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The mkiv Supra Owners Club

Fuel-cut and how not to defeat it


Guest Ash
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I've had a number of questions from various members on the subject of fuel-cut over the past couple of days, which mainly seem to have been prompted with my experiences with Justin's car.

 

I've not enough time to reply personally to each individual email. So I thought I'd answer the points in a post to the BBS. Hopefully, I will have answered everyone's questions. If not then please let me know.  

 

Is Justin's problem caused by something inherent to the HKS unit?..........

 

The Fuel-Cut Defencer that was fitted to Justin's EVO happened to be an HKS device. But any make of device that tinkered with the pressure signal in basically the same way as the HKS one, would have caused the dangerous leaning-out problem we experienced.

 

How does it all work, basically?..........

 

A sensor is attached to the intake manifiold which has the job of measuring the intake pressure and then instantly converting its reading to a voltage. This voltage is fed by wire to the ECU. The change in voltage is directly proportional to the change in manifold boost pressure, i.e. if pressure rises then the voltage will rise: pressure falls and the voltage will fall. And it will all do so at the same rate. So if pressure were to rise quickly, then the voltage would rise equally quickly, and so on.

 

The higher the boost, the more fuel you need; the lower the boost then the less fuel is burnt by the engine. It's as simple as that.

 

Therefore, as boost pressure rises, the intake-manifold pressure-sensor immediately detects this rise, which raises the voltage being fed to the ECU. In turn, the ECU interprets this rise in voltage, and allows the injectors to flow more fuel. The ECU can also make adjustments to other vital features at the same time, such as the ignition timing. But here I'm solely concerned with the fuelling.  

 

What is fuel-cut and why is it fitted?..........

 

Manufacturers fit fuel-cut to turbo cars for good reason. And that is: turbo motors have the capacity for rapid self-destruction.

 

The actual principle of fuel-cut is the same on all cars that have it fitted. All that simply happens is, at some stage in the boost curve, the engine ECU calls it quits and cuts the fuel pronto.

 

With a turbo engine a wastegate opens, at a particular boost pressure, in order to stop the rise in boost. The actual point at which the wategate opens is set by the manufacturer.

 

Let's say the point at which the wastegate operates is 10 psi intake-manifold pressure.

 

So to guard against potential engine destruction, through something like wastegate failure, manufacturers will set a fuel-cut at a boost pressure a little higher than the pressure at which the wastegate opens. In our example, let's set a fuel-cut to a pressure of 12 psi.  

 

So how does the HKS FCD defeat fuel-cut?..........

 

Well, technically, it doesn't actually do away with the fuel cut. All the HKS unit does is to alter the voltage of the pressure signal being fed to the ECU.

 

As you know, the pressure-signal voltage is proportional to boost pressure. Which means, the higher the voltage then the higher the boost (and the more fuel the ECU feeds to the engine).

 

The typical range is from zero to +5 volts. With a 2-Bar sensor, zero will relate to zero pressure and +5 volts will relate to 1 Bar of turbo boost.

 

Okay, so in our example we have the wastegate opening at 10 psi boost pressure and we've set fuel-cut to occur at 12 psi. With a 2-Bar sensor, 10 psi will result in a pressure-sensor voltage output in the region of 4.16 volts, and 12 psi relates to a pressure signal in the region of 4.5 volts.    

 

Right, so all we do is program our ECU to initiate a wastgate-open process when the pressure signal reads 4.16 volts, and a fuel-cut process when the pressure signal reads 4.5 volts.

 

So as boost is rising, the pressure-signal voltage rises and the ECU allows progressively more fuel to enter the engine. When the pressure-signal voltage reaches 4.16 volts, the wastegate opens, boost stops rising, which means the pressure-signal voltage stops rising too. And never, therefore, reaches the 4.5 volt fuel-cut threshhold.  

 

So lets say you fitted an electronic gismo that gave you in-car control of your wastegate, such as the Apexi AVC-R. On the front panel you set the wastegate to open at 14 psi boost.

 

Okay, 14 psi relates to a pressure-signal voltage in the region of 4.83 volts. So this time, as you accelerate, the pressure-signal voltage rises with the boost pressure. The voltage passes through our old wastegate operating threshhold of 4.16 volts. At which point the ECU will still attempt to initiate a wastegate-open process. But as we have overidden it with our boost controller, the voltage will now continue to rise... until... it reaches 4.5 volts at which point blammo! We hit fuel-cut. Which is quite frustrating.  

 

So what companies like HKS have done is to invent little gizmos that tinker with the pressure-signal voltage.

 

With the HKS unit, you cut the pressure-signal wire and feed both ends into a small black box. What this unit does is to scale the pressure-signal voltage to a lower amount. The actual amount of the reduction in voltage is set by an adjustable switch.

 

Let's say the output from the intake-manifold pressure switch is 3 volts. This 3 volts, instead of connecting directly to the ECU, is now being fed to the HKS FCD. We set the FCD to scale the this voltage to 90% of the original. What comes out of the FCD is, therefore, 90% of 3 volts which is 2.7 volts. This 2.7 volts is fed to the ECU pressure signal input.

 

The effect is the same regardless of the input voltage. Basically, whatever you feed into the HKS FCD, will be scaled by 90% (or whatever scaling factor is set, within the capacity of the unit).  

 

Let's now fit an HKS FCD, scaled by 90%, and install it into our theoretical example.

 

Our goal is to reach a boost pressure of 14psi or 4.83 volts pressure signal. Fuel-cut is initiated at 4.5 volts and we are scaling by 90%. Therefore, at 14 psi boost the pressure signal actually being fed to the ECU will be 90% of 4.83 volts which equals 4.347 volts. Which is 0.153 volts below our 4.5 volt fuel-cut threshhold.  

