Alright... I've always thought that I had a good grasp of what's happening (physics wise) with an intercooler in a turbo setup, but the more I think about it the more confused I'm getting.

I understand that you want cooler air going into your motor for the simple fact that the cooler air will resist detonation/pre-ignition. (less heat = less chance of combustion due to heat/compression before the firing of the plug)

I also understand that cooler air is more dense; so cooler air will have more oxygen going into the motor to combust.

But, the air going to an intercooler has already been compressed by the turbo and forced into the piping. The intercooler will cool the charge of air directly after the turbo itself... but there will be no more oxygen molecules added to the charge of air because the turbo has already compressed this air and passed it on to the intercooler (there is nowhere for any new oxygen to come from); so the intercooler simply cools the air flowing through it.

Now, if my reasoning above it correct (in the sense there will only be X molecules of oxygen in a stream of air after a turbocharger on its way to the motor, intercooled or not), then why would you have to re-tune your car after adding, say, a more efficient intercooler? In my little theory here, I can only see benefits of possibly adding timing due to the reduced chance of detonation. I oftentimes see posts of people who say that you have to re-tune your car or risk running lean when adding a more efficient intercooler/air cooling system, but I don't understand how that's possible, because the amount of air will always be the same...

This leads to question #2:

Now, I've heard of intercoolers creating a pressure drop in the compressed air; I can see that if the number of molecules of oxygen is not changing, and you cool the air, the pressure of the air will decrease..... so is the benefit of an intercooler to drop the pressure as much as possible so that you can increase the boost and effectively be adding more molecules of oxygen at a given psi of boost (i.e., add more oxygen by upping the boost without upping the actual boost seen by the motor)?

But, I've read that the lowest pressure drop across an intercooler is better (lower pressure drop = more effiecnt intercooler)? If this it true, where hasmy reasoning gone wrong? What exactly is the efficiency of an intercooler?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by servion &raquo;</TD></TR><TR><TD CLASS="quote">But, the air going to an intercooler has already been compressed by the turbo and forced into the piping. The intercooler will cool the charge of air directly after the turbo itself... but there will be no more oxygen molecules added to the charge of air because the turbo has already compressed this air and passed it on to the intercooler (there is nowhere for any new oxygen to come from); so the intercooler simply cools the air flowing through it.</TD></TR></TABLE>


this is wrong. there are more molecules of oxygen leaving the intercooler for a given unit of volume than there are entering the intercooler. Cool compressed air has more o2 in it than hot compressed air. So what I am saying is that more air by volume enters the IC than leaves it, because it becomes cooler and therefore more o2 in the same amount of space.

but of course, the same NUMBER of molecules leave the IC that enter it, they're just closer together, and at the same psi (aside from the dynamic flow losses of the IC).

 

for question #2, the pressure drop is not caused by the cooling of the charge, but rather by the resrictiveness of the intercooler itself. its harder to force air through all those slots than it is to push it through a pipe. so you get a pressure drop on the outlet since the IC is slightly restrictive.

you want minimal pressure drop across the intercooler, so the turbo doesnt have to work so hard to push air through it, and the effeciency is a reference to how much thermal engery it can take out of the compressed air only, nothing to do with flow characteristics.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by falconGSR &raquo;</TD></TR><TR><TD CLASS="quote">


this is wrong. there are more molecules of oxygen leaving the intercooler for a given unit of volume than there are entering the intercooler. Cool compressed air has more o2 in it than hot compressed air. So what I am saying is that more air by volume enters the IC than leaves it, because it becomes cooler and therefore more o2 in the same amount of space.

but of course, the same NUMBER of molecules leave the IC that enter it, they're just closer together, and at the same psi (aside from the dynamic flow losses of the IC).
</TD></TR></TABLE>

How is it that more o2 molecules leave the intercooler per unit volume than enter the intercooler? Where do they come from... are they being backed up and collected from the entrance of the intercooler to the exit somehow? I don't see how an intercooler can "add" molecules of o2 to a charge of air... the only place it can get more o2 is from the turbo.

Its true that cool compressed air has more o2 than hot compressed air... at a given pressure. So, X psi of cool comressed air has more o2 than X psi of hot compressed air... but it you let them both reach the same temperature, then the air that was initially cooler would be at a higher pressure.

I guess I still don't see how an intercooler can add more o2, since the only place it gets air is from the turbo.

 

The original reply is 100% correct, and it was a good post too.

The o2 dosen't come from somewhere... you need to understand the relationship between pressure, volume, and temperature.

PV=nRT

or

<U> [Pressure] x [Volume]</U> = Constant
. [Temperature]

So, as the air is cooled down. the pressure does not change, so the "volume" must decrease... or to put it another way, less colder air comes out.

Get it?

 

Ok... I see what you're saying with tht relationship in the equation.

So, if temp goes down, then volume goes down (as long as the pressure stays the same). But, I always thought that the volume would stay and the pressure would drop.

For example... if you check your tire pressure before and after a burnout, the pressure will be higher after the burnout. The volume stays the same because its all contained in the tire, and then temp increases due to the burnout. So in this case, temp goes up so pressure goes up with constant volume.

Why would the pressure be the variable to change in the tire example, but not the intercooler example?

BTW, I'm not attacking either of you or your responses (I appreciate your help). I'm simply trying to clear up my misunderstanding. Thanks

 

hot pressurized air enters the IC from the turbo. it cools down, and the molecules are more tightly packed because cooler molecules vibrate less and can be closer together without banging into eachother (yes, this is essentially where the concept of heat comes from. its just molecules banging together).

read what i said carefully. the volume of a given number of molecules that enter the IC occupies goes down when the set of molecules passes thru the IC, cools down, and therefore takes up less space, i.e. the charge is more dense. same number of molecules comes out that what came in. they're just closer together (cooler).

since the turbo is always pushing more air into the IC, the pressure stays the same. the pressure doesnt drop because the charge is getting more dense, as theres always more air coming into the IC to keep the pressure up. (aside from the flow losses for the IC).

its important to understand that theres two ways to make a substance more dense: 1) pressurizing and 2) removing heat. thats the best way pack the most oxygen in your intake charge.

 

Ahhh... the lightbulb upstairs just turned on.

So (to ensure my proper comprehension here)... the volume of air is decreasing due to the temp. drop.. but since the wastegate dumps at X psi (no matter what the intake temp is), the turbo is allowed to pack more air into the intake air stream at the same pressure. So, the function of an intercooler is to cool the air, which allows the turbo to put more air into the motor (to keep the boost at the same pressure), effectively increasing the output of the system. Makes sense to me. Sound right?

 

yeah thats it man. same boost level, more oxygen, more power.

 

Cool... now that I understand where the additional o2 is coming from, the whole setup make a lot more sense. Thanks all!

 

Where did you get the Manifold from?

 

Great discussion ... i hope to see more posts like this in the future... minus the B.S.