Heres a reproduction i did ages ago on RsBB about why a T4 makes more power than a T3 or T34 at the same boost level.

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Let’s go back right the way to basics…

Why is it, that a turbocharger is universally is recognised as the ONLY form of forced induction that COSTS power?

Well let’s look at this shall we?
We have a decently specced engine, say an Rs2000 I4 and we are making 170bhp through our otherwise std setup with some head mods and cams.

To make this power we are utilising the air pumping ability of our 4 x 500cc cylinders.

They are drawing in enough air and fuel at the correct ratio and importantly, expelling it again to produce this power. This ability incidentally is related to its volumetric efficiency which i will touch briefly later.

The I4 is making its 170bhp with its nice, well-designed STANDARD 4-2-1 exhaust system.

Let’s now redesign it in the great Ford RsBB fashion and remove the nice exhaust system and stick a tiny pathetic straw sized .48A/R turbine housing on it with a closed wastegate and the turbocharger actually welded tight so it can’t spin.

Does ANYONE on this BB think this engine will now make 170bhp? Well It will be very lucky to make 120bhp now.

So whats happened?

WE HAVE INCREASED PUMPING LOSSES DRAMATICALLY.
This is the biggest issue. The reason this costs us power, is a great proportion of the energy produced from the power stroke of 1 cylinder burning our nicely presented charge of fuel and air is now wasted trying to push the spent gas out of the previously active cylinders exhaust valve and through the tiny turbine housing.

We now have some detrimental knock on effects:
The friction on components caused by this pumping loss now adds heat to our engine too. This heat was part of our power strokes energy.

This pumping loss has also caused the overlap events effective scavenge volume to drop massively as the nice, meticulously calculated pulsation vacuum that designers spent hundreds of hours to create has now been exchanged for BACKPRESSURE.

This backpressure has now also decreased the amount of air the exhaust pulse drew through the inlet valve at overlap when piston speed was at its lowest so maximum cylinder fill has reduced, down goes VE.. Things are looking bad for our power curve now.


So conversely, as we now have LESS airflow on overlap, we are going to start dumping heat through our exhaust seat and port and are heating our soft alloy head up.

WHY? Very simple ,
Designers use scavenging on overlap as a very simple and very effective way of cooling valve seats, guides and ports.

How? Also very simple ,
As we reach overlap in our cam timing event we have both COLD inlet and HOT exhaust valve open, this gives the exhaust valve and relative components time to be cooling from their grievous job only moments ago of shifting a mass of immensely hot air through its system so it’s a great relief to sit in some nice cold flowing air for a second and transfer a bit of excess heat away!!

Our fancy new engine / design isn’t looking too hot now is it? (Well actually, its getting damn hot.. )

So,
I hope this large and simply worded explanation helps you to understand that a turbo DEFINATELY costs power by its very presence on your engine and also why it does so?

Ok? Good!!

So what the hells that got to do with today’s BB argument? Oh yeah, sorry........

Exhaust backpressure caused by the turbine housing assembly’s restriction is our key element between T3 and T4.

But let’s deal with the delivery of our air first and we will use the good old YB for our examples.

A T4 produces far more VOLUME of air at a given pressure from its HOUSING than a T3.THATS UNIVERSALLY AGREED.

Now if we quickly use this bizarre volume of air example supplied by someone earlier...


quote:
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Foolishly, sorry, Originally Posted by erm.. someone
Look at it this (simple way so you understand ), imagine a 1 ft diameter balloon inflated to 15psi. Then imagine a 2ft ballon inflated to 15psi - are you trying to tell me that both have the sane volume of air in them?
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That balloon is actually bigger so yes it holds more volume, but this is not applicable to an engine because even if we take all the inlet valves out and pressurise the system with our turbochargers, we are still presenting each turbocharger with the same volume to fill, lets say 2000cc for the cylinders and 1000cc for the intake of the head and the plenum / hoses.

So we have a 3000cc volume to pressurise with air. This does not change unless the engine begins to operate. An engine will only generate more power by shifting more air at the correct AFR. SIMPLE.

The engine will only shift more air if we do one of the following:

1) Improve the airs route into the head.
2) Increases the pressure we push it in with.
3) Improve the mapping.
4) Improve the volumetric efficiency.

So,
We still know a T4 WILL make more power than a T3 at the same boost so why is that?

We don’t appear to have done any of the above mods do we?
The head hasn’t been ported.
We are running the same boost.
We aint touched the chip cos PHIL cant remember what spec it was for!
So have we changed the engines VE?

