I was discussing this with my dad last night (a mechanical engineer) about turbo upgrades. I've seen a whole bunch of people on this forum talk about gains from upgrading turbos at a given level of boost. Now, I understand that forced induction is all about increasing mass flow into the intake manifold.

Now, if we go back to high school chemistry, PV = nRT. n is directly proportional to mass, so n = PV / RT. Given that the intake manifold doesn't change, Volume (V) can be assumed to be constant. So we can either increase the pressure or decrease the Temperature to increase mass flow.

So here's the thing. I've got a 1.1 bar tune on my K16 turbos and, while I haven't dynoed it, I'm assuming that i'm putting out something like 400-420awhp. Now, I've seen people with A28's putting down 500awhp on a 1.1 bar tune. How can that be? Is it the water cooling lines preventing heat soak into the cold side of the turbos or is it something else that I'm missing?

 

I believe it has to do with the volume of air moved for a given compressor wheel at a given pressure. A smaller turbo will pass a smaller volume of air than a larger turbo given the same pressure. While the volume of the intake is static, the amount of compressed air in that volume can be variable.

 

It can only be variable with pressure and temperature. I'm taking pressure to be fixed, so it can only vary with temperature. I suppose it's possible that the intake manifold has to pull timing to keep things at a constant pressure but that doesn't seem quite right to me.

 

It's way more than just PV=nRT. You need to think about the engine as an air pump so you're talking about air flow, not just pressure and how efficient your set-up is in moving that air.

 

It's actually pretty close to PV = nRT, the mass flow equation is:

Qm = ρ * C * (∆P)^n

ρ is the air density and is given by:

ρ = P / (RT)

Since C is constant, again, we're left with an equation that only varies with Pressure and Temperature. The engine can change it's resistive load by pushing or pulling timing, but what would cause it to do that other than changes in Temperature and Pressure?

 

The difference is in the density of the air filling the cylinders. A bigger turbo tends to heat air less as it compresses it than a small turbo, all things being equal. A turbo that heats up air less at a given pressure will be more efficient and gain more power. Also cooler denser air lets you inject more fuel for a bigger bang. The only trade off from usingwhat bigger andthe bigger turbosthe generally is more turbo lag or narrowing of powerband.

Talk to any board tuner for more details in comparing turbos.

 

That's what I've been thinking Morden. Thanks.

I really just wanted to understand the physics. I'm building up my car to AutoX in ASP right now (though maybe not in the future) which precludes Turbo upgrades, but I wanted to know where the differences came from. More being inquisitive than needing information with purpose.

This means that bigger turbos and bigger ICs will work hand in hand...

 

Why are u holding volume constant? As pressure goes up volume in the system goes up to a certain point. I'm sure the engine can take more volume than you can safely cram into it! Think of an air compressor. As long as u have a strong enough pump u can cram more and more volume into it. Cooling it down will help some but not proportionately more! Pressure is the biggest factor and that's what the larger wheel is able to do.

 

In an air compresser, volume is most certainly constant. If the volume changed you'd se a big bulge in your tank.

 

Yes but, there certainly are varying levels of compressed air that fit in that volume. You cannot look at air as constant as a solid or a liquid.

 

If that were the case you would have the same tune for all cars regardless of what turbo is installed. Using your logic we should be able to make our intake manifolds smaller, thus creating more pressure (PSI) and make more power. The answer has more to do with flow than pressure.

 

That's why he mentioned 1.1bar in his first post

He's saying the volume of the intake piping is a constant, and that the air pressure is at a constant 1.1bar for both turbo set-ups

 

Thumbs up

 

Well... I'm holding volume constant. Same intake manifold, just a different turbo. As i understand it, flow will be related to intake timing. And intake timing will be related to temperature and pressure (and, of course, volume, but i'm holding that constant).

Just trying to understand the physics from an engineer's perspective.

Tony, that article you sent is awesome. I think (from my initial scan) it looks like exactly the info i was looking for.

 

Its easy to get caught up with psi, but its more than that. An extreme case could be a bad roots blower generating 1bar but heating up the air so much that the density of the air is very low, resulting in little gain. A nice set of k16's flow much more effiently with same psi but generate more power because they dont heat up air as much lowing the density of the air. A bigger turbo could be even more efficient since it spins slower to generate same psi, heating air less, more density = more power with same psi. This is to answer why the difference in power between two turbos generating the same psi.

A bigger turbo also gives you more flow capacity to generate higher boost safely than a smaller turbo, downside is lag, assuming all else is equal.

Nitro adds little psi and gains huge power by adding lots of cold/dense O2 allowing you to add a bunch of fuel and zoom you go.