Same boost different turbo\'s...

[Archbish]

OK - this is where I look even dumber than usual...

How can 0.7 bar of boost on a T78 produce more horsepower than 0.9 bar of boost on sequentials.  The manifold pressure is higher in the second case, yet considerably more power seems to be available from the first.  Surely pressure is pressure, irrespective of source?

This has been confusing me for ages...

Cheers

OJ

[grinder]

this is a long shot ... ( like always )

the T78 is a bigger turbo - that can move more air at any time

so it simple moves more air quicker - thus more power

then you mention pressure - if the T78 has a bigger diameter output then xx PSI will be more

imagine a hose pipe - blow thru it at 5PSI , then blow down a drain pipe at 5 PSI - they are both the same - but you would get more air down a drain pipe

imagine a ton of bricks and a ton of feathers - both the same weight but the actaul area will be bigger

gonna stop now incase i am well off the mark ..... whistle\'s innocently as i run away

[Archbish]

But if the maifold pressure is being maintained equally, surely an equal amount of air is being passed through it??  with a consistent outlet and a constant head of pressure, the flow through the outlet must be consistent irrsepective of source??  ((10 year old university fluid mechanics theory there...))

[Fish]

Bish, the best thing you could do, is try and get your hands on a book by Corky Bell.

This will explain everything that you could ever want to know about boost.

Fish

[maximtaylor]

Also - another item to consider is the exhaust. Any single with a half decent sized exhaust housing is going to flow a great deal more exhaust gas through it. The exhaust pathway is far more direct as well. More exhaust out, more air allowed in, more flow.

Also - air temperature is a factor. A single will produce air at a given pressure in a more dense (cooler) form.

Also - inlet pathway. Look at the route air has to take through both turbos. Path of flow is again more direct with a good single.

Flow is the key to most of the gain

So, in summary - the pathway is not consistent on the two types.

Or I could be talking rollocks

Max

[Archbish]

Righto - *strains brain*

Given      PV = nRT
where
  P = Pressure
  V = Volume
  n = Number of Mols
  R = Reynolds Number
and T = Temperature

if P and V are constant, then for the number of mols (air particles if you like) to increase, T must be lower (R is a constant).  But surely the temps can\'t be that much lower to allow over 100 hp more for the same pressure and volume.  Also, surely the flow AFTER the engine is the critical path to allowing higher hp, as the manifold pressure for both setups would be identical.  So, for the twin turbo setup the impellor must offer more resistance to the flow??

[MikeLMR]

a larger turbo will also be correctly sized for the higher boost pressures and air flows so it will be working more efficiently and still within its design specification. Upping the boost on standard turbo\'s pushes them outside of the peak efficiency range and raises charge temps etc. which will make for less power.

(I think)

Mike

[Archbish]

I understand the temps raised issue, but prior to the manifold, it doesn\'t matter if the boost is coming from Darkest Peru if the manifold pressure is consistent.

Gurj is running ENORMOUS hp at less than standard boost, and without freezing air to 0 degrees kelvin, slicing it up and dropping it into the lower manifold I can\'t see the how the temp can be low enough.  From the engine back, the manifold and impellor are the only differences, but is this enough to cause the couple of hundred hp extra Gurj gets??

If Gurj ripped out his single (and gave it to me), and whacked in stock twins (inc 3 miles of vacuum hose), and ran exactly the same boost, he\'d still lose about 12389129365 hp (at the flywheel).  maybe an extra 10 - 20 degrees intake temps, but no more.  Infact, if he stuck on a little T04, at the same 10 psi, he\'d still lose power.

it stinks of witchcraft to me.....

[maximtaylor]

Right, for comparison then - what power is Gurj getting on 0.7 bar?

0.9 bar on twins properly tuned is somewhere around 280 atw\'s 340 fwhp - agree?

[Glenn Butcher]

Originally posted by Archbish
Righto - *strains brain*

Given      PV = nRT
where
  P = Pressure
  V = Volume
  n = Number of Mols
  R = Reynolds Number
and T = Temperature

if P and V are constant, then for the number of mols (air particles if you like) to increase, T must be lower (R is a constant).  But surely the temps can\'t be that much lower to allow over 100 hp more for the same pressure and volume.  Also, surely the flow AFTER the engine is the critical path to allowing higher hp, as the manifold pressure for both setups would be identical.  So, for the twin turbo setup the impellor must offer more resistance to the flow??

Bish!

My understanding has always been that a large turbo will flow a larger volume of air.  I think in the above formula, it is V=Volume that is NOT constant for a larger turbo.

Another example, a small turbo providing 15psi of pressure (boost) - will have a much smaller volume of air compared to a TRAIN turbo providing 15psi of pressure.

