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Turbo Flow And Pressures

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As topic States.

I have looked for threads regarding this and although there is a mountain of reading on turbo flow rates and the likes, nothing i found that made it clear to me, as to which makes the most impact on the engine so, I was wondering;

Just an Example

If you had 2 turbos; For Example; Lets say a GT-RS and a GT30R;

One comes in 52 Trim with a smaller compressor wheel and One comes in 56 Trim with the larger wheel.

@ given RPM, the 56 with the larger compressor wheel and housing would flow more air Per/Liter, Than a 52 Trim, with smaller housing and wheel.

For the smaller Turbo to match the flow of the Turbo to make the desired hp output, the smaller Turbo would have to work harder, to match the specified power at a higher PSI, and may not equally flow as much as the larger in comparison.

Lets say you wanted 270 rwkw from your car. And either of the turbos could do it. But you Might Drive on 220rwkw daily,

- If the Higher flowing larger Turbo, could flow lower Actuated pressure (PSI) for your desired rwkw

- And the Smaller turbo needs more pressure (PSI) to match this rwkw desired figure..

What puts more strain on engine internal components, such as pistons, conrods, bearings, values, etc. whilst Underload, between either of the following.

more PSI Pressure ?

Or more FLow at a lower PSI pressure ?

or does it have the same effect either way? And I'm way off the ball here :laugh:


Edited by silverbulletR33
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What car is this for 2.5L or 2.6L ?

I've been thinking about this question for a while. (re 2.5L)

This issue seems to be more to do with terminal velocity at the outer edges of the blades as far as I know.

If the outer edges of the blades hit terminal velocity you will get compressor surge where the air will backflow through the compressor.

I'd have to assume that you need to find a BALANCE of compressor to turbine ratios. Then consider the total flow you need for the power level you are looking for.

The ratio of air flow being the most important issue. So size of turbo not as important other than being small enough to run on the exhaust output and big enough to keep flowing enough air at high rpm to give you constant boost at the level you want.

Bigger blades represent bigger pipes = more flow. More flow capability means more total air supply.

I would expect the terminal velocity to be hit earlier on big blades so better to run these with lower speed.

The thing is that this does not necessarily mean lower boost.

While bigger pipes represents more turbo lag as more volume to boost up.

Bigger turbines may make boost come on later. But bigger compressors will make boost come on earlier.

The smaller turbine side is usually better for bringing the boost on earlier and doesn't really restrict exhaust flow as the wastegate gives way once it reaches the correct boost.

it is however limited to the amount of topside.

The big power can only be acheived with a bigger compressor flow. And to power the compressor you need more direct exhaust onto the turbine. With a bigger turbine, instead of bypassing the turbine though the wastegate more of the exhaust gas will go directly through the turbine.

In the end though you simply need the exhaust flow to be enough to bring the turbine up to speed sufficient to run the compressor at the output you require. With 2.5L engine we have enough exhaust flow to run bigger turbine side, and along with it can therefore choose bigger compressor.

If you don't bother increasing the exhaust valve lift at the top of the rpm range you may find a smaller turbo is better,

while if you set up your engine cams to flow heaps then go for a bigger lower pressure turbo with more flow.

Directly answering one of your questions. The lower actuated pressure will occur best on smaller turbine, with larger compressor

However a larger compressor wheel will hit terminal velocity sooner if you use a smaller turbine, meaning less total flow!

Regarding the question on strain you asked...relevant is pressure as opposed to flow: The capability of the manifold to hold pressure is up to 20 psi without modifying gaskets as far as I know in a skyline.

As for the internals, on hearsay conrods may go in first 2 years at power output 250rwkw. While 230rwkw is considered safe for your conrods. Not sure any factors influence this other than purely the power strain.

Now for my own question...

What is the advantage if any of 6 blades over 5 on the compressor?

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After reading my own reply, I realise that I don't really understand cams.

I want to know if the compression ratio is lowered through raising cams.

Also if so does this mean more or less exhaust output at lower rpm?

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Thank you Edge, for your post, regarding turbo dynamics and flow.

Hmm, I think the answer is A). PSI pressure (but this is my opinion, and i wasnt too sure)

As during the compression stroke, with more PSI added you are raising the amount of compressed air in the chamber giving more compression during the compression cycle, although your physical compression on the stroke is the same.

anyone? I was already leaning on A for this reason. But just Wanted some Clarity.

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Hi all , no time ATM but back later on .

The thing to remember firstly is how much charge air the engine in question can swallow . The right capacity compressor can supply the required air flow by mass (lbs/min) and not too much more . Any more is a waste and critical turbine shaft power is wasted powering a larger compressor and its extra capacity may not have been of any use anyway . Result = lag and added turbine inlet pressure (backpressure) for zip .

Pumping capacity will vary with a compressors trim size and a large trim ie 56T 71mm compressor may have the same approx maximum as say a 76mm 48T wheel but the maps will look different and 101 things like housing style A/R etc all play their part .

Compressor surge is not a factor of exducer tips going supersonic , thats a sign of the compressor being spun beyond its mechanical speed limit .

More later cheers A .

