Detailed look at turbo sizing (compressor maps, VE, etc)

[Joe]

OK heres the deal, we all have a general idea on what turbo will get you to what power goals BUT a properly sized turbo can make a WORLD of difference on your car. shorter spool, longer engine life, etc. i always love to use the example of my friend who has a stupid large turbo on his RX7. @10psi the intake temps are over 180 degrees vs. at 16 psi they are 10% away from ambient air temps! thats because the turbo is so inefficent at such low boost/power the air is compressed differently and gets ridiculously hot

anyway i am by no means an expert so i need help from you, the members of NICO who do understand this stuff. im gonna ask these questions and would love to see them answered by people who know this subject. over time this will evolve into a great FAQ thread for all other RB owners.

What is a compressor map and how do you read it?

What is volumetric effiencey? (VE)

How do the two corrolate?

if anyone has had questions on turbo sizing and such please ask them in here too!

[Carl H]

if you dont have it corky bell's maximum boost, very good read.and for info on the intarweb http://www.turbomustangs.com/turbotech/main.htm

[RB20DETodd]

OK heres the deal, we all have a general idea on what turbo will get you to what power goals BUT a properly sized turbo can make a WORLD of difference on your car. shorter spool, longer engine life, etc. i always love to use the example of my friend who has a stupid large turbo on his RX7. @10psi the intake temps are over 180 degrees vs. at 16 psi they are 10% away from ambient air temps! thats because the turbo is so inefficent at such low boost/power the air is compressed differently and gets ridiculously hot

anyway i am by no means an expert so i need help from you, the members of NICO who do understand this stuff. im gonna ask these questions and would love to see them answered by people who know this subject. over time this will evolve into a great FAQ thread for all other RB owners.

What is a compressor map and how do you read it?

What is volumetric effiencey? (VE)

How do the two corrolate?

if anyone has had questions on turbo sizing and such please ask them in here too!

VE, haha i learnd this in A&P school but like most things i forgot the exact meaning, ill put it this way a boosted motor is close to being 100% volumetric efficient depending on how much power its putting outBecause the max potential of each cubic inch is used. im retarded
Modified by RB20DETodd at 12:27 PM 5/9/2006

[FattyMcBaggins]

Corky Bell's Shop is literally in my neighborhood. Gated airpark community, and his shop is 10 seconds from my house, within the gates of the neighborhood. He's a great guy, and he's the most knowledgeable car guy I've ever known. Maximum Boost is a GREAT read by the way. It will explain tons of things. Sorry for the threadjack!

[MAGILLA]

Here is another site that has a jack load of info if you have the time to read it. http://www.turbobygarrett.com/....html

[SeVa-S13]

I'm sure this is all covered in the links but I'm bored so I'll type a little because people seem to ask me about this alot and I end up plotting tons of compressor maps for people, ha.

VE (Volumetric Efficiency) is basically a ratio of how much air makes it into the cylinders versus how much air would be in the same volume at ambient temperature. Most modern DOHC, 4 valves per cylinder engines have around 80-95% VE, according the illustrious Mr.Bell. Byt the way,"Maximum Boost" is quite old by now and not necessarily the bible of modern day turbo tech and theory, but a great basis I recommend to everyone except novices, as good ol' Corky likes to assume his readers have certain basic knowledge and doesn't bother to explain some of his abbreviations in particular. (It's been a while since I read it.) /tangent

Of course all NA engines are going to have a VE <100%, whereas supercharged engines can attain well over 100% because that's obviously how they make more power; more air. And yes your turbo engine is supercharged; a "turbocharger" is just an exhaust gas-driven centrifugal supercharger, unlike those internally geared Paxtons and Vortechs you see hanging off the front of domestics.

VE relates to compressor maps because it's a necessary variable for the formula to calculate your engines airflow rate (CFM or the more proper, and modern lbs/min) so you can plot on compressor maps. The basic formula is:(cid*RPM*.5*VE*PR)/1728cid: Cubic Inches of DisplacementRPM: Rabid Pelvic MovementsThe ".5" is due to using 4 stroke engines; one power stroke per revolution. VE: Volumetric Efficiency, obviously...PR: Pressure Ratio in terms of bar; your absolute pressure (ambient + boost), divided by ambient pressure. My RB20 ran 12psi, so (14.7+12)/14.7=1.82, which would be my PR.The 1728 is just to convert cubic inches to cubic feet.

