High Speed Boost Question

Hiya peeps,

Since I bought my car two weeks ago; 96 Supra GZ (JDM Twin Turbo – BPU), when flooring it up to high speeds i.e 100mph +, when my boost gauge gets to just over 1 bar, the boost gets cut back and I experience a big jerk almost like a slow gear change (which at first I thought it was as my cars an Auto). As the speed increases further, the boost gets back up to just over 1 bar and the same thing happens again, until my blitz boost controller (which I don’t really understand just yet, so I have it set to the “off setting, although its still on if you know what I mean, showing boost pressure) beeps at me at about 130 / 140, which I presume is warning me of something, at which point I’m shitting myself anyway and ease of the throttle.

I’m presuming this is a safety mechanism that stops the boost from getting too high, but should it be so violent?! Kinda feels a bit wrong.

Any advice on what’s going on would be great.

Thanks in advance

Matt

Do you get the warning triangle light up on the dash? If so that's fuel cut - get an FCD.

Otherwise could be a boost controller setting?

Hiya peeps,

 

Since I bought my car two weeks ago; 96 Supra GZ (JDM Twin Turbo – BPU), when flooring it up to high speeds i.e 100mph +, when my boost gauge gets to just over 1 bar, the boost gets cut back and I experience a big jerk almost like a slow gear change (which at first I thought it was as my cars an Auto). As the speed increases further, the boost gets back up to just over 1 bar and the same thing happens again, until my blitz boost controller (which I don’t really understand just yet, so I have it set to the “off setting, although its still on if you know what I mean, showing boost pressure) beeps at me at about 130 / 140, which I presume is warning me of something, at which point I’m shitting myself anyway and ease of the throttle.

 

I’m presuming this is a safety mechanism that stops the boost from getting too high, but should it be so violent?! Kinda feels a bit wrong.

 

Any advice on what’s going on would be great.

 

Thanks in advance

 

Matt

Do you have a fuel cut defender fitted ? I just had mine fitted (Thor make them), your ECU is probably telling the car to stop over-boosting...

does sound like fuel cut, it comes in after so many seconds above a certain boost pressure, around 1.0 bar or just over, and it cuts everything so it is very violent as you describe.

this is the one you want http://www.thor-racing.co.uk/Fuel_Cut_Defencers-152.asp

No i dont get the warning triangle, not sure about the fuel cut defender, but shouldnt the dump valve / waste guage just let of the additional boost to maintain a certain level instead of what is happening?

No and fuel cut at 1.0bar is VERY violent - happened to me after I decatted going up a sliproad, basically lost all power for about 3 seconds - it's not nice. I had the warning triangle flash up though I thought that happened with fuel cut (maybe you didn't notice it?).

No and fuel cut at 1.0bar is VERY violent - happened to me after I decatted going up a sliproad, basically lost all power for about 3 seconds - it's not nice. I had the warning triangle flash up though I thought that happened with fuel cut (maybe you didn't notice it?).

Yeah maybe, I'll have a look tonight when i get back to my car. Altough it only lasts for a split second. It happens all the time tho, whenever i floor it too high speeds.

If it is that, did you sort the probelm out with the THOR product that Chilli kindly suggested? (Cheers Chilli).

Will my boost controller stop this from happening tho? maybe thats what the original owner had it installed for?!?!

Thanks

Matt

the boost controller is to control (i.e. raise) the boost by a certain amount. The ecu has a safeguard to protect the engine from overboost damage. you need the FCD whenever you boost is higher than the stock limit for whatever reason (i.e. j-spec decat or Uk spec with electronic boost controller).

Try it again and look for warning indicator on the dash, if it's violent, always at/over 1bar and you get the warning indicator on the dash, it must be fuel cut. Anything else and maybe you have a fault somewhere.

as tbourner says, you usually know it's fuel cut mainly because the first time it happens you tend to 'crap your pants' - I thought I'd blown something badly for a moment until I realised what it was the first time it happened to me.

O.k then, i'll check that out tonight. Just one more thing to note tho,

The boost keeps rising with the increase in speed (is that o.k / normal), then as the pressure reaches the 1.1 bar this "cut" if you will, occurs and the boost goes down to sort of 0.8 bar, then as the speed increase further and the boost reaches 1.1 bar the "cut" happens again, and so on until i ease of throttle. I reckon i could experiance 3 "cuts" from accelerating (foot to the floor) from about 80mph to 140mph.

hmmm that sounds different. Fuel cut is complete loss of power (i.e. no fuel! so no boost either of course). So if when it happens you still have some power and boost then this is not fuel cut. Sounds then like what you have is something going on with the electronic boost controller perhaps? what boost do you get with it totally switched off? does it drive ok then?

hmmm that sounds different. Fuel cut is complete loss of power (i.e. no fuel! so no boost either of course). So if when it happens you still have some power and boost then this is not fuel cut. Sounds then like what you have is something going on with the electronic boost controller perhaps? what boost do you get with it totally switched off? does it drive ok then?

Its all ways in the off setting, just showing the pressure rating in "bar". The car drives perfect, under normal accereration, i.e just pulling away from the lights at a steady pase the boost is between -0.5 to 0, then under heavy load the boost goes up and keeps rising as the acceleration rises, it would keep going if it wasnt for this "cut" at around 1.1 bar.

