paw140

I just have a general turbo question. I've heard a lot of people say that it doesn't matter what psi your turbo is pushing, but what matters is the *flow* of the turbo. For example, I've read people saying that an engine will make more power at 12 psi with a single than with 12 psi on the stock twins.

Lets compare running 12 psi with the stock twins versus running 12 psi with a large single.

Assuming both turbos keep the intake manifold charged with 12 psi of pressure throughout the entire RPM range, I don't see the difference. Now, if the twins could not flow enough air to maintain 12 psi at all RPMs, and the boost began to drop, I could see how this could be a factor. But if this happened, then that person would no longer be running 12 psi anyway.

Do you see my point? If the intake manifold always has 12 psi of pressure, what difference does it make if that pressure is supplied from the stock twins or a single?

Nathan Kwok

Remember power is proportional to the amount of air flowing through the engine. "Boost" is only a measure of pressure, which is essentially air density. So, at a higher boost level, you may have higher air density, but that tells you nothing about how quickly the air is moving . Thats the important part. Now, you may think each combustion event is a static event and that power is then determined from rpm, thus making density the only factor. However this is again untrue. The reason is the combustion chamber never completely fills to its theoretical maximum in the first place, and the speed of the incoming column of air then determines how well the engine gets filled. A "big" turbo creates a large column of moving air that is then necked down to whatever your port size is. A "small" turbo creates a smaller column of moving air that is also necked down. The difference is the air velocity at the neck will be higher for the large turbo then the small turbo, so even if your intake pipe (or manifold or whatever) is the same for large or small turbo, you'll still see a benefit from a larger turbo, at the same psi, same rpm, and same size intake port. This even assumes that both turbos heat the air the same amount, which they don't, but that effect only magnifies the benefit of a larger turbo. Of course, there is a limit to how fast the air can move through the neck, but then thats what porting is for . Make sense?

poss

iTrader: (5)

i have always wondered this as well. if i'm not mistaken, in a given system, pressure and volume are directly related to one another. like the Ideal gas law (PV= nRT, where P=pressure and V=volume)


the main advantages that I would see are:
a. operating the turbo in the maximum efficiency range (less power needed to create the boost)
b. less heat due to higher efficiency, which would make more dense air available

paw140

Ok, I buy that the more efficient turbo (large single, or whatever) will heat the air less, and this will create more power. That makes perfect sense.

Nathan:
I understand what you are saying about how much air the turbos are capable of flowing. So what you are basically saying is that the smaller turbos will not be able to supply the air as fast as a larger turbo, and thus less air will enter the combustion chambers. This implies that the boost pressure in the intake manifold, just outside the intake ports, fluctuates a lot more with a smaller turbo. Is this the basic idea? In essence, the smaller turbo cannot keep up a constant 12 psi, whereas the larger turbo can keep up better.

Is that right?

13brv3

The lower heat is certainly a valid advantage of a larger turbo, running in a more efficient range. If pressure is really the same between single and twin turbos, I can't see how there will be any difference in power (disregarding temp).

The question is- is it really the same? When the intake port opens, there is going to be an instantaneous drop in pressure as the air rushes into the chamber. The manifold will then recover to it's normal pressure quickly. This would all happen way too fast for our pressure instruments to see. I would speculate that the larger turbo could recover the pressure more quickly, even while the port is still open, thus flowing just a bit more air. This is pure SWAG on my part, and I just can't believe it would be enough difference to really notice.

Cheers,

paw140

This is exactly what I tried to say, but I think you said it a lot more understanably.

Nathan Kwok

Yeah you can think of it like that. Even the biggest turbo is not going to keep a constant 12psi right at the intake port, in fact, you don't even have positive pressure at the intake port when its opening, its a vacuum. As the intake stroke comes around, don't think that there is a steady stationary 12psi resevoir of air simply waiting around for the port to open up and let itself in, its not like that. The engine spins so fast that its really better to think of the air as a continuous flow, and the engine is always trying to suck air in faster than what is available. If you have to break it into steps, imagine this: right as the intake port opens, you have all this air backed up in the manifold at 12psi waiting to get in. What makes it go in? Well, the vacuum for one thing, but remember a vacuum alone isn't enough to completely fill the chamber with 12psi worth of air. In fact, a vacuum would only give you 0psi worth of air. So you have the turbo in the back of the room helping by pushing from behind. The port is only open for a split second, so if you're going to cram all that air in, as close to 12psi worth as you can, you want a big turbo to give that big shove and get as much in there as possible, right? Even then, you STILL don't get 12psi worth. No matter how big your turbo is, you can never achieve this. However a bigger turbo will still give you closer to 12psi worth than a small one. Yet, with both turbos, you see 12psi, because your boost gauge doesn't measure the chamber pressure right as the intake port closes, that would be impossible. Instead, the gauge measures pressure inside the waiting room. Whats the point in that anyway? Who cares about the waiting room? Well there you go, thats why boost isn't the whole story.

P'cola FD

Think of it like this: A stock car, with 2 cats and stock catback making 10psi throughout the rpms(theoretically) makes less power than a car with full 3" exhaust running 10 psi. Like when the turbos transition (assuming the boost stays at the same level after the dip) there is a huge increase in power, due partly to the revs climbing, but even more so to less exhaust restriction (i.e. TCA door). Does that make sense?

rynberg

Picture it this way:

stock twins = a 1/2" diameter hose under 12 psi of pressure

large single = a 1" diameter hose under 12 psi of pressure

Notice that while both volumes (of air) are under the same pressure, the 1" diameter hose (large single) is flowing four times as much air.

spurvo

OK, had a little wine, but want to weigh in.

First, PV=nRT requires that pressure and volume hold an INVERSE relationship. The tighter you squeeze (higher pressure) the smaller the volume, PROVIDED all else is equal (temp T, number of molecules per given unit of volume n, and the gas constant, which doesn't ever change R)

Next, from my days as a piston guy, the thing we want to compare is volume of fuel air mixture pumped through a a given orafice. The limiting factor here is the intake port (valve cross section, sive and durationopen in piston engines). It is the smallest cross sectional area that the fuel air mixture must travel through in most high perf engines. Ever seen the carb in a six cylinder dodge dart?. The port is open for a fixed period of time for a given rpm. This gives us a flow velocity at that rpm, measured in cubic feet (or meters for our friends up north! And everywhere else int he world, for that matter...) per minute, calculated as the cross sectional area multiplied by the period of time the port is open, represented as a linear value (feet/meters) per minute. So the flow through the engine at a give rpm will BE CONSTANT NO MATTER WHAT, unless you change the rpm, or change the port size. Thus the wish for a street ported rebuild.

So we have what amounts to a fixed volume of air moving through the engine for a given rpm, provided that while the engine is running the port size does not change. According to PV=nRT, we rearrange so that V = (nRT)/P. How do we make more power from this?

Increase n. When we spool up the turbo, we are increasing the number of oxygen molecules contained per unit volume. Thus, at 10 psi, the cubic foot of air that is moving through the engine now contains.. ok, the partial pressure of oxygen at 14.7 psi (pressure at sea level) is 0.21 ata. 10 psi = 68% increase over sea level pressure (note this is why less power is seen by turbo thrust in the mountains), so ppO2 = .353 ata @ 10 psi. Basically, at 10 psi, there is 68% more O2 to mix with fuel for the same flow rate as would be seen in a N/A engine. More O2 to combine with gas = more power. Further note, 12 psi = 82% increase over sea level!!!

But wait, what about T? Temp is directly proportional to pressure. So as the pressure goes up, the temp MUST go up, given that the volume of flow at a given rpm is fixed. Thus the intercooler. We are using this device to reduce the temperature of the air that was compressed (i.e. when n went up) by the turbo. Why? If we didn't, then the increase in O2 we saw from the turbo compression would be COMPLETELY offset by the increase in temp, less the SLIGHT reduction in temp as the gas moves from the turbo outlet to the intake manifold. (power gains were seen in cars without intercoolers by just increasing the length of the piping, and making that piping out of aluminum rather than plastic for heat absorbance factors in twin turbocharged Chrysler Hemi engines. But I digress). The intercooler is radiating heat of the compressed air into the surroundings through the great surface area provided by the intercooler. That's why bigger intercoolers make more power. AND why intercoolers that DON'T sit right ON TOP OF the liquid radiator make more power. They have greater capacity for heat dissipation. Interestingly, given that the port is the limiting factor in flow, I recall that intercoolers that gave less absolute flow numbers but greater access to surface area gave MORE power that those that flowed well. Get the housings ported.

So. Volume increases through porting. Number of molecules per unit volume (n) increases by compression. Temperature rise caused by compression is reduced by intercooler. We make more power, until the Y-pipe coupler gives way. Then n is decreased, as seen by the resultant loss in P. The V STAYS THE SAME!!!!!!! Temp probably goes up as you overspin the turbos, but goes down as you lean out the fuel/air mixture less while overboosting on the stock ECU. Ooops...

OK, if you got this far, you need more of a life than even I need, 'cause I'm drunk and not so happy with the girlfriend!!! Anyway, airspeed is NOT increasing. Just air DENSITY...

spurvo

Sorry, numbers a little screwy. 10 psi = 40% increase in ppO2. 12 psi = 45% increase. Check my numbers, somebody, please?

maxcooper

12 psi in the manifold is 12 psi in the manifold, no matter what turbo is pumping it there.

The single makes more power for two reasons:

- The 12 psi in the manifold from the single is cooler than the 12 psi from the stock turbos.

- The 12 psi in the manifold from the single is fighting less pressure in the exhaust system to enter to the combustion chamber, so you flow more with the freer-flowing exhaust afforded by the larger turbo (and less convoluted exhaust manifold).

Backpressure and air temps are responsible for all of the power you add to a turbo car if you do it without raising the boost or changing the ports. Freer-flowing exhaust means less back pressure and more power, even at the same boost. A freer-flowing intake means the turbo has to compress the air less, which yields lower intake temps and less backpressure (if the turbo only has to increase intake pressure by 14 psi rather than 14.5 psi, it will be less of a restriction on the exhaust side). A cool-air intake means more power at the same boost pressure because the intake temps will be lower (even after passing through a big IC) and the turbo has to compress it less to get the same manifold pressure and temp. An IC with lower pressure drop means the turbo has to make less boost, which means less backpressure. IC efficiency also plays a role in intake temps, but if the IC has lower pressure drop, the air entering the IC will be cooler than it will if the IC has a high pressure drop. It all comes down to intake temps and backpressure, given a fixed level of boost and port configuration.

-Max

HeX

I have a question that is somewhat but not really related to the discussed topic above. I was wondering why people never ran two larger turbos than the stock twins? I'm new to fds and all this turbo stuff so don't slam me yet.

-HeX

ps. How would the car run if it did have twin upgraded larger turbos?

RICE RACING

I would say in this order......

Space, price, complexity These are the main reasons why there are not many real twin turbos around.

The other big one is that you realy want to be aiming for over 600bhp to justify running twin turbos over a properly sized single turbo, which I think would rule out over %95 of RX7 owners, even hardcore performance orientated ones.

The biggest thing is that you can only push so much power from a 2.6lt engine on pump gas regardless of your turbo set up, this tends to be the limiting factor for many owners.

BoostCrzy

Maxcooper is the winner!!...congrats..lol...