In the spirit of perestroika and glasnost I welcome the comments here and am happy to play in the spirit of these comments.

I too find it highly frustrating with the self-righteous hypocrisy and specious argument in here in the name of goodness.

I am happy to post my experiences as and when I get them. My ambition to run a more powerful car than I have today remains. My two hours of the most powerful Scoob ever run at PE two years ago have paled into insignificance and the power level I achieved to do that is almost laughable now - 346bhp. Things move on - and many others have moved it on in ways that were entirely unexpected. All credit to them - however I find it a shame that it is not more of a collective joy of taking Subaru tuning further forward.

In the past I have been as guilty as many - however even when innocent have been frustrated by much of the behaviour here.

One point of slight correction in this post - when I commented that Steve of SMG also has a full time job - this is also build related and much of the prep work - e.g. for the blocks and liners is done as part of his day job. The assembly is done in his own time.

Rannoch

 

Well worded AlanG

David-
I think this is yet another myth ?? If you check the rotational speed of the standard turbo you will find that the compressor exducer tangential velocity is frequently above Mach 1.
The air speed however is likely to remain subsonic due to the blade camber.

Centripetal acceleration forces are likely to create a bigger challenge for hybrids fitted with larger diameter compressors.

Andy

 

doesn't the high back pressure at points where boost is being generated increase the speed of sound in the exhaust housing anyway?

would have thought the speed would also increase with temperature too.

has this been considered in peoples calculations?

how close to supersonic do the tips therefore actually get?

 

The speed of sound will increase with temperature due to decreased density (I think ?) and decrease with pressure. I'm sure there is a calculator on the net somewhere ?
Even the inducer section of the blade on a std TD04 can run supersonic and thats at atmospheric P&T

Andy

 

Andy,

would have thought speed of sound will increase with increased pressure.

especially since it is zero in a vacuum.

Speed of sound increases with density too, and attentuation decreases, hence putting your ear on the desk at school and hearing how loud it is.

Suppose it depends how much density decreases, but I would ahve thought overall, speed of sound would be higher than on compressor side.


[Edited by Adam M - 9/11/2003 3:33:46 PM]

 

Andy,

thats a myth would have done..

I got lost just after..

David

 

QED.

 

I can't see what the difference would make with the blades going supersonic? Yup there's a shockwave, but unless the blades are already on the edge of their capabilities - in which case it could just chuck'em over it - there shouldn't be much of an effect. And, apparently, the efficiency doesn't necessarily drop at this point, nor does the heat definitely increase, nor any other nasty side-effects. Shearing, which I thought could happen, doesn't need to happen either (apparently...) because of the angles used in modern exducer designs.

Hmm, having read that again, I think I could just say 'what Andy said' .

Now, spinning the thing in a vacuum (so nothing to go supersonic ) could still lead to the blades disentegrating, purely from the force of its own mass acting upon itself. But I don't think the movement of air has much to do with it.
It still doesn't explain how the blades could bend forward, though! Into the thing (air) that's producing the pressure against them? Even if the material gets far more ductile when hot (presuming it's getting over-hot in the first place), they would bend outwards. Unless there's some seriously weird dynamics going on in there Seems unlikely...

[Edited by nom - 9/11/2003 5:58:29 PM]

 

Ah - and the speed of sound increases with density, so lower temperature and/or higher pressure will raise it.
The temps at that point will be high, presumably the air at pretty much atmospheric (static pressure-wise, anyway) as the air hasn't got to the expansion bit of the turbo by then, so the speed of sound is likely to be much lower at the exducer.
That is, if that makes any difference anyway, which may or may be the case...

 

David, you & me both

Speed of sound also changes with relative humidity BTW, hence why a world class music studio will do everything to keep humidity under control.

</nerd alert>

Pure poetry that

 

the effect of humidity is tiny.

nom, the exhaust gas back pressure is not atmosperic, you can see my exhaust gas back pressure from the dyno print out of when my engine failed.

It gets to a point where it exceeds boost pressure, so is certainly not atmospheric.

 

Rannoch,

Re : "My two hours of the most powerful Scoob ever run at PE two years ago have paled into insignificance and the power level I achieved to do that is almost laughable now - 346bhp. Things move on - and many others have moved it on in ways that were entirely unexpected."

Maybe you have understated that figure? One car made that same figure way back in december 1999, almost four years ago.

http://dyno.scoobynet.co.uk/jap/arthur_dent.htm

 

Andy, had a thought about compressor blades bending the "wrong" way ... dump valves, any thoughts ?

