Mazda Rotary Club

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

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

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...))

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

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

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??

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

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.....

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?

QuoteOriginally 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.

QuoteOriginally 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:

Quote"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 !!!!"

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...

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  ;)

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

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

QuoteThe 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.

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.

night night Glenn (poor bloke\'s gonna be dreaming of turbo housings and stuff now.... *grin*)

Presumably, if the compressor turbine is clipped (the blades of the turbine being shortened) air will flow round the blades better, resulting in less backpressure, so higher flow through the engine.  Although lag will increase, the power will be higher for the same pressure, as more flow will be required to maintain the manifold pressure.  The stock twins must result in higher backpressure, so for the flow required to maintain the manifold pressure is lower than for a single.

IT\'S A MIRACLE - I SEE HOW IT ALL WORKS NOW!! *grin*  Thanks everyone, you\'re all wonderful.

QuoteOriginally posted by 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.....

Hi Mate,

There is not that much difference depends on how the car has been tuned. e.g gurj I think Has about 347 flywheel horsepower at .75 bar
smifta has stock twin turbos at .7 bar got 326 flywheel horsepower on a non ported engine also.
If you have a injector say 850cc with at 45psi of fuel pressure it will flow a given a mount of fuel for a given time frame put a 1600cc injector in it\'s place have the same fuel press the same time frame it will of flowed more fuel in the same period of time.

Same for a bigger turbo for a given amount of pressure a much larger turbine will flow a larger volume of air at the same psi than a smaller turbine would in the same given time period.
Volume is the key not only pressure.
Another example on my car with downpipe,  catback, induction, ecu, and intercooler running at the same psi my injector duty cycle was 100% under wot consistently. when I removed the intercooler and put the stock one back in running the same psi my max injector duty cycle was 84.6 consistently. This was because at a given psi the larger intercooler affected volume a lot mor ethan the stock one does, again same thing applies for the twins(small) turbine and the singles (large) turbine scenario.

rgds

I think I understand how the different turbo\'s work, and (if I\'ve got the hang of it) that explains how the intercooler affects it too.  For consistent turbo\'s, engine, exhaust and intake, the flow reduction of a given intercooler will result in less air getting to the engine, so the injector duty will be reduced.

the difference with the turbo\'s is the different EXHAUST flow characteristics as the impellors flow much more air than the stock twins (speculation on my part), so more air can get through the engine.  

It\'s not so much the turbo\'s can put out much more air, it\'s that they are less restrictive AFTER the engine, before the exhaust.

QuoteOriginally posted by Archbish
I think I understand how the different turbo\'s work, and (if I\'ve got the hang of it) that explains how the intercooler affects it too.  For consistent turbo\'s, engine, exhaust and intake, the flow reduction of a given intercooler will result in less air getting to the engine, so the injector duty will be reduced.

the difference with the turbo\'s is the different EXHAUST flow characteristics as the impellors flow much more air than the stock twins (speculation on my part), so more air can get through the engine.  

It\'s not so much the turbo\'s can put out much more air, it\'s that they are less restrictive AFTER the engine, before the exhaust.

It\'s actually a bit of both. The larger compressor wheel on s single turbo will flow more air into the engine than the stock twins would and the larger exhaust housing on the turbo as  you mentioned does the same trick in a different way however porting of the exhaust ports and size of the exhaust system plays a big part as well.

rgds

good god - the most technical thread i have ever read on here - and to make it worse OJ started it

what is this world coming to ??

its been a good read though :)

In theory, if the compressor side of the stock turbo\'s were kept as original, but the impellor turbines were clipped, they\'d flow better, so more power would be produced - albeit in a smaller power band as they\'d take longer to spin up.  Right?

See Grindy, I can be techy too!

Of course, neons increase the flow rate too...

well of course neons do - and go faster stripes add 20BHP !

so are you thinking of changing your turbo - or just gaining info so that you can call gurj\'s turbo ?

Bish
if you get a chance to have a look at a stock turbo unit, you will see just how small and restrictive it is. The manifold and opening into the the turbine wheel really are small especially compared with a to4.

& OJ if yer guna "call gurj\'s turbo" like Grinder suggests i suggest you watch yer back i hear he is very protective lol
Si

The intake temp and the backpressure will be completely differnt on the T78 compared with the twins. Therfore you will get more air into the engine at the same boost and this will equal more power. HOORRAYY
 
Basically you can get different flow rates through the engine at the same pressure due to changing one or more parts within the inlet or exhaust.

QuoteOriginally posted by Archbish
night night Glenn (poor bloke\'s gonna be dreaming of turbo housings and stuff now.... *grin*)

Presumably, if the compressor turbine is clipped (the blades of the turbine being shortened) air will flow round the blades better, resulting in less backpressure, so higher flow through the engine.  Although lag will increase, the power will be higher for the same pressure, as more flow will be required to maintain the manifold pressure.  The stock twins must result in higher backpressure, so for the flow required to maintain the manifold pressure is lower than for a single.

