Delica Canada | Mitsubishi Van Club

I've just finished explaining turbo upgrade options to someone, and in the process made some new turbo maps in a more understandable format. Might as well share them here...

First, though, some turbo basics:
Without a turbo, your engine would ingest a fixed volume of air depending on its speed. At 1000 revolutions per minute (rpm) a 4D56 takes about 40 cubic feet per minute (40CFM). At 4000rpm it takes 158CFM. You can only burn so much fuel in a fixed volume of air, so your power output would be limited. Luckily for us air is compressible- if you double its pressure, you double the amount of fuel you can burn. That's what a turbo does.

A turbo ingests a large volume of air and compresses it down so it'll fit in your engine. The amount of air the turbo needs to ingest depends on how big your engine is, and how much it's being compressed. Our un-turboed 4D56 at 1000rpm takes 40CFM. If you want to double the air, the turbo needs to take in 80CFM. 1.6 times the air = 62CFM. 2.6 times the air = 101CFM. And so on.

Flow maps describe how efficient a turbo is across its range of possible airflows and compression, so you can choose a turbo that will be most efficient on your engine, at the speeds and pressures you use most often. There's much more detailed nerdery here if you're interested/insane/insomniac :http://www.stealth316.com/2-3s-compflowmaps.htm. If you read and understand that, you'll realise I'm simplifying things here quite a bit.

And so, on to the pretty pictures.

Here's our stock turbo. What the hell are we looking at? The whorls are the important bits, the centremost island is the turbo's maximum efficiency (a whopping 76%), each band out from there is about 2.5% less efficient. The coloured lines represent the turbo's airflow requirements at various engine speeds (e.g. red=1000rpm). From here we can see what sort of boost levels will make use of the turbo most efficiently.
Looks like peak efficiency happens around 2250rpm (100kph freeway speed in Japan), up to 12psi (factory boost limit). Mitsubishi definitely know what they're doing- this turbo is perfect for fuel economy and low EGTs. The blue line (representing 4000rpm) does go off the right edge of the map, which indicates the turbo is overspeeding, but only a little bit. Unfortunately, if we turn the boost up to even 14psi you can see that the turbo will overspeed at a mere 3500rpm, and the engine will be operating outside of the turbo's efficiency peak. If you want more power efficiently, this is not the turbo for you.

Let's have a look at the next turbo up in the range, the 13G:
Peak efficiency happens around 3000rpm (green line), but you can go up to 20psi without risk of overspeeding the turbo. The red line (1000rpm) is off the left side of the map, which indicates that the turbo isn't going to work down there, but at 1300rpm it'll be making 14psi which is not bad. This would be a good choice if you wanted more power for towing, though you'll have decreased efficiency at freeway speed.

The last one is the 15G- this is a big turbo, and the one all the boy racers talk about. It's not great on our engines though: Peak efficiency at 4000rpm is ridiculous. Up to 23psi is nice, I guess, but there's even less available below 1500rpm, and freeway cruising speed is still well outside the peak efficiency zone. Great for drag racing, I guess? Not so great for torque.

(I might shift this into the Technical Reference section once it drops off the front page)

Great info as usual Growler--
I have a couple questions which will let me better understand the info you've provided. The first is what constitutes a turbo overspeed condition? The other is between the 9B and 13G I would presume that more air and greater pressure for equivalent rpm's should provide more power. How do you see this in the charts? Hopefully it's not a bunch of complicated calculations.

Once I can get my head around this I'll try to have better questions for my next group!!

Thanks

Larry

"what constitutes a turbo overspeed condition?"

The turbine inside a turbo floats on engine oil bearings. Oil bearings are great up to a certain speed, and above that they fail pretty spectacularly- so manufacturers specify an upper limit to how fast you can allow the turbine to spin. The top curved line on the map indicates maximum safe turbine speed. On the 09B map, the green 3000rpm line crosses the danger line (marked 180000rpm) at about 19psi- that means that if you try and make more than 19psi at 3000rpm, the turbo will be damaged. Annoyingly, the 4000rpm (blue) line hangs off the right side of the chart*, but if you extend the top line down you'll see that the blue line would cross it at about 12psi. Trying to make more than 12psi at 4000rpm is bad.

"more air and greater pressure for equivalent rpm's should provide more power. How do you see this in the charts?"

You're right: more pressure at equivalent RPMs means more power, but as we've seen, too much pressure will damage the turbo. To prevent damage, the wastegate limits boost to a safe level- usually the maximum boost you can safely make at the maximum RPM. Wastegates are dumb. They limit boost to the same pressure at all RPMs, regardless of what the turbo is capable of.

Look at the 09B chart- at a top speed of 4000rpm (blue line), the most boost we can safely make is 12psi. So our wastegate is set to 12psi. 12psi at 1000rpm, 12psi at 2000rpm... That's nice for not blowing up the turbo, but it means that much of the turbo's potential is unused**.

Now the 13G chart- at 4000rpm we can make 20psi safely. Set wastegate to 20psi, and now we have 20psi available at all RPMs. Much better.

the 15G chart shows you can make 26psi at 4000rpm, which would be awesome. Unfortunately, maximum pressure isn't the only thing to consider when choosing a turbo, and the 15G has other drawbacks (like how much of the 1000 and 2000rpm lines are off the chart entirely***, and how the peak efficiency island is way up around 4000rpm.)




*(The right edge of the map indicates that the turbo is so inefficient you might as well not have it: looks like an 09B turbo really runs out of puff by 4000rpm).
**The fancier electronic boost controllers allow different boost limits at different RPMs, which is awesome. They're also really expensive.
***On a diesel engine, don't worry too much about the left edge of the map, it just means the turbo won't be going fast enough to make any usable boost. On a gas engine, bad things happen to turbos when you cross that left edge too often.