just wrote this:

 

Boost Jet FAQ

 

so no more questions on boost jets until youve read it!! :D

Why?

Actually, Paul, this is a top effort and I think we need a separate section on the forum for such things, the question was inspired by my 3 year old who says 'why?' everytime you tell him to stop asking questions!

erm, there is a seperate section - its in the "FAQ section" !!

why?

maybe I shouldn't have missed my nap today!

...nurse!

you say they hold the wastegates shut till a higher manifold pressure, but doesnt the boost level eventually creep down back to stock as the pressure actuator-side of the jet equalizes? Ive just watched one of Stuarts videos again just showing his boost gauge giving it some beans after installing boost jets the boost falls from 15psi down to 12psi in second (and afaik he was flat out, sounded that way anyway).

no, mine drops 1psi or so, & TBH i think that may be the turbos running out of puff higher in the rev range than anything else.

 

lower gears it drops at higher rpm, shift into 5th & boot it & it holds a steady 16psi!

 

its like crushing a hose pipe so less water flows through.

no, mine drops 1psi or so, & TBH i think that may be the turbos running out of puff higher in the rev range than anything else.

 

lower gears it drops at higher rpm, shift into 5th & boot it & it holds a steady 16psi!

 

its like crushing a hose pipe so less water flows through.

If you crush a hose pipe less water flows through it but the pressure stays the same (apart from frictional losses and tuberculation). Also, you shouldn't have any significant flow through the boost jets as there's nowhere for it to go. I must admit I've always been intrigued with how the boost jets actually work. It doesn't make sense to me either.

Paul C, very interesting mate well done, also found some more good technical info from your thread, keep up the good research.....

If you crush a hose pipe less water flows through it but the pressure stays the same (apart from frictional losses and tuberculation). Also, you shouldn't have any significant flow through the boost jets as there's nowhere for it to go. I must admit I've always been intrigued with how the boost jets actually work. It doesn't make sense to me either.

 

hmm, not so sure about that.

 

we have an air compressor at work that puts out 120psi pressure, on the ends of the air pipes we have a regulator (basically a variable restrictor), the pressure after this point is 90psi, open the regulator giving less restriction & the pressures back up to 120psi.

Boost jets provide a restriction in the hose.

 

To overcome this restriction takes pressure (its like trying to breathe through your nose when you've got a cold - harder work!!!).

 

If the wastegate opens at a fixed pressure (or thereabouts) and you have boost jets providing restriction, then the upstream (pre-jet) pressure needs to be higher to open the wastegate than if the jets were not there.

 

Hence higher boost!

 

Simple really!!!?

Makes sense to me

just wrote this:

 

Boost Jet FAQ

 

so no more questions on boost jets until youve read it!! :D

 

Nice work Paul, just one thing though 1 Bar does not equal 14.7 PSI, it's 14.508.

 

Cheers

...not that we're being picky or anything eh!

Nice work Paul, just one thing though 1 Bar does not equal 14.7 PSI, it's 14.508.

 

Cheers

 

LOL!! oh yeah, must have been thinking about the stoichiometric fuel ratio at the time!! :o

Boost jets provide a restriction in the hose.

 

To overcome this restriction takes pressure (its like trying to breathe through your nose when you've got a cold - harder work!!!).

 

If the wastegate opens at a fixed pressure (or thereabouts) and you have boost jets providing restriction, then the upstream (pre-jet) pressure needs to be higher to open the wastegate than if the jets were not there.

 

Hence higher boost!

 

Simple really!!!?

Not quite. I used to work writing computer simulations for water pipes and I also had problems getting to grips with this when I first started.

 

A restriction in a pipe provides an increase in pressure upstream and a decrease in pressure downstream only while there is fluid flowing through it. In the wastegate pipe there is no flow of gas, apart from the marginal flow as the pressures are equilised on either side. I've never seen a boost jet but I would presume the restriction is absolutely tiny if that's how it works. If that's the case, it's going to provide a VERY variable amount of boost.

Not quite. I used to work writing computer simulations for water pipes and I also had problems getting to grips with this when I first started.

 

I am an experienced chemical engineer - I know a little about fluid flow in all its many forms ;)

 

A restriction in a pipe provides an increase in pressure upstream and a decrease in pressure downstream only while there is fluid flowing through it.

 

From looking at the diagrams around the turbo (headache central :rolleyes: ) it looks like a flow would be maintained through the pipe the jets are installed in, which do not feed the wastgate actuator directly, but through a 3 way piece that feeds back to the turbo and actuator together!. The flow recirculates to the turbo, and allows pressure to build until it gets to waste-gate levels. Thus with a restriction in this line, a higher pressure drop is achieved, and hence a higher boost pressure is required (or obtained depending on how you look at it!) to meet the wastegate pressure.

