turbo knowledge anyone?

[2.5bluenob]

Can anyone educate me please!
If two different turbos are both running the same psi but one turbo is larger than the other does it mean they are producing the same power? or is the larger one flowing more air and making more power? or not as the psi is the same? this is something that has been puzling me for ages and my little brain cant work out the answer.
Any help appriciated
Steve

[mgbond]

2.5bluenob said:

Can anyone educate me please!
If two different turbos are both running the same psi but one turbo is larger than the other does it mean they are producing the same power? or is the larger one flowing more air and making more power? or not as the psi is the same? this is something that has been puzling me for ages and my little brain cant work out the answer.
Any help appriciated
Steve

I'm no expert but you are correct. if they are both running 1bar then the bigger unit is pushing more air so more power.

Bondy

[JLSELAN]

Sorry--if you appeal to basic thermodynamics for ideal gases (i.e., Boyle's Law) which relates pressure, volume and temperature through a universal constant, you will find that, for a fixed temperature, pressure and volume are inversely related by a constant. That means if you fix pressure you fix volume, so the amount of gas (air) you get at 1 bar of boost is independent of the source of boost. For example, you would have the same power (or essentially volume of air) if your source of boost were a supercharger rather than a turbo. The dynamic response (boost (HP) vs. time vs. rpm) is what varies with the characteristics of the source.

[andygtt]

if you have 1 bar then your turbo is able to provide enough air to cover the engines consumption, but only at that given rpm...

Most probably a large turbo wont produce 1 bar at low revs and a small turbo is wont produce 1 bar at high revs... were the boost is made makes a difference to power... so the question has to be where is each turbo making 1bar.

if a turbo makes 1bar till redline, then a larger turbo set to just 1bar will only loose low down and gain nothing at the top I believe.. or at least that is my understanding.

[Gadgeroonie]

think of a hosepipe and a drain pipe

if the pressure in them both was one bar, you would get a lot more water flowing out of the drainpipe

[andygtt]

You measure boost at the inlet after the throttle body rather than the turbo though? So isn't 1 bar at the inlet (at as given rpm) the same regardless of turbo size?

[JLSELAN]

Gadgeroonie said:

think of a hosepipe and a drain pipe

if the pressure in them both was one bar, you would get a lot more water flowing out of the drainpipe

True, but not really relevant here, there is a huge difference between the characteristics of compressible and incompressible fluid flow .

[JBL930]

Heat comes into it too, a bigger turbo will run cooler, and also cause less back pressure in the exhaust

[2.5bluenob]

thanks for the help everyone. i was kind of thinking along the same lines as Andy.
the reason i asked is i did a few upgrades all at the same time and a remap and i cant tell if changing my turbos to bigger t25's was actually a worth while job. Im running 1 bar which i could have done anyway which i could have have run on the smaller turbos. So im trying to guess which one is the best performer but the old ones were not running as much boost so unless i put the old ones back on running the same boost ive got no comparison.
As the boost gadges on our cars dont seem to be very acurate its hard to tell from looking how early im making full boost an how its holding through the rev range.
Cheers
steve

[ivanhoew]

the temperature is one variable that creates a power difference for the same boost . a large turbo putting out 1 bar at 180c wil make more power than a small turbo putting out 1bar at 200c ..but only if the intercooler and the ex back pressure and the turbine efficiency are the same on both .vary any one of those and it all goes in a different direction .

take a small turbo with a greater efficiency in the used air flow range ,and it will produce air at say 120c so make more power ,,now add a smaller turbine that may be part of the same turbo design ,and you may have a higher turbine inlet pressure to get the same shaft power ,and so the bhp goes down ,now ad a big turbine housing so the turbo comes in a bit later ,and this may reduce back pressure and so power goes up ...

take a big turbo ,with a bit les efficiency in the used power range , temp goes up ,less power ,now find the turbine pressure is lower cos of the big wheel , power goes up ,now stick on a small housing to bring it in sooner ,power goes down ..

etc etc .

[Gadgeroonie]

to decide if the bigger turbo is worth while you need to carefully look at the compressor map

by resding the map you can decide how many pounds of air are produced at 1 bar of pressure

then look at the bigger compressor map and see how much more air is produced at the same boost

the gains may not be that large - in which case you can consider running more boost on the original turbo to get the same results

there is always a compromise - more boost may mean more higher turbine speeds and higher temperatures

try and think of airflow and not boost when choosing a turbo

[andygtt]

Gadgeroonie said:

try and think of airflow and not boost when choosing a turbo

here is a nice example of the airflow difference between a T28 and my turbo

[andygtt]

Gadgeroonie said:

try and think of airflow and not boost when choosing a turbo

I understand why airflow is important, but I have to be hounest I thought airflow is related to boost, ie an engine producing 1bar with all other things the same will be consuming the same amount of air regardless of turbo size?

