Understanding Compressor Maps & Choosing A Turbo

[TrisK]

Hi Guys,

Reading and understanding compressor maps was something that baffled me for a long time, so hopefully with this I can help you to understand them, and use them for some of the helpful information.

Please be aware this is just a basic for dummys guide, and is by no means 100% correct, and is definately not 100% of the information that you can get from a compressor map. Theres a reason this sort of stuff is a several year long degree!

Now, one bug bear of mine is when people say 'OH yeah, im running a TD04'. A TD04 is a family of turbos. We also need the 2nd part, which is what gives you information on the compressor. This is key. As this will tell you how/when it will spool, and if it will make the power that you want.

This is a compressor map for a TD04-15g. A pretty common TD04 used on our cars.


Calculating Theoretical Max Power.
Now to make approxiamly 10hp at the flywheel, you need to move 1LB of air a minute.
So using this knowledege you can roughly calculate the maximum power that a turbo could make.

To do this take the far right point of the graph, in the TD04H-15G above, this is nicely marked for us as 428CFM.

BHP = (CFM/1.427)
BHP = 299.9BHP

So this tells us the turbo maximum airflow supported is for approximately 299.9BHP. This can be used to roughly tell if a turbo will supply for the figure your hunting.
Please note: This doesnt mean your engine will spin the turbo fast enough for this power, it also doesnt take into account any loses across the intercooler and doesnt take into account the effiency range of the turbo. Its purely a quick and dirty way to find if a turbo is roughly sized correctly.

Calculating Potential Spool.
To calculate theoretically when you could acheive full boost. First choose what boost your targeting.Your pressure ratio is calculated by
(TARGET BOOST+14.7)/14.7 TARGET BOOST IN PSI
A simpler way for us bar guys to work it out is
TARGET BOOST + 1 TARGET BOOST IN BAR

So lets take an example of 1 bar (14.7psi) on our TD04H-15G, that gives us a pressure ratio of 2.0.

Draw a line across at 2.0. Where you hit the graph draw down.


As you can see this hits, at around 110CFM.

To find the RPM that a starlet will spool this at you use the forumula

RPM = (5660(CFM) )/ (CC * PR * VE)

CFM is the airflow from the compressor map, that we found before. This where full boost will be achieved.
CC is 1.3 (The Cubic Capacity of our engines)
PR is Pressure ratio, or target boost, as calcuated before
VE is volumetric efficency, I havent been able to find a figure for 4EFTE efficiency, so I've been guesstimating at 88%. Which is what most websites recommened for a healthy engine.

RPM = (5660(110)) / 1.3 * 2 * 88)

RPM = 2721

What this is telling us, is theoretically, the TD04H-15G on our engines, could achieve 1 bar of boost, by 2721RPM.

Now, lets try this with a TD05HR-16G6, a turbo commonly found on the Evo 8.


Heres the graph, so for 1 bar again, so a Pressure Ratio of 2.0, we read across til we hit the map, then look down, and I get a reading of approximately 150CFM.

So plug it in to our calculations
RPM = (5660(150)) / 1.3 * 2 * 88)

RPM = 3710

So the TD05HR-16G6, on our engines could acheive 1 bar of boost by 3710RPM

How much Boost?.
Again, this can be calculated mathematically based on your target power figure.

AIRFLOW = HP * AFR * (BSFC/60)

Airflow is the amount of air in Lb/min that is required

HP is the target flywheel horsepower

AFR is the target Air/Fuel ratio, you want to be around 11:1, on boost, roughly.

BSFC is brake specific fuel consumption, this is a figure that tells you the fuel flow rate required to generate each horsepowe. This will change at different RPMs and for different engines. Again, I cant find figures for the 4efte, but a rough guess figure is 0.5-0.6. Race fuels, and methanol etc will lower this figure, meaning you need less boost.

Let's stick with the TD04, so we will aim for a target of 240hp, which is roughly what people seem to make on average TD04 family turbos, such as our TD04H-15G we are using for these examples.

AIRFLOW = HP * AFR * (BSFC/60)

AIRFLOW = 240 * 11 * (0.55/60)

AIRFLOW = 24.2lb/min

You can then convert lb/min back to CFM by multiplying by 14.472, so

AIRFLOW = 350CFM.

We decieded in the first part of this the max flow for our TD04H-15G was 428CFM, so it is possible to get this power from this turbo.

TO BE CONTINUED

[Zeldoz]

That is confusing, but brilliant!

[triple j]

Great info man, thanks.

[Phil]

Just throwing this out there.... feel free to decide your beliefs yourself lol

http://www.clubwrx.net/forums/engine-modifications/92306-stock-wrx-turbo-not-13g-13t.html

Trisk i trust this is suitably on topic, the link strongly suggests that what we call a td04 (currently -g) is not actually the correct turbo trim at all when checked.

as it was named as hearsay they may be right.

