Intercooler theory and practice Part 1

This is a reprint of a small three part series I wrote on intercoolers so some may have read it before.

Abbreviations:

IC = Intercooler

Ambient = The temp of the air outside the car

Charge air = the air coming from the turbo to the engine

Tubes = These are the passageways that the charge air goes through within the intercooler.

AKA = As commonly known as...

CFM = Cubic feet per Minute (A measure of airflow)



I don't like imperial measurements, we (in the UK) have been Metric since 1971. BUT for the sake of simplicity here I am going to write in inches (") because it makes the numbers easier to digest for you.


Ok, let's start with the basics, the turbo on an engine compresses the air so more of it can be forced into the engine: More air (and petrol to match) = more power.

The main trade off is that compressing air also makes it hot, this is where the intercooler comes in - to cool the charge air. Cool air = more power AND reliability - hot air causes uncontrolled detonation within the combustion chamber which in turn leads to major engine damage and failure. Colder air is more dense (It contains more oxygen) so it gives more power.

The intercooler must meet two criteria, the first we just covered, the second is where a lot of people mess up: *It must be able to flow enough air for the engine to breathe properly*, get it too small and you will strangle the engine and throw away potential power.

Here is how you label the dimensions of an intercooler:











Let's look at a 'cooler in detail, there are two types of core: Bar & plate and tube & fin, it refers to the way they are manufactured and the shape of the tubes, B&P is more desireable as it flows better.

Bar and plate detail:



Tube and fin detail (an integrale IC):







Take a look at the zig-zag fins, these are known as 'Turbulators' or just fins, this is because they cause turbulance in the air. Basically the hot air hits and bounces over the turbulators as it goes through the tubes, when it does this it loses its heat to them, the fins then pass this heat (by convection) to the outside of the cooler, a similar process then happens - cooler ambient air passes through the external fins and cools them.

It is all down to heat transfer, basically an intercooler is a large heat sink, it takes the heat from the air within it then holds it until it is cooled by the air outside. The next facts are very important:

Turbulators slow the air to take the heat from them, the longer the tubes, the more you slow down the air, (velocity) do we want the air to slow right down? Of course not!! Slow lazy air = bad throttle response, AKA: Turbolag. Loss of power, response, economy etc.

FACT:
In a good efficient core the majority of air cooling is done in the first few inches of tube.

The rest is mainly waste, yes it drops the temp a little bit more, but nothing worth shouting about and is more than offset by the fact that that the air has practically gone to sleep....

What is a good figure here? About 7 to 8 inches, 12 - 16 as a max.

7 - 8"? Really?

Yep.

So why do aftermarket manufacturers make 'double pass' intercoolers which make the air go through 48" of tube and a U bend?



To make money out of people who don't know anything about ICs. Take a look here:





Will there be any flow figures (CFM)? Pressure drop? MPG? Power? Nope.
If you see a double pass intercooler for sale then just leave it on the shelf please, hopefully someone will melt it down and make something useful from it.


Please go to Part 2.

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Intercooler theory and practice Part 2

Fact:
It's the number of tubes, not the length of them which matters.

To make a basic comparison, you have a guy who can run a marathon and you stick a drinking straw in his mouth, tell him to breathe through it and send him off, he gets 1/2 a mile before collapsing with exhaustion, why? Because he cannot get enough air to his lungs, so you pick him up, dust him off and give him a drink and a rest then stick a 3" pipe in his mouth (!!) and send him on his way again, he finishes the 5 mile marathon..... Your engine is just like the marathon runner.


Now for some basic maths, if you look inside a 'cooler:




You will see that roughly half is tube and half is blank, so only about 50% of the total area will actually flow air. Lets take the integrale cooler and compare it, a quick sum gives us sq" remember, only is half is tube so:

8 x 3 (x .5) so = 12sq"

This:



Is a pathetic 7.5 sq" (6 x 2.5 x .5) So although it looks bigger it will actually flow less air...











And then we return to this complete mess of an attempt, oh yes, the good old 3 x 12 x 24 double pass! (oh my god):



The air has to travel a whopping 48" (4 times further than an integrale IC) to get through it and it has a flow area of 9".
It flows less air than the OE cooler.
Jeez, I bet you could measure the turbo lag with an egg timer.

Turbo lag

Yes we just brushed on it there, you wanna know where lag comes from where tube area and pipe diameters are concerned? Well I'll let you into a secret, lag comes from the fact that not enough air can get through in the first place, not too much volume.....





How do we know for certain we are on the right track? Where to look for good designs? Other newer cars of course, here are a few good examples:
















The first one is a VW, check out the well shaped end tanks, designed to give well balanced flow to all the tubes. Also note Subarus nice job, short tubes, compact and the idea of twin pipes to equalise the flow again. The Pug one is just good basic design, tapered end tanks, lots of tubes, a very bad 90' bend on the end pipe, but still, you get the idea.

So, an off-the-shelf good design? Let's look at this:



Core size is 3 x 18


This looks like a good design, 3" thick x 12 wide and 16" long tubes:




Now you've just got to do the hard bit - fitting it in the cramped space......

Intercooler theory and practice Part 3

Ducting is very important, especially if your IC is small and/or thick. Some people remove the duct from the integrale IC and then wonder why there car slows down as the intake temps go up...

FMICs cannot always be ducted easily, the air can go around them instead of through - which it does as it will always take the easiest path.

























Grp A integrale, this has a small compact intercooler so they had to make the best of it with an excellent duct, see how it is smaller at the entrance than where it meets the 'cooler face, the air has no way of whirling round and coming back out, it is forced through the intercooler matrix where it cools it down:







Ducting on a race car:





See how the rad is fitted at an angle? This makes the air struggle to get through the core and gives better cooling, it also allows a bigger core to be fitted in the same height.

And finally here is what happens on the front of an integrale:



The cooling air goes throught the rad and intercooler and is then dragged back out of the engine bay by the air passing over the louvres.



So does the theory work?



Of course it does ;-)

http://www.youtube.com/user/Evoluzione2?feature=mhw4#p/u/4/mYEyAaqBbd4