HydroStream6

iTrader: (16)

But it's not only about volume, it's about pressure as well. 1000 CFM at 5psi boost may make 400hp on a 300ci motor while 1000 CFM at 30psi may make 500hp on a 130ci motor. There's many factors at work here. But it's true, a turbo has no way of knowing or caring about RPM. It's just an easy way for people to convey what they mean.

RedHotG8

iTrader: (1)

What part of higher rpm's produce more exhaust flow don't you understand? With a supercharger you'll never choke it out, but with a turbo it CAN happen if it's not large enough.

SATAN

iTrader: (2)

What makes you think that I don't understand that the higher you wrap out an engine, the more exhaust it produces? I GET THAT.

Still has nothing to do with what I am talking about, which has clearly gone over your head.

I'm done here. I've made my point and I'm glad at least a few people understand where I'm coming from.

qqwqeqwrqwqtq

At this point I'm not sure what is being asked/debated.

Does a turbo "care" about RPM? Well a turbo doesn't care about anything as far as I know but if you choose incorrectly it will certainly choke early and have a severely limited RPM range.

The following example is roughly correct, the figures are from memory-

A PT7675 on a 346. It will make around 900rwhp at 6,000rpm

Same turbo on a 370 will make around 860rwhp at 5,500

Same turbo on a 408 will make around 800rwhp at 5000 then drop off hard. Will not make more than 17psi with a blockoff plate in place of the wastegate.

Even at lower boost/power levels the trends will be the same. (408 peaking very early compared to the 346). In this case both combinations will easily make 600rwhp but the 408 will still peak and drop off approx 1000rpm earlier than the 346. The turbo is not maxed out at that power level but since it is not matched well to the larger engine the power/torque curve gets shifted far to the left.

This all leads back to what has already been stated. Pressure ratios.

Sure you may have a compressor wheel capable of moving 100lbs/min but that doesn't mean anything if your backpressure is 4x intake pressure by 5k rpm.

Proper turbo selection is key to achieving the desired power curve and usable RPM range.

SATAN

iTrader: (2)

Ok, well here is exactly why I say RPM maxing out a turbo makes no sense. Lets say someone asked the following question (Which is a question I see a lot and the reason I made this thread).

"I have a Garrett GT47R turbo and a built LQ4. Is this turbo too small for my engine?"

There is NOT enough information in that question in order to answer it. One would have to know the HP goals to answer the question. Yet time and time again, I see people saying...

"No, that turbo will be a restriction at higher RPM's."

To which one would say, "Oh, thanks for the info. I was only planning on making about 900whp with it and spinning the engine to 7,000rpm's. Guess I will look for a different turbo then... "

That last little bit of info that constantly gets left out changes everything now, doesn't it?

qqwqeqwrqwqtq

Did you happen to read my previous example? You don't need to know the horsepower goals because "X" turbo on "X" displacement is going to produce peak torque and peak horsepower at roughly the same RPM at every boost/power level. (Though will be even worse/exaggerated as you approach the limits of the turbo).

If someone said "I have a Precision 7675 and a built 408. Is this turbo too small for my engine?"

I would say- Yes, it is not ideal and will be restricted on the exhaust side at higher rpm regardless of the power/boost level. It will peak early at 500rwhp and it will peak early at 700rwhp because it is not a good match for that displacement. It will basically choke after 5000rpm no matter what boost level or power level you are at.

So yeah, you can safely say "X" turbo will not produce a favorable power curve on "X" displacement without even discussing the power level.

RedHotG8

iTrader: (1)

I don't think he does either. With that said I'm done here.

Orr89rocz

iTrader: (13)

If you are talking about the compressor side, RPM has everything to do with that situation because of pressure ratios. To calculate pressure ratios in the first place you need an RPM value.

Have you ever used a compressor map before? To make X hp you need Y lbs/min. The other variable on the compressor map is Pressure ratio which is calculated by determining the absolute pressure required for a said motor to make the desired HP. It involves air temperature, cubic inches, and RPM. Then you just ratio that to atmospheric pressure.

Varying cubic inch and rpm will result in different absolute pressures and thus different pressure ratios.

The same compressor wheel can flow Y lbs/min at various pressure ratios. Where you fall on the compressor speed map is going to determine how much power you can make out of that turbo.

Just because a turbo has a max capability of 90 lbs a min does not make that a 900 hp turbo for ALL motors. Smaller cubic inches have higher pressure ratios unless turning super high rpm.

Case in point, a 200" motor turning 8000 rpm will have same pressure ratio as a 400" motor turning 4000 rpm assuming VE and bsfc is the same. So same turbo wont choke out on a 200" motor as quickly as it would on a 400" motor.



That GT4202R has a peak flow of ~95lbs min at PR of 3.3 or so. For a 200" motor to operate there, figure 7500 rpm to get it to work. For a 400" motor to operate at that point, you'd need rpm of 3750. Thats why that turbo wont make 900 hp on a 408" motor but can on a 4g63T evo provided the turbine side is sized correctly

The turbine wheel is similar situation. Too much exhaust volume will choke flow a wheel and backpressure will build up. Exhaust volume/mass comes from HP made. What goes in comes out! More power is more air/fuel. You can make more power with more RPM provided the heads/cam/intake are there to support it. So RPM does have its effect.

05HD

Damn, all you people are stooooooopid.

A turbo makes torque. What RPM it happens to make that torque at coincides magically with how much horsepoooowwwah it makes, almost like there is a mathematical formula involved or something.

