pressure vs power when setting boost psi - Page 2 - Miata Turbo Forum - Boost cars, acquire cats.

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Old 03-10-2009, 03:21 AM   #21
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Quote:
Originally Posted by JimAtFSU View Post
Aren't we talking about raising the boost on a turbo rather than comparing one turbo to the other?
yes i think hustler is just emo on how many threads we get on "should i keep my 1556 turbo or get a 2456 i wana run the same psi whats the diff". u know what i mean
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Old 03-10-2009, 03:27 AM   #22
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Yes and no. The issue is how much it can be raised by and Hustler is pointing out a huge point - that a big turbo's 12 psi effects the motor WAY differently than 12 psi from a small turbo, so a given psi could be fine or terrible for the motor.
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Old 03-10-2009, 04:01 AM   #23
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Yeah I know. I mostly just ignore such posts. If I raged at everything stupid on the internet I wouldn't do anything else.
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Old 03-10-2009, 10:37 AM   #24
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Originally Posted by ThePass View Post
Yes and no. The issue is how much it can be raised by and Hustler is pointing out a huge point - that a big turbo's 12 psi effects the motor WAY differently than 12 psi from a small turbo, so a given psi could be fine or terrible for the motor.
exactly^^^ , although other people have already said to look at the compressor map for his turbo. but the op might not know what they were getting at.
basically it's cfm's at a given psi. 12 psi might be safe with a t25 that flows (i'm just making up a number here i didn't look it up) say 275cfm. but a t3/t4 60-1 turbo at 12 psi might be pushing 400cfm, enough to pretzle your rods.
so i think the concensus is, if your running a small setup t25,t28,14b 12 psi is plenty, especially for trackday use. on the street 15psi would probably be fine since you would rarely be at that psi or for more than few seconds. unlike a trackday where your at wot for 20mins. straight 3-4 times a day. i personally only run 9-10psi at the track. it's plenty fast, i pass porsche's and vettes on a regular basis, and hang with modified porsche's and vettes on hoosiers with my ra1's. remember our car's strong point is lightweight. that let's me outbrake and turn faster than alot of ''badass'' cars. hell, i see fast spec miata's passing stock vettes and other's. with my extra 100whp i'm just that much faster. have fun.
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Old 03-10-2009, 11:02 AM   #25
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Thanks for all the input. I put a post in the tuning section with a 4th gear pull. I'll also post pdf images of my timing map and fuel map. I sent the datalog to FM and was told that it was pretty good, and he made some comments on where to adjust. If anyone from here would look at them and give me some feedback I would appreciate it.
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Old 03-10-2009, 08:15 PM   #26
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There aren't "different kinds of boost" in the way you guys are thinking.

Lets pick a magic rpm, say 4000 rpm.
X psi of boost from any turbo is going to produce the same amount of flow into the engine. The pressure differential between the intake plenum and the cylinder is the same so the flow should be the same. And it is.

The difference in power comes from:
-look at the compressor map and find the pressure multiplier and the CFM at that RPM. The higher the efficiency at that point, the cooler and denser that air is going to be. This means more power.
-newer compressors have much higher peak efficiencies. My old turbo peaked around 70 percent efficiency. My current turbo peaks around 80 percent. This means cooler and denser air and thus more power at the turbo's peak efficiency.
-newer compressors have much wider efficiency peaks. At values of CFM that would leave me around 50-60 percent efficiency on the old turbo, the current turbo is still around 80 percent. This is a HUGE difference in power. It also means that the intercooler doesn't heat soak as quickly since it has much less heat to dissipate.
-if you are running a very small turbine this can choke off flow at higher rpms but this is IMO a relatively minor influence compared to compressor size.
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Old 03-11-2009, 05:26 PM   #27
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Quote:
Originally Posted by JimAtFSU View Post
There aren't "different kinds of boost" in the way you guys are thinking.

Lets pick a magic rpm, say 4000 rpm.
X psi of boost from any turbo is going to produce the same amount of flow into the engine. The pressure differential between the intake plenum and the cylinder is the same so the flow should be the same. And it is.
100% wrong.