 

In the wastegate pipe there is no flow of gas, apart from the marginal flow as the pressures are equilised on either side.

 

The wastegate actuator has not "exit flow". However, I believe that there has to be flow through the jets for them to work (and it does back to the turbo, if I'm interpreting the diagrams correctly :rolleyes: ). With a restriction the air has to flow through this narrower channel which is harder work, therefore requiring a higher driving force (i.e. pressure). The narrower the channel, the harder the work, the higher the required pressure. This is why smaller jet sizes lead to higher boost (albeit at different rates for different peeps).

 

I think this also explains why boost jets "spike" - eventually the air pressures equalise around the restriction, but initially the pressure will increase higher than final until "steady-state" is obtained.

 

Also if the pipe was direct to the wastegate actuator, with no flow, then ultimately boost jets wouldn't work. With no flow, at steady state the pressure would be maintained at design pressure regardless of the jet size. So if this were the case no-one would see boost increases with jets, just speed of boost increases (if you smell what I'm cooking!) However, many people get good results with boost jets!

 

I've never seen a boost jet but I would presume the restriction is absolutely tiny if that's how it works.

 

Consider a 1mm jet in a 1cm body. This gives an area reduction of 100 times over the "pipe". This is substantial in the scheme of things! Consider trying to breathe through a straw!

 

If that's the case, it's going to provide a VERY variable amount of boost.

 

I wouldn't know - I haven't got any :tongue:

 

Enough pedantry (nearly). I may be interpreting the diagrams wrong, but the evidence is that boost jets do increase boost, the smaller the jet the higher the boost, so restriction must play a part!!! The restriction must affect the boost pressure (as they work) so must cause a pressure change in the system. This must be related to flow since we agree that pressure change and flow are inextricably related (you can't have one without the other).

 

You may now give me a dry slap :p

 

StuartR

Ok ok ok enough guess work :D

 

The standard plumbing does NOT go directly from the intake to the actuator! There is a T-piece in the line which has a bleed valve in it. This is switched in to go OUT of safefty boost - all it does is bleed off some of the pressure fed to the actuator, so the pressure in the manifold must be higher to achieve the 7psi required to open the actuator fully. When this valve is closed, the t-piece is effectively removed from the pipework, so the pressure at the actuator is the presure at the intake manifold - ie 7psi is achieved.

 

NOW, when you install boost jets, you increase the restriction in the feed to the actuators. When the solenoid is closed this makes no difference to the boost level since the wastegate does not leak any pressure - 7psi at the intake will cause the actuator to be open fully. HOWEVER, as soon as the solenoid valve opens the t-piece, the bleed valve is switched into the circuit and it starts to bleed off the pressure from the actuator feed. Since there is a decrease in flow to the actuator, caused by the boost jets, a higher percentage of the air is bled off by the bleed valves. The net effect of this is you need to achieve higher manifold pressure in order to achieve 7psi at the actuators.

 

Incidentally, the stock bleed valves simply feed back into the turbo intake pipes. This is because the air in the manifold has already been metered by the MAS so it must stay in the system - otherwise its whats called a boost leak ;)

 

I hope this made some sense....

 

Oh and top job Paul!!! ;)

 

CheerZ,

 

Andy

 

Oh and top job Paul!!! ;)

 

 

cheers :)

From looking at the diagrams around the turbo (headache central :rolleyes: ) it looks like a flow would be maintained through the pipe the jets are installed in, which do not feed the wastgate actuator directly, but through a 3 way piece that feeds back to the turbo and actuator together!.

Ahaa! Of course. Forgot about the stock bleed valves as I don't have any anymore!

So after all of the debate does this mean that by using some 1mm/1.2mm welding tips you can actually achieve higher/quicker boost levels ? (Simplistic answers please) no more proffessor lingo.

yes, but we've known that for years ;)

Well I've learnt something new today!

 

Actually two things - how the boost system (inc. jets) works - and that Andy Duff is actually God!!! :bow:

 

Any chance of world peace, Andy?

what size jet would i need to get for say increase from stock 7psi (jap import) to 10psi don't want to go to the limit's of stock ecu just want a bit more power, 1.4 mm ?

 

thanks alot

 

Matt

Stock is around 9 PSI already - 7 PSI is safety boost.

If you are going to try boost jets then buy a boost guage so you can measure what you have - otherwise you run risks with the engine. You will get an initial spike then it will settle down a bit.....You need to make sure the spike isnt too high and the stock guage wont really help you enough.