Should boost rise with speed?!

Mine goes up in more of a logarithmic pattern ie: it goes up quickly from 4k revs to almost full boost within 1k revs ish, then the rest of the revs it goes up slightly. At high speed like in 5th the boost goes up to max almost as soon as number 2 is online, then stays there and maybe goes up a bit more as the revs rise. I'm almost certain it's not completely linear with revs/speed though.

Could be a funny setting on the boost controller - some simple boost cutoff point where it stops giving boost increase if it goes over a certain limit? Are you decatted or just forcing the boost up with the controller?

This really does sound like fuel cut.

Should boost rise with speed?!

Yes and no, it depends on the gear and speed you're doing. You really need to have a good long run down a motorway in a high gear (3rd for auto, 4th or 5th for manual) and look at the boost gauge.

From around 4krpm boost should rise almost immediately to say 1 bar and stay at this level up to 6krpm. If you hit fuel cut at any point, the boost will drop off again and take a few seconds to return to 1 bar.

Fuel cut occurs when the fuel cut boost level has been exceeded for a certain period of time 3 seconds maybe.

Easiest thing to do is to pull back the carpet in the passenger footwell, remove the black cover and look for any little boxes wired into the ECU. Most FCD's are about the size of a matchbox.

Edited to add: see below for a graph showing boost vs rpm (top red line). This was done on a dyno but the result is the same on the road.

but surely it can't be fuel cut if he is still making 0.8bar boost during it? for mine, for the 3 seconds or so it was cut the whole engine was down, i.e. no boost at all, because the whole engine is cut off

The engine nevers cuts out, the boost just drops to a lower level. Yeah the cars BPU.

I'll have a closer look tonight at whats going on and the things you chaps have pointed out.

In the meantime, thinking caps on please people, worried about my car

The FCD is a piece of garbage. The Greddy BCC is a far superior product. I would highly recommend that product to you.

Joe

The FCD is a piece of garbage. The Greddy BCC is a far superior product. I would highly recommend that product to you.

 

Joe

LMAO, you're kidding right?

The Greddy BCC completely removes fuel cut, the THOR FCD raises it to a safer limit 1.25bar. I'd rather keep a safe limit than remove the safety of fuel cut completely.

Of course, if youy have a UK spec or hybrid turbo car then it doesn't matter so much...

Back on topic...

The engine nevers cuts out, the boost just drops to a lower level. Yeah the cars BPU.

 

I'll have a closer look tonight at whats going on and the things you chaps have pointed out.

 

In the meantime, thinking caps on please people, worried about my car

So, when this happens there is no violent jolt then a quick regain of boost? If thats the case then it could be something simple like a split hose that it blowing open when high boost is acheived. Have a really good look around the IC pipework, especially around the area it goes into the wing. Do you have a standard IC?

LMAO, you're kidding right?

 

The Greddy BCC completely removes fuel cut, the THOR FCD raises it to a safer limit 1.25bar. I'd rather keep a safe limit than remove the safety of fuel cut completely.

 

Of course, if youy have a UK spec or hybrid turbo car then it doesn't matter so much...

No I'm not. On the US Spec Supras the BCC is the best option for a Supra equipped with a stock ECU. The FCD has proven to cause problems. With proper tuning, the BCC is an awesome unit.

Back on topic...

 

 

So, when this happens there is no violent jolt then a quick regain of boost? If thats the case then it could be something simple like a split hose that it blowing open when high boost is acheived. Have a really good look around the IC pipework, especially around the area it goes into the wing. Do you have a standard IC?

Yeah there is a violent jolt. Not sure about quick, i woud say it was quick but then again thats a matter of opinion. But the jolt is defo violent without question! Wish i could take someone out in it to show them what i mean!!!!!

Yeah there is a violent jolt. Not sure about quick, i woud say it was quick but then again thats a matter of opinion. But the jolt is defo violent without question! Wish i could take someone out in it to show them what i mean!!!!!

That definately sounds like boost cut.

That definately sounds like boost cut.

Whats boost cut mate? sounds like it could be cos the boost does get cut for a split second! lol. whats the remedie for it?!

Thanks

Matt

boost cut = fuel cut = get a Thor VFCD (do a search on hear to read why it is better than the alternatives...)

Thats strange my last message didnt get posted, but anyway here it is again;

O.k, well last night i did a bit of investigative work (i.e driving a bit fast!), and found out the following

1.The warning light does not light up when the boost gets dropped - does that rule out fuel cut then?

2. I have to say the kick is rather quick actually, happens for a split second.

3. the boost gets knocked down when it reaches 1.15 bar (which incidently is what my blitz boost controller is set to on the warn setting, altough i have it set to the off position so ??????)

4. from about 90/100 to 120/130, i epxeriance say 2 of these kicks.. the boost rises to 1.15 bar then drops to about 0.8 (maybe slightly less) then climbs again to 1.15 bar then drops again. It happens again at about 140 then my boost contoller bleeps at me ??

Thanks for the help again people............

ok, that is not fuel cut lol, as mentioned fuel cut = complete loss of power = no boost at all for about 3 seconds and a warning indicator...