Turbo efficiency question

[dunc]

In 200sx related as we all have turbo's

Basically, we all know that more boost = more power and more stress on the engine. But it can't just be down to boost pressure as a T25@15psi is a different kettle of fish to a T28@15psi so it must be down to the volume of air forced into the cylinders as apposed to the actual pressure. Is this right?

So if you were wanting to limit the stress on an engine to say the amount of a T28@13psi what would this be equivilent to on a T25? Around 15-16psi at a guess? How do you work out how one compairs to another?

Thanks in advance for any replies,

Duncan.

[Alany]

I would have thought 15psi in one turbo is the same 15psi in another turbo, after all it is still 15psi.

The difference between the two would be that the T25 would have to work alot harder to make that 15psi so heating the air alot more where as a larger turbo would be spinning alot more slowly to make the same psi hence lower charge temps.

Im not sure if there is a chart to comapare size to charge temps at certain psi as there are so many variables.

[Phil LS]

Nope 15 psi on one turbo is not the same as 15 psi on another turbo. It all depends on the size of the compressor wheel, compressor housing etc.

And the thing that determines how much power your engine can develop is the flow of air that it can draw.

[SM]

The differance is the amount of backpressure the turbine gives to create the boost.

There are 2 ways in which a turbo isnt "free power" one is the back-pressure from the turbine, the other is having to have a lower CR and therefore a more inefficient chamber.

[dunc]

And the thing that determines how much power your engine can develop is the flow of air that it can draw.

Does anyone know how to compair/calculate the differences? I'm interested in the difference betweena small turbo running high bosst and a larger turbo running lower boost. Also the fact that the small turbo will loose effeciency due to spinning too fast and the lardger turbo will loose effeciency due to more energy to spool.

Cheers,

Dunc.

[Phil LS]

Does anyone know how to compair/calculate the differences? I'm interested in the difference betweena small turbo running high bosst and a larger turbo running lower boost. Also the fact that the small turbo will loose effeciency due to spinning too fast and the lardger turbo will loose effeciency due to more energy to spool.

Cheers,

Dunc.

I think for a rough estimated guide is every 10 lbs/min that a turbo flows is 100hp. Only a rough guide though.

For example one make of GT3037S can flow 55lb/min of air at x psi, so at x psi the engine could make about 550bhp.

Ish

[Alany]

Nope 15 psi on one turbo is not the same as 15 psi on another turbo. It all depends on the size of the compressor wheel, compressor housing etc.

And the thing that determines how much power your engine can develop is the flow of air that it can draw.

How can 15psi be any different in what ever form ?? Yes the compressor wheels may be different and what not and the turbo blades will be spinning at different speeds, obviously the bigger one will spin more slowly. How on earth can 15psi be different from 15psi, I can grant you that to get that pressure will be different, but the end pressure will still be the same all be it more heated ?

edit: answered my own question. A cooler 15psi charge has more denser charge hence more power despite the same pressure

[SM]

edit: answered my own question. A cooler 15psi charge has more denser charge hence more power despite the same pressure

Its not heat .. its VOLUMETRIC EFFICEINCY, or VE of the engine configuration, and with the same block/head just different turbos, same boost, the VE will change dependent on backpressure from the turbine. on a T28 it will be spinning slower @ 15psi than on a T25 with a smaller compressor wheel.

[stevec]

How can 15psi be any different in what ever form ?? Yes the compressor wheels may be different and what not and the turbo blades will be spinning at different speeds, obviously the bigger one will spin more slowly. How on earth can 15psi be different from 15psi, I can grant you that to get that pressure will be different, but the end pressure will still be the same all be it more heated ?

edit: answered my own question. A cooler 15psi charge has more denser charge hence more power despite the same pressure

It is that 15psi in a barrell would be a different VOLUME of air than 15psi in a coke bottle.

the pressure may be the same but the volume isn't.

the T28 is a bigger turbo so it flows more air for the same pressure. I have no idea how to work it out though.

there must be a way as the engine knows from the air flow meter how much air is going in.

perhaps someone somewhere has hooked up the AFM to something to log the differences between the two tubbies ?? finding it would be nigh on impossible though.

cue the turbo gurus.... anyone

/steve

[Jonny]

It is that 15psi in a barrell would be a different VOLUME of air than 15psi in a coke bottle.

the pressure may be the same but the volume isn't.

