OBVIOUSLY (unless your a fookin dumbass ) 20psi with a small turbo is less volume air than 20psi with a big ol turbo.

BUT what about in cyl pressures? I guess as its pressure and not volume that stays the same?

As in a T2 running 20psi means the same in cyl pressure as a T4 running 20psi.

Yea?

The pressure is caused by the exhaust backpressure from the small T2 ex housing, so it same in cyl pressure as the T4, even tho the T4 is flowing LOADS more air, due to lot less backpressure cause of the bigger housing.

That right? No matter how big or small the turbo, a set boost pressure would give a set in cyl pressure. 90% sure it right, but somtimes you missing out the obvious so best to ask...

 

eeeeeeeeeeehhhhhhhhhhhhhh?

methinks u hit the new topic buitton by accident

 

Nah i'm with ya there steve surely 20psi is 20psi no matter what produces it?!

 

Graceland---

By accident? What you on about you belfry? Course i started it on purpose, its a new topic

A new topic titled-

 

d'oh - excuse my insulence, for i';m only 20 and still half pissed

 

higher cylinder pressures with greater volume or air being consumed surely??

exhaust housing is irrelevant cos cylinder pressures are surely measured when all the valves are shut

i could be misinterpreting what you mean tho....

 

Are we talking compression pressure or actual internal cylinder pressure at any given time?

 

actual internal cylinder pressures post ignition i was on about

 

Basicly i was implying that say for example you had a engine with a T34 and for it to safely run 1.5bar it needed to be 7.5:1comp, would it be the same comp to run a T2 or T4 at the same 1.5bar boost.

Ignoring temperatures, which would vary, id say yes as the in cyl pressures would be the same as its the backpressure that governs the in cyl pressure...

Possibly

 

Is it not the air mass in the cylinder that denotes the pressure in the cylinder when it's ignited?

 

peak cylinder pressure or BMEP is not conteolled by the size of the turbo :s


alex

 

Peak cylinder pressure is however reduced if the head/exh housing/exh/inlet etc cannot flow the air mass.

PJay

 

yes imo pjay so the bigger the turbo, the bigger the volume of air, the greater cyl pressures

 

VE

alex

 

Agreed, as long as the head etc can flow enough air.

i think people get hung up that turbo pressure is the be all and end all when in fact its air mass/flow that's what is required and more boost isn't always the best way to acheive it.

PJay

 

Yea but matt you fool, the bigger the turbo the bigger the exhaust housing so the less the back pressure so more gasses can get out (greater scavenging) so the cyl pressure wont change..

Alex just agreed with me...

 

But surley if the turbo is too small it will provide a resriction that will effect the cylinder pressure, so then the turbo will effect the cylinder pressure?

 

Think about it on cossie engines.....

Your average 400bhp engine running 30psi+ on a T34 and greys---- 7.3comp
Your average 500bhp engine running same boost as the 400bhp T34 lump, but on a fuckoff T4 and 8 greens--- 7.3comp AGAIN.

Same boost pressure, same comp ratio needed, no matter how big or small the turbo is....

 

No, as the in cyl pressures would directly affect the inlet manifold pressure, which is where boost level is measured, so its the same no matter what...

 

The 500bhp lump would have higher cylinder pressure post ignition than the 400bhp lump regardless of boost pressure.

 

wait a sec...

bigger turbo bigger volume of air?

i'm going to actually disagree (and yes i DID read that big turbo essay from stu and it was my orignal Q about this on RSBB that sparked it all off )

you have a certain rea to pressurise right? you got your combustion chamber, inlet ports on the head, inlet manifold+plenum, boost hoses, intercooler etc.

now whe you pressurise these to 2bar absolute (ie 1bar bost) then lets say for example you have 1 million air particles (imagine theyre FOOKIN big particles)

why would it matter which turbo is used to pressurise this air?

1 million particles @ 2bar is 1 million particles at 2bar nomatter what way you look at it.

surely?

or am i missing something obvious?

i can see how a larger turbo has the potential to flow more volume of air but i cant see how a T34 cossie running 20psi can physically have more air running thru the inlet tract (which is what you are measuring with your boost gauge) than a T4 cossie running 20psi. i CAN however see how they create more power for the same volume of air tho...

 

You've just answered your own question.

 

jims right,,, if you have the same volume in your inlet track,,, it would take the same volume of air from your turbo to get it too the pressure you want!

boils law states it

p1 x v1 = p2 x v2

there is soo many factors to this question its mad!

but cylinder presure changes if you change the turbo and run the smae boost because you will reduce pumping looses,, and as alex said VE

 

i'm gonna save this post gareth as shit like that dont happen too often!

 

Steve,
Go read my turbo essay again you "Belfry"

 

who stu?

 

Just for you:

------------------------------------------------------------------

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:
--------------------------------------------------------------------------------
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?
--------------------------------------------------------------------------------



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
----------------------------------------------------------------

Hope this helps the 10th time round

 

sneaky edit

 

so the extra 100bhp is made from the bigger exhaust housing on the t4? if everythign else is the same? if you get more waste out of the chamber then you have to put more fresh air in.

also compressor efficiency has got to come into it hasnt it - to get a smaller turbo supplying the same boost as a bigger one the effiecincy is going to be poorer therefore temperature higher therefore less MASS of air for same volume = less power and less fuel required.

unless you say bigger intercooler with the smaller turbo?

hmmm actually i think im with same cylinder pressure theory, but with waste gases in there so leaves less power - but doubt thats 100bhp worth for the exhaust housing only.

 

yep of cause it does thats why a t4 tourcar set-up could make more hp if a different turbo was used

 

id wrote my reply before stu posted his big turbo little turbo essay...

im now going to re-read this to see if ive retained any info.

steve just bleedin confuses matters as his question is... are the cylinder pressures the same.

so not sure that stus post answers that directly... as far as I can see then yes.
but to me that doesnt mean that is the reason for the CR been kept the same?

 

The reason steve was showing the CR as the same was to provide a standard engine as it were between the 2 comparisons

 

yes but are they comparable?

do they really exist like that - as I would of thought that they had bigger inlet ports etc on the 500+ engine

so more air in for same measured boost?

 

by the way ive just realised that i was wrong, and i know why...

i think i'm wrong becasue what we measure as boost is the EXCESS pressure the engine is not consuming that just fills up the inlet tract...

if we make the engine consume more air and keep that pressure in the inlet tract the same, then yes we are indeed flowing a higher VOLUME of air

 

fuck im getting dulll in my old age

 

its not really excess pressure though.
everything accept a vacuum has pressue

hence why you didnt seem to understand vacuum leaks the other week


here's a twist on it then.. i think we've done it before but...
external wastegate - lets say can flow whatever you want exhaust gas wise
little compressor wheel turbo and little exhaust housing, and external wastegate, can the t3 for example be set to run 30psi volume, with the wastegate open taking away the greater exhaust gases.

and what would happen theoretically then (as in supposing the t3 was still in its effiecency range and not overspeeding or what have you)

 

jim, example of it not been excess pressure.

erst running 9psi --- up the boost to 14psi with everything been the same - you are forcing the engine to consume more air therefore it makes more power -- hence its not EXCESS pressure

that is correct though

 

it is cos its not in your bores

 

why isnt it? its measured back from the bores back isnt it?

its a measure of flow not a static pressure just in the inlet.

see my example above...

if it was just in the inlet then and excess why does an erst set at 14psi make more power than at 9psi?
with no other factors of improving the engines air consumption - you just keep the wastegate closed longer to create higher inlet boost pressure.

 

i havent got my thinking cap on today