IT\'S A MIRACLE - I SEE HOW IT ALL WORKS NOW!! *grin*  Thanks everyone, you\'re all wonderful.

You were right! was dreaming of turbos!!!  :)

So - does this mean in the formula, that \'V\' is not constant as I thought?  Would it be fair to say that flow would be the same as volume in this case?

Quotedoes this mean in the formula, that \'V\' is not constant

Yes and no...  in a snapshot of the 2 setups, the volume will be identical (I was only talking about the intake manifolds), but given a period of time, the higher flow will result in a higher volume of air...

My understanding is that the intake manifolds are not the same for stocks and singles (as Ross was saying I think) therefore, while V is constant for a given set-up, V was not equal for the two examples.

Iain

The upper and lower intake manifolds are different?  I thought it was the exhaust manifold that was different - as that provides the feed to the turbo\'s impellor housing...

Certainly the upper intake manifold is the same, and that\'s where the boost is measured, and the lower intake manifold is simply the connection between the UIM and the rotors...:confused:

Hi All,
If the single and twins are on an identical  highish boost,say 12 psi, then answer is simple.(This is just for the sake of argument)

The twins heat up the incoming air more than the larger single does,which means even though they are at the same boost pressure the single is more efficient due to more air molecules being present per square inch of incoming air.

There are many other parameters, such as compressor maps, that come in to it but that is the simple answer as to why the right single turbo can make more power at the same or sometimes even less boost.

Kind regards
Carl;)

QuoteThe twins heat up the incoming air more than the larger single does

That suggests Water Injection, or a nitrous spray onto the intercooler would result in similar hp??  but even Yorkies most powerful stock twin UK car is struggling at 400hp with daft low temps, whereas single turbo\'s are breaching this with ease at similar or lower boost.  Surely this must be a result of \'post-engine\' items, as the boost forced into the engine is at an identical level, with very similar boost temps?

QuoteOriginally posted by Carl
Hi All,
If the single and twins are on an identical  highish boost,say 12 psi, then answer is simple.(This is just for the sake of argument)

Carl - you are talking standard FD Twins I\'m assuming?

The exhaust housing must play a huge part in this also, as Bish has mentioned above?  On that theory Carl - having a VERY large compressor housing and a standard exhaust housing may make you a little more power.  But large of both with make a stack more...

Would this comment following be correct?...

The smaller the exhaust housing on a turbo, the more back pressure in the engine as the turbine housing/wheel cannot extract the gasses fast enough.  Therefore the harder it is to force more air through from the intake side, and as the turbine wheel in the exhaust housing is driving the compressor - it wouldn\'t be able to do this efficiently because of the exhaust restriction.

PV=nRT

R here is the gas constant.
Reynolds number is a fudge factor used by chemical engineers to determine when fluid flow will change from laminar to turbulent.

Hi Glenn,
Of course the compressors are not the only complete factor,and the turbines and their sizing has much to answer for in this discussion.
But as you will see from my words quoted below I was trying to simplify the point and not go in to the other factors too much.
If we were then fueling,intercooler,intercooler piping,water injection,air temp ambient,type of fuel,porting,state of the engine int terms of condition like is it fresh or tired,and much much more come into the picture,I just did not want to go there.
Kind regards
Carl

QuoteOriginally posted by Carl
Hi All,


There are many other parameters, such as compressor maps, that come in to it but that is the simple answer as to why the right single turbo can make more power at the same or sometimes even less boost.

Kind regards
Carl;)

My Brain is fried reading this lol:D

tell me bout it but is good stuff....:god
Si

I kinda thunk that using a bigger single turbo over stock twins (on anything) would have the following benefits.

1. That the plumbing for a single will not be the same as an OEM setup, and the OEM setup has to allow for such niggling things as reliability, economy (hah!), emissions, and packaging. An aftermarket setup can be as big and bad as the clown paying for it wants it to be. The \'clown\' just wants neddies and is not overly concerned with other things, are they? So all those things that a factory setup must attain in some way can be altered or removed for best performance. Short version: OEM\'s are always like a garden hose, good stuff is like a sewer pipe. Big pipe flows better (most of the time).

2. The smaller turbos will spool up quickly, and as such can\'t keep spinning up for as long as a larger turbo could at a given RPM/gas flow. For example, the twins may be at max RPM at say, 5500 engine RPM, where the bigger single is only at 2/3 of max RPM at the same engine rpm. They are both flowing about the same amount of gas, the single just has more ability to move it in and out. It\'s why an undersized turbo will spin up fast, but the thing will run out of boost pressure at higher engine RPMs. It just can\'t keep spinning forever.

