RomRaider

It's a simple question that I can't find the answer to.

How do you know what the target boost should be in each load/rpm cell?


The only thing I can think of is figure out what you want your max boost to be and taper down from there in all directions. But how long should you expect to hold max boost?

What should min boost be?


If there is no set limits, how do you figure it out through testing?


Thanks for the help
-Tom

I can't speak of anything but 02-03, but Throttle position determines target boost, not load.

You're right, my mistake, i was thinking of timing.

But still, how do you know at what throttle position and rpm you want your boost to be?

What you want to do is set boost so that the turbo is efficient and your egt's don't get too high.
The MAF determines how much air is getting into the engine. If you raise boost and your MAF readings aren't significantly higher than your probably just blowing hot air into the engine that is less dense and at higher pressures. I would compare MAF readings at different pressures to see when the turbo becomes less efficient.
Just an example on my car 2.0 VF30 at >5000 feet above sea level. After 18psi I had no real increase in MAF readings.

You can do this by comparing MAF g/sec at the same RPM at different boost levels.

Someone (Freon?) said that he liked to set boost targets in accordance with a compressor map, which makes a lot of sense to me. However with a stock turbo you may have a hard time finding a compressor map. In that case you sort of have to guess, as eminehart described.

However I suspect there may be cases where MAF goes up but horsepower goes down because the increased heat output forces you to remove timing to stave off detonation.

I've been reading compressor maps all day trying to figure out the relationship.

I figured out that your max psi that your turbo could run would be the point on the map that's furthest to the right. That's where the most air will be moving, so you look at the y value at that point and see what the compressor ratio is.

So, say we have a Td04


So you look at the point furthest to the right, which is somewhere around 370 cfm. At that point, the value of the y axis is 2.1

So your absolute psi = 14.7 * 2.1 = 30.87. Then subtract 14.7 to get your relative boost = 16.17. Which is pretty close to the max you should be seeing for a TD04

Which is pretty close since your target boost for a cobb stage 2 map is 16.1psi.

Doing this for a vf39 yields 22psi, which also seems correct.



The thing I can't figure out is how to read the map to figure out how much boost you should be making at other points or cells. I think it's dependent on the CFM per rpm/throttle position. Then you could look at the map and have a good clue since you would stay in the efficiency range.

But how do you figure out what CFM is required per cell?

you want to be efficient as possible. That's what those overlapping fingers represent.

More efficient, less it heats up the charge air.

I would think you want to force as much air in as possible (max cfm). That's the only way to make power. Keeping the charge cool won't help. The intercooler will take care of the extra heat.

I mean you can push 18psi if you want, but you'll only be pushing about 280 cfm. That's less power

Here is an Excel spreadsheet I think I found here. I found it to be very useful for determining best turbo efficiency for different RPM's. It should help you out.

Be sure to go to the top left corner of the spreadsheet and fill in correct engine size and ambient pressure.

Jeff

topic1926.html

The compressor maps are not that useful for a variety of reasons. They might be good for some general comparison but that's about it.

Keep increasing boost (and of course adjust timing to prevent knock) until your acceleration stops getting better.

There are diminishing returns when you start getting into the upper end of the turbo's efficiency range. The intercooler can only reduce the air x degrees... For arguments sake, lets say the intercooler can cool a charge 40 degrees. 240-40=200. If you up the boost and the charge gets hotter, 300-40=260. (this is not exactly how it works) - The charge is just hotter than it would be if the turbo wasnt pushing hot air... The intercooler cant magically reduce all the air that passes through it to the perfect temperature...

It's hard to determine actual pressure ratio - pressure is dropping at intercooler, stock WRX IC has significant drop. So actual PR numbers will be sligtly higher - be afraid of going too far on surge side.

Edit: I see, that's already discussed on mickeyd2005 thread

I just whipped up a spread sheet for calculating Airflow at different boost pressures at different RPM's. You can compare the output to the compressor map to help calculate your best target boost.

I am not an expert with spreadsheets but I think this could be helpful.

^ thanks for the spreadsheet. I'm not sure how to use it. It's exactly what i was looking for, but I'm not sure where the rpms are listed, I don't see a label.

It would be perfect to compare the required CFM at a given RPM to the compressor map. That would be a huge help.

