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You have to deliberately be dicking around to get a 454 to not make much power. Even with a 2 barrel carb.

I rather not go into that subject either... As that's a WHOLE different story from what I originally asked.

Screw it. Ill touch it.

12 psi is 12 psi is 12 psi

There is no "high flow 12 psi". There is no "low flow 12 psi". You pointed out a vital point Matt, the pipes are exactly the same. If you have the same psi, you are moving the same amount of air. Air ONLY moves if/when theres is a pressure difference. If you are moving a larger volume of air, it is because you are creating a larger pressure. As long as ALL other factors are the same, if both turbos are making the same boost at the same rpms, they WILL make the same power. The only time this isnt true, is when other key factors are different, namely exhaust flow.

Ill even touch this one. Lets clear some things up, 454 ci is 7.4 liters, this will help make all this a little easier to understand later.

Motors are nothing but air pumps. We move more air, we make more power. On an NA car when we open the throttle we move 1 atmosphere of pressure through it, this is often represented as 1 bar, or if we are at sea level (Which will assume for the rest of this point, 14.7 psi at sea level- which im going to round up for 15 psi to make the math easier to follow).

Now lets start playing with boost. lets take that cute little 2 liter motor, and add 1 bar of boost to it. This doubles the air its moving, this is equivalent to making it a 4 liter engine, and will essentially double its power, not perfectly mind you due to factors such as extra heat being added to the whole system (air/cylinders/etc), but this is damn spot on until we get to some crazy factors, and this "rule of thumb" can be applied to get some -damn- close ballparks. At least this is my experience of playing with cars on our dynos for the past couple of years.

Now notice something, even with double the air through that 2 liter, its only moving the air of a 4 liter. Still not close to that other engines base of 7.4, and worse- if we move say, only 5 psi through that 7.4, were only going to add 1/3 an atmosphere to it, make it move 1/3 more air, it will now act like it has almost 10 liters of displacement. This is why it doesnt take as much boost on a large motor, to make a lot more power than a smaller motor.

This post has been edited by sideways on Nov 12 2010, 01:41 PM

Spot on. Especially the part about motors being air pumps, or vacuums actually. This is why the compressed air from the larger turbo is not possible to go through the smaller turbo, there's always compressed air coming from both turbos, and when the piping is connected to the throttle body, air will ALWAYS go to the lowest pressure of its environment, which means that it only really has one way to go, and that's in the engine.

Kinda sorta. He wasnt completely off in what he said. Its not so much that the air from larger turbo is going to "leak out" through the smaller turbo. In the simplest way i can think to describe it, the air pressure inside the pipes will be too much for the smaller turbo, and itll get compressor surge trying to push air into the air stream.

Think of trying to use your mouth to blow air into a system thats got something else adding pressure to it (and a lot more than you can at that). As you blow air will just escape around the egde of your lips. Same idea.

I don't get it. In your example, there is no area where the pressure can go, like the engine. I've looked deeply into this, and it doesn't really make sense that the smaller turbo would surge at all, unless the throttle was closed and there was no blow off valve.

Break it down. On a single turbo set up, what causes compressor surge? Let off the throttle (well assume no bov/recirc valve for this application) and the pressure builds up within the intake pipes until theres so much pressure that the turbo can no longer add air to the system (again well use a modified blowing example, blow into a bottle of soda until its pressurized as much as you can, and keep blowing- air will just escape around the edge of your lips), however the turbo does not completely stop spinning instantly (momentum and more importantly exhaust flow keep the turbo spinning), when this happens the air its compressing backs along the edge of the blades and makes that lovely "gobble gobble" compressor surge noise.

This all happened because there was too much pressure within the intake pipes for the turbo to add air too. The throttle being closed is not the only thing that can cause this. Compressor surge can happen at full throttle. Can and does. Ive seen a hand full of examples in person while operating the dyno where cars make so much boost they literally reach the "limit" of what the motor can take in, but because the turbo is "so good" it keeps trying to spool up and increase boost, then goes beyond what the engine can take in- which over pressurizing the system, and results in compressor surge. Again the point is, compressor surge happens whenever the turbo can not add pressure to the system, there are various things that can cause this.

