuel pressure behind you MIGHT be able to boost in the 14psi range without burning holes in yer pistons.....

 

have fun

 

garner

[rallyruss]

well I am definately more confused now than I was before this thread started. so much conflicting info. the haynes book has another version.

 

ok so I have drivin the car without the switch for the light hooked up for close to 30,000 miles. I haved tested to make sure that my system stays rich under boost with out any problems there. I have been workingon the Saab APC all morning and after work I will be able to test it.

 

just a thought. while out working on the car I figured I would look for the second sensor for overboost protection. I could not find any thing that looked like a second pressure switch. I was disconecting the stock waste gate controls and figured mabey its built into that? dont know. normaly I am confident in understanding the function of my own car but this has me a bit confused. I thought I understood it but now I am not too sure.

 

I will get back later tonight with APC results. :cool: