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Guys,

Hopefully someone out there can clear up something that I've been wondering for a while. The easiest way to raise horsepower on turbocharged cars is to increase the boost pressure. So the first question is, does this mean that the more you increase boost the faster the turbo spins? I think it's safe to assume it does. We've all heard time and time again about fuel cut on Evo's where the ECU cuts fuel if it detects more boost than about 1.3 bar to save the engine being over stressed. So to get more boost pressure than 1.3 bar you uprate the internals and change the ECU and hey presto more power.

But doesn't the wastegate also release pressure to stop the turbo spinning above a certain rate and exploding? Therefore, if it does, this must mean that there is also a fixed boost pressure you can get to before the wastegate becomes the limiting factor and to get more power from here you need to uprate the turbo?

Glad to get this off much chest at long last! Just tell me if it's all b******s and put me straight :)

Dave
 
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Discussion Starter · #2 ·
Dave,
The turbine part of the turbo , that is the impeller powered by exhaust gas rotates and on the other end of the same shaft is the compressor wheel , which pressurises the inlet air 2 the engine.
The wastegate bypasses part of the exhaust gas when the required boost pressure is attained , so by reducing the amount of gas bypassed by the wastegate the turbo receives more and will spin faster , thus developing more boost pressure.
However , the wastegate is not the limiting factor , the various parts of the turbo , ie exhaust impeller , compressor wheel , wastegate etc , r a matched system.
A given turbo will be capable of developing a certain boost pressure , if made 2 exceed this by overunning , it will operate outside it's efficiency map and will develop excess heat in the boost air , the engine will then detonate .
Pressure is one thing , volume of air another, a larger turbo can pressurise a larger volume with less induced heat than a smaller unit.
Turbos r a hugely complex subject , u shud see the maths involved in designing the turbine and compressor wheels , I'm always amazed how long they can last given the operating environment.
 
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So the first question is, does this mean that the more you increase boost the faster the turbo spins?
You are quite right, increasing boost pressure means the compressor spins faster (or rather its the other way around). The output of air from a turbo compressor rises as the square of its rotational speed or in other words, double the turbo shaft spin speed and the output of air increases by a factor of 4.

But doesn't the wastegate also release pressure to stop the turbo spinning above a certain rate and exploding?
The wastegate is to control boost level. The boost control mechanism detects a 'preset' level of boost (set by the restrictors in the case of the Evo) and gradually opens the wastegate to 'waste' or bypass the exhaust gas around the turbine.

Therefore, if it does, this must mean that there is also a fixed boost pressure you can get to before the wastegate becomes the limiting factor and to get more power from here you need to uprate the turbo?
This is where it gets a bit more complicated!
I think the limiting factors on the factory turbo are to do with the internals (flow of the compressor and turbine wheel and the A/R ratios of the housings) and not necessarily the wastegate itself. There will come a point on very high power cars where an internal wastegate will not pass enough exhaust gas so an external one is required but that won't be a problem on the factory unit before the other considerations apply.

Have a look at '21st Century Performance' if you want a good grounding on turbochargers and superchargers. 44 pages and think that is only the tip of the iceberg! Hugely complex subject!
 
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Evoboy sneaked in while I was writing my response, I wouldn't have bothered if I had known ;)
Time to retire I think ;)

If anyone wants to know about modern cars, how they work, how to tune and modify them correctly then can I suggest the book '21st Century Performance' by Julian Edgar. It is now become my bible along with 'Fundamentals of Motor Vehicle Technology' and 'Hillier's Fundamentals of Automotive Electronics' both by V.A.W. Hillier.

'21st Century Performance' is available from http://www.autospeed.com http://www.whsmith.co.uk 'Fundamentals of Motor Vehicle Technology' and
'Hillier's Fundamentals of Automotive Electronics' are available from http://www.amazon.co.uk

P.S. I know Evoboy has just ordered '21st Century Performance' surely recommendation enough ;)
 
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Thanks for the answers guys. Got a few more now if you don't mind!

Evoboy,
Good point about detonation due to excess heat from the turbo. What's the most common approach from curing this problem? More efficient turbo, better cooling or perhaps raising the point at which detonation can occur within the cylinder (is this possible?).

HH6,
To what degree can the wastegate control the exhasut flow over the turbine. Is it from virtually no flow to maximum flow?


Do either of you know what pressure the standard turbo can generate before it fails or heat becomes an issue? Also, if the standard turbo pressure is 1.1 bar (almost 16psi) and normal air pressure is 14.7psi then the standard 280bhp 4G63 engine would produce 134bhp (?) plus whatever power is normally lost through driving the turbo? Not a bad performance figure for a 2 litre, 4 cylinder engine, but plenty of room for improvement.

