resistor for boost cut

[avinesh]

hey guys just wanted to know what resistor to use for removing boost cutt in a 1jzgte, how many volts what resistance looked over the internet couldnt get any good answers

[the fallen303]

just as a note, won't work with the soarer 1jz, i've got it on my jza70, but can't remember the resistor, will try and find out for you.

[Crampy]

Wouldn't you need a voltage clamp and not just a resistor?
Why not just buy a FCD?

[the fallen303]

resistor works fine just think the colours have come off mine because it's been in there for several years now 2c for a resistor, or $100+ for a fcd?? to do the same job? you do the math

[solitaire]

If its map sensored will it mess with the fuelling?

[the fallen303]

the resistor allows the map sensor to only read up to a certain boost level, then it flat lines. the standard fuel pressure regulator seems to handle this quite well, and we haven't had the a/f ratio be a problem even on 20psi.

[sergei]

resistor does not "flat line" as you describe.
It moves the curve up or down (or changes the angle, depending where you put and how many). Also if you disconnect turbo pressure sensor and put resistor in there it will pull up/down to certain voltage (which would be obviously constant). In my GT4 I used ~2kOhm resistor to pull down (which resulted in about 1.5V, from memory). It was safe to do that because the celica (ST165 which was running ST185 ECU) had AFM.
With Map Senored ECU (no AFM) resistor/zener/FCD is a big NO-NO.

P.S. Standard fuel pressure regulator has nothing to do with "adding extra fuel", what it does it keeps pressure difference between fuel and inlet manifold constant, thus making fuel flow a bit more predictable.

[Zak]

Wouldn't a simple zener diode make it a bit more reliable?

That way you wont get a voltage drop accross the whole pressure range, but it wont go over the boost cut level either.

[sergei]

As I said above, you can do anything you like if you have AFM.
On other hand if you have MAP sensor as main sensor, you better not mess up with it unless you have another ECU/injector(s) combo to provide fuel above the pressure on which MAP sensor was clamped. Anyway this is so dodgy that you are far better off investing in aftermarket ecu.
Clamping voltage at MAP sensor will result in cracked pistons for sure.

Also if you clamp the voltage at MAP sensor, you will not gain power above certain boost as the mixture will be very lean after certain point and will result in increasing boost while the car will slow down.

[Zak]

I didn't realise you'd posted before me

I've only just started learning about electronics in detail, so it's still alittle confusing.

I imagine it wouldn't be too bad, if you know what the fueling is like above the fuel cut, and you didn't go over board (im only talking 2 - 3psi at most). But it would remove any saftey measures that the ecu had against over boost, making the engine rather vulnerable.

[sergei]

On stock car 2 or 3 psi above boost cut will not do much of performance gain, as usually turbo will run out of puff at that boost. With CT26 you can weld shut the wastegate and it will not run more than ~21psi. While CT20b (steel) happily go over 2Bar (as it did unintentionally due to boost creep on my ST165 until it had 2 externals fitted).

[avinesh]

i dont intend on putting the boost up but the normal boost of the car with intercooler, exhaust an intake has gone a lil over boost cutt thats why i wanted to fit a resistor the car sends 4.9 volts to cutt off boost i wanted to drop this voltage by a lil bit so can anyone help me by telling me what resistor i should use like the ohms an watts

[postfach]

did you do a full exhaust from the turbos back? if you replaced the factory downpipe you will definitely overboost and without the boost cut you'll kill your turbos pretty quickly, they have a ceramic exhaust wheel and it falls off at around 16psi (boost cut is 14ish)

if you did replace the downpipe, i suggest putting some kind of restriction in the exhaust, perhaps a sheet of metal between 2 flanges with a 2.5" hole in it or something

[avinesh]

did you do a full exhaust from the turbos back? if you replaced the factory downpipe you will definitely overboost and without the boost cut you'll kill your turbos pretty quickly, they have a ceramic exhaust wheel and it falls off at around 16psi (boost cut is 14ish)

if you did replace the downpipe, i suggest putting some kind of restriction in the exhaust, perhaps a sheet of metal between 2 flanges with a 2.5" hole in it or something

the downpipe is stock just 3inch exhaust from the downpipe back an i dont think it boost 16psi more like 14.8ish

[the fallen303]

it's funny people saying that the turbo's die above 1bar... mine have been running stable at 16+for 10 years of very hard driving, and they're still fine. running with custom 3" dumps and straight through 2 3/4 exhaust, no mufflers.

oh well, it works *shrugs* just talking to dad, it's a zener diode, sorry, i thought it was a resistor from a conversation we had about it a few years back, memories shot, lol. dad says it's to allow about 4.7 volts? if that makes sence to anyone.

