Hi I have just bought my first zx today its a twin turbo model. The seller told me before I bought the car that it was not boosting, this didn't put me off too much as it was a bit of a bargain.

 

However I obviously want to try and sort the problem out and was wondering if anybody has any ideas of what is the likley cause or any checks I can do to try and diagnose the fault. Any help would be great :)

My dump valves will go off if i rev the car stationary

 

As will mine, gotta be a hefty prod of the throttle mind from about 2k

 

So will mine but that's not because I'm boosting, it's because they are opening on deceleration.

thats not right is it..anything other than boosted air would not lift the valves other wise every saxo and corsa would have dump valves and the "backed up air" would lift them:confused::confused:

 

The boosted air comes from the close of the throttle bodies, like slamming a door shut in a wind tunnel, the air compresses because it temporarily has nowhere to go, and this activates the dump valve.

standed tubs here and iff i rev my motor while under the bonnet i can feel my boost pipes filling out and getting hard(

 

so your boost gauge goes into plus when revving in nuteral ?

so your boost gauge goes into plus when revving in nuteral ?

 

fvk ime even doubting myself now:headvswal ime going to re-check next week but ime sure i can get mine into+ hmmmmmm

Maybe a daft question but why can you not get positive boost when not under load? ive noticed my recircs dont go off if i just rev it and i dont build boost unless driving?

every turbo car ive had will let of the BOV's during a big hard rev when stationary including the zed, dont know about positive boost tho, never done it sat in the drivers seat

Maybe a daft question but why can you not get positive boost when not under load? ive noticed my recircs dont go off if i just rev it and i dont build boost unless driving?

 

On some exceptional cars you can, some of the variable vane turbos can achieve it.

 

What happens IIRC is the engine reaches max rpm before the turbos can reach boost.

 

Imagine an athlete running 100M, he will use a certain amount of fuel and air to get to his maximum speed. Now make the athlete pull a 300ZX (1500KG) 100M, he will be using more fuel and air, and be slower.

 

It is the same with the engine, it is not moving the weight of the car and requires minimal power to reach max rpm, because it is free of weight restricting its revolutions per minute.

makes sense

On some exceptional cars you can, some of the variable vane turbos can achieve it.

 

What happens IIRC is the engine reaches max rpm before the turbos can reach boost.

 

Imagine an athlete running 100M, he will use a certain amount of fuel and air to get to his maximum speed. Now make the athlete pull a 300ZX (1500KG) 100M, he will be using more fuel and air, and be slower.

 

It is the same with the engine, it is not moving the weight of the car and requires minimal power to reach max rpm, because it is free of weight restricting its revolutions per minute.

 

So basically if the car is not drawing as much air/fuel in as it would under load then there isnt enough back pressure to spool the turbo's until reaching max rpm?

The car reaches max rpm before the turbos spool to reach normal driving boost levels!

 

Imagine you are driving along the motorway and without changing gear you floor the accelerator, it takes time to reach max rpm, because it is powering the weight of the car.

 

If you are in neutral and at a standstill the engine revs very quickly before the turbos have time to boost and would be bouncing off the rev limiter, before you get any boost.

ok well if its not actually boosting at all as you say, then why does the gauge read 5-6 psi and you can hear the dumpvalve dumping? surely it won't dump if theres nothing to dump??? and why does the piping that side go hard when revving it up?

when i say revving, i don't just mean slowly increasing revs i mean pumping the throttle to the floor and nearly hitting the limiter.

 

tbh i only posted on this thread to try and help someone out by saying what i did to find, not to have everyone tell me that what my car does can't happen and basically get called a liar. I won't bother trying to help in future!

 

Gone and put the car on e bay so there !!:D

http://cgi.ebay.co.uk/1991-NISSAN-300-ZX-TWIN-TURBO-UK-CAR-MANUAL-300ZX_W0QQitemZ120348975203QQcmdZViewItemQQptZAutomobiles_UK?hash=item120348975203&_trksid=p3286.c0.m14&_trkparms=66%3A2%7C65%3A7%7C39%3A1%7C240%3A1318

Edited December 12, 200817 yr by jimmer

The car reaches max rpm before the turbos spool to reach normal driving boost levels!

 

Imagine you are driving along the motorway and without changing gear you floor the accelerator, it takes time to reach max rpm, because it is powering the weight of the car.

 

If you are in neutral and at a standstill the engine revs very quickly before the turbos have time to boost and would be bouncing off the rev limiter, before you get any boost.

 

Ahhhhhhhhhh i see now, sorry another stupid question........

so if you were to hold the revs at say 6,000rpm for a few seconds whilst in neutral will the turbos eventually build boost or the same as above?

Ahhhhhhhhhh i see now, sorry another stupid question........

so if you were to hold the revs at say 6,000rpm for a few seconds whilst in neutral will the turbos eventually build boost or the same as above?

 

Not really the engine needs very little energy (air+fuel) to reach max rpm, so a very small increase in boost means the engine reaches max rpm before any noticeable boost is attained.

 

Think of it another way, if you think of all the rotating parts of the engine, they probably weigh 30lbs (just a guess!), think of the energy needed just to move the components of the engine, not much in fact we can turn it over by hand. So the energy needed for the engine to rev to max rpm is a fraction of the energy needed when it is connected to and trying to move 1500kg of car!

Edited December 12, 200817 yr by TopLess

Not really the engine needs very little energy (air+fuel) to reach max rpm, so a very small increase in boost means the engine reaches max rpm before any noticeable boost is attained.

 

Think of it another way, if you think of all the rotating parts of the engine, they probably weigh 30lbs (just a guess!), think of the energy needed just to move the components of the engine, not much in fact we can turn it over by hand. So the energy needed for the engine to rev to max rpm is a fraction of the energy needed when it is connected to and trying to move 1500kg of car!

Thank you for the knowledge i now understand:hyper:

although by the time your car starts to get near the redline, even at a standstill your turbos will probably just start to boost

I don't think it works like that pete. The turbo's have no reason to spool without load.