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Here' my uninformed response :)

I'm gunna guess that from the engine point of view, if your creating the same kws, that it doesn't make a difference to the lower/higher psi, except the fact that the air inlet temperature will be higher due the that littler turbo spinning faster and creating more heat.

But if that turbo is built to handle it, like the GT-RS for 270rwkws, it would be fine.

If your ultimate goal is 270 (or whatever) you are better getting the smallest turbo that will reach that limit so to reduce your lag.

If you get a bigger one which runs a lower psi, you will have some lag issues and if you aren't planning on going any bigger, it's a bit of a waste of power potential.

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I suggest getting a turbo that is only a LITTLE bigger then your desired power level (no more then 50rwkw) and then tune it to the boost level needed to achieve that power.

If you can run your charge air temp much lower then thats allways a bonus.

Flow is much better then PSI IMHO

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I suggest getting a turbo that is only a LITTLE bigger then your desired power level (no more then 50rwkw) and then tune it to the boost level needed to achieve that power.

If you can run your charge air temp much lower then thats allways a bonus.

Flow is much better then PSI IMHO

Ok So flow would be better than PSI, possibly because of Heat distribution from the Turbo, Any other Reasons?

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I gather what you want to know is which (GT3071R/GT3076R) will make the 270 Kw/360 Hp

with best response .

The first aspect is what can you afford to spend because either can get you there , arguably the GT3071R would be more responsive in its best configuration which is twin scroll and external gate .

The bottom line is having an equal pressure balance across the engine and its not possible to achieve this and have responsive power delivery (throttle/boost response) with a single scroll system . When you collect all of an engines exhaust putts and merge them before a single turbos turbine nozzle you create an area of high pressure for the cylinders to vent into . The problem is that the exhaust manifold pressure ends up being high when it should be low (at the exhaust ports) and low when it should be high (at the turbine nozzle) . With a true twin scroll system each turbine nozzle only see's an exhaust putt half as often so it gets twice the time for its exhausting cylinder to blow down through the nozzle , also on an I6 there are only 3 cylinders connected to each half of a divided manifold so the chances of cross contamination is far less . This is the way to reverse the situation and have low pressure when it should be low (at the exhaust ports) and high pressure when it should be high at the turbine nozzles . Low pressure for the cylinders to vent into and high pressure across the turbine nozzle to accelerate the gasses into the turbine blades . Venting into an area of low pressure means much more of the exhaust gases thermally driven expansion energy (high velocity) is available to power the turbine . Its the low energy loss communication between the exhaust valves and the turbine blades that does the trick .

You can sort of have low energy loss venting the cylinders into a large A/R single nozzle turbine housing but having the volute and nozzle that big makes for lower gas velocity into the turbine blades and lazy turbine response .

Turbo wise the real GT3071R is a good thing in twin scroll form though not so responsive in single scroll form . This is because the GT30 turbine is reasonably large for the 71mm 56T compressor and its a juggle to make it spin the compressor fast enough to make the "wind" without compromising exhaust flow and ever increasing turbine inlet pressure . Single scroll turbochargers are very sensitive to turbine housing A/R ratio where twin scroll ones are not .

The real GT3076R has a fair bit larger compressor (76.2 vs 71.1mm) and in 56T is borderline missmatch particularly with the smaller available A/R GT30 turbine housings . Its cartridge was originally a HKS spec Garrett one and it was intended to be used with large ratio HKS turbine housings ie 3037 56T 0.87 A/R , GT3037S 56T 1.01 A/R or 1.12 A/R .

with the base model GT3037 the comp trims go 48/52/56 and turbine housing A/R's 0.61/0.73/0.87 .

I don't think there's any cheap easy way to get a responsive 270Kw though efficient intercooling and headwork/cams should enhance whatever you go with on an RB25DET . Some say the RB25 GCG Hi Flow with the VG30 turbine housing can get within reach but the other engine enhancements are probably a big part of getting there .

Cheers .

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Thank you guys, for your replys, But my question was not directed properly obviously, and i shouldve left out the 270rwkw part, out of my initial post, less to confuse everyone.

forget response, and 270rwkw for a second, and lets concentrate on this part,

What puts more strain on engine internal components, such as pistons, conrods, bearings, values, etc. whilst Underload, between either of the following.

more PSI Pressure ?

Or more FLow at a lower PSI pressure ?

given said that im aware there are a few variables to consider, like Heat, etc. But i just cant wrap my head around it properly. and looking for clarification, And disregarding even heat for a second.

using those variables, which of the above would be more characterisitcally imposted on internals and why.

does compression ratio fit into this equation? along with heat generated from the smaller turbine?

what are the fundermentals on forced induction physics against compression ratios and the likely hood of detonation occuring.

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yep dicopotato03 - I was talking about the compressor wheel outer edges going supersonic or what ever you want to call it. Are you trying to say it is caused by physical vibration or something - what do you mean by mechanical speed limit? As far as I know it can spin even faster such as mach 2 for example!

Bigger compressor wheel outer edge can go supersonic faster. Then cavitation (air compression from air being thrown to the outside) effect is lost and you get back flow of the pressurised air through the compressor blades which is obviously really bad.

