[Apollorx7]

OKI am reading "Forced Induction Performance Tuning" by A. Graham Bell. All of the information I am giving comes directly from this book. Unfortionatly this book doesnt seem to pertain to rotarys.

A/R:

a small a/r means quick spool up time but the turbo will choke the exhaust at the higher rpms
a large a/r means a slow spool which equals lag but there is more potential for power in the high rpms.

a/r is determined by dividing the area of the turbine nozzle by the radius from the center of the turbine axle to the centroidof the housing throat.

Now on to the compressor maps:

pressure ratio(pr) is calculated by adding boost pressure(bp) and air pressure(ap), and then dividing the sum bye the air pressure. the equation would look like this:

bp+ap = pr
ap

lets say you are running 14 psi of boost at sea level and you want to find the pressure ratio, the equation would look like this:

14+14.7 = 1.95
14.7

so the pressure ratio is 1.95

now lets say you are running 14 psi at lets say 1000 feet above sea level. According to the book, you would subtrast .5psi for every 1000 feet above sea level you are. So the equation would look like this:

14+(14.7-.5)
(14.7-.5)

this simplifies down to this:
14+14.2 =1.98
14.2

so the pressure ratio is 1.98

the pressure ratio is shown on the verticle axis(the one going up and down on the left side) of compressor maps.

more in next post...
hopefully i explained this so everyone can understand it :)

This post has been edited by Apollorx7: 01 May 2005 - 06:04 PM

[Apollorx7]

Now on to calculating engine airflow(which is the horizontal line, the one going across the bottom of the compressor map):

First of all CFM means Cubic Feet per Minute

to find cfm you multiply(x) the engine capacity in liters(L) maximum engine speed(rpm) engine volumetric efficiency(ve) and pressure ratio(pr) and then divide the sum by 5660. The equation would look like this:

L x rpm x ve x pr =cfm
5660

according to the book the VE of a
stock 2-valve = 85%
stock 4-valve = 90%
street modified = 93%
competition = 105%

These rating seem kind of generic to me but the book doesnt go farther into it(I'm only on page 90 right now, if the book goes farther into VE i will updat this). Plus it gives nothing for our motors :sad:

Lets say you are driving a lightly modified 4-valve 2 liter engine that runs up to 7200 rpm, operating at a maximum boost of 11.4 psi. (This is at sea level btw) Now using the equation i provided in the prievious post, you can find the pressure ratio(pr) which is 1.78 ( rounded up from 1.775).

But anyways the equation would look like this:

2 x 7200 x 93 x 1.78 = 421.2 cfm
5660

Now, not all compressor maps use cfm, others use pounds of air per minute (lb/min). The book says to convert from cfm to lb/min, multiply by 0.07.

421.2 cfm converts to 29.48 lb/min.(the book says 29.51lb/min, but my math gives me 29.48. I am assuming they just made a mistake multiplying in the book. If somebody else wants to check my math go ahead :) )

I'll plug the numbers into a compressor map in my next post.

[Apollorx7]

I just used the first compressor map supplied by 93r1 on the first page of this thread.

compressor map

I pluged in the numbers I got from my last post to plot the points, the numbers were: 1.78 pr and 29.48 lb/min.

The blue line is the plotted points. Idealy you would want the points to meet inside the inermost circle. This would mean that the turbo is flowing at maximum effiency at the highest rpm you would take the engine.

But on this compressor map our points fall well to the left of this circle. They dont even fall inside any circle, which means this turbo is not the one you want if you are only running the 11.4 psi. Now because the lines fall to the left of the circles I would assume that you would want a smaller turbo, and if the points fall to the right of the circles i assume you would want a larger turbo.

Also on the compressor map is the surge line. There is a way to calculate that but as of right now im not sure how to do it. When I have a better understanding of it I will post some info.


Another thing I want to add is that I am still learning and my posts shouldnt be taken as facts. I am just going by what I read and theorize. So if I am wrong about something please dont bite my head off.

This post has been edited by Apollorx7: 01 May 2005 - 07:51 PM

[Capt.Moe]

Sorry to jump in on everyone but, I am new to the whole turbo understanding scene and it's late and after work so bare with me.

I know that a T04 is a garret turbo.

but would someone please care to break this down for me.

To4E 57 trim, 360 thrust bearing upgrade

I'd greatly appreciate it, as my mind is just toast from work today.

Thanks.

MOrgan

[CGeek2k]

Quick question:

Does anyone have a good estimate on what the VE of an S5 N/A with a moderate streetport would be? I don't really want to set up something to manually measure the flowrate on my car. Dont harass me about the turbo + N/A stuff, I've read it all already.

Thanks
Andy

This post has been edited by CGeek2k: 02 November 2006 - 06:21 PM

[CGeek2k]

Also: Theres a slight problem with the CFM equation above. It assumes a 4 cycle motor that only displaces half the total engine displacement per revolution. Since a rotary displaces both rotors every revolution, the denominator should only be 2832 instead of 5660. IIRC, the displacement volume of 2 rotor faces is ~1.3L, which is displaced each revolution. This is why ricers like to say a rotary is a 2.6L engine.

Peace
Andy

[piescrank]

here I am looking threw all your replys to one another and for someone like me that doesnt realy pay much attention to turbo tech stuff you all seam to be covering it pritty dam well.
Ya got to remember that there is a lot of maths to do when you are dealing with matching and applying them to engines.
Compressor maps and what not are made by turbo engineer's for engineer's. Im not saying you guys are to thick to understand it all. In fact you are all doing a mighty job of something that is realy hard to grasp.
But I think for the most part especialy a lot of people get stuck turbo shoping. Todays Garret turbos will make more HP than most people can ever aford to make provided ya pick the right frame i.e GTxxr .
I dont remember the last time I herd of a street car 13B maxing out his GT35r running on pump fuel. (corect me if im wrong.)
I work in a turbo shop mainly on large frame marine turbos yea there application is compleatly different but principle is the same.
Nice work fella's