davey

The actual amount of air molecules you can get into a cylinder of a set volume at a set pressure is dependent on the temperature of those molecules.

The colder their temperature the more you can squeeze in for the combustion process. Now consider the volumetric efficiency (VE) of the turbo, all turbos have a sweet spot and there is a zone where the turbo is most efficient. The td04 on a 2.0 litre at 1.3bar is not as efficient as a GT30 in the same conditions compressing the air to 1.3bar.

So in my example the inlet temps coming from a GT30 will be less than a td04, hence more air to burn hence more power.

Thats my simplistic take on it, I'm sure neether turbos are at their max VE in my example

 

Lots of bad info here and confusion - but it is a confusing subject..."edited to say agree with Jay - his post was not up when I started writing"

If the pressure is the same in the manifold through the rev range then the size of the turbo doesn't make any difference, except for, as has been said, the temperature of the air.

If you run a small turbo at very high speeds to attain 15psi through the rev range then the turbo itself gets very very hot which adds significantly to the charge temperature, which means less available oxygen and more timing retard.

A big turbo at the same psi would only produce more power because of the cooler charge temperature.

The main reason you run a bigger turbo though is to get MORE boost pressure, especially at the top end of the rev range - which equates to a lot more power.

A TD04 will run 16psi but I think you will find that the boost drops off significantly once past 5000 rpm, especially in first and second gear (less time for turbo to spin up).

The same car running a TD05 set to 16psi will be a lot quicker because it will easily hold that boost to the redline AND the charge temperature will be l slightly less. It will suffer from a lot more turbo lag though.

The max torque between these two cars may not be much different but the TD05 car will hold it to much higher in the rev range

 

There is a little more to performance V turbo size than just air temp.

Even if the air temps, and boost appear identical, the bigger turbo would still out perform the smaller turbo on the same engine, like, for like.

In reality, the smaller turbo's whilst appearing to hold boost, don't, and struggle to fill the cylinders to anywhere near 100% VE.

If you've ever seen what happens to a boost gauge if it isn't damped, and how it fluctuates, this will give you a good idea of what's really happening.

The same is true for the fuel system, and why some people think they need bigger injectors, than the power they have would actually suggest, assuming the mapped AFR is appropriate.


Mark.

 

IMHO most of the heat is caused by the act of compression its self, air has resistance so when you "squash" it down the air particles generate heat as they move around.

And that makes zero sense, air pressure is bassed on a volume of air in a volume of space so you cant have MORE air at the same pressure.. Ofcourse lower temp. air is more dense but the charge temp's shouldnt vary that much.

 

If compressing air makes it hot, why does the air out of an compressed air line (in a garage or factory) feel very cold and the air line its self feel cold???

 

Because the air is expanding into the tyre. Touch the compressor end and see how warm that is

 

Air gets hot when compressed, and cold when released from a compressed state.

Davey,

That's fine if that space is static. Introduce spaces that change over 100 times per second, cams, valves, exhaust system, and all the other things that effect VE, and it's a different story.

You will even find a difference in boost pressure if you measure boost at the turbo exit, to boost at the throttle body, let alone what you think you're getting into the cylinders.


Mark.

 

air does get hot when compressed, and the intercooler is designed to try and cool it as much as possible.

the heating of the compressed air in the turbo heats the turbo itself, and if the turbo is spinning very fast (because it is small) then the bearings create a shed load of heat. Thus the turbo gets very very hot and may even glow! this extra heat does effect the temperature of the air being forced into the manifold.

Cool air enters induction, and then
it is warmed by very hot turbo components (housing, blades etc), and then
it is heated even furhter by act of compression, then
it is cooled as much as possible by IC, and finally
it enters manifold.

The heat from the turbo components on a small turbo running too much boost to be efficient is as significant a problem as the heat created by the compression itself.

 

Don't forget what actually drives the turbo ! EGT's will easily reach 700-900C on boost, and there is a fair amount of heat transfer through the turbo.


