Porsche Enthusiasts Club Forum

I know i nipped home around 9am Dave wanted the camera for the cam cover pics for Scho. But if it was around 8:30 Dave was driving, but you should tell them to wave especially now i know what they drive.

How far down/up are they? (if thats not being too cheeky )

Coming up the hill there's a row of five old terraced houses on the left, they're in there, just before the bit of land that is their car park near the retirement home.

I know where you mean, go past there everyday whether it be for work or otherwise. but get them to flash or wave i dont mind, not as if you can miss my baby, think she is the only white 44 if not the only 44 in congleton. Have still yet to see another one (Not including yours Scho yours doesnt live around here just being mended )

turns out i'ma t1t!

i wasn't that far out but after plenty of research it appears that......

a turbo needs heat as well as pressure. the more heat you can get to the turbine the better but this has to be coupled with pressure but heat aids pressure so.. i'm big enough and man enough to admit and accept i was wrong!

sorry all

but the turbo needs to be closer so it gets more of the hot stuff <-- this i wont back down on.

should get enough pressure after a curry to get the turbine spinning, but isn't it a waste trap ?

Right, a few things to pick up...

Firstly I haven't stopped posting for any reason other than it was night time and then I had meetings first thing when I got in to work.

Secondly I take exception to the suggestion that I need apologise for anything. I have taken care not to throw insults nor be personally offensive and I have merely stated facts. That the facts are the unpalatable truth that someone has a very poor understanding of a subject they think they are expert in does not detract from the fact it is true.

Thirdly I could just drop this, but unless people using this forum are happy for it to be filled with misinformation and untruth I think it is important to get the facts correct.

Now I assume as I'm on his ignore list I'm no longer talking to the one poster who really needs to understand, but for the sake of anyone else who is trying to learn something here the facts are as follows:

A turbocharger uses otherwise wasted energy in the exhaust gas to pressurise the inlet charge in an engine, thus effectively increasing its displacement. The Wikipedia quote above is great as far as it goes, but it's over-simplified such taht what it fails to convey, and what the OP fails to grasp or be educated on, is that the energy in the exhaust gas is mainly present in the form of heat, and to a lesser extent flow rate, and that the pressure present is merely a function of flow through a restriction (the turbocharger exhaust impeller).

In other words if the exhaust gas was cold a turbocharger would not be viable as there is insufficient energy from flow alone to make the system work. This is why when the engine is not under load (i.e. in neutral) there is no boost produced regardless of engine revs.

A very high level overview of the reason (as I understand it - while I know heat is required the following is my reasoning as to why that is the case and not something I have been told by someone who fully understand the physics involved) is that just as compressing a gas creates heat so the drop in pressure of the exhaust gas in the header compared with that in the exhaust pipe causes a cooling effect. The thermal energy "lost" in this cooling is converted to kinetic energy which drives the impeller. Clearly the more stored thermal energy present in the form of higher temperature in the higher pressure gas the more energy is available to be converted to drive the turbine.

The crux of the initial misconception that I felt needed to be corrected is that a turbocharger runs purely on "pressure" in the headers. Note here that Pauly and Jim have both confirmed that heat is part of the requirement also, so I am not alone in understanding how this works.

In the ensuing discussion several further statements have been made that could not be more wrong.

For example it was stated that cooling the crossover pipe temperature before the exhaust gas reaches the turbo is "not a bad thing". That could not be further from the truth as doing so would be to waste valuable thermal energy that you want to use to drive the turbocharger.

In a similar vein it was stated that lagging the manifold is done to prevent surrounding components going brittle from the heat present. That might be a nice by-product, but why would you lag something that is hot when you believe it is not a bad thing to cool it down? This is one of a number of inconsistencies in the argument posted against my description of how things really work. In truth there is no logical reason why you would and the reason to lag and/or thermal coat headers and in the case of the 944T the crossover pipe is to keep that valuable thermal energy in the pipe until it reaches the turbo where it can be of benefit.

Another misleading assertion is that pressure drops dramatically in an open-ended pipe. This is incorrect for a number of reasons, most significantly that pressure in an open ended system is somewhat abstract and the consideration is flow. Regardless it is irrelevant to the discussion as the part of the turbocharging system under discussion lies between the exhaust ports and the turbocharger impeller, which is not open ended as the turbocharger presents a restriction. The system only becomes open ended (relatively at least) after the gas has passed through the turbocharger and when we are no longer interested in it. The pressure differential between the exhaust ports and the turbo exhaust inlet I would contend is negligible and at any rate far from significant to the extent that almost all of the pressure has been lost as was suggested. This I would suggest is backed up by Brett's experience of a rear-mounted turbocharger.

It has also been stated that turbochargers work most efficiently when they are cold. For the inlet side (commonly called the cold side) that is true, but the suggestion was that it is also the case for the exhaust or hot side, and again that is the opposite of the truth - the heat should be retained in the housing and outside the impeller. Once through and into the outlet the temperature is largely irrelevant. As an aside it is not possible to run a turbocharger cold as it will heat up very, very quickly due to a combination of the temperature of the gases passing through it on the hot side and the temperature it produces in the cold side as a by product of compressing air, which I mention purely as it highlights that some figures stated are nothing but pie in the sky fabrication.