If that large turbo is pressurizing the intake pipes beyond what the little turbo can contribute to (Just doesnt make enough force/pressure to add air to), itll surge.

Make more sense?

This post has been edited by sideways on Nov 12 2010, 06:25 PM

Definitely, Thanks for taking the time to explain it more in depth. I did know the definition of compressor surge and that it does happen when the throttle is WOT. So how do I know if the small turbo will surge..?

Basically need to look at the compressor maps of the turbos and see how well they will or wont work together. If the efficiency is drastically above the other, chances are it wont work. This is probably why most people just use twins, no issues haha.

True, thanks man. I'll have to do my homework, although I've done quite a bit so far...

I can take a video of my boost gauge with this happening if anyone wants a demonstration...

I think you should, someone could learn a lot from just reading this thread, and that would be a visual help.

Thanks man, I didn't feel like arguing with people.

Only one thing I would like to clarify ( because it makes more sense when trying to visualize it ) :

The first sentence is correct, but the second makes it hard to understand.

Pressure is only generated ( talking about static pressure in the manifold here ), when there is an airflow difference between what the motor is taking and what the turbo is moving. The bigger the difference in flow, the higher the pressure. A turbo that's not connected to anything doesn't generate any measurable static pressure, it's just moving air. Now I want to say I wish it were as simple as that, it is not, but it is a good approximation.

Either way, air flow is only caused when there is a difference in pressure between two zones, and it always moves high to low. Its not difficult to understand I made that second sentence to address the idea of "Same psi, more cfm" when dealing with a larger turbo (ie that HIGH flow 12 psi, it doesnt exist). Same psi, same cfm. If youre moving more cfm, its because there is a larger pressure zone moving more cfm.

Gotta look at the sentence following it, helps make things mucho clearer. Least to me, of course

This post has been edited by sideways on Nov 12 2010, 11:01 PM

I know what you're talking about, but it's just confusing the way you worded it, because you don't even need to look at that part if you want to understand it.

Also, I don't want to get into impact pressure, so let's throw out the "airflow travels from high to low pressure" part.

Wasnt confusing to me

That's because you wrote it.

I didn't write it and it wasn't confusing for me. I thought it was laid out pretty simply.

I am talking about that one specific sentence.

"Air only moves when there is a pressure difference."

So in the context of the entire post which deals exclusively with static manifold pressure, dropping a comment like that ( which deals with impact pressure of the turbo itself ), isn't confusing?

"Either way, air flow is only caused when there is a difference in pressure between two zones, and it always moves high to low."

So what are the zones of pressure we are talking of here?

To make a statement like that, in the context of the static pressure of the manifold, serves only to confuse people ( how the heck does that apply to the air going from the low pressure side of the intake of the turbo, to the high pressure side of the outlet of the turbo ? )

Remember, I am not asking because I don't know, quite the contrary.

Edit : This is why I didn't want to touch it, because even people who have an idea are just being stubborn.

If you want to mix up static pressure and impact pressure, be my guest. I still say that comment, regardless of the fact that it is true, served no purpose other than to have the potential to confuse people. It's not me who's overthinking it if there was extra stuff added that has no relevance.

This post has been edited by Möbius on Nov 13 2010, 12:47 AM

Air only moves when theres more pressure. More pressure. More air. Same pressure. Same air. It was explaining how/why air moves, and explaining that the idea that a big turbo at 12 psi "moves more air than a small turbo at 12 psi" is silly. I found the idea of understanding "why" air moves from location to location not just relevant to a discussion talking about air moving through a system, and how to move more you need more pressure, but interesting.

This post has been edited by sideways on Nov 13 2010, 01:15 AM

Now that I agree with 100%.

Before people nitpick about it, and it was covered in sideways' post before, that statement is only with all other factors considered constant.

This post has been edited by Möbius on Nov 13 2010, 01:12 AM

I barely have a minute to add after-thoughts and edits to a post before you get to it Your admin habits have not left you, haha

I understood Sideway's explanation totally, and it makes sense. And besides I looked it up, and it was true.

Please point out where I said it was untrue.

This post has been edited by Möbius on Nov 13 2010, 01:42 AM

Haha, please point out where I ever said it was untrue. You said the content was confusing, and I disagree. Stop putting words in my mouth.