Mark.

 

And?? Those are set variables if we are talking about the same engine..

 

Davey,

I tried to explain using what happens to an un-damped boost gauge as an example. I guess if you haven't seen one, it doesn't help.

Maybe someone else can explain it to you better, although I'm not certain it will help.

The fact remains, that a bigger turbo will produce more power, at the same boost, as a smaller turbo on the same engine.


Mark.

 

Yes it does - the 2 turbos you are talking about have totally different size exhaust housings - the smaller will create much more exhaust gas back pressure whereas the larger will offer much less. This will make a difference to how much the engine will flow.

Cheers

 

VE is VOLUMETRIC efficiency, so it is completely independant of changes in density caused by turbo compressor and intercooler efficiency.

The exhaust side of the turbo has the biggest effect on VE for a given inlet manifold pressure, you can pretty much ignore the compressor side. If you assume the cams and head flow are constant, then increasing back pressure caused by restrictive housing, turbine and wastegate (ie primarily closed instead of primarily open) will have a huge effect on the gas trying to get out the cylinder.

A larger turbo works in 2 ways to increase power. Increased charge density from increased compressor efficiency, and reduced back pressure from more efficient turbine/housing combo, and also allowing the wastegate to open because you're not trying to drive the turbo beyond it's efficient operating window.

Looking at the bigger picture, then head flow in and out of cylinder, exhaust flow to and from the turbo, charge air flow in and out of the turbo, valve opening (ie cam timing) all have effects that will vary with boost and RPM.

Paul

 

Thanks very much for the info guys, very imformative.

We will see at the dyno day tomorrow.

 

Thanks Delboy and Paul, I had never considered backpressure differences with different sized turbos. I has often heard people talking about marger turbos 'flowing more' but never really understood the whole story - you learn something everyday!

 

Hmmm I have to say I had forgotten to add the increased OUTPUT efficiency (exhaust flow) (as mentioned by ZP) into the equation.

 

Davey,

Paul did explain it better than I did, but he explains about the bigger turbo reducing "back pressure", which increases the "Volumetric efficiency" of the engine.

An engines "Volumetric efficiency" being the % of 100% you can fill the cylinders with air. An engines VE will vary across the RPM range.

It is possible to get 100% VE (cylinder fill) but it's normally limited to a small rpm range.

Mark.

 

VE=MAP/EMAP

This really sums it all up for practical purposes. Obviously this is a ratio, I don't need to tell anyone here how to convert this to a percentage

 

I am pretty sure I have seen about 126% VE if the MAF sensor is to be believed.

 

Paul,

I was going to say "100% or more", but decided against it, on the basis of having to explain how you can get over 100% VE, when I didn't do a very good job of explaining why most struggle to get close to 100%.......

I'll let you do that


Mark.

 

Interestingly (perhaps) I have observed that the compressor efficiency has a major influence on the power generated at a particular boost pressure, every bit as much as the exhaust turbine size/efficiency.
During back to back tests on my TD04 hybrid Vs a TD05-16G, the TD05 couldn't match the midrange torque of the TD04 hybrid at the same boost of 1.4 bar The only explanation i have for this is that the 04 hybrid compressor was working at a higher efficiency and as such requiring less exhaust energy (read as flow limiting backpressure) to maintain the boost level.
Ultimately the 05-16G made more power and torque once the boost was cranked up in excess of the level that could be efficiently supported by the 04 hybrid.
Bigger is not always better !

Andy

 

I found something similar when changing from standard td04 at 1.3bar to VF34 at 1.3 bar. The td04 made better midrange torque, but lost out at the topend. I wondered if a relatively higher EGBP in the midrange maybe helped with scavenging (or should it be reversion?), so forcing charge that has spilled out into the exhaust ports on overlap back into the cylinder? Similar to pulse tuning exhaust headers iyswim.

Hadn't considered Andy's explanation, but that probably is more realistic

Simon