Over Boooost!

Hi Guys, I have not long purchased a 1995 twin tubo 6 speed manual, when i floor the car in 4th gear she over boosts about 5k revs and does the same in 5th and sixth, can feed in the accelerator pedal and still reach pretty much max revs though, any ideas?

Mike.....

Not sure what you mean here by 'over boost'. Are you saying that the boost pressure spikes at higher RPM, or that the rpm's rise inconsistent with speed increase?

Hi Homer thanks for getting back so quick, the car just basically shuts down in a big way but if i take my foot of the gas and feed it in slowly then she will still accelerate but not at full throttle.

If its quite a violent cut off then it sounds like the car is hitting fuel cut. I think this happens at around 0.9-1 bar but this shouldn't really be possible unless a cat has been removed, bleed valve fitted (hope not) or it has a boost controller.

Does the car have one (or more) cats removed and do you have a decent boost gauge fitted - if so what boost are you seeing?

The car has an after market HKS dump valve and a full stainless steel exaust fitted, super dagger or something like that, sounds like the cats have been removed then, do you know what the solution is Homer old man?

If it is fuel cut then you need a fuel cut defencer, Thor do a great one.

First you should check if you have both cats in, if one or both are missing then you can confirm this is the cause of the trouble. You need to get right under the car too see, one is mounted on the downpipe from the manifold (bit tricky to see) and the other is on a 2nd pipe with the cat mounted a bit back from where your feet would be.

If you do have a cat removed then aside from getting the Fuel cut defencer (FCD) you need to make sure the correct grade/gap spark plugs are fitted and that you have an uprated fuel pump at the minimum (ideally a few more things!), otherwise you run a risk of running lean.

Agree with Homer, fuel cut. Find out what boost this happens at. You NEED a boost gauge if you're running above stock boost. If the boost levels are no more than 1.25 bar you can stop it hitting fuel cut by fitting a Fuel Cut Defender from Thor Racing. If it's going above 1.25bar you must do something to restrict the boost, e.g. fit a restrictor plate in the exhaust.

Get a boost gauge and go from there.

Just had a look and the exaust is called a Mongoose 3" stainless steel system, it does look as though it has two in line boxes before the back box, maybe these are cats?

The first cat is up by the exhaust manifold, the second is roughly under the driver's seat

it has def got a central cat about three feet from the back box and a some what slimmer longer bulge in the pipework just beyond the bellhousing, it is an extremely well made system, the welding is superb. is this a well known system for Supras and could the fact that it is 3" be causing the fuel cut off?

it has def got a central cat about three feet from the back box and a some what slimmer longer bulge in the pipework just beyond the bellhousing, it is an extremely well made system, the welding is superb. is this a well known system for Supras and could the fact that it is 3" be causing the fuel cut off?

I have attached a photo showing the stock catalytic converters. Mongoose is a known quality brand for the Supra, it is one of the more quieter aftermarket exhausts available, the 3" diameter piping should not be a problem, but will mean the exhaust is more free flowing than stock. This is a good thing as it means that the turbos will spool up faster and car will make more power.

As has been said, it sounds like you are hitting fuel cut, probably because one or both cats have been removed. If you are only getting fuel cut in higher gears when the engine is under the most load, then you are probably only just hitting 1bar. As has been said already, suggest you do the following:

1. Fit a boost gauge, mechanical are cheaper, electronic more expensive but offer other features such as alarm, peak boost, etc. (see links in traders section)

2. Go to link Jake posted and get a Fuel Cut Defender (Thor call it VFCC.) This will raise the fuel cut to 1.25bar.

3. Check what peak boost you are making, 1.1-1.2bar should be the maximum safe limit you want to run.

4. If you are going above 1.2bar then you will need to fit a restrictor ring in the exhaust (contact Chris Wilson for restrictor ring and expert advice) this is basically a plate with a smaller diameter hole that restricts the exhaust flow and so lowers peak boost to safe limit.

Some other things you may need to do are:

1. If the car has still got original fuel pump fitted, it is now nearly 10 years old and may be past its best. Would suggest fitting new higher flowing Walbro fuel pump (see Vortex in tarders section.)

2. If you feel a slight mis-fire at peak boost, you will need to fit new cooler grade plugs. Do a search for recommended plug specs.

3. Check the condition of your intercooler (mounted inside drivers side airduct) Again if it is the original IC then it may well be past its best (broken, crumbling cooling fins etc) and need replacing.

Hope that helps. Let us know how you get on,

You need to get right under the car too see, one is mounted on the downpipe from the manifold (bit tricky to see)

Isn't it easier to see this one from under the bonnet? Rather than climbing under the car!!!