Quote:
Originally Posted by JimAtFSU View Post
The difference in power comes from:
-look at the compressor map and find the pressure multiplier and the CFM at that RPM. The higher the efficiency at that point, the cooler and denser that air is going to be. This means more power.
-newer compressors have much higher peak efficiencies. My old turbo peaked around 70 percent efficiency. My current turbo peaks around 80 percent. This means cooler and denser air and thus more power at the turbo's peak efficiency.
-newer compressors have much wider efficiency peaks. At values of CFM that would leave me around 50-60 percent efficiency on the old turbo, the current turbo is still around 80 percent. This is a HUGE difference in power. It also means that the intercooler doesn't heat soak as quickly since it has much less heat to dissipate.
-if you are running a very small turbine this can choke off flow at higher rpms but this is IMO a relatively minor influence compared to compressor size.
Compressor efficiency is important, but not for the reasons you mention. Your points are valid, but you're missing the big picture. Cooler charge exiting the turbo is nice, but when we have an IC that's 90% efficient, the difference between 88*F post IC temps with an efficient comp. vs 91.13*F IC temps with an average efficiency comp. is trivial and NOT the reason why the engine with 3.13*F cooler temps made 20whp more.

The turbine is the main thing that dictates flow in the engine. It's the restriction. So why do we have a turbine? Because we need to supply power to drive the compressor. It's a struggle. The more power we want to make, the more air we need to move, the more power the compressor needs to move more air, the more restrictive the turbine becomes to supply said power and this restriction limits flow.

In your example, say the higher efficiency comp. is using 25hp where the lower efficiency comp. is using 30. The difference in power is not from the 3*F post IC temps, but from the fact that the turbine only needs to supply say 25 hp to the comp. instead of 30 so it opens up the wastegate some more and you operate at a lower delta P across the turbine to make 5 less HP than before, and the net result is less back pressure pre-turbine. This is where power is made.
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Old 03-11-2009, 07:54 PM   #28
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First off, I'm measuring pressure at the intake manifold plenum, not the turbo. So I'm not wrong. The laws of physics are pretty straightforward- cavity > passage > cavity with a pressure differential between the two cavities = a certain amount of flow between them. Unless you change the characteristics of the passage between the plenum and the combustion chamber, this isn't going to change. The variables that determine how many moles of air gets into the combustion chamber are temperature and pressure. (edit: for a fixed shape of passage, obviously)

Also, intercooler isn't a magic bandaid. It requires a much larger volume of very hot air than warm air to produce a given volume of cool air. This means an inefficient turbo has to work harder to maintain a given pressure within the manifold. This means more power taken from the exhaust and more heat for the intercooler to eject into the ambient air.
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Old 03-11-2009, 08:00 PM   #29
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I just reread your post I think we agree but we are expressing it differently.

Ok, I see what you were angry at in my original posting. I left the turbine out of it completely.
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Old 03-11-2009, 08:08 PM   #30
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Quote:
Originally Posted by JimAtFSU View Post
I just reread your post I think we agree but we are expressing it differently.

Ok, I see what you were angry at in my original posting. I left the turbine out of it completely.
Yeap. That and you said that mass flow through the engine is constant for a given temp and pressure, regardless of turbo size. This is just obviously not true. I mean, if this were so, there would only be one size turbo.
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Old 03-11-2009, 08:11 PM   #31
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No, I was talking about flow into the combustion chamber, not through the engine. I just sort of took it for granted that any slack due to inefficiency would be taken out of the exhaust flow. I left the exhaust/turbine side out of it because for the most part we all run similar size and efficiency manifolds/turbines. The only big variable IMO is what is happening between the compressor inlet and the piston.
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Old 03-11-2009, 08:40 PM   #32
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Originally Posted by JimAtFSU View Post
No, I was talking about flow into the combustion chamber, not through the engine. I just sort of took it for granted that any slack due to inefficiency would be taken out of the exhaust flow. I left the exhaust/turbine side out of it because for the most part we all run similar size and efficiency manifolds/turbines. The only big variable IMO is what is happening between the compressor inlet and the piston.
Is the mass flow rate going into the engine different than the mass flow rate going out? Conservation of mass? First law of thermodynamics?

Turbine size is what's critical. It's what ultimately dictates flow. You can have a T2 comp. or a T3 comp. Putting a bigger comp. doesn't magically make the engine flow any more air. It will only make more power if the new comp. is more efficient at the given range where it will be operated. But putting a larger turbine will ALWAYS make more power because that's reducing restriction, which is what mostly governs power.