Moving on there was a statement made that an intercooler is used to reduce the temperature of the compressed inlet charge to stop the fuel evapourating before combustion. I think virtually everyone knows that the purpose of cooling the inlet charge is to increase its density and nothing to do with fuel or the evapouration thereof.

Some other statements were made relating to ancillary components such as wastegates, boost controllers and bleed valves. I find it difficult to even follow what was being said in some of these cases, but to dexcribe what these components do in order:

A wastegate is used to allow a proportion (from none to most) of the exhaust gas to bypass the turbocharger. It is needed as that is the only means of modulating the pressure created in the inlet of the engine; were all of the exhaust gasses to be passed through the turbocharger it would spin faster and faster as it would force more and more air into the cylinders resulting in more energy in the exhaust gasses which in turn would produce more pressure in the inlet. It's a viscious cycle in other words and would eventually lead to a component failure at some point in the system. Note that it is not a perpetual motion machine as the energy that drives it is derived from chemical energy in the fuel/air mix that is converted to thermal and kinetic energy during combustion.

A boost controller is simply a device that controls the wastegate. In basic terms it senses the pressure in the inlet side of the engine and when it reaches a preset level it causes the wastegate to open thus allowing some of the exhaust gas to bypass the turbocharger and preventing the pressure in the inlet rising still further. A boost controller can be a simple valve or a sensor connected to an electronic device which actively manages the pressure in the inlet to match other conditions.

A bleed valve is a crude means of gaining extra boost. It works by introducing a leak between the inlet system and the boost controller. This has the effect of delivering a reduced pressure signal to the boost controller compared with the actual pressure in the inlet which causes the boost controller to actuate the wastegate later than it otherwise would.

I'm sorry that this has become so wordy, but I feel quite strongly that it is important to be factually correct in discussions such as this. I am quite happy to continue discussion on the subject as it interests me and I would be keen to learn more, or indeed be challenged if anyone thinks I have it wrong.

The above was cross posted - and (if yuo are reading this) I appreciate it that yuo are able to admit you were wrong.

I don't diasgree in general terms that the turbo is best placed near the exhaust ports, but specific to the 944T I don't think it's a better solution than the one Porsche employed and I also don't believe it's the source of the majority of lag the cars have a reputation for.

fen,
after admitting i was wrong i thought it right to remove you from my foe list.

lastnight i got at best frustrated. yes i was adamant that turbo's drive of pure pressure and the cooler they was the better....i had this blown clean out of the water by asking the question in another forum (group of friends) who i know know...if you get my meaning and they was on your band wagon but in simpler terms and NOT trying to belittle me. this is simply because upto 3 hours ago if you and i was stood side by side in a thunderstorm and you stated that fact (that we was in a thunderstorm) i wouldn't have believed a word you said. because you have accused me of begging for work. dude i got cars coming out of my ears. i'm doing scho's car because he needs it asap and scho has seen how busy i am. so your statement is out of line and very degrading.

to simplify what you have said regarding turbo's and rather than post up an afternoons worth of key bashing is this...

a turbo relies on heat coupled with the pressure of the spent exhaust gases, the heat aids the flow, this is becuase heat expands everything including air forcing the gases out of its only exit...turbine.

oh and a turbo will build pressure without a gear being engaged but only on WOT. you are correct that a turbo will not spool on a constant held 4,500 rpm in neutral but will boost when blipped.

I'm glad you got your head round it and respect for coming straight out and saying you were wrong.

On the other hand if you want me to apologise for a comment which I made not only at a time when you were insisting black was white, but also after telling me I was an illiterate sub-human who's entire post count was worthless, incorrect or irrelevant, then you will be waiting for a long time.

Don't get me wrong - it's all water off a duck's back to me and I genuinely don't give two hoots what you called me, nor indeed what you think of me, but what was said was said and I have no remorse for the way I have conducted myself in this or any other thread on this forum.

fair do's

water under the bridge now i hope


i hate getting into arguments and like most when i think i'm right i'm adamant but when i find out i was wrong i'm man enough to stand up and admit it.

All cool on my side. I try to do the same and I meant it when I said respect for fronting up - I know how it feels.

for what its worth - your argument actually taught me a fair bit about how turbos work

so thanks - to both of you

Hi

Wow I know a lot more about Turbos now thanks to Fen....

Extremely interesting, I always wondered why racers lagged turbos. I "thought" it was to keep the heat AWAY from other things, I now know its to keep the heat IN....

Personally I wont be getting one as I think Nitrous is a lot easier and produces more power per £.


But a great post none the less

Thanks Fen...

All the best Brett

Phew - feel the love once more

The most important thing with Turbos is to TURN THE BOOST UP

i was quite confident playing with boost and rebuilding turbo's but now that i TRULY understand them it makes them a little more interesting