[avinesh]

it's funny people saying that the turbo's die above 1bar... mine have been running stable at 16+for 10 years of very hard driving, and they're still fine. running with custom 3" dumps and straight through 2 3/4 exhaust, no mufflers.

oh well, it works *shrugs* just talking to dad, it's a zener diode, sorry, i thought it was a resistor from a conversation we had about it a few years back, memories shot, lol. dad says it's to allow about 4.7 volts? if that makes sence to anyone.

what value zener diode did he use

[Malcolm]

sounds like he used a 4.7V one

[Crampy]

Yep, now that makes a whole lot more sense.

If you use a zener diode rated at 4.7 volts it'll stop the voltage going higher than that.

You need to put it reverse biased and shunt it to earth. that way the MAP sensor voltage will remain normal up until the Zener voltage, above that the voltage is clamped at the zener voltage as the extra current is shunted to earth. Wahlaaa a voltage clamp.


An avalanche diode will do the same thing, provided you can get one rated to the correct voltage.

From the internet:

A common application is protecting electronic circuits against damaging high voltages. The avalanche diode is connected to the circuit so that it is reverse-biased. In other words, its cathode is positive with respect to its anode. In this configuration, the diode is non-conducting and does not interfere with the circuit. If the voltage increases beyond the design limit, the diode suffers avalanche breakdown, causing the harmful voltage to be conducted to earth. When used in this fashion they are often referred to as clamper diodes because they "clamp" the voltage to a predetermined maximum level. Avalanche diodes are normally specified for this role by their clamping voltage VBR and the maximum size of transient they can absorb, specified by either energy (in joules) or I2t. Avalanche breakdown is not destructive, as long as the diode is not allowed to overheat.

[avinesh]

Yep, now that makes a whole lot more sense.

If you use a zener diode rated at 4.7 volts it'll stop the voltage going higher than that.

You need to put it reverse biased and shunt it to earth. that way the MAP sensor voltage will remain normal up until the Zener voltage, above that the voltage is clamped at the zener voltage as the extra current is shunted to earth. Wahlaaa a voltage clamp.


An avalanche diode will do the same thing, provided you can get one rated to the correct voltage.

From the internet:

A common application is protecting electronic circuits against damaging high voltages. The avalanche diode is connected to the circuit so that it is reverse-biased. In other words, its cathode is positive with respect to its anode. In this configuration, the diode is non-conducting and does not interfere with the circuit. If the voltage increases beyond the design limit, the diode suffers avalanche breakdown, causing the harmful voltage to be conducted to earth. When used in this fashion they are often referred to as clamper diodes because they "clamp" the voltage to a predetermined maximum level. Avalanche diodes are normally specified for this role by their clamping voltage VBR and the maximum size of transient they can absorb, specified by either energy (in joules) or I2t. Avalanche breakdown is not destructive, as long as the diode is not allowed to overheat.

so i cant just get a 4.7 volt diode an stik it on an map sensor already has a ground wire

[Crampy]

Put the Zener diode reverse biased onto the MAP sensor output that goes to the ECU and shunt it to earth.

Eg, connect the Cathode to the MAP sensor output and the Anode to an Earth. Any earth will do, as the car is negatively earthed. You can use the MAP sensor earth if you want, or connect to any metal bolt or whatever in the engine bay.

NOTE:
As already mentioned though, this will clamp the voltage that goes to the ECU. So, this means as the boost increases and the voltage is clamped, the ECU thinks it's still only seeing that boost level when clamping occurs, so will only fuel to that level. It probably wouldn't be wise to boost much over the clamping level without dealing with this.