Fuel Cut And You, An Infomative Guide. (An expansion on Fubar231's OT)

[Jeeves]

Measuring Air Flow
In the 7M-GTE, the Amount of air is read as a frequency signal, measured in Hertz (hz), generated by the Karman Vortex device in the intake pathway. The ECU in turn, uses this signal (and many others) to caluclate how much airflow the engine is getting. The engine will then add fuel for THAT amount of air (real brief aside, this is why when you have a leak past the Air Flow meter it's a horrible thing. The ECU believes it is getting a certain amount of air and is in realit not, so the air/fuel calculations will be -horribly- off in one way or another).
Now, in that vein of discussion:

Fuel Cut Defensers (FCD)'s are dangerous for a simple reason. They operate by limiting (chopping off) the air flow signal at a level that is just high enough that the ECU will never reach Fuel Cut. So, if you install an FCD; Once you run into the FCD's limit the ECU no longer knows how much more air the car is getting, and thus cannot add any more fuel to compensate. Essentially, it will look like the car is continuing to receive exactly the same amount of air, when in reality the amount has increased beyond where fuel cut would occur. This can cause a less than wonderful instantaneous lean condition and destroy everything from spark plugs to pistons. If you have Additional Injector Controllers (old school hardware) or these days things like the Maft Pro or other devices, you can obviously get around leaning out your motor however an FCD by itself is roughly equivelant to Pandora's box.

Tech Notes: For those interested, see the blurb at the end.


Sneaking Past 12PSI:
The stock turbo and fuel system in the 7M-GTE is good for 12PSI. The turbo is very near the peak of it's heat/output efficency and the fuel pump and injectors are very near their output maximum. In order to exceed this, one would need the following upgrades:

- Walbro 255LPH Fuel Pump (or Similar)
- 550cc Injectors + Wiring Clips
- Air Flow device to use with 550cc Injectors (commonly the Lexus AFM upgrade, although if using a VPC or a MAFT Pro this whole component isn't needed and others are)
- Drilling out the J-Tube
- Modifications to fuel pressure

The Lexus AFM / 550cc Injector Upgrade Path
The reason this works is truly simple. It's not because Lexus has better engineers. It's not because the 550's are better or the afm has a bigger 'bypass screw', it's because these two balance each other out. The Lexus AFM and the 550's are almost exactly bigger than the stock Mk3 Supra AFM and the 440cc's by the same percentage amount (if someone wants I'll go dig up the Reg Reimer documents), so they balance each other out (e.g. One adds 25% more fuel, the other adds 25% more air). With the AFM by itself you'll lean the engine out and destroy it. With the Injectors and not the AFM, you'll flood the engine with fuel and it won't run right possibly doing harm as well.

At that point, you will need a method to get around fuel cut. As I've never gone the Lex + 550's route, I've not done it and I can't help there. Sorry to say I can't help out.

Tech Notes
Lots of Information has been gained about Fuel Cut as of late.
Reg Riemer wrote a tech article back in January of 1997 under the filename of "LEXUSair.txt" talking about "The 7M-GTE TCCS, Lexus Air Flow Meter Upgrade and Vf Data". In this document he writes:

If the TCCS reads an air flow meter square wave frequency value equal to a pre-programmed value stored in the memory of the TCCS the TCCS will shut off the fuel injector pulse to save the engine from detonation. This is called Hitting the Fuel cut.

Several people (myself inclued) used to believe that fuel cut was actually cutting spark. This was proven false (and Reg proven correct) by a SupraForums.com member running a Haltech standalone fuel controller. The Haltech managed the injectors and the TCCS was left to manage spark. The user ended up without a rev. limit and without fuel cut. It would seem that spark is not really modified except in the case of ignition timing. The Greddy Rebic LC (which eliminates rev. limit as well) drives the injectors directly but is an old product and must be found used AFAIK.

Fuel Cut, if you are actually watching the frequencies (using either an S-AFC or something that can monitory the outgoing hertz) going TO the ECU, is commonly around 1500hz. Several variables are believed to alter this up or down (including the High Altitude Compensation Sensor (HAC), IAT, Coolant Temp, and RPM).

Edit List:
- Original Post: July 30th, 05:29 PM
- July 31: Clarification on FCD's, the KV system, leaks, and the 550's/Lex Upgrade.
- April 15, '09: Added Tech Details

[Jeeves]

Originally Posted by Fubar231

Lol, whys my name in the thread. Whats the J tube?

You kept trying to interrupt my thread and taking it Off Topic and I got tired of it and made a thread to address your questions.

Originally Posted by willtotheumm

the J tube is the pipe on your intake. I dont remember it's usage, but its literally a pipe that looks like a 'J' thats closed.

Umm. No and No.

If you look under the intake manifold there is a metal tube. There is a metal plate bolted to the side of your engine, attached to this metal plate, is a metal tube formed in the shape of a J. On either ends of the tube is a rubber fuel line. If you pull it off you'll notice that the J tube is nearly sealed shut except for pinhole openings at either end. This is to increase fuel pressure. When you go to a walbro fuel pump and 550cc injectors's people drill this out so that it is the full width of the tube. If you leave it as a pinhole it has a bad habit of leaning things out horribly and making the system nigh impossible to control. Some people will replace the whole section of fuel line entirely and delete this bit.

Well, Let's go a'drillin' (aka Drill Baby Drill!):
Not so fast. If you're stock or on the stock fuel pump/injectors you don't need to be drilling this bad boy out. Your flow rate hasn't changed from stock and you'll actually throw your fuel calculations out of whack in the wrong direction because you'll be changing fuel pressure to the rail/injectors.

Photos of the J Tube:

[img]http://www.**********s.com/supra/jtube-1.jpg[/img]
[img]http://www.**********s.com/supra/jtube-2.jpg[/img]

Here's a diagram, c/o Shaeff over at Supramania.com:
[img]http://www.**********s.com/supra/jtube-3.jpg[/img]

Tech Notes:
One of the things that some people may not realize is that in the fuel system, pressure in the rail is controlled by means of allowing/preventing flow through the return line. While many people believe that the pressure regulator is on the supply side, it's actually on the return. The J Tube's function is one of fail-safe. If the fuel pressure regulator fails, the J-Tube restricts flow enough to maintain a constant pressure in the rail, rather than dropping to dangerous levels/flow rates.

Edit List:
- April 15, '09: Re-worded reasoning for drilling for clarity, added photos.

[Jeeves]

Added tech details and edited to make things clearer.