 

The upshot of which, our target boost of 14 psi will be reached whereupon the wastegate will operate.

 

So what's the problem with the FCDs?..........

 

The key thing to hold in mind at this point, is that the amount of fuel entering the engine is ALSO directly proportional to the pressure signal.

 

In other words, not only does the ECU use the value of the pressure signal in order to determine things like when the wastegate should open, or to initiate fuel-cut: it uses this signal in order to calibrate precisely how much fuel to feed into the engine.

 

Fact is, for a high-power turbo engine, and *especially* an engine that is running higher boost than stock, fueling is darned critical. At wide open throttle, correct fuelling is CRUCIAL.

 

If you were to compare the WOT fueling range, of between 11.9 to 12.2 A/R, as a percentage of the total range 11.9 to 14.7 A/R. A 10% reduction in fuelling is about 0.3 A/R. So even just a small scaling ratio on the HKS FCD can cause your fuelling to be outside tolerable limits... and at the most critical fueling stage of an engine's operation, namely, wide open throttle.  

 

The table below is the scaling range of my old HKS FCD that I once fitted to my MKIV.

 

Input Voltage: 10.00 volts

 

Output Voltage -versus- switch position:

 

(Note: position 1 is the highest cut.)

 

1) 4.54 volts

2) 4.82

3) 5.10

4) 5.38

5) 5.66

6) 5.94

7) 6.23

8) 6.51

9) 6.79

10 7.08

11) 7.36

12) 7.64

 

So as you can see, the very *least* amount of possible cut (or scaling range, depending on which way you look at it) is at position 12: which is a whacking 76.4%!!!

 

Looking at it in fueling terms, set at the very minimum FCD scaling possible, there is a potential leaning out of the engine air/fuel ratio of 23.6%. Which moves the A/R ratio at WOT from a mean point of 12.05 A/R (being the mean point of the range 11.9 to 12.2 A/R) to somewhere in the region of 12.7 A/R.

 

Consider that the maximum recommended A/R for a high-power turbo engine at WOT is 12.2. So the very *minimum* setting on the FCD takes the fuelling way too lean.

 

Note: the minimum setting was not a sufficient scaling factor to defeat fuel-cut at stock boost on Justin's car (Hiper exhaust and no cats).

 

Additional note: I'm making this post-edit to make clear the car referred to in the para above is the EVO.

 

Because of Justin's insistence (after all it is his engine), to defeat fuel-cut I had to set the HKS FCD to position 8 or 9 (forgot to make a note as we were running out of time). This gave a reading of 13.4 A/R at WOT which is frightening really. But not half as scary as the 15.4 we recorded on one of the runs.

 

So if fuel-cut is a good thing then why defeat it?..........    

 

Manufacturers have different aims and objectives than some types of individual owner. Manufacturers have to take account of future circumstances like potential warranty claims, adverse publicity (particularly concerning safety), being sued by an army of irate owners, etc., etc.

 

So by defeating the fuel-cut I may be potentionally damaging my engine?..........

 

In a word, yes. In the sense you are going beyond the boost pressure which has been determined to be a safe upper limit by the manufacturer of your particular vehicle.

 

So how can I justify boosting beyond what a manufacturer has deemed safe for my car?..........  

 

Simple! The MKIV (for example) doesn't have a particularly brilliant stock intercooler. Increasing intercooling efficiency, can allow you to run more boost yet still hold engine conditions within safe operational limits.

 

But it's not just with intercooling: there are a wide range of tuning options that can be applied to an engine that will allow it to produce more power, yet have the engine's vital signs still operating within logical limits.

 

What are the engine's vital signs exactly?..........

 

An ideal analogy here is the Human body. We have an optimum operating temperature; an optimum heartbeat; an optimum rate of breathing; and so forth.

 

If one of these vital signs were to stray outside the fairly narrow range of high and low limits, the effects could be very damaging... quite possibly fatal.

 

Engine's too have inherent vital signs that must be measured and kept within fairly tight limits. And a crucially vital sign (especially in a high-power forced induction motor) is called the air/fuel ratio (expressed A/R)  

 

What is air/fuel ratio exactly?..........

 

Over the decades, scientists have determined that there are optimum ratios between the amounts of fuel and air entering the engine.

 

It has been found that optimum burning occurs at a fuel to air ratio (A/R) of 14.7. Any higher than this and the mixture of fuel and air is said to be lean, lower than 14.7 A/R is said to be rich. For example, 10.2 A/R would be very rich, and at the other end of the scale, 19.1 A/R would be very lean.    

 

Under steady-state cruising conditions, for minimum emissions and minimum fuel consumption, manufacturers aim for an A/R of 14.7. But the engine ECU will change the A/R ratio dependant on certain conditions.  

 

With a high-power turbocharged engine, under full load, i.e. foot to the floor and accelerating like mad, the air/fuel ratio should be in the region of 11.9 to 12.2. A/R. Under the same conditions, with a normally aspirated engine, the A/R ratio can be slightly higher, in the region of 12.7 to 13.1 A/R.  

 

If the mixture settings fall outside these limits, by any significant degree (particularly with a turbo engine) severe engine damage can result.

 

Okay, well, I think I answered everything. Thank you all for your kind comments and here's to the future success of the BBS.

 

Yours,

J

 

 

(Edited by Ash at 4:49 pm on July 3, 2001)

 

 

(Edited by Ash at 8:06 pm on July 3, 2001)

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Guest Martin F

Excellent post  JA.

 

Reckon i'm one step closer to understanding how my car is running now!!

 

Thanks

Martin

 

P.S. Did you work those convenient figures out first??  ;-)))

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