[Joe]

AWESOME info guys! keep it up im having horrible flashback to school when i learned this stuff.

[rb20detsilvia]

So how do you figure out what the best turbo would be for like an rb20

[rbsileighty]

Key point to note on VE is that it goes down as engine speed goes up... so when you run the numbers keep this in mind.

That Garrett link is one of the better ones out there... it should get just about anyone by in trying to figure out turbos

For simplicity with comp maps here's the deal:

Left column (y-axis) is pressure ratio between your boosted pressure and the atmosphere around it... so if you are running 1bar of boost (being that atmosphere is 1 bar) your P2/P1 is 1... so you trace this value out with the calculated Mass Flow (lbs/min... which as someone said before includes your VE which is an estimate since most of us don't have the equip... I usually assume .96-.98... I'd rather choke the turbo than oversize it... just my opinion)

I think there is a grand national site that has a calulator somewhere... I'll look for it later unless someone can find it and post it up... gotta work so we keep producing Nissans!

I'll post some more on this stuff later...

[Joe]

So how do you figure out what the best turbo would be for like an rb20

what does "best" mean?

it ALL depends on what youre doing with the car. the "best" for a drag racer is completley different than the "best" for autocross.

also, lets try to keep off topic discussion to a minimum. there is some GREAT information in here already i dont want it dragged through the mud.

[rb20detsilvia]

i just meant if there is a formula to figure it out than somone could show how to figur out what the most efficient turbo would be for the rb20 on stock internals sorry if that sounded dumb

[Joe]

i just meant if there is a formula to figure it out than somone could show how to figur out what the most efficient turbo would be for the rb20 on stock internals sorry if that sounded dumb

you missed the point! there is no 1 most efficent turbo because it all depends on what you are doing with the car! 50 turbos can be damn near perfect effcencey but will all perform differently and make different power.

[Wulfgang]

I'm going to dispute a few things...

First, SeVa-S13, you've got a typo. NA engines have VE < 100%. Also, I see people make the comment that Bell's book is "old" all the time, but then they never say why. I think that is an unfounded concern because (from a tuner's perspective) turbo technology hasn't changed much. You still read the maps the same, you still need to match the turbine AR to your needs, you still need good oil supply, good intercooling, etc. Corky is a bit eccentric (aren't we all?), and I think that people sometimes dismiss his book as "old" because of it. It's still a good book, and I would wager that there will be nothing in this thread that contains "newer" info on setting up a good turbo system.

I also dispute rbsileighty's comment that VE falls with rpms. If the fuel and ignition maps are set up properly, peak VE will actually occur somewhere near peak torque in the rpm range. This is why short intake runners perform better at high rpms... they achieve peak VE later in the rev range.

[rb20detsilvia]

you missed the point! there is no 1 most efficent turbo because it all depends on what you are doing with the car! 50 turbos can be damn near perfect effcencey but will all perform differently and make different power.

ok i see what you are saying

[DanCouga]

Here is a pretty good read on VE

[Reignman]

Here are a couple compressor maps I did for the RB25. One with a Garrett GT30r and one with a GT35r. You can see that with the larger turbo you need to run it at higher boost to keep it in its efficentcy range. These maps have helped me pick which turbo I want based on how much power I want and how much boost I want to run.

[BoostFab]

Useful info from another site: Terminology:

Compressor - This is the "cold" side of the turbo that sucks in intake air and compresses it for the engine to later combust with fuel.

Turbine - This is the "hot" side of the turbo. Hot exhaust gasses pass through it, expanding and cooling. This expansion spins a turbine wheel that drives the compressor wheel via a shaft. Unfortunately turbo manufacturers don't make turbine maps available to the general public.

Absolute Pressure - This is pressure referenced from a pure vacuum. Most calculations done involving compressors use absolute pressure. Note - 1 atmosphere = ~14.7 psia (Absolute pressure in pounds per square inch) = 0 psig (gage pressure in pounds per square inch). Your boost gauge reads in psig, referenced to local atmospheric pressure.

