Why does the Supra have a fuel cut system?

[RedM]

You couldn't stop the turbos overboosting though. A stand-alone ECU can't compensate for the undersized j-spec wastegate.

 

So, can the JDM wastegate be replaced with a larger one or would that cause more issues in itself?

 

If this is you're feelings go single then, save up £6k ish iirc for a small single. Job done.

 

No. Going single would be a waste of money and power. I reckon that BPU for an auto running the sequential twins systems is about as close to ideal for daily driving as you can get.

 

What I'm trying to work out is if there is another way to go about it.

[Nic]

Could the whole FCD, restrictor ring, boost controller route be avoided by using an aftermarket ECU that is set up properly?

 

You don't need an FCD if you fit a piggyback (eg. emanage) or standalone ECU.

 

An ECU change cannot prevent overboosting on stock J-spec turbos and you still need a restrictor ring. The stock j-spec wastegate is just to small to control the boost of the turbos when the cats are removed.

 

I'm not sure what is happening to cause the damage that the ECU/FC is seeking to avoid but it does seem that the BPU bits are just a fudge to get more power for minimum expense but with a risk of reduced reliability/longevity.

 

Any increase in performance over stock will reduce the life and reliability of the engine/turbos to a certain extent. Yes an FCD is a fudge to fool the stock ECU.

 

Removing the cats and fitting a freer flowing exhaust is a good way of getting more out of the stock setup, it's not the best option, but for the cost, it is relatively cheap to get quite a bit more power and response from the stock setup. It is a calculated risk though, more so on a high mileage older car.

 

It is certainly worth doing for the relatively cheap power gains, but I'd recommend that anyone considering it, first ensures that their engine and cooling system (intercooler, radiator, oil pump, water pump, etc) are in good condition and capable of dealing with the extra power.

[Wez]

Bloody good question. I'm guessing it over fuels. Right?

 

A UK/US motor should be ok due to them using MAF for fueling but a J-Spec which is MAP based the fueling will be out as the signal to the ECU will be modified.

 

As mentioned already the search button brings up loads of info on this.

[Ian C]

Fuel cut is just a failsafe. When we defeat it we take responsibility for preventing the engine from overboosting on ourselves. That means we can run higher boost but we can also merrily blow our engines up if the boost goes too high

 

-Ian

[Wez]

Good answers Nic

[carl0s]

A UK/US motor should be ok due to them using MAF for fueling but a J-Spec which is MAP based the fueling will be out as the signal to the ECU will be modified.

 

I don't understand this MAP/MAF business, but I've been reliably informed that both cars run the injectors at 100% d/c at anything abouve 15psi (1bar).

[Wez]

On a UK car the MAF sensor is used to calculate airflow into the engine which fueling is based upon.

 

J-Spec motors use a map sensor for fueling and the FCD alters this signal to the ECU so fueling is the hands of the gods but generally works ok, this has been discussed many many times.

 

 

100% dc at 15psi doesnt sound right to me as UKs have 550cc injectors and JS 440cc injectors, plus 15psi is just over 1bar. That would equal at rough guesses about 500hp at 1 Bar on a stock UK motor which is not the case.

[Angarak]

What would you set it to? 1.25bar? 1.3? How often is 1.2 bar reached in a BPU car?

 

The THOR FCD comes pre-set for Supras at 1.25 bar, which is ideal if your going BPU on a JDM with a 1.2 RR.

 

If you decat, you probably wont need a boost controller to up the boost to 1.2 bar.

 

I see 1.2 bar on my BPU JDM consistently from 2nd gear through to 4th (I've never had the need or desire to bring tubby #2 online above 4th to see what boost I get on WOT)...I've never given if bifters in 1st so dont know what boost Id achieve there.

 

Also, I'd have thought that the engine/turbo wear issues with BPU cars would only occur when your giving it death above the 0.8bar stock boost levels. I cant see how 'normal' daily driving below stock boost would cause excessive wear just because your BPU spec.

[Ian C]

J-Spec motors use a map sensor for fueling and the FCD alters this signal to the ECU so fueling is the hands of the gods but generally works ok, this has been discussed many many times.

 

It clamps the signal, not really alters it. Everything reports right up to a certain point and then the signal goes no higher.

 

100% dc at 15psi doesnt sound right to me as UKs have 550cc injectors and JS 440cc injectors, plus 15psi is just over 1bar. That would equal at rough guesses about 500hp at 1 Bar on a stock UK motor which is not the case.

 

Early on when I first got the E-Manage Blue and I was still on hybrids I started the datalogging and tuning exercise. I pretty quickly found out that you hit fuel cut once you got over 1bar of boost. By setting the boost cut limit (fuel cut limiter) to 4.40v you stopped fuel cut. 4.4v = 0.98bar of boost according to the stock MAP sensor. At this point you run 100% duty cycle, because the stock ECU is going "hmm, I'm over 0.8bar but it's not far enough to initiate fuel cut, I'll just open the floodgates and make sure it's rich as hell for safety reasons".

 

That is for a MAP-based Jspec. The MAF based UK spec is similar except the MAF sensor runs out of range at 4.4v anyway, and it still uses the MAP sensor purely for fuel cut. You can defeat it by disconnecting the reference hose and blocking it. Free FCD

 

Now for the big fat confusing caveat:

When I say 100% duty cycle what I actually mean is around 18ms of duty. It doesn't lock the injectors open once you reach 0.98bar, it just delivers it's maximum duty it'll ever need to deliver. 18ms is 0.018s, and you can have 55.5 of these 0.018s long injection events per second. So when the injector is fully open all the time, you need to be doing 3330 injection events per minute = about 6660rpm = injector open all the time (100% duty) and bang into the rev limiter around there.

 

So they are more and more flat out as the revs increase once you need a fuel cut defender. By about 6000rpm you are on 90% duty.

 

You won't see 500bhp just by using that rule of thumb for injector sizing as a) although the turbos give a marked performance increase over 0.8bar to 1.2bar, they are still getting less and less efficient, b) it's too rich, and c) stock fuel pressure is around 38psi and most injectors are rated at 43psi, so factor in 5 to 10% loss of power right there.

 

Just running BPU you'll get 10:1 AFRs or maybe even lower. You can actually tune this by running 1.2bar instead of 1.0 or 1.1! The injectors are at full chat so you add air instead

 

-Ian

[Wez]

Great info, thanks Ian