One last question, is it normal to think about this stuff ;)
Dave
 
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Discussion Starter · #6 ·
I dont know if it's normal but I always think about it as well. So does MAXI. And a lot of other people here I guess. Probably we could call it passion or the need for boost (doesnt rime, so what?)

I am not too good at very technical stuff, but what I can tell you is that the more you cool the intake air the better. So when the turbo spins faster and generates more heat, cool the air by for example spraying water on the I/C. Not sure if that's good enough though. I was told to get a Trust intercooler with 3 layers, keeps the intake temp equal to outside temp at all times with standard and even uprated small (2835) turbo.
 
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Dave,
Yr just trying 2 save squid by not buying books , rn't u m8. ;)
Heat,
Bigger intercooler (correct term , aftercooler) , water injection , water spray on aftercooler , r all methods used 2 remove heat from the charge air.
Preventing detonation totally is not possible , any turbo engine will have a point at which it will detonate.
Raising that point beyond the problem threshold 4 a given installation is the practical gole.
High octane fuels , optimising gas flow , creating best possible combustion , accurate mixture control , thoro ecu mapping etc etc will all raise this threshold.
However, when going 4 big power ups from larger turbos and all the attendant mods required with it come other problems.
Bigger aftercoolers and larger induction pipe systems create larger inlet air system volume , this together with the larger turbo with more inertia can cause drivability problems thru boost lag.
So , I think u may c , there is no absolute solution , once u start power ups , it can b an endless cycle 2 resolve the issues as they become evident.
I don't know what the AR ratio or compressor and turbine maps r for the standard turbo , but from wot I do know and have discovered the practical max boost wud be about 1.6-1.7 bar , but it wud need a bigger aftercooler..........see wot I mean :)Boost pressure,
When boost pressure is discussed , we generally refer 2 the BOOST pressure the turbo develops OVER atmospheric.
So if u see 15 psi on yr boost gauge the ABSOLUTE pressure seen by the engine is 29.7 psi , or about 2 bar.

Potentially , these motors can produce 500 bhp with not an extreme amount of work , but IMO around 850 bhp is achievable with ultimate mods .
Want a surprise ? U may know about Tractor Pulling ? These nutters use triple compound turbos 2 develop upto 250 PSI boost.........whoa :)

Thinking about this stuff is the usual symptom of advanced petrolheadism
 
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Discussion Starter · #8 ·
Evoboy

You already mentioned this before, can you explain why the intercooler should be called aftercooler? Just curious...

On HKS packaging it says intercooler. How come they dont know the difference? Or is this just a difference between English and American English?

Maybe you could more easily post a link for me to understand the difference.

Thank you

Claudius
 
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Claudius,
The correct term has always been aftercooler , ie , a cooler AFTER the turbo compressor output.
Intercooler has just become the colloquial term in tuning dialogue.
Checking anywhere else , such as boat , aircraft , industrial , railway etc
aftercooler is always used.
This is typical of the automotive tuning world , other terms such as pot when refering 2 brake pistons are equally incorrect , shud b 6 PISTON calipers, but thats life :)
 
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Discussion Starter · #10 ·
Could be wrong but isn't an intercooler where you use the engine cooling system (water jacket) to cool down the charge? An aftercooler in my books has always been an external component that uses outside ambient air temperature to drop the charge temp.

Cheers,

Mark
 
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Discussion Starter · #11 ·
Mark,
Errhhh , don't think the charge air wud be cooled much by the engine cooling system , ideally u want charge air as much below 40 deg c as possible , engine coolant runs at around 80-95 c.
U probably mean a CHARGE COOLER , this uses it's own independant cooling water and radiator , so like an aftercooler but using water instead of air , ie air 2 water instead of air 2 air.
 
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Evoboy,

I'll have a look when I get back to work tomorrow :) . I'm sure some of our lower powered engines use water as the cooling system for the charge air cooler but I didn't think it was on a different system like my Toyota GT4 used to have.

Could be wrong - like you say, the coolant is at a much higher temp than you're trying to bring charged air down to on the EVO's small 2 litre engines. At work, our engines (4 litres upwards) are running a tad more boost than the EVO's ;) so 80 degrees might be a good target for the intake air - I'll find out and let you know.
 
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80 degrees intake temp??? Are you talking Fahrenheit?? :)

I heard aftercoolers (see I actually learn stuff here) that use water to cool the air down are not as good as air to air aftercoolers.
 