Phil

Edited January 17, 2014 by Philm

[TrisK]

Just throwing this out there.... feel free to decide your beliefs yourself lol

http://www.clubwrx.net/forums/engine-modifications/92306-stock-wrx-turbo-not-13g-13t.html

Trisk i trust this is suitably on topic, the link strongly suggests that what we call a td04 (currently -g) is not actually the correct turbo trim at all when checked.

as it was named as hearsay they may be right.

Phil

Just for you Phil. Here is the TD04-13G and TD04-13T compressor maps layed over each other, scaled the same, and animated to fade. (Dont say i never do anything for you)

I would say for us, at 1.0/1.2 bar there is a lot in it, but it does start to make a different at 1.3 bar and above!

[Phil]

Just for you Phil. Here is the TD04-13G and TD04-13T compressor maps layed over each other, scaled the same, and animated to fade. (Dont say i never do anything for you)

I would say for us, at 1.0/1.2 bar there is a lot in it, but it does start to make a different at 1.3 bar and above!

Haha thanks for that trisk, that was the guy hotrod(see the link) point.

two different turbos and the one we call td04-13g is actually a 13t when measured.

So the issue though we have the comp maps, is that we have no reliable info about which turbo we have.

Phil

Edited January 17, 2014 by Philm

[AdamB]

Where did you get 110 CFM from?

Remember you need 2 points to plot a graph and I can only see you calculated the pressure ratio.

I worked out that at 1 bar the 1.331 engine with an 88% VE will flow 298 CFM. Which is about right as more boost than this the compressor starts losing efficiency.

Edited January 17, 2014 by AdamB

[TrisK]

Where did you get 110 CFM from?

Remember you need 2 points to plot a graph and I can only see you calculated the pressure ratio.

I worked out that at 1 bar the 1.331 engine with an 88% VE will flow 298 CFM. Which is about right as more boost than this the compressor starts losing efficiency.

I was just calculating where in the RPM range the turbo would/could be on full boost.

What RPM are you getting 298CFM at?

[TrisK]

Also added a section of theoretical max power

[AdamB]

I was just calculating where in the RPM range the turbo would/could be on full boost.

What RPM are you getting 298CFM at?

But how can you put in the forumla 110 CFM if you don't know without testing the turbo on a gas stand that that is what the compressor will flow at that pressure ratio?

Essentially your trying to solve an equation with 2 unknowns which isn't possible in this case.

Only asking where you got 110 CFM from as its not clearly stated mate. Although your not far off, if I reverse back the formula I get 112 CFM.

I used 7200 as the RPM limit.

Edit:

I see what you done, just gauging the surge line as the point in which the turbo starts making positive boost pressure.

Edited January 17, 2014 by AdamB

[TrisK]

Edit:

I see what you done, just gauging the surge line as the point in which the turbo starts making positive boost pressure.

Yeah mate, was starting with the really basics of could this engine even spin this turbo. Going to start with a picking a turbo, then hopefully gonna carry on writing this to a 'what power will it make'

Glad somebody is checking my work, im not qualified in this, its all self taught

[AdamB]

I go the opposite way and just change the RPM in the formula until I get to the surge line, probably make more work for myself but I can gauge and plot that way how efficient the compressor is at a range of RPM's.

Just to add, do take the numbers you get with a pinch of salt, don't expect a 1.05 AR turbine housing to spool the same as a 48 AR housing just because a compressor map says so lol.

[TrisK]

I go the opposite way and just change the RPM in the formula until I get to the surge line, probably make more work for myself but I can gauge and plot that way how efficient the compressor is at a range of RPM's.

Just to add, do take the numbers you get with a pinch of salt, don't expect a 1.05 AR turbine housing to spool the same as a 48 AR hoYusing just because a compressor map says so lol.

Yeah. Although this is all theoretical as the engine isn't 88% efficient across all rpms. 88% was also just a average figure plucked from the air

Also to remember this is entirely based on compressor not on housing, or lressure drops or a multitude of other variables

[TrisK]

does anyone have any interst in me writing any more on this?

[TurboTobz]

It's useful/interesting imo

[Ryan_EP82 GT]

Yea keep writing Trisk!

[dangeestarlet]

Yes interesting ,

[richardc9052]

Yes, keep going

[morgey]

It's a good read dude. Keep it up

[seab]

If you need any more in depth info or maps give me a shout guys. I work in aerodynamics for Holset so do this for a living :-)

[gorganl2000]

excellent post. keep writing if you can

[harerace]

Agree, this is awesome, we are looking at maybe turbo our Starlet with the 5EFE forged motor we built, Wiseco Pistons, SCAT Rods, has had a heap of work done, we also run the 4AGE bridging caps on the main bearing caps and 4AGE main bearing stud kit in the 5EFE block

Edited February 13, 2017 by harerace

[5e colin]

a tfo35hm-19T compressor upgrade is about the best turbo you can have on our stock 4efte engines fast spool and much stronger top end then a stock tdo4l-13t we normally use

[glanzanut22]

I inderstand none of this ☹️