By the by, what kind of retard spins a 2JZ to 10 grand on purpose?

Professor_speed

iTrader: (8)

This thread is driving me up the wall

A turbine of any type works because of Delta, period. (Delta = difference of pressure) Heat in a turbo can affect the volume of air and therefor the pressure, but that's not what makes it work, ignore velocity for now.

Now where the op went wrong was how compressor pressure ratio affects how a turbine will interact with the engine. running a compressor at 15psi is fine, but if it takes 60psi of turbine pressure to drive the compressor the engine will not be happy. (as stated by Hellbents10)

All a turbine cares about is flow? Turbine flow is very misleading when comes matching a compressor. It helps us compare turbines but what we really need know is how much work a turbine can do. Who gives a **** if it flows a ton of air if it cant drive the compressor? clipped turbines tend to flow more air than none clipped turbines but does that mean they drive the compressor with more force? NO you can increase the drive force by increasing delta or, by increasing leverage, (larger turbine). Lets take gt4788 build for example, put that turbo on 180ci and it pretty laggy but the exhaust pressure to compressor pressure will be favorable, great for a race car not the greatest for street car. same turbo on 320ish to 350ish engine and now you can drive the compressor to max flow and still have great street manners, the best of both worlds. 360ish plus and not the turbine cannot drive the compressor without excessive back pressure. not to say it wont work in less than max effort cars because it will, but you will never get all that the compressor is capable of due to the fact that the turbo that is not matched to the engine, it will choke early. put a turbine that can drive the compressor at a lower back pressure and you have made the engine happier and the turbo happier, the spool will barely suffer and the engine can now make more power.

look at a 346 with a t76 turbonetics with 68mm turbine vs. a precision 76gts cast wheel, the compressors are basically the same but the 75mm precision turbine can drive the 76mm compressor with less back pressure due to better leverage. both spool plenty fast.

I think people forget that turbos have two sides and they work in unison. there are pressure ratios that always working with, or against you depending how you designed your turbo SYSTEM. Pick your compressor based on how much power you want with your engine in mind (displacement,VE,etc.), figure out what pressure ratio will be needed to make that power, match you turbine to that compressor based on what pressure ratio the compressor will need to run.

The other thing to consider it how changing the compressor p/r will move you around on the efficiency of the turbo. class turbo guys are doing what you are talking about, to eke out every bit of power they can, slower engine speeds and more boost, and vise versa spinning the engine faster with less boost, to try and find the sweet spot of the turbo. tons of testing goes in at this level.

slow67

iTrader: (2)

Sorry but your wrong here, turbines NEED heat. Yes they can work with cold flow, but they are MUCH less efficient. You might want to check into the formulas for turbines, they all have heat in them all.

http://en.wikipedia.org/wiki/Turbine

Professor_speed

iTrader: (8)

Heat can increase delta did I not say that? reread what I wrote a turbine can work with out a heat differential, they cannot work with out delta, My point is for people to think in terms of delta not just heat

slow67

iTrader: (2)

Increasing the heat drop across the turbine means less pressure drop required, which also means less backpressure, and more power.

Professor_speed

iTrader: (8)

Its all about Delta, temp drop is a byproduct of delta "Ideal gas constant"

SATAN

iTrader: (2)

If a turbo is capable of supporting 1160FWHP, guess what? It has to be able to flow the amount of exhaust produced while making that 1160FWHP.

So by your logic, a 7675 is capable of making over 900whp, but can only flow 500whp worth of exhaust before it becomes a restriction; SIMPLY BECAUSE you put it on a 408 and revved it high?. I'm sorry but that makes no sense.

Holset HX35... Comes on a Dodge diesel pick up. It peaks down low and the engine does not rev high. It is this way from the factory. It is a tiny turbo for this engine. The reasoning is because it has certain goals it has to achieve. Yet people take those turbo's and put them on little 2.0 liter engines, rev the crap out of the engines and run 10 second 1/4 miles.

Both vehicles are VASTLY different. Both are running the same turbo. Yet both are achieving the VASTLY different performance goals they are designed to do.

When either of those vehicles starts to max out the flow of the HX35 turbine, guess what? They are both producing roughly the same amount of exhaust volume regardless of RPM.

slow67

iTrader: (2)

Did you not read what I read? If you do the math you will understand. Turbines extract energy, from both heat and flow. If you increase the heat drop across the turbine, you need less flow, which means a lower delta pressure across the turbine. It is not a byproduct. Efficiency of turbines goes up with heat.


You are saying lb/min of air ALWAYS = HP, which it does not. They shouldn't even make compressor maps in lb/min, they should all be in CFM.

HP is a formula derived from RPM and torque. This is why you can take a diesel turbo that only makes 150hp, and put it on a high revving gas motor and it makes 300hp. The higher the RPMs, the more the HP you have the potential to make.

Oh also, diesels have different exhaust volumes than gas motors (because of the lower EGTs and different fueling characteristics), so thats apples to oranges as well.

skinnies

iTrader: (33)

The real question is how far can I push my billet 76 on a 5.3.

Professor_speed

iTrader: (8)

We are off topic here. the OP could probably care less about the theory of turbine operation.

again if the compressor moves x amount of lbs/min,(horsepower) the engine size, efficiency and rpm will dictate what p/r is need for x amount of lbs/min once you know that you can figure out what turbine will be suited to drive your compressor.

NemeSS

iTrader: (127)

LS1NOVA

iTrader: (12)

Yes, yes it is.