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RS Pilot is correct but I have only heard that some turbo diesel engines fitted to lorries use the engine coolant system for the cooling the Water/Air intercooler. Obviously its not very efficient. I don't know of any petrol engines that take this approach or why they would want to.

To put the cat amongst the pigeons Intercoolers should be called Charge Air Coolers but that is being pedantic (15) ;) I think we all know what is meant by the term 'Intercooler' so that is what I will use.

Water/Air intercooling can be more efficient than Air/Air Intercooling as the specific heat capacity of water is about 4 times better than air but it is not widely used probably due to the extra expense involved of the plumbing/pumps and control systems etc. Lotus Esprit Turbo, Subaru Legacy, Toyota Celica GT4 to name but a few all have water/air intercoolers and all have a seperate water supply/radiator for the system.

There are designs of intercooler that have ice, air-conditioning or even nitous oxide sprays as the cooling medium but that is getting way too exotic.

Anyway back to answering the question that was asked of me by DaveG (Evoboy couldn't you have answered it ;)
To what degree can the wastegate control the exhaust flow over the turbine. Is it from virtually no flow to maximum flow?

To answer this it is best to show you what a typical internal wastegate looks like:



When a 'preset' level of boost pressure is sensed by the wastegate actuator it starts to open the wastegate swing valve (the crank for which is on the left hand side of the picture). As soon as the wastegate opens the turbine will start to slow, the compressor thats attached to the turbine will also slow and boost pressure reduces. As the boost pressure reduces the wastegate actuator will sense it and start to close the wastegate swing valve. It will reach an equilibrium where the wastegate is open just enough to balance the amount of exhaust gas passing through the turbine for the given boost required. In answer to your question, the wastegate will pass all exhaust gas through the turbine when shut but only some of the gas when openned. There is never a point where there is no gas going through the turbine.
 
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oooo nice piccy hh :)
to add my 2 penneth worth (and that's about all it is worth) I think that a turbo bigger than you need is worse than a smaller turbo that will give lower peak power. This is because the too large turbo will have greater lag and less boost (therefore less power) at lower rpms for whilst we all like talking about peak power it's the spread of power which is much more use, particularly on the road (unless you have zillions of gears). The 21C performnace book shows this well with a turbo normally fitted to a 2 l motor having been fitted to a 3 l engine. Peak power was not that impressive but the spread of power was awesome. Hence I personally would not fit a larger turbo unless i was going for major ( and amp;gt;450hp) top end power.
 
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Discussion Starter · #16 ·
Oil Goon,
Yep quite agree and thats why the likes of the Skyline, Supra etc have twin turbos in order to get better lower end response combined with efficient performance at high boost. Once you start looking into turbos and turbo matching then you soon realise what a vastily complicated subject it is.
I think the ultimate evolution of turbocharging was acheived by the Lancia Delta S4 which was not only turbocharged but also supercharged to eliminate turbo lag completely! Highly complicated and expensive to set up but they managed it, never to be repeated though by any manufacturer.
 
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Thanks for the responses guys. I can sleep a lot easier now that I know all that stuff. :)

Seems to me that there could be a lot gained from a more efficient 'after' cooler? I know some of the scooby owners have benefited hugely from the use of more efficient, usually front mounted, intercoolers. I'm surprised there isn't more use of better intercooling on Evo's or is the standard intercooler just good?

Dave

P.S. The book is on order so HH6 you should be getting the commission soon :)
 
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Discussion Starter · #18 ·
sorry sir,but intercooler is technically correct,as its between the turbo and engine.are you getting confused with pre-heating old side valve engines ?

also bear in mind that a pump (in this case the turbo) creates flow,
restriction to flow gives pressure.

you might find this site educational !

www.turbofast.com.au/TFmatch.html
 
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Discussion Starter · #19 ·
HH6

Giwasa in France made an engine conversion on a Lancer Evolution with a compressor and a bigger turbo, 530 bhp. I wanted to get them to do that on mine but I was afraid not to be able to make it to the closest petrol station. I would, however, expect throttle response to be satisfying (provided your slicks can cope with the power).

Regards
 
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Pluto,
Sorry m8 , u r wrong!
With single turbo installations , aftercooler is the correct term.
On double compound turbo installations ie 1 turbo blowing into another,
where a cooler is used BETWEEN the turbos , that cooler is refered 2 as an INTERCOOLER , then the AFTERCOOLER is the one after the final turbo stage into the engine.
This also applies 2 turbo supercharged installations.
So there :)
 
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