But surely, if you are measuring pressure at the manifold, then the volume is going to be the same, as the pressure is the same (and I guess you could assume that as long as you have a good enough intercooler the temp will be the same for both tubbies).

The ability to shift larger amounts of air would only take effect when you are flat out at high revs, so the turbo can't shift enough air to keep the pressure constant (which is I guess why the boost starts to drop for a T25 when the car is revving)

Maybe it's worth asking Papa or one of the other guru's for a proper explanation, cos I'd like to know as well

[Stainy]

I'm with Jonboy on the theory......it's the same i/c and plumbing and it's measured at the same place on the plenum. Therefore 15psi is 15 psi no matter what, surely.

For example, you inflate a tyre with a foot pump to 30 psi. Now inflate the same tyre to 30 psi using a bicycle pump....it's the same! Surely this is the same for the 15 psi measured in the plenum

[SM]

There is a good explantion somewhere, a turbo guru copied from a Cossie site, saying how the differances between T3's for Cossies were not the same power at the same psi due to the VE.

Cant remember where ..

Same Q
http://www.sxoc.com/vbb/showthread.php?t=60683
http://www.sxoc.co.uk/vbb/showthread...ght=Volumetric

FOUND IT !!!
Lets go back right the way to basics…

Why is it that a turbocharger is universally recognised as the ONLY form of forced induction that COSTS power?

Well lets look at this shall we?
We have a decently specced engine, say an Rs2000 I4 and we are making 170bhp through our otherwise std setup with some head mods and cams.

To make this power we are utilising the air pumping ability of our 4 x 500cc cylinders.

They are drawing in enough air and fuel at the correct ratio and importantly, expelling it again to produce this power. This ability incidentally is related to its volumetric efficiency which i will touch briefly later.

The I4 is making its 170bhp with its nice, well-designed STANDARD 4-2-1 exhaust system.

Lets now redesign it in the great RsBB fashion lets remove the nice exhaust system and stick a tiny pathetic straw sized .48A/R turbine housing on it with a closed wastegate and the turbocharger actually welded tight so it cant spin.

Does ANYONE on this BB think this engine will now make 170bhp? It will be lucky to make 120bhp.

So whats happened?

WE HAVE INCREASED PUMPING LOSSES DRAMATICALLY.
This is the biggest issue. The reason this costs us power, is a great proportion of the energy produced from the power stroke of 1 cylinder burning our nicely presented charge of fuel and air is now wasted trying to push the spent gas out of the previously active cylinders exhaust valve.

We now have some detrimental knock on effects:
The friction on components caused by this pumping loss now adds heat to our engine too. This heat was part of our powerstrokes energy.

This pumping loss has also caused the overlap events effective scavenge volume to drop massively as the nice, meticulously calculated pulsation vacuum that designers spent hundreds of hours to create has now been exchanged for BACKPRESSURE.

This backpressure has now also decreased the amount of air the exhaust pulse drew through the inlet valve at overlap when piston speed was at its lowest so maximum cylinder fill has reduced, down goes VE.. Things are looking bad for our power curve now.


So conversely, as we now have LESS airflow on overlap, we are going to start dumping heat through our exhaust seat and port and are heating our soft alloy head up.

WHY? Very simple Mike ,
Designers use scavenging on overlap as a very simple and very effective way of cooling valve seats, guides and ports.

How? Also very simple Phil
As we reach overlap in our cam timing event we have both COLD inlet and HOT exhaust valve open, this gives the exhaust valve and relative components time to be cooling from their grievous job only moments ago of shifting a mass of immensely hot air through its system so it’s a great relief to sit in some nice cold flowing air for a second and transfer a bit of excess heat

Our fancy new engine / design aint looking too hot is it? (well actually,its getting damn hot)

So,
I hope this large and simply worded explanation helps you to understand that a turbo DEFINATELY costs power by its very prescense on your engine and also why it does so?