ONTO THE MAPS

Surge - This is lowest amount of airflow a compressor can supply at a given pressure ratio(getting to that). Any pressure above this at this airflow, the compressor will "gulp" air. This is not good for your turbo, or your power output. Fortunately you have to saddle a pretty huge compressor with a small turbine to really worry about this effect.

Here is a compressor map with the surge line highlighted in red.



On the X-axis(horizontal) you'll notice the mass airflow of the compressor in lbs/min. On the Y-axis there is the Pressure Ratio. Pressure ratio is defined as follows:

Atmospheric Pressure + Boost Pressure = Pressure RatioAtmospheric Pressure

So the astute read will notice a pressure ratio of 1.0 is the exact same as atmospheric. A pressure ratio of 2.0 is equivalent to 1 atmosphere or ~14.7 psig in your intake manifold. Without concrete data proving otherwise, it is always the best course of action to assume the pressure is ambient at the compressor inlet and make note of the pressure drops of the system will in the end cause less horsepower to be produced than the mass flowrate of the turbo would suggest.

The oval shaped rings on the compressor map are efficiency islands. They are regions where the compressor has approximately the same efficiency at compressing the air. Of course, the higher the efficiency the better since the compressor will be introducing less unneeded heat into the charge air. Note that as you go away from the maximum efficiency island you always go down in efficiency. By the time you're off the map you're usually in the <60% range, which is not a good thing.

The lines that slope from the surge line to the right and down across the efficiency islands are constant speed lines. This would be really useful if you could match up the speed of the compressor to the speed of the turbine and find out its efficiency and mass flowrate for that shaft speed, but since we don't have turbine maps we're kind of at a disadvantage there for picking the ultimate turbo match. These maps aren't too detailed. Some maps will have loads of data like putting numbers on the speed lines, more efficiency islands etc.

I won't go into the hard equation to calculate the mass airflow of the engine, as it really doesn't gain anybody any further insight into the turbo selection process. The only important things to understand that the big factors in how much mass airflow an engine is consuming are:Engine DisplacementVolumetric Efficiency(how well the engine breathes)Pressure at the inlet valves(BOOST!)RPMBy altering these things(more displacement, cams to increase VE, more boost, more RPMs) you can make the engine combust more air and make more power. I'll be attaching a spreadsheet that makes calculating the airflow of an engine an easy matter. It does over simplify things since it doesn't vary VE by RPM and whatnot, but it is a reasonably close approximation. I use a VE of 90% in most my calculations. It is pretty close to what a modern 4 valve engine gets in high RPMs, and tends to be conservative on less modified engines.

So go ahead and download the spreadsheet and we can look at a compressor map.

Here I'll look at a GT30R turbo on an S52B32 engine with a VE of 90%, displacement of 3.2L and maximum RPM of 7000. For the first go, we'll see what happens at a modest boost level of ~8.7 psi(pressure ratio of 1.6).



How I evaluate compressor maps is to note the airflow at 2000 RPM. Find that on the X-axis and draw a straight line from that point at a PR of 1 to the airflow at 3000 RPM at your desired PR(1.6 in this case). This gives you an idea of how a typical turbo will look when spooling up, and let you know if it's at a risk of surging. From there, the line should stay at that PR all the way to the airflow at redline ~39 lb/min here.

As you can see, surge is not a problem here, but this turbo sure does look a bit too small for this sized engine! It goes off the map just before redline, so that means it is very inefficient at higher revs.

Let's see what happens when we up the boost to ~17.4 psi(PR of 2.2).



No risk of surge due to this being a large engine for the turbo, but boy does it ever get REALLY inefficient at higher revs. Past 6000 RPM it is again off the map.

Let's go to a slightly larger turbo to see the difference. Same boost of 17.4 psi(PR of 2.2).



Now that is more like it! Notice how the engine spends a good amount of time in the really efficient islands, and the turbo is still at 72% efficiency all the way to redline. I'd think this turbo would be putting out in the 450-500rwhp range at this boost on this engine, and that's probably being a bit conservative. If the VE of the engine was even higher(which it can be), this turbo could still put out even more power. The compressor map also suggests it has a bit more headroom on this particular engine.

[REDLN]

this is nice info yah put up but can we start from the view of a noob that doesn't understand this. I know someone see this and read it and in the head it's like blah blah blah. I at one point in time just looked at the hp ratings and didn't look at the A/R and other things.

