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Discussion Starter #1
At the same boost, a bigger turbo makes more power than a smaller one. Why is that?

Doesn't boost/pressure dictate the power of the engine no matter how it is made? Or are there other variables like maybe smaller turbos heats the air more?

Can anyone explain?
 

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same question

must admit the same question has been puzzling me for a while.
hence my hesistation to change turbo chargers.

JamesW
 

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SURELY GIVEN 2 DIFFERENT TURBOCHARGERS BOTH BLOWING
AT THE SAME PRESSURE, THE VOLUME OF AIR GOING INTO
COMBUSTION IS THE SAME.

JamesW
 

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Think about it.

Q: If you have a tap running at a given pressure, say 1 bar, how much water are you going to get compaired to a drain pipe running with the same pressure?

A: A lot more for the drain pipe. Its all about volume!

Edited to remove confusion.
 

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ARE YOU ASSUMING THE DIAMETER OF TAP AND DRAIN PIPE ARE THE SAME?
IF YOU ARE, THEN WITH THE SAME PRESSURE, BOTH VOLUMES
ARE THE SAME.
 

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Eh?

If I wanted to illustrate two 'things' with the same diameter I would have said a tap and a tap!

:confused:
 

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Re: Eh?

Howsie said:
If I wanted to illustrate two 'things' with the same diameter I would have said a tap and a tap!

:confused:

ROFL! Good quote. :D

Seriously though, a bigger turbo will be less restrictive internally. Therefore, for a given boost level the air charge released from it is done so with less friction and, consequently, less heat. A cooler charge is denser and, therefore, for a given volume contains more oxygen. Oversimplifying, the more oxygen, the greater the combustion.

Hope this makes sense. Please feel free to add/correct any details as neceesary.

Cya O!
 

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all about volume?

so it's not all about volume. it's about temperature.
does anyone know by just changing from stock turbos to
hks 2530 what power gain could be had?

BTW, Howsie, i was thinking more on the combustion chamber
end not the turbo end.
 

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James,

It's not that simple I'm afraid. To run 2530s at anything over I guess something like 0.8 bar, you'd need to sort out fuelling first. To do the fuelling, you'll need larger injectors, pump and a remap either using a programmable ecu (Power FC, etc.) or a remapped ecu (Abbey ecu, Mines, etc.). That's just the start....

Peter.
 

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Pressure

With pressure comes temperature and hence the intercooler. I was just trying to illustrate a point.

:smokin:
 

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It is all about air flow rate at the turbo inlet.
More air in = more power.
You can only pump more air by....

1) Increasing boost levels (more dense air - but only if charge temperature is controlled)
2) Increasing rpms
3) Increasing capacity
4) Improving the volumetric efficiency of the engine - ie. how well it pumps air through it - ie. bigger cams, valves, intakes, exhausts, etc.

As far as temperature rise goes it all depends on which point of the turbo compressor map you are operating at.

Temperature rise under given boost and flow is easily calculated from a compressor map and thermodynamics equations.

High temperature intake charge will lose power because the air is less dense. The intercooler lowers the air temperature and increases the air density to the intake. Denser air through the "fixed capacity pump" -the engine- (2.6 liters) at a fixed rpm will produce more power (assuming correct fuelling).

Just swapping turbos without changing anything else only gives changes in power due to volumetric efficiency changes (turbine side a/r) and inlet temperature changes (compressor efficency).

The important thing is to decide what power at what rpms you want. The approximate required boost levels and air flow rates can then be calculated and the turbo(s) chosen accordingly.


"At the same boost, a bigger turbo makes more power than a smaller one. Why is that?"

It doesn't unless the engine is spinning faster or pumping air more efficiently.
Smaller turbos have lower air flow capacity than bigger turbos - only a pump after all and limited by the size of the compressor (shovel!) and the speed it spins( eventaully they explode!)
 

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lightspeed

i don't suppose you can translate your last post into
mathematical equations? i tend to find them easier to follow.
i especially like the section about deciding which turbo.

regards
 

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What about exhaust manifold pressure?

In addition to the lower charge temp (as mentioned above) by using a compressor that is more efficient at the flow rates you're now getting ...

... as far as I understand it, if you can achieve the same boost with a lower exhaust manifold pressure, the engine is going to be able to breathe more easily and the gas flow (and therefore the power) will be increased. Similar effect to fitting a free flow exhaust.

A larger turbo may flow exhaust gases more easily (having a different nozzle area). Consquently, exhaust manifold pressure may be less at the expense of needing a little more exhaust gas to get the thing spinning in the first place ... resulting in needing a few more rpm before it comes on boost but more power once it actually HAS come on boost.
 

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Big turbo will have bigger ports, larger turbines and compressors and so on, so at high power levels will flow more generally. But the conundrum is that we measure the boost at the manifold. So surely 1bar in the manifold is 1 bar?

Yes an no.

If you make high boost on a small turbo, you revs the nuts off it. The problem with that is, the exhuast turbine will still act as an impellor (centrifugal compressor) if you spin it enough. So there comes a point where you're spinning the turbo fast enough to make say 1.5bar boost. Except the exhaust side is spinning so fast, that the has trying to get out the middle is being forced out, and back into the turbine housing. This will create a big increase in exhaust back pressure.

Symplistically, the exhaust back pressure will rob power. Firstly it will leave burnt ex gases in the cylinder so you get contamination of the inlet charge. Secondly, because there's gas in the cylinder when the inlet charge is entering, there is already a higher pressure in the cylinder to overcome. So although you're reading high boost at the manifold, you're net boost, relative to the cylinder pressure is lower.

But what about density? The harder a turbo has to work to compress the air, the more heat it will put into it. There will always be a rise in temperature due to compression, and while you might get it back post intercooler, it's still extra work. So if you have a smaller compressor side, like on a small turbo, and you do eventually make the boost, you are going to have to drive the compressor harder to do it, this means more work, and nearly all the work done on turning the shaft goes into heating the air, which will artificially raise its pressure. You may get some of that temperature back after the I/C, but unless it's very efficient, you're more than likely going to have a reduction in air density, and therefore mass flow, when compared to a physically larger turbo.

Paul
 

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Pavlo

Very interesting - I had not considered the effect of the exhaust turbine acting as an impeller.

So what we want is a wider exhaust nozzle and a correspondingly smaller radius on the exhaust turbine wheel? As long as the radius isn't too small as to become a huge restriction to flow, would that not then help?

How much do good flowing external wastegates help with power, as long as enough exhaust gas pressure is still present to spin up the turbochargers enough?
 

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Sorry you can't get something for nothing!

If you reduce the radius (in effect the lever) you must do something to increase the force on the turbine. Increasing area will reduce gas speed so actually decreasing the force. SO to use such a turbine to drive the compressor, you would need high gas speeds, leading to lag at low rpm, and increased pressure drop (back pressure) at high speeds.

To reduce back pressure you can make the flow area larger, but increase the radius at the same time, thereby increasing the whole lot. However, then you increase the mass and drag of the turbine, increaseing lag.

Or you could just increase the radius (thereby decreasing the A/R ratio) which would mean higher torque, but lower ultimate speed potential. Or by increasing the area alone (increaseing the A/R rario) you will decrease gas speed and back pressure, but lose grunt to drive the shaft at low speed, due to the smaller radius however, the same gas speed will relate to higher shaft rpm (at increase flow over all though).

Not very clear, I am sure someone can decypher it

Paul
 
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