3. And while we are talking about spinning turbos, those smaller turbos are going to make the air turn lots of corners through small pipes, the smaller compressors are going to chop the air up a heck of alot and the impellors are going to do the same trick on the way out. All that bending, chopping and squeezing reduces possible flow, raises temperatures and generally make a pest of itself. A larger turbo won\'t smash and bash the air so much and it will have larger pipes to send it\'s gas through so it\'s doing about the same work but it\'s being kinder to the gas. Good things happen to kind folks. ;)

4. Finally, the factory setup will try to meet the needs of all owners as best as it can. Some will want lotsa bottom end, some will want a healthy top end, some prefer a well rounded package. The factory will attempt to keep everyone happy to some extent. With something aftermarket, you can choose what to sacrifice and by how much. Those nutters running 1,000bhp Skylines think they have anything resembling driveability? Of course they don\'t, their engines are all top end and who gives a rodents bottom about the, well, bottom end. Of course, you could squeeze 1,000bhp out of a 13b with the factory turbos but you would need some pretty interesting things to make it happen. Like a track on Antarctica, prolly neat Toluene for fuel and liquid nitrogen as coolant. And deep pockets because the thing would pop PDQ. :p

I hope that\'s readable AND understandable.

The whole turbo thing is a mixture of science, art and black magic, all doing the high wire act above a crocodile infested desert. It\'s also why guys with no official training get good results, and why folks with a bazillion letters after their name still can\'t get it right.

Oh yeah, if you want big power, lock the wastegate shut. Works really REALLY well and makes great smoke clouds soon afterwards... ;)

Basically, a larger turbo doesn\'t have to work as hard to produce say 0.9BAR as a couple of smaller ones do.

The flow is far better through the bigger turbo due to the exhaust gases being able to escape more easily from the engine, which as mentioned already cuts down operating temperatures in the engine and improves efficiency - this is why we fit free flow exhausts.

The maximum power produced by an engine is totally dependant on how easily (or efficiently is probably better) air can be drawn in and expelled.  The more air (& fuel) you can process per minute = the more power that will be produced.

Since the single turbo can flow a much greater mass of air  more easily it doesn\'t heat up the gases as much - which is effectively what Ross said - at the same pressure the air is denser from the big turbo giving you a bigger bang and the exhaust gases can get away more quickly as there isn\'t the same bottle-neck as with the twins.

Efficiency is the key!!!!!:D

Please explain why the two smaller turbos heat the air up more than one large one?  all the sensible heat is going to come from the compression, plus a small amount conducted over by the turbo from the exhaust gases. Also note that in the PV=nRT equation, T is in absolute units, K, thus air at 27 Deg C will be about 300K; air at 87 Deg C will be at 360 K, representing a 20% reduction in mass for the higher temperature if pressure and volume are the same.


I believe the opinions expressed along the lines of single turbos having bigger exhaust pipes/bigger inlets are correct. Physics/chemical engineering shows that mass flow of a gas through a pipe is dependent on the fourth power of the radius of the pipe (ie a 2inch pipe will flow 16 times the amount of gas  as a 1 inch pipe.)  However, bends, especially sharp ones restrict the flow much more than a slightly smaller diameter.

QuotePhysics/chemical engineering shows that mass flow of a gas through a pipe is dependent on the fourth power of the radius of the pipe (ie a 2inch pipe will flow 16 times the amount of gas as a 1 inch pipe.)

Exactly,  so say the outlet of the large turbo is 2 inches and each of the small turbo\'s is 1 inch - does this not mean that the large one is capable of flowing that much more gas?
As a result the compression of the gas in each of the smaller turbos is that much greater and  the temperature of that gas is increased proportionally.

Also, I may be wide of the mark here, but I would assume that two turbos working near their peak efficiency would generate a lot more heat mechanically than one that is well within its limits and easily flowing the same mass of gases.

The smaller turbo(Taking one individualy) when pushed outside of its optimum range(Compressor map) to make higher than stock boost does the following..............
The compressor starts to agitate the air more creating more turbulance than at lower boost settings and as a result it becomes less efficient.The compressor thus starts to heat up the air more instead of moving it smoothly through itself.
A larger single turbo compressor can maintain a smoother air delivery for higher boost settings by virtue of its not having to work so hard which means less heating up of the air,of course when the turbo is asked to work outside of it\'s optimum compressor map range then it too starts to agitate the air creating turbulance and heats it up more(Albeit at higher boost than smaller turbo\'s).
Kind regards
Carl

QuoteOriginally 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

Okay ignore all the technical explanations.....In terms you understand

It costs more so it must be better.:)

I was gonna suggets it was magic :(