Thanks for the info guys, I guess I'm confused about the topic because when I looked through the target boost maps of the protunes I had done, they are night and day, but for almost the same setup.

one map is pretty much the stock TD04 map that's been modified in the > 60% throttle and 3000 rpm up area to run 22 psi on a vf39 by 4200 rpm and hold it till redline.

It's like the tuner said, OK, I'm gonna run 22psi on this setup and hold it till redline, so I'll throw those numbers in and then just interpolate across a bit to smooth it out.

the other map is an entirely new map. But the differences from stock are huge. Like in the low areas the map has a target boost of about -11 at 800 rpm and -13 at redline in the first throttle angle column, while the stock has -1 to -11.

That's a difference of about -10 below and -3 up top.

I'm wondering why that was done and how those numbers were determined.

the other major differences between the maps are that the stock like map has no AVCS adjustment while the new turbo maps is accompanied by a whole new AVCS map as well.

...

Hey guys, I'm trying to do a writeup about everything I learned about this topic, but ran into a road block.

Take a look:

Target Boost:
This is where you will tell the car how much boost you want to make. The ecu will do what it can to help you to reach these numbers. They are broken down by RPM and throttle position.

So why not just enter 10000psi across the board and be done with it?
Each turbo has an compressor efficiency range. This is the area where the turbo produces a usable air flow. Let's take a look at the compressor map for a TD04-13G turbo

The pressure ratio on the y axis is how much ambient air pressure is compressed. At sea level that's 14.7psi. So if you look at the inner most ellipse at 200 cfm on the y axis, you see a 1.8. That means 1.8*14.7 = 26.46 psi, now subtract out the 14.7 and you get 11.76, which is what you would see on your boost gauge.

What do those circle thingys mean?
Those are a measure of how much of the energy input into the compressor is being used to compress the air and how much is used to create heat. So if you look at the inner most ellipse you see 75%. That means 75% of the energy is being used to compress the air and 25% is used to create heat.

What what does that mean for me?
It means that the more psi you try to run, the hotter the air will be.

So what?
Well, the hotter the air is, the less of it there will be per cubic foot. So even though you're moving 200cfm in the 75% efficiency range at 11.25psi and 65% efficiency range at 17.7psi, there will be more air molecules per square foot at the 75% efficiency range because the air is colder and therefore denser.
In other words you are moving more air (mass) at the lower psi because of the higher efficiency range.

But isn't that what an intercooler is for?
Yes, but it can only move so much heat. So let's do some math for a minute here to show why it's better to run a lower psi and stay within the compressor's efficiency range.
Here is the equation used to calculate the mass for air that is moving into your engine:


psia = absolute pressure comming into the engine (so atm + boost)
V = CFM from the map (we'll use 200 as an example)
DegR = Air Temp at throttle body + 460

so the only thing we don't know is the Air Temperature going into the throttle body. Well to figure this out we need to figure out 1.) what the air temperature will be after the compressed air leaves the turbo and 2.) how much the intercooler cools the air.

To figure out the air temperature after the turbo we use the following:




Tin= (ambient air temp + 460)
Pin= atmospheric pressure - intake resistance
Pout= atmospheric pressure + boost psi
Pout/Pin = this is the compression ratio
efficiency = efficience of the compressor.


for our example we have
Tin = 70F + 460F = 530F
Pin = 14.7psi - 0.5psi (guessed on the intake resistance)
CR = 1.8 for one and 2.2 for the other
efficiency = 75% for one and 65% for the other

So now



The next setp is to fund the thermal efficiency of the intercooler so we can estimate how much heat will be removed. To do this we will have to use the following formula:




Here is where we get lucky. TurboXS actually went out and tested the capabilities of the stock intercooler with the following results:
14psi max; Temp into IC 92.5C; Temp out 41.5C; IC Efficiency 73.4%
16psi max; Temp into IC 106C; Temp out 49.5C; IC Efficiency 68.1%
18psi max; Temp into IC 103C; Temp out 50C; IC Efficiency 66.3%.

So to figure out our final air temperature going into the trottle body we just rearrange the equation to:


So we apply this to our high and low boost numbers:



Now that we have all the parts, lets see what the final mass of air going into the engine is at both conditions:



Wow, holy crap, it turns out I'm wrong about this assumption. That kinda leaves me stumped. So the question becomes, when is it better to turn down the boost?