Basically you're saying what common wisdom says, which is wrong. Read up.
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Old 03-11-2009, 08:48 PM   #33
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what kind of boost am i using?

lol
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Old 03-11-2009, 08:49 PM   #34
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Originally Posted by hustler View Post
what kind of boost am i using?

lol
The good kind.
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Old 03-11-2009, 08:58 PM   #35
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Quote:
Originally Posted by JimAtFSU View Post
Since the 1.6L has the same rods and stroke but with less bore, shouldn't a higher combustion chamber pressure result in the same pressure upon the rod and bearing?
Umm, no...

P=f/a

multiply that by a larger area (ie larger bore piston) and you get a larger force.

More explicitely, pressure is force over UNIT area, so for the same pressure, you get more force applied to a larger area.

Ex:

P=50lb/in^2

On an area of 40in^2, force will equal 2000lbs.

On an area of 30in^2, force will equal 1500lbs.

This is why it's harder to hold a larger umbrella in a stiff breeze, or harder to push a large suv through the air at 80mph than a Lambo. The second example has other factors, but you get to the idea.

So to answer your question, the 1.6L rod would actually be under less compressive force than a 1.8L rod for the same given peak cylinder pressure.
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Old 03-12-2009, 12:10 AM   #36
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PHYSICS
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Old 03-12-2009, 12:38 AM   #37
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Originally Posted by johnhanson View Post
Is the mass flow rate going into the engine different than the mass flow rate going out? Conservation of mass? First law of thermodynamics?
The mass flow rate is the same, but the amount of work done by the piston during the exhaust stroke is greater. The orifice it's flowing through is "smaller" because the wastegate is closer to closed, to force more exhaust through the turbine.

Turbine sizing doesn't have that big an influence, at least in my limited experience. It still comes down to how much work is needed to turn the compressor wheel to produce a given amount of cooled, compressed air at the intake manifold. If you run a large turbine, less exhaust will go through the wastegate. If you run a small turbine, more exhaust will go through the wastegate. All you're really controlling with turbine sizing is the behavior at the edges- rate of low rpm spool-up versus high end power.
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Old 03-12-2009, 12:42 AM   #38
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Quote:
Originally Posted by JimAtFsu
Since the 1.6L has the same rods and stroke but with less bore, shouldn't a higher combustion chamber pressure result in the same pressure upon the rod and bearing?
Quote:
Originally Posted by gospeed81 View Post
So to answer your question, the 1.6L rod would actually be under less compressive force than a 1.8L rod for the same given peak cylinder pressure.

I said that with higher combustion chamber pressures in the 1.6, the stress on the bottom end would be the same.

You said that with the same combustion chamber pressures in the 1.6, the stress on the bottom end would be lower.

You just spent a page "correcting" me and came to the exact same conclusion I did!
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Old 03-12-2009, 12:58 AM   #39
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Quote:
Originally Posted by JimAtFSU View Post
I said that with higher combustion chamber pressures in the 1.6, the stress on the bottom end would be the same.

You said that with the same combustion chamber pressures in the 1.6, the stress on the bottom end would be lower.

You just spent a page "correcting" me and came to the exact same conclusion I did!
First, you weren't very clear, but apparently we all do that from time to time.

Second, I didn't come to any conclusion, I already knew this ****.

I did figure out after reading a few of your above posts you probably know enough about physics to not make such an elementary error.

The explanation was however worth it for newbs that may misread your post as I did, and come to a poor conclusion.
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Old 03-12-2009, 04:35 PM   #40
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Quote:
Originally Posted by JimAtFSU View Post
The mass flow rate is the same
I'm glad you understand this now.


Quote:
Originally Posted by JimAtFSU View Post
The orifice it's flowing through is "smaller" because the wastegate is closer to closed, to force more exhaust through the turbine.
Uhhhhh....


Quote:
Originally Posted by JimAtFSU View Post
Turbine sizing doesn't have that big an influence, at least in my limited experience. It still comes down to how much work is needed to turn the compressor wheel to produce a given amount of cooled, compressed air at the intake manifold. If you run a large turbine, less exhaust will go through the wastegate. If you run a small turbine, more exhaust will go through the wastegate. All you're really controlling with turbine sizing is the behavior at the edges- rate of low rpm spool-up versus high end power.
Ok, I guess you don't quite get it. You acknowledge that:

Quote:
It still comes down to how much work is needed to turn the compressor wheel to produce a given amount of cooled, compressed air at the intake manifold.
Might as ask where said work to drive aforementioned compressor comes from?

Food for thought: Net work out of a turbine is the difference in enthalpy across the turbine. Enthalpy, h, is U + P*V where U is internal energy, P is pressure