Choice depends on balance of speed and wheel sizes for the amount of air required by the engine, and on whether the engine supplies enough exhaust volume at low rpm to spin a larger turbo's turbine.

Variable cam shaft in the 2.5L helps to give this.

Some turbo's attempt to prevent compressor surge through the blades by enabling backflow surge to go around the outside of the compression wheel instead of through it.

I would think you would have more loss from a small turbo going supersonic as you attempt to acheive higher power than it is designed for.

As for whether compression in the cylinders is increased due to higher manifold psi now that sounds like it could be the case and is a good question, hope someone can help with an answer.

It would also make sense for more heat to be generated with higher psi.

However in general a small turbo should spin up producing boost earlier due to smaller exhaust turbine housing taking same initial air volume = more speed and less pressure due to flow dynamics of circular air flow in a smaller housing.

The idea that a smaller turbo is better thus has its appeal.

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The no1 enemy is exhaust gas over temp generally caused by restrictions on the hot side . If you take most of the restrictions out of the exhaust side the detonation demon is easier to cope with . Everyone likes to think that high charge temps are mainly caused by elevated inlet air temps but reversion sends post combustion gasses back into the chambers and this is very likely to be at higher temps than post compressor air .

If you need mega revs out of a compressor then chances are that the exhaust gas velocity required to drive the turbine at that speed comes at the price of elevated turbine inlet pressure .

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I'm looking for clarifaction on this too

My guess would be that the higher pressure on the intake manifold generated to produce the required volume of air with a smaller turbo may cause detonation.

If variable cams can be adjusted to lower compression ratio it should aleviate the problem mostly shouldn't it?

My limited understanding of cams is that cams control valve lift duration and distance and thus probably also control compression ratio through the 2:1 directly proportionate effect on piston duration? At least it makes sense that the compression can be lowered through allowing valves to let some air escape at the beginning of the compression stroke to compensate for the higher pressure and temperature.

With the same power output being in the operating range of each turbo:

It is probably equally efficient regardless of bigger or smaller turbo and the only real difference is the speed of initial low rpm spin up on one hand; and the potential for increasing power output on the other.

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At least it makes sense that the compression can be lowered through allowing valves to let some air escape at the beginning of the compression stroke to compensate for the higher pressure and temperature.

Although This would seem the case, physically this doesnt change the compression does it?

Its not designed to be there to get out the higher pressure from the turbo, because Ive seen this kind of profiling in not only turbo cars, but even n/a cars, or v8's with high comp + high valve lift. Cam profiles are normally ' these days' designed to scavenge the chamber to expell all the used air it can, without going into the next cycle, to effeciently discharge the chamber, to a certain degree depending on the type of cam it is ie. nitrous, turbo, na. vs duration.

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do some research on static compression ratios and dynamic compression ratios.

static compression ratio is determined by the volume of the combustion chamber at btc compared to the volume at tdc.

dynamic compression ratio is determined by the cam setup. if your cam keeps the exhaust valve open for a short period of time during the compression stroke, then some fuel air mixture is lost, and the dynamic compression ratio is lowered.

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Dynamic or effective compression ratio is determined by many things though the free flow abilities of the inlet tract and the trapping efficiency of the cam timing have I think the most say . If you have any notable pressure drop in the inlet system ie poor filter/s or inadequate throttle/s porting valve size etc then cylinder filling won't be the best . Long duration cams open the inlet valves earlier and close the exhausts later so generally the overlap period is longer and trapping efficiency less - at lower revs anyway . This because gas has mass and the faster it goes the less it wants to stop and go backwards (reversion) , ever seen a big cammed N/A engine "come up" on its cams and take off ? Thats because now the gasses have enough velocity to travel in the intended direction .

So worst case scenerio poor inlet tract and long cams means low effective compression pressure .

On N/A high performance engines higher static or measured compression ratios are used to raise the dynamic (effective) CR when the trapping efficiency is low because of long duration cams used to allow the engine to breathe at high revs .

Turbo engines are different because the compressor supplying air a greater pressure than the atmospheric gives us better cylinder filling so the effective CR is higher than the measured or static CR .

Personally I think cams that give more valve lift and modest duration is the way to have the engine breathe and have high trapping efficiency . The BUT is that the whole exhaust side has to be low restriction from the exhaust valves south so as to not create high turbine inlet pressure . My old broken wheel (twin scroll turbo system) has a large say in this and if you can keep the turbine inlet pressure down , trapping efficiency up you have modest boost pressure/lots of airflow/good turbine response and much more controllable power delivery . Systems like this seem to make a lot of torque over a fairly wide range and don't need horrendous rev ceilings to achieve it .

BTW I was talking about a single twin scroll system earlier but much the same is achieved with parallel twins .

Cheers A .

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Are you refering to single compression or a turbine wheel when you speak of a single twin scroll system? Or a single turbo that has two wheels with seperate exhaust inputs for each wheel?

So to try to answer silverbulletr33's question,

The bigger turbo would be kinder as the exhaust side flows better and requires less (turbo) rpm: This results in as discopotato3 described above, the lower likelihood of exhaust gas backflow into the cylinders raising air temp under compression which can lead to detonation.

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