I think Grinder is on the money.

[Ric]

Originally posted by Archbish
OK - this is where I look even dumber than usual...

How can 0.7 bar of boost on a T78 produce more horsepower than 0.9 bar of boost on sequentials.  The manifold pressure is higher in the second case, yet considerably more power seems to be available from the first.  Surely pressure is pressure, irrespective of source?

This has been confusing me for ages...

Cheers

OJ

I thought manifold and boost pressure were different things but then again I don\'t pretend to understand all the witchcraft which is engines :rolleyes:

The last paragraph from here (http://www.turbonation.com/turbo.htm) might explain though:

"The amount of boost produced by the turbo is controlled with another device called a waste gate. The waste gate is a large valve that sits at the exhaust inlet to the turbo that, when opened, causes the exhaust gasses to bypass the exhaust turbine instead of through it. The further the waste gate is opened, the more exhaust is bypassed and less boost is produced. A spring in the actuator closes the waste gate (by pulling on the rod). The back-side of the actuator diaphragm is connected to the intake manifold. As pressure builds up in the manifold, the actuator rod pushes out and the waste gate opens. This pressure is bled off by a solenoid that is modulated (switched on and off quickly) by the logic module. The longer the duty cycle (amount of time it spends turned on) of the solenoid, the higher the boost pressure that is produced. There were two different configurations used to accomplish this, depending which turbo charger was installed on the vehicle. The function of both is the same and the only real difference is where the manifold pressure comes from.

So just remember, when it comes to turbo chargers, size DOES matter !!!!"

[Archbish]

Hmmm - I\'m taking Volume to mean the volume from the entrance to the upper intake manifold....  If you take an instant snapshot of the UIM, sealing the inlet from the elbow instantaneously, it doesn\'t matter where the boost comes from - twin or single.  The volume of the upper intake manifold is the same, as is the engine given the same porting.  If the single is pushing in more air, as the volume at the manifold is consitent, then the pressure HAS to go up (given equal air density).  The only way the single to provide more air would be for the manifold to \'process\' air more efficiently - ie the impellor offering less resistance - allowing a higher flow rate.

So for a single turbo to produce more power, the path AFTER the manifold has to flow more air, otherwise the manifold pressure would be higher...

[Glenn Butcher]

Bish,  yep - I understand where you are coming from.  Give me a few mins. I\'ll find out some more.

.ps Just using //www.google.com with PV = nRT  

[Glenn Butcher]

This is a pretty good site, looks like there is more science to it! LOL!

http://www.turbocalculator.com/turbo-works.html

The Turbo Charger Turbine
At the end of combustion, as the piston is traveling back up the cylinder, the exhaust valve(s) opens and hot air is forced out of the cylinder. This exhaust is wasted in a naturally aspirated engine, simply being released into the atmosphere. A turbo charger, however, recycles this energy. In a turbocharged setup, the exhaust gases are forced through the turbine of the turbo charger. This causes the turbine wheel to spin.

The Turbo Charger Compressor
The turbine wheel is directly attached to another wheel in the compressor of the turbo. On this side of the turbo, often referred to as the compressor or the "cold" side, the spinning compressor wheel compresses the intake air. This pressurized air is then forced into the engine. Since pressurized air has more oxygen per unit volume (PV = nRT), there is more opportunity for combustion. This additional oxygen can either accompanied by additional fuel for increased power or in some diesel applications, the advantage is in the excess oxygen which allows for cleaner and more complete burning of the fuel, resulting in reduced emissions and increased efficiency.

Compression Generates Heat
When air is compressed so that more air can fit in the same volume, its temperature is increased. This is compounded by the fact that the turbo is not 100% efficient and generates heat when compressing the air. Since the air is hotter, it is less dense, which means less oxygen is in the cylinder than if the air were cooler. This explains why a 14.7 psi turbo gauge pressure will not double the power of an engine. The heat also explains why an engine under boost is more prone to knocking (predetonation or auto-detonation). In order to cool the charged air, an intercooler or water/alcohol injection is often used.

The bit I put in bold kind of explains it.

[Glenn Butcher]

Thinking out loud about it more now, usually large exhaust housings go hand in hand with a larger compressor housing...

I\'m assuming that a similar principal applies here, with a small exhaust housing the heat produced would be a lot greater through the exhaust turbine wheel - making it inefficient (not getting the exhaust gasses out of the engine - thus creating back pressure), and extra heat in the engine itself.  A larger exhaust housing would reduce this heat, making the turbine wheel spin more efficiently, and the engine could make better use of the colder more dense air coming from the compressor housing/intercooler.

Wonder if I\'m on the right track?

Anyways - off to bed now.