New Zealand Datsun Club

So i thought it was about time to open the lid on a tough subject.

Turbo Compressor maps & how suitable a particular turbo will be. You want to buy a turbo but you don't really know how well its going to flow and cool its going to run.
Before you begin need to know the followings things.

*Your Engine Cubic Inch Displacement
*Your Maximum working RPM ( say 500rpm under the red line)
*How much boost do you think you will run ( don't bullshit yourself because it wont help you make a good choice )

Before you Read your Turbo's compressor map you need to calculate 2 key numbers

Pressure Ratio
And airflow in CFM or lbs/min

Pressure Ratio PR

PR = atmospheric pressure + boost pressure divided by 14.7

this looks like:
14.7 + 12
14.7

=1.81

What does this mean ?
It means atmospheric pressure at sea level is 1bar , and 1bar = 14.7 psi
so you always start with 14.7, then you just add your boost pressure of 12. this means you are pushing 0.8 of an atmospheric pressure into your engine, and that is BOOST.

Airflow Rate

CID x RPM X 0.5 X Volumetric efficient divided 1728

and example for a VG30ET is

183x6000x0.5x0.85
1728

=267 cfm

this figure of 267 cfm is for Zero Boost

So now you take the pressure ratio of 1.81 multiply it by 267 cfm

and this = the airflow rate for 12psi

1.81x267
= 483 cfm

Now you have your 2 numbers to read a compressor map

PRESSURE RATIO 1.81
AIRFLOW 483 CFM

Some compressor maps are read in LBs/min in which case you take your CFM and multiple it by 0.069
so you do 0.069 x 483
= 33.3 lbs/min

now is the easy part, look for 1.81 on the left hand side, then find 483 or 33 on the bottom

match up the two and you have successfully plotted compressor efficiency

you want to be somewhere near the middle Island to be as efficient as possible, in the example i just posted the plot lands on the left hand side of the middle island which means somewhere near 79 % efficient telling us this compressor is a good match for this engine and this boost pressure and also with some room to move.

If you are way over to the left hand side off the graph your turbo is too big and you will be surging just to try and spin it
If you are way over to the right hand side your turbo is spinning really fast and is too small for your engine, you are loosening efficiency and creating more heat.

If anybody needs help with this feel free to post or pm me

nice write up mate that turbo you have looks nice and big

Nice man, I've read a couple of explanations before but could never really get the hang of it. The volumetric efficiency is just rough estimate, there's no easy way to calculate it?

I did my motor and turbo and the intersection would be just to the right of the middle island, looks like it would be about 77% efficient.

Yea the volumetric efficiency is just an estimate.
I figure that corky bell used 0.85 in his book for an example of a 302 ford so that number is also good enough for me.

there is probably a fancy way to work it out but really we are not engineers so we can be happy with that estimate.

this and more is covered in maximum boost,its definitely worth buying or borrowing