Understanding Compressor Maps
 

Hi. I plan on going down the turbo route later this year, and I have been doing endless amounts of research in regards to which turbo kit to go for. That I am pretty much set on; full race.

Now I am trying to figure out what turbo size to go for. The kit offers the EFR 6258,6578 and 7163. I have a sound understanding on how turbos work and smaller turbo equals less lag, but I am trying to get a deeper understanding to properly figure out what turbo is best suited to my needs and goals for my future build. After being put off by just the sight of compressor maps and the calculations, I decided to dive in the deep end and try to gain some sense.

What are my goals? To make a balanced fun weekend/track car. It will definitely spend most of its time on the streets with the occasional track day, with the aim to eventually go to track days once a month. I want the car to be as reliable as can be; yes i recognise it won't be as reliable as stock but I don't want to be blowing parts often. I have no fixed power goal, although I would see the car hitting around 350whp on pump gas (93 for those in the US, 98 Octane for me down under).

When I first buy the kit, it will be a straight bolt on. No upgrades to fuel pumps, clutches, engine internals. Bone stock. I would expect to be around the 270whp and 220 ft lb. Again, these are ball park numbers as I intend to upgrade the necessary components. After about a year of owning the kit and creating an 86 fund, I plan on upgrading the necessary parts to handle the extra boost and go to around 350whp and 270 ft lb. I have already worked out all the parts needed; been researching for years; originally going supercharged route but the flexibility of a turbo kit got me.

So, what I would like is for you guys to hopefully help me to gain a greater understanding of how to read compressor maps, and whether they are a great tool to allow not just me, but anyone new to turbos, to figure out what turbo is best for them. I have done hours on the internet and most forums over complicate things and use jargon and just try to prove who has the bigger ego. Those with advice, could you please try and dumb it down so everyone can get a benefit out of this.

My first stupid question is how to i get images that I have put onto photo bucket onto the forum. I would like the actual image to appear on the forum rather than just a link so everyone can easily see what we are talking about.

Years and years of articles online...

http://lmgtfy.com/?q=how+to+read+turbo+compressor+map

http://www.turbos.bwauto.com/afterma...6_wrsin=92044&

S&S: Pressure Ratio = Boost. 1:1 = atmospheric (no boost), 2:1 = 14.7 psi boost, 3:1 = 25.4 psi boost, 4:1 = 40.1 psi boost

Mass Airflow is just that - how much air the turbo can flow *by weight* at a given pressure
Why use mass or weight? Because in engines, different temperatures affect/change/manipulate volume. Weight is the only equalizer between air & fuel.
Every 13.07 cubic feet of std. atmospheric air weighs 1 pound.

RPM x CID x VE = CFM
CFM * 60 / 13.07 = lb/hr Air
lb/he Air / AFR = lb/hr Fuel
lb/hr Fuel / BSFC = FWHP

With a little more reading and a calculator, you should be able to put all the pieces together.

^ what he said.

Simply put, a turbo is only capable of so much boost if it is moving a fixed amount of air (by mass). That is the pressure ratio vs mass flow portion of the compressor map.
Compressor maps also have other useful information in them, like
1. for small flow numbers, turbo compressors have a boost boundary, above which the turbo surges and gets damaged (surge line)
2. Efficiency islands: at your boost vs flow operating point, it helps you estimate what the efficiency of the compressor is, which helps to calculate theoretically how hot your air coming out of compressor will be
3. Turbo speed numbers: estimates turbo shaft speed at operating point. you can use turbo speed sensors to control your turbo or your engine mapping, it is a nice fast easy method of ECU control logic
4. Mass flow limits (choking): direct indication of max power limit of a turbo on gasoline

Thanks guys. This was one of the articles that I came across. Found it quite useful as it was comparing which two turbos would work.

http://www.lovehorsepower.com/joomla...id=4&Itemid=88

The calculations I have managed to get my head around and are pretty straight forward. What I guess I am struggling with is looking at two compressor maps and think, ok this turbo a is better for me than turbo b. Is relying purely on compressor maps a good way to figure out which turbo is best, or should it be used in conjunction with other tools?

http://i86.photobucket.com/albums/k1...psyrqhlnyg.jpg

http://i86.photobucket.com/albums/k1...pszdf73iqt.jpg

http://i86.photobucket.com/albums/k1...psgz3vzoim.jpg

http://i86.photobucket.com/albums/k1...psfyfwn00x.jpg

Ok so this is what i came up with earlier. At the moment, I have completely removed the variable of the engine and whether it will handle the power. All i want to know first is will the turbo be able to create boost at certain RPMs.

When i look at the 3 compressor graphs, all 3 will be able to create boost at the 3 different boost levels calculated. The only one I have found that won't work, is the EFR 6758 @ 20psi at 3,000rpm, as it is to the left of the surge line. the 7173 is pretty much right on it. So from what I can gather, all three are still options, so no less clearer as to which is best.

So a couple of questions;
1. In regards to the efficiency islands, what is deemed to be efficient? This i could not find anywhere. I know you want to be at the central island, but is there any tolerance to say over 65% is deemed efficient?

2. @ 3,500RPM, all 3 are below the 64% efficiency island. To me, this says it is inefficient in creating boost at that RPM. Is that correct or does it mean it won't make boost then. Reason I ask is I have heard many owner's of the full race kit making 13PSI by 3500RPM with the 7163.

3. Can the compressor map assist in determining what the power band will look like? For the 7163 and 6758, at peak RPM, they are pretty much smack bang in the 74% island. This to me says, the turbo is now the happiest and will easily make the boost. For the 6258, it is within the 74% up to about 7,000RPM and by 7,500RPM, drops to 68%. Does this mean power will drop off, or it will be able to maintain the power, but it less efficient?

4. What should my goals be when looking at the compressor map. To make sure I am to the right of the surge line and to the left of the far right? Or to spend as much time in the efficient islands as possible?


5. From what I can gather, the 6758 and 7163 look near identical with my plots; so between the two the 7163 is a no brainer. But looking at the 6258, it seems it has very good mid range and dies off towards redline. So it looks like 7163 vs 6258?

I got excited when i clicked the link!! Have to admit it did make me laugh!!

I have already done that, hence I am here to learn from your wonderful minds!!

Is this something I should be really paying attention to. Yes the more revolutions the more heat, but should my first concern be where my points lay within the map?

No, if you will be using ECU tuning methods like ECUTEK or OFT, there is no point in paying attention to turbo speeds

Efficiency is not an indication of how "happy" a turbo is or how much power it will make. High efficiency = colder air at intercooler entry; Low efficiency = hotter air at intercooler entry, so efficiency is used mostly for intercooler design.

If you're not comfortable jumping off that cliff yourself, run your options past whoever is going to be fabbing and/or tuning it for you.

Yep that makes sense. And colder the air the more power you can make.

All I was wondering is should I be looking at anything else other than compressor maps to help me pick between the 3 turbos.

Too much thought IMO. Go small for less lag or quicker power delivery n bigger for more power but with lag. Spend more on a ball bearing for quicker spool. Spend more on a twin scroll turbo and manifold for more efficiency and quicker spools. Spend even more on variable twin scrool turbo to have the small and big turbo effect with a wider and flatter torque curve. Advantages to the latter would be more power at every RPM with less boost needed. Why not just run a small turbo for quick spool but go at high boost for power? Less efficiency means more heat.