So where should a noob start at?

[Joe]

[noob]how do you determine VE for different engine RPM's? (what is the formula)

i know best idea is to plot them with different boost pressures on the maps to get an idea of how the turbo will work, but i guess i overlooked this info[/noob]

[SeVa-S13]

Thanks for catching the typo, I'm sure there are more.

[noob]how do you determine VE for different engine RPM's? (what is the formula)

i know best idea is to plot them with different boost pressures on the maps to get an idea of how the turbo will work, but i guess i overlooked this info[/noob]

I don't really think calculating the exact VE for every RPM step that you're plotting is needed. RB's a pretty modern engines with decent stock flow characteristics so I usually just play it safe and use .9 across the board. With cams you'll likely jump up a bit in the higher rpms so maybe a .95 if you wanted to be that thorough. But I doubt such nitpicking is going to be necessary to pick which turbo you want as the changes in airflow will be relatively small.

And don't forget people, just because your engine plots out a certain way on a compressor map, doesn't mean you'll be making that boost at that RPM; choose turbine's carefully. (This is more an area of preference than set rules)

[Carl H]

30r+rb20=tehwin!

[rbsileighty]

I'm going to dispute a few things...

I also dispute rbsileighty's comment that VE falls with rpms. If the fuel and ignition maps are set up properly, peak VE will actually occur somewhere near peak torque in the rpm range. This is why short intake runners perform better at high rpms... they achieve peak VE later in the rev range.

- Fluid friction occurs between the molecules of a gas or liquid in motion, and is expressed as shear stress. Unlike solid friction, fluid friction varies with speed and area. In general, lubrication is the substitution of low fluid friction in place of high solid-to-solid friction

You were missing my point, and I agree with what you are saying...

I'm saying that if you don't retune your motor with cams/intake mani/exhaust mani/head work... and try to run comp map calculations saying you can make more power with your stock setup at 8250 on a RB20... you're kidding yourself b/c the VE falls off quickly after ~6100 in stock form

[Wulfgang]

Ah, ok. You meant "engine speed" as in "top engine speed." I thought you were talking about the whole useable rpm range. Sorry for the confusion.

Yes, I know a bit about fluid mechanics....

[l0nestar]

Reignman,

What did you use to generate the graphical map?

Reason: I began reading this thread and then to the other links and then thought of writing a piece of code to help generate all of the data (mostly from the Garrett site). How do you plot the points for your map? What did the plotting for you? Theoretically I could whip something for giving 'raw numbers' in just a few minutes, but it would be quick-n-dirty. Is there already programs to do this? Thanks.

l0nestar.

[l0nestar]

I think there is a grand national site that has a calulator somewhere... I'll look for it later unless someone can find it and post it up...

really. If not I could whip something together shortly, let me know! I was thinking of some more 'advanced features' and would appreciate any info if NICO would like me to write this. At first I was going to write like a VB.net app, but then decided it would be more universal to do it in either CGI or (eeww) Java so it could be viewed as a web page, instead of having to actually install an app. Let me know if you guys are interested. I am just unsure of how to create the graphs. Like I said to Reignman, I could write a quick-n-dirty app / script, then get more advanced with time.

l0nestar.

[Darius]

There is a program out there that already does all of this including compressor maps. It's called "turbocalc" and I have a copy of it from somewhere. Can't remember where I got it because it was so long ago, but it basically runs through the exact sequence of inputting VE, CID, rpm, etc and spits out a few maps with your three points plotted on it. Then you can flip through and see which map fits your application best. I'll e-mail you a copy and you can do what you want with it.

[l0nestar]

Awesome! Yes Please!

[Reignman]

I have a spreadsheet that does all the calulation once I enter the engine info and I just used Photoshop to plot the points on the compressor maps

[l0nestar]

Hrm. Interesting. I must learn of this 'Photoshop' that you speak of. *kidding* I have 0 ps skills. I am trying to think of a way for the script to generate plot points itself. Thanks for the info!

[l0nestar]

Hrm. Interesting. I must learn of this 'Photoshop' that you speak of. *kidding* I have 0 ps skills. I am trying to think of a way for the script to generate plot points itself. Thanks for the info!