Good question, and back to turbo’s.
A T3 50trim with our desired pressure ratio of 2.4 (14.7psi inlet +20psi outlet divided by 14.7psi inlet) will be spinning at 134,000 rpm with a compressor efficiency of 70%.

A T4 60 trim with our desired pressure ratio of 2.4 (14.7psi inlet +20psi outlet divided by 14.7psi inlet) will be spinning at only 97,000 rpm with a compressor efficiency of 82%.

Now lets go to the turbine housing.
The T4 P trim wheel flows a lot more air than the Std T3 trim rear wheel. (They are all the same as std on T3) but it conversely takes more to spin it to speed.

We now have an exhaust backpressure IMPROVEMENT due to a better flowing rear wheel!!!!!!!

Secondly,
We now have a wastegate that will open much sooner and much wider than it would on the T3 as less exhaust volume is required to spin the turbine as we have a 37,000 rpm improvement in efficiency at our 20psi.

HEY, we have another exhaust backpressure IMPROVEMENT.

If our T4 is using a bigger housing, and it IS if compared to a T3 we have another exhaust backpressure IMPROVEMENT!!

So lets go to boost at the intake:
Now our exhaust backpressure is reduced, our cylinders demand for air has increased. We have overlap efficiency gains, we have thermal efficiency gains so we can suck more air and we can suck it with a greater pull because we are actually revving more freely so our peak piston velocity has increased.

So we are CONSUMING more air and this T4 can supply it for fun… But… we aren’t making more power because the T4 pumped more air at 20psi.

We are making more power because this turbo improved the volumetric efficiency of our engine mainly through exhaust backpressure reductions and an improvement in outlet temperatures at the compressor outlet due to Adiabatic Efficiency which we may or may not discuss later.

Goodnight folks,
If anyone wants to know what the compressor maps do and how to decipher the turbo specs, let me know when ive slept cos this essay took me four hours to write in a fashion that anyone can understand!! It would take about 15mins using techie terms

Best Regards and goodnight,
Stu

 

highlight, right click, copy, open word, paste, save.

 

 

excellent read

 

Good read that. Nice one Stu

 

 

good read.

 

 

Moved to here for a while to provoke more technical discussion...........

 

Dicussion?

About what?

fook me you covered EVERYTHING in one go!


LOL

 

Another good read

So a bigger turbo puts less stress on a engine, but you must also get a longer spin up time before its boosting So all gains must be top end ?

How do you improve mid range which is more important ?

Thanks

Steve.

 

Well in my case i raise the limiter to 8K, so that 5K is now the midrange

Problem solved


Thats actually not as much of a pisstake as it sounds if you think about it, as with a raised limiter you change up gear and land with the rpm a lot higher in the next gear so it becomes less important what happens at lower RPM

 

I don't understand at all , but cheers for the information, it's really interesting

Thanks

Gonna have to do a few more laps with this one

 

But i think my engine won't like 8k

Also my Rev limit is 7100rpm

What i want is a solid 400bhp with as much torque as possible and to be reliable, not stressed to the max

I have a ported head, will bd10s + 400s and a T38 do the job ? also would like good MPG.

Steve.

 

you will get far more than 400bhp from that i should think.

for MPG, talk to stu about "closed loop" its the only way to go really TBH

 

honestly stu you should sit down and right a book

as your method of getting the topic across is alot easyer than 95% of the books i read

 

yep, very easy to understand thanks stu

 

good read

 

chip and stu should collaborate on a book - together they've been posting some cracking tech topics over the last few days

 

Good read mate.

 

FUnny that I was thinkg the other day should I pimp for the T3 or T2 when I get the conversion done on the car.

T3 I think now

 

Fair play Stu.you know your shit....


























But can you tell me why,when i cook spag bol,my sause taistes better when i use tesco mince,insted of asda???

Pete..

 

You don't want much steve good fuel economy with 400 horses

 

Another excellent read

 

very interesting & have a free bump from me

 

Was worth a read again

Steve

 

nice info thanks stu.

 

i do not think i was ready for you to put words in quotation marks. i'll think about what you really want when you put your search in quotes and try to return something that makes sense.

 

Good info, Can you explain How VNT turbos work? they dont have wastegates and have variable AR exhaust housings? i understand that they basicaly tighten up (small ar) and open up (big Ar) but how do they stop the turbo over speedeing with no wastegate? Ive had one in bits and seen all the vanes and how it moves.

 

sounds good to me

 

vtn work by using a too bigg an exhaust housing ,so when the vanes are wide open
the exhaust it too big to drive the turbine wheel at too much speed ,
when vanes are closed down gas speed hitting the turbine wheel tips drive it quick and spool quick

 

nice read