Ok? Good!!

So what the hells that got to do with todays BB argument? Oh yeah, sorry........

Exhaust backpressure caused by the turbine housing assemblys restriction is our key element between T3 and T4.

But lets deal with the delivery of our air first and we will use the good old YB for our examples.

A T4 produces far more VOLUME of air at a given pressure from its HOUSING than a T3.THATS UNIVERSALLY AGREED.

Now if we quickly use this bizare volume of air example supplied by Mike earlier...


quote:
--------------------------------------------------------------------------------
Originally Posted by Mike Rainbird
Look at it this (simple way so you understand ), imagine a 1 ft diameter balloon inflated to 15psi. Then imagine a 2ft ballon inflated to 15psi - are you trying to tell me that both have the sane volume of air in them?
--------------------------------------------------------------------------------






That balloon is actually bigger so yes it holds more volume Mike, but this is not applicable to an engine because even if we take all the inlet valves out and pressurise the system with our turbochargers, we are still presenting each turbocharger with the same volume to fill, lets say 2000cc for the cylinders and 1000cc for the intake of the head and the plenum / hoses.

So we have a 3000cc volume to pressurise with air. This does not change unless the engine begins to operate. An engine will only generate more power by shifting more air. SIMPLE.

The engine will only shift more air if we do one of the following:

Improve the airs route into the head
Increases the pressure we push it in with
Improve the mapping
Improve the volumetric efficiency.

So,
We still know a T4 WILL make more power than a T3 at the same boost so why is that?

We dont appear to have done any of the above mods do we?
The head hasnt been ported.
We are running the same boost.
We aint touched the chip cos PHIL cant remember what spec it was for
So have we changed the engines Ve?

Good question, and back to turbos.
A T3 50trim with our desired pressure ratio of 2.4 (14.7psi inlet +20psi outlet divided by 14.7psi inlet) will be spinning at 134,000 rpm with a compressor efficiency of 70%.

A T4 60 trim with our desired pressure ratio of 2.4 (14.7psi inlet +20psi outlet divided by 14.7psi inlet) will be spinning at only 97,000 rpm with a compressor efficiency of 82%.

Now lest go to the turbine housing.
The T4 P trim wheel flows a lot more air than the Std T3 trim rear wheel. (they are all the same as std on T3) but it conversely takes more to spin it to speed.

We now have an exhaust backpressure IMPROVEMENT due to a better flowing rear wheel!!!!!!!

Secondly,
We now have a wastegate that will open much sooner and much wider than it would on the T3 as less exhaust volume is required to spin the turbine as we have a 37,000 rpm improvement in efficiency at our 20psi.

HEY, we have another exhaust backpressure IMPROVEMENT.

If our T4 is using a bigger housing, and it IS if compared to a T3 we have another exhaust backpressure IMPROVEMENT!!

So lets go to boost at the intake:
Now our exhaust backpressure is reduced, our cylinders demand for air has increased. We have overlap efficiency gains, we have thermal efficiency gains so we can suck more air and we we can suck it with a greater pull because we are actually reving more freely so our peak piston velocity has increased.

So we are CONSUMING more air and this T4 can supply it for fun But we aren’t making more power because the T4 pumped more air at 20psi.

We are making more power because this turbo improved the volumetric efficiency of our engine mainly through backpressure reductions and an improvement in outlet temperatures at the compressor outlet due to Adiabatic Efficiency which we may or may not discuss later.

Goodnight folks,
If anyone wants to know what the compressor maps do and how to decipher the turbo specs, let me know when ive slept

[dunc]

I think for a rough estimated guide is every 10 lbs/min that a turbo flows is 100hp. Only a rough guide though.

Does anyone know how many lbs/min the T25 and T28 flows?

Cheers,

Dunc.

[Stainy]

I actually follow this now!! (I think)

The 15psi on a T25 is artificially high (to some extent) due to the restriction of flow caused by it's very presence.

The same 15psi on a T28 is less restricted and therefore is flowing more air 'through' the engine = better power.

I think.....................

