Jump to content
The mkiv Supra Owners Club

Fuel cut ?


thedirtycat
 Share

Recommended Posts

I did a search but couldn't really see anything on the forum about fuel cut. Now I know when people want to go full bpu and raise the boost levels most the time they need a fuel cut defender so they don't hit fuel cut.

Now I typed in fuel cut into google and read a post on a forum about why you should not try and get rid of fuel cut because its there for a reason which is what I thought too.

What they were saying is the way to eliminate it is to get a better fuel system i.e bigger injectors. Basically saying that you hit fuel cut because the fuel system cant cope with the amount of extra air coming into the engine. They said don't try and eliminate fuel cut by fitting some kind of defender.

I also remember a friend saying not to use a fuel cut defender too.

 

Are supras different and hit fuel cut too early ?As it seems that jspec injectors cant actually handle 1.2 bar of boost but are maxed out and I guess uk specs would be able to handle a bit more. Is it just some design fault that they hit fuel cut way too early ?

Do uk specs not hit fuel cut due to bigger injectors ?

Link to comment
Share on other sites

its all to do with afr. the stock supra setup is rich so by fitting a bigger pump it allows yet more room to up the boost into. its not to do with the fuel system struggleing. a bpu at 1.2 bar on a 255 pump is mid 11s on WOT witch is pretty ideal afr on boost.

Link to comment
Share on other sites

No it's not a design fault.

 

A stock Supra is designed to produce around 0.8bar as standard, Toyota obviously felt this was a safe and reliable limit for the life span of the car. To prevent the turbos from overboosting the stock ECU is programmed to completely cut the fuel to the engine if the MAP sensor detects the turbos are boosting above 1bar. This is a safety feature designed to prevent the turbos from over boosting - which could potentially cause damage to the turbos/engine - in the event of a fault in the system.

 

The only way you can run above 1bar with the stock ECU is to fit a Fuel Cut Defender, the FCD is wired in between the MAP sensor-ECU and basically fools the ECU into raising the boost limit that the fuel is cut.

 

A stock Supra is setup to run quite rich, again this is a safety margin built in by Toyota, but this also means that the boost can be raised to say 1.2bar and the air fuel ratio will still be okay. Raising the boost will produce more power but it will also potentially reduce the life/reliability of the turbos and engine. Toyota could have sold the Supra with a higher boost limit but this would have had a big effect on reliability.

 

For more info click -> BPU

Link to comment
Share on other sites

No it's not a design fault.

 

A stock Supra is designed to produce around 0.8bar as standard, Toyota obviously felt this was a safe and reliable limit for the life span of the car. To prevent the turbos from overboosting the stock ECU is programmed to completely cut the fuel to the engine if the MAP sensor detects the turbos are boosting above 1bar. This is a safety feature designed to prevent the turbos from over boosting - which could potentially cause damage to the turbos/engine - in the event of a fault in the system.

 

The only way you can run above 1bar with the stock ECU is to fit a Fuel Cut Defender, the FCD is wired in between the MAP sensor-ECU and basically fools the ECU into raising the boost limit that the fuel is cut.

 

A stock Supra is setup to run quite rich, again this is a safety margin built in by Toyota, but this also means that the boost can be raised to say 1.2bar and the air fuel ratio will still be okay. Raising the boost will produce more power but it will also potentially reduce the life/reliability of the turbos and engine. Toyota could have sold the Supra with a higher boost limit but this would have had a big effect on reliability.

 

For more info click -> BPU

 

Oh I understand. I was a bit confused about fuel cut because I thought you would never be able to boost past 1 bar with the cats in anyway. I understand it being a safty thing now so I guess in a way its dangerous eliminating fuel cut but required to get more boost.

 

You say they could have sold the supra with a higher boost limit but it would have a big effect on reliability but isn't running up to 1.2 bar still consider to still be very reliable ?

 

I know I spoke to Keron recently and he did actually not recommend running 1.2 bar as you never know the condition of used turbos unless you take them apart and check them out.

 

I'm guessing its mainly turbos that will break as everything else should withstand more boost/power.

Link to comment
Share on other sites

You say they could have sold the supra with a higher boost limit but it would have a big effect on reliability but isn't running up to 1.2 bar still consider to still be very reliable ?

 

I know I spoke to Keron recently and he did actually not recommend running 1.2 bar as you never know the condition of used turbos unless you take them apart and check them out.

 

I'm guessing its mainly turbos that will break as everything else should withstand more boost/power.

 

In terms of fueling you can safely run up to 1.2bar, it's a popular modification because it makes a significant difference to power and response and really brings the Supra to life. The down side is that it will reduce the life of the turbos and ultimately the engine - although there are many high milers still going strong at BPU levels.

 

The main reliability issues are with the turbos, the J-spec ceramic turbine wheels can let go, some last many 10K's miles others can be less reliable - I had the factory fitted turbos blow on the dyno and then around 7 years later the brand new set I fitted went pop. In comparison the UK spec turbos seem to suffer more from bearing and oil seal wear, probably due to the narrower diameter shaft and bearings.

 

The better FCD's don't eliminate fuel cut altogether but allow the user to raise the level when the fuel is cut. So you could set the fuel cut level to say 1.3bar and run the car at 1.2bar, then should something happen to cause the boost to spike there is still a margin of safety.

Edited by Nic (see edit history)
Link to comment
Share on other sites

In terms of fueling you can safely run up to 1.2bar, it's a popular modification because it makes a significant difference to power and response and really brings the Supra to life. The down side is that it will reduce the life of the turbos and ultimately the engine - although there are many high milers still going strong at BPU levels.

 

The main reliability issues are with the turbos, the J-spec ceramic turbine wheels can let go, some last many 10K's miles others can be less reliable - I had the factory fitted turbos blow on the dyno and then around 7 years later the brand new set I fitted went pop. In comparison the UK spec turbos seem to suffer more from bearing and oil seal wear, probably due to the narrower diameter shaft and bearings.

 

The better FCD's don't eliminate fuel cut altogether but allow the user to raise the level when the fuel is cut. So you could set the fuel cut level to say 1.3bar and run the car at 1.2bar, then should something happen to cause the boost to spike there is still a margin of safety.

 

 

Unfortunatley, if you wanted to use a FCD to cut in at 1.3 bar you would also need to look at finding a replacement MAP sensor, the 3 stock ones I have tried give meaningless inconsistant outputs once you go past about 1.2 bar. I cant remember if it was CW or Ian C or someone, Andy Blyth maybe who posted a really nice table showing his findings with the stock sensor. I did play with a Cossie sensor on mine with a little bit of circuitry that emulated the output of the stock sensor voltage vs boost up to the point just below fuel cut and then clamped it until the sensor saw 1.2 bar or whatever I wanted to set it at and then the clamp was dropped and fuel cut introduced. Worked ok but was messy and never had the time to refine it from the breadboard stage and ended up putting the oem sensor back.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. You might also be interested in our Guidelines, Privacy Policy and Terms of Use.