 

All too technical for me, but what about a simple question about that power graph.
Most I have seen so far from peoples cars, have peaked at around 6500rpm. Why is that one so high at almost 7500rpm?? STI 3 cams?? Even big turbo cars, like Andys, peak around 6500rpm. few seem to even make good power after 7000rpm.

 

Andy, pretty tough wheels I think, I will be mapping a '141 in the very near so I'll let you know about surge.

cheers

bob

 

My dump valve is fed off silicone pipe from intercooler to throttle body approximately 2-3" from throttle body and recirculates back to inlet pipe approximately 2" from compressor.

Have ran two different dump valves on the car, both std issue and Forge with varying springs and shims.

At the moment i've put on the std dump valve.

Can you share your thoughts on the effect different dump valves may be having with the turbo Bob?


Alan

[Edited by AlanG - 9/11/2003 11:07:37 PM]

 

ustolemyname??stevieturbo

Can't answer for that specific car but on my own (UK99) my peak power was always at 6100rpm until the headers went on where it now peaks at 6800rpm.

Alan

 

Adam - I was thinking actually at the blades, rather than 'later on'? As the design functions by accelerating the fluid to very high velocity and then expanding it in a 'controlled' way to exchange velocity for pressure, the actual pressure at the compressor blades is very different to at any point it is actually measured.
Erm. I think that makes sense?

 

nom, think i know what you mean, except that my egbp mas measured at the end of the uppipe. So are you saying that the pressure at the tip is atmospheric and highly localised to the tips of the blades?

its irrelevant as we arent going to measure exactly whats going on, it was justa theory.

Andy,

I was saying that the speed of sound increases as density increases as the speed of sound in wood is over 1000m/s due to the ease with which the molecules transmit energy to eachother its also why the sound does not attentuate much in a solid.

 

Andy - probably quite right with the temperature/speed of sound thing - but it depends whether the fluid is contained at the same volume or pressure for the different temperatures I suspect in your stated case the volume was kept static? In which case it's a measurement of whether increasing pressure or decreasing temperature makes a bigger difference - and temperature wins! Sort of makes sense when you think about the way efficiency works (or rather doesn't ).
Basically, the closer the molecules are together, the faster the soundwave will travel. So however you can manage to get the molecules closer together will raise the sound barier.
Anyhoo, back to the point...

 

Erm, Adam, I was thinking about the cold side of the turbo rather than the hot side?

 

oh,

I thought we were talking about the damaged blades after changing the exhaust housing.

Doh.

 

It was the compressor housing that was to be changed (Adam, please pay attention )

Still disagree with this.

I suggest you don't directly compare a gas to a liquid or solid and try to draw conclusions !

Regarding gasses (which we were discussing)
Example -
Air at 20C. Density = 0.0289 g/mol. Speed of sound = 769 mph
Helium 20C. Density = 0.004 g/mol. Speed of sound = 2256 mph


Andy

[Edited by Andy.F - 9/13/2003 2:16:35 PM]

 

You Spotty-Tw*t

 

andy, you are right, but we aren't talking about switching from a high mass gas to a low mass gas.

In two gases at similar temps and pressures, the number of molecules per unit volume is the same (1 mol occupies 24 litres at r.t.p. or soemthing). The number of molecules per unit volume and the speed that they are travelling is surely what determines the number of collisions between molecules, which is directly related to the speed of sound.

I was merely saying that increasing the number of molecules per unit volume in a gas, is analagous to increasing the density in a solid, which does increase the speed of sound.

You were right, better not to mix solids and gases.

 

Especially accidentally... Can cause a right mess...

Can we get back to the informative, tuning related bits of the thread?

 

Alan, yes no problem, since the blades were "bent" the "wrong" way it occured that this could be caused by reverse pressure pulsation, if the compressor wheel heat treatment was not up to scratch then a possibility, given a recirc its less likely dependant on the spring pressure of the dump valve of course.

Probably complete twaddle but there you go. I've seen VF wheels snap blades due to that.

cheers

bob

 

Therefore with a high pressure spring in the re-circ dump valve a situation such as you mentioned could happen due to the high back pressure through the intercooler and pipework due to pressure acting on a partially opened butterfly?

Alan

 

Not been in "the zone" with this topic (just read the posts on this last page)



Simon

Edited to remove p****d rubbish. Soz
Ooo me head...


[Edited by SiHethers - 9/13/2003 8:41:49 AM]

 

Read page seven now, and need to lie down (and sleep).

Ignore the last post, if you like, as it is completely off-topic!

Simon

Edited when realised drunken ramblings aren't particularly useful

[Edited by SiHethers - 9/13/2003 8:37:09 AM]