Of cause 1 turbo may be working a lot harder than the other hence heat etc etc.

Boost is what gives us the power, if the turbo cant provide the air flow then boost drops... so are we not correct that air consumption is the same for both turbos given all other things the same ie temp, revs, boost etc???

TBH over the last year I too have been trying to understand this all fully, i'm not quite there yet hence all the questions.

[Mike Tuckwood]

In this and similar instances for Boyles Law to apply, there needs to be a fixed volume, and with different flow rates of Turbo's there isn't a fixed volume (even if there is a fixed pressure).
As for power production, Turbocharging is all about good thermal management, with a larger Turbo (more lb/min) you will move the same volume of air, but at a lower pressure, lower pressure = lower temps.

As for speccing the right Turbo, that's very much more complicated.
Have a look through the Garret "Turbo Tech" articles Here http://www.turbobygarrett.com/turbobygarrett/tech_...


Mike.

Edited by Mike Tuckwood on Tuesday 16th August 10:56

[andygtt]

so larger turbo transfers less heat to the charge... but if the charge temp is the same the engine will not consume any more air with a larger turbo unless you increase the boost so no extra power would be made?

there is lots of considerations to specing a turbo for sure... very easy to get wrong (and a lot of the time is got wrong hence turbo cars have a bad rep) but lets try to separate all the other considerations and concentrate on this one question.

Will 1bar on a small turbo at the same revs and air temp produce the same power as a larger turbo with exactly the same boost, air temp revs etc?

[ThatPhilBrettGuy]

andygtt said:

Will 1bar on a small turbo at the same revs and air temp produce the same power as a larger turbo with exactly the same boost, air temp revs etc?

More or less yes (assuming the exhaust isn't being effected). The engine doesn't know what's on the end of the pipe. Might be a big bloke with some massive bellows for all it knows.

[anonymous-user]

Clarify please do you mean:

1) 2 different sized turbo's fitted to one engine with a common intake plenum? (i.e for a given boost pressure, which turbo is doing more work)

or

2) 2 comletely different engines, that are identical but happen to be fitted with different physically sized turbochargers? (i.e. for a given fixed boost pressure which engine would make more BMEP (assume everything else was the same)

[andygtt]

exactly the same engine, intake, intercooler etc... just swop the turbo, this is what the original poster has done.

[951]

Mike Tuckwood said:

In this and similar instances for Boyles Law to apply, there needs to be a fixed volume, and with different flow rates of Turbo's there isn't a fixed volume (even if there is a fixed pressure).
Mike.

But there is a fixed volume isn't there? The capacity of the intake & cylinders, and it's all about getting molecules of oxygen into the cylinders.

I think it's easiest to think of it like this:

Any turbo can pump air into a static cylinder and reach say 1 bar - implying there is twice as much air, and so oxygen, crammed in there. The flow of the turbo just defines how fast it can do it, and affects it's ability to still achieve that 1bar at high engine revs when it has milliseconds to replace the air just exhausted.

As for the temperature of the air, consider a sealed container full of air. If we apply heat to the container the air will expand and the pressure will rise. When it gets to 1 bar though, as before, there will not actually be any more molecules of oxygen in the container. So if we pump the cylinder up with air at that same temperature, there will also not be any more oxygen.

[Mike Tuckwood]

2.5bluenob said:

Can anyone educate me please!
If two different turbos are both running the same psi but one turbo is larger than the other does it mean they are producing the same power?

No it doesn't it would be unlikely that they are, if you applied the strict regime necessary to comply with Boyles Law on a static test bed, then yes they would be. In situ on the car there are a whole host of other considerations to allow for which is why so many people get it wrong with Turbo cars by just thinking bigger is better. You need to pay meticulous attention to the compressor maps particularly.

Remember, a Turbo is just a 'simple' air pump, capable of only moving a certain volume of air (rated in lb/min) at a certain rated Turbo rpm. That's how it's possible to approximate HP limits for Turbo's.

In simple terms, you will get different results at 1 bar with different Turbos.



Mike.