[stevec]

seems to me that the gains are made by the bottleneck that is the exhaust turbine. by making it more efficient you actually free up "space" on the outlet by getting air out faster, therefore you can get air in faster, and increase your power.

that is the best explaination I have ever read on this.. and the only one. you learn something new everyday.


so to take that a step further it must be a fine balance between getting a good outflow from the exhaust ports to actually turning the turbine shaft on the tubbie.

I also agree with what you said RE at the point you were talking about 1.2 bar being 1.2 bar. good thread guys

[Alany]

Ahh I understand now, I also now understand what the guy over at fresh alloy was talking about when he was talking about exhaust and turbine wheel size balancing.

[Papa Lazarou]

So if you were wanting to limit the stress on an engine to say the amount of a T28@13psi what would this be equivilent to on a T25? Around 15-16psi at a guess? How do you work out how one compairs to another?

Stress on the engine is difficult to measure really. As 15-16 psi on a T25 is operating beyond its limits of efficiency at higher RPM, I would say that would be more stressful on the engine than the T28 at 13psi. Even though power output would be less with the T25. The thing would basically be overspinning and blowing very hot air, as it struggles to keep up with the engines air demands to maintain 16psi. This in turn increases charge temp and hence risk of detonation increases.

Say if you looked at a different example. Say someone running a T28 at 25 psi and a T04 at 25 psi, both on a modded CA. The T28 is at this point not very efficient, so its heating up the intake air pretty well. At the same time, assuming its a normal T28 the chances are the small restrictive turbine housing and wheel are causing very high pressures in the exhaust manifold, which will cause the power levels to drop off. Read the bit in the post above about the effect of backpressure on exhaust gas scavaging and the effect this would have on temperatures in the cylinder head. Your engine is in danger of melting something at this point!!

With the T04 on the other hand, while it would certainly be operating far more efficiently at this boost level, and exhaust manifold pressure would be much lower, temps would be lower, VE and power would be higher, I reckon there would be other forces acting which would be far greater than with your T28... The torque at high rpm, when the T28 starts to strangle itself, would be greater (which obviously results in more power) but which would put more load on the rods and rod bolts, big ends, crank etc. Also due to the nature of the larger turbo it would be far more likely that you keep it revving to that red line, where the benefits of doing so on the small turbo are less. High rpm = more stress.

Just a few random thoughts anyway.

[dunc]

Sounds like thats the idea of running my T25 @ 18psi out the window then

[BARKY]

Very indepth posts!

What you need are compressor maps to determine compressor efficiencies and like wise for turbine efficiencies.

The extra stress imposed on the engine is much less than you think as the engine RPM is not going to be significantly increased.

Its the cylinder pressures that increases which causes main problems such as blown head gaskets, overheating, knocking etc etc.

Engines create power with air mass, the concept of turbocharging is basically to increase this mass of air. Which in turn allows you to feed in extra fuel and hey presto increase power ouput.

However increasing air mass by compressing it also generates heat therefore you want a compressor stage that is efficient i.e. air mass with as little heat as possible generated.

Its not true to assume 15PSI for a small turbo produces same power as 15PSI for a large turbo in the same engine, yes the pressure is the same but the large turbo at 15PSI would have much more air mass flow. Remember it is air mass we are interested in to gain power not the boost pressure!

The engine uses a fixed amount of air to generate a particular power output, therefore a small turbo may supply this at 15PSI, while a larger turbo will supply it at less pressure lets say 10PSI.

However running at 15PSI on the small turbo maybe at the limit of its compressing efficiency, therefore increasing boost to try and get more flow basically doesn't happen as more heat is generated and the air mass does not increase. The Large turbo on the other hand may run upto 25PSI before the efficiency is reduced therefore supplying lots of air mass efficiently upto 25PSI.

Turbo boost pressure is used as a unit of measurement because it allows easy monitoring of what is happening with the turbo without resorting to complicated flow measuring devices. In reality turbo boost pressure all links and relates back to air mass flow, you can find out what the flow rate at a particular boost pressure by using a compressor map for the turbo.

I could carry on but I'm working, on turbochargers as it happens! Must go now!