96fd3s

hello everyone..here is my stupid question for the day!
ok first off a stock FD's boost pattern should be 10-8-10..
I understand the theory of sequential turbocharging (or at least i think i do) and basically it uses two turbos, usually of different sizes so the small one spools first, almost laggless then the larger one comes online later to provide more boost up high. I always figured a sequential car might boost 5 psi on primary turbo then up to 10psi on secondary for example...

SO what i dont understand is how the FD creates more power with both turbos going but its still only boosting 10psi.
I understand the first turbo boosts to 10psi, then there is a slight drop in boost pressure to 8psi, then it builds back up to 10psi as the second turbo spools up.
is it creating 10psi through BOTH turbos so esentially creating 20psi or what? if not i cant understand how or why there would be a power increase with both turbos running if the boost pressure remains at only 10psi..

Dont get me wrong, i have a working sequential system and i really like it. And there is deffinitly a "kick/ suck you back in your seat" when the second turbo comes online and thats what confuses me!
i do not understand how you get more power out of the same amount of boost and why it would not be better to just have a small single turbo boosting 10psi the whole time.

anyone care to explain?, i hope that made sence..believe me it was hard trying to put my thoughts into writing!, thanks!

dhays

iTrader: (1)

Here is my ignorant $.02.

Two turbos give a more responsive car than one single.

Both turbos are the same size.

The drop you get at transition is because the output from the primary is starting to spool up the secondary, taking some of the pressure from the primary to do it.

Total boost is still 10psi with both turbos. Keep in mind that the 10psi is relative air pressure. Atmospheric air pressure is a bit over 14.5 psi at sea level IIRC. What your boost guage measures is how much greater than that 14.5psi the turbos are creating.

I view it this way, but may be wrong. The engine is sucking in air while it runs. The higher the rpm and the greater the fuel supplied, the more O2 it requires. This creates a relative vacuum which is why your intake shows a vacuum when it is not under boost. When load and throttle position call for it, the ECU adjusts the wastegates to shunt more of the turbo output into the intake system. As load and rpm increase the ECU increases the output to try to keep the boost level (relative intake pressure) at the levels required to keep the mapped AFR. The problem is that as the RPM increases, the engine is sucking more air in creating a stronger relative vacuum. This means that you need more turbo output, hence the secondary kicking in to keep the relative pressure up.

Does that make any sense at all?

BTW: You did ask for "anyone" to explain. That even means ignorant folks like me.

Mazda Family

It's all about flow. The turbos in the rx7 are more like hair dryers than real turbos. The first turbo can only flow a small amount of air and maxes out just before 4.5Krpm. Thats when you get a small dip in the boost. Now the first turbo is running out of steam (remember max flow not pressure) as the second turbo catches up and then you have twice the flow there, hence the big jump in power.

Turbo's work by moving more air into the engine than it would normally be able to suck in by it's self The boost thats created is the "extra" air that having to be forced into the engine. More air = More boost but once you start to spin the turbo fast enough the inlet of the turbo can't bring in enough air and the pressure starts to fall because the engine is going faster and needs more air but the turbo can't bring anymore in.

I think that covers the basics and I'll be back if you have anymore questions on how it all works.

Chas

96fd3s

i think that makes some sense, so basically you need to create more pressure in order to create the same psi of boost at a higher rate of RPM"s?,
as the higher the RPM's, the more the turbos have to fight to force that same amount of psi through the system?

so if i understand this correctly this would be why a small turbo works well at low rpm with no lag but will run out off puff and become ineffient at higher rpm as it has to fight too hard in order to force that same amount of psi though the system?
thanks for the reply BTW!

Upgrayedd

iTrader: (1)

Correct the jump in power is all about air VOLUME, hence the reason a gt42r at 10 psi makes so much more power than stock turboes at 10 psi... More air = more power

dhays

iTrader: (1)

Yeah, but both Chas and Tim explained it much better than I did in terms of flow and volume.

Pressure is the result of air flow (and volume=flow x time) that is higher than what would be normally drawn into the engine. You can think of the rotary engine as an airpump. The faster it turns, the faster it expels exhaust and quicker it draws fresh air in. The turbos simple push air into the engine faster than the engine would draw it in, creating a pressure head.

Cgotto6

iTrader: (8)

Tim nailed it. Its all about the added volume of air caused by the second turbo comming online and boosting along with the primary.

Nat6c

iTrader: (1)

easy way to look at it.

2 small turbo's = turbo 1 does 10cfm + turbo 2 does 10cfm at 10psi together = 20cfm. and less lag due to smaller turbos.

vs.

1 bigger turbo = same pressure at 15cfm with more lag.


both at 10psi per say, numbers are not certain.. just a quick equation to make a point.


i.e. my gt35r-.74 at 12psi lags just a bit more.. but produces more cfm at the same boost equalling more power.


mazda used the 2 small twins as allowing great response wish a 1-2 combo... while keeping the pressures low and still allowing a great power gain. make sense? i hope so.


Clos

Cgotto6

iTrader: (8)

Uhh...None of this has anything to do with why more power is created while the turbo(s) stay at the same psi...

Monkman33

iTrader: (28)

the turbos staying at the same psi has nothing to do with why an engine makes more power as it goes through the RPM range. the resulting power is based on rpm.


However, this is a whole different can of worms.

a single small turbo does not have the capability to push enough volume to maintain 10 psi at higher rpm. It requires more volume the higher you are in the rpm range. thus, a second small turbo kicks in and "supplements" the first turbo... thus giving you a [i]volume][i] capability equal to approximately twice that of only one small turbo.

it's all about volume. an engine at 6000 rpm needs twice the volume of air feeding it as an engine running 3000 rpm. boost or no boost. boost just makes it more complicated.

dhays

iTrader: (1)

OK. I always thought that power was related to the amount of fuel that you could convert to energy through combustion. The amount of fuel that you can burn is dependent on the amount of O2 available. As rpm increases, you are adding both fuel and O2. As i understand the FD (and my understanding is really weak), peak power is around 6500+rpm. So even though the turbos are keeping air flow high enough to stay at 10psi over what the engine would naturally draw, power is increasing along with fuel consumption up to about 6500+rpm. So the discussion does have something to do, i think, with why more power is created while boost is constant. Of course, I'm likely wrong.

I'm not sure, but it seems to me that what we actually feel is torque. Doesn't it peak at a much lower rpm, around 5500rpm?

arghx

iTrader: (3)

The stock FD twins aren't THAT small. Ever seen the stock turbos on say a VR-4? Or the stock turbos on a 13B-RE Cosmo motor?

rossc

Are you kidding me? This may be the silliest thing I've ever heard. Air at 10psi in your intake from one turbo will have the same volumetric flow rate as 10psi from any other turbo. Even fundamental science shows us that. Shall we take a trip back to the ideal gas law? The only things that are going to affect the density of the air (density is really the key) going into that engine are pressure, and temperature.

If your intake sees 10psi at all times then it doesn't matter if you're using a gt42r or blowing air out your ***, assuming your *** air is the same temperature as the gt42r air, you will get the same power.

arghx

iTrader: (3)

^ have you ever read a turbo flow map?

96fd3s

so you need a bigger turbo / more small turbos to create the same amount of PSI at higher rpm's because you must compress a larger VOLUME of air into the same psi?
so basically its harder to compress a larger VOLUME of air to the same psi (10) then it is to compress a smaller volume of air to 10psi hence the need for bigger/more turbos?

dhays

iTrader: (1)

^^^ Works for me.

arghx

iTrader: (3)

stop thinking in terms of PSI. PSI isn't airflow, it's a force in your intake manifold.

Joe13BT

Delete pls...

Joe13BT

Then why I made more power when upgraded my .70 trim turbine to 1.00? Care to explain? PSI is just pressure of that particular amount of air in that area. Is not the same as volume moved by the turbine. Those are two different things and why BIGGER turbines will make more power at same PSI that smaller ones...

dhays

iTrader: (1)

I don't understand this. It seems to my simple mind that the intake is much like a water hose. Increase the pressure while keeping the nozzle the same and you increase flow as well as the velocity of the water coming out of the hose. If you keep the nozzle the same size, and the psi the same, it doesn't matter how large a hose you have, you will still have the same volume of water coming out of the nozzle.

Unless you changed the piping of the intake, changed the throttle body, or something else, it doesn't make any sense to me that you would get a power increase if the psi was constant. Same pressure + same nozzle = same volume.

What am I missing?

rossc

So hypothetically, lets say my engine is running at 1000RPM, and per 1 complete RPM the car uses 1L volume.

n=PV/RT

volume is fixed (unless you wanna port your intake, but that's besides the point, and even then it would still be fixed, just at maybe 1.1L let's say).
Temperature we will assume the same from any source
The only thing that can change is pressure.

The only thing that can affect the mass of air in your combustion chamber is pressure.

If your turbo can't produce the flow to keep your pressure maintained, that is when you will see loss of power.

rossc

Your increase in performance was actually related to the restriction on your exhaust as opposed to the amount of air in your intake. When you upgraded to a larger trim you most likely relieved backpressure on your exhaust ports.

Or just simply producing a cooler intake charge.

Joe13BT

Look a this charts. Why the smaller turbine achieves a smaller air flow quantity than the bigger turbine at same p/ratio

NeoTuri

iTrader: (2)

In your example, the motor is consuming L/revolution. Since a revolution takes a certain amount of time to cycle, we are talking X liters per unit time. If we are to maintain the intake pressure, we have to add the same amount of air in the stream as the engine sucks out.

That means, that the turbo must (regardless of pressure) output X liters of air per unit time that matches the consumption of the inlet port at Y RPM. That sounds to me like air flow (cfm) instead of pressure (psi) creating power.

I've personally experienced this when I swapped my stock IC with a larger one. My intake temps only changed slightly, but because the turbos could move the air more efficiently, I gained lots of power at the same boost levels.

ptrhahn

iTrader: (9)

Or in short, 10 psi @ 4500 rpm requires less turbo flow than producing 10 psi @ 7000 rpm. Were 2nd turbo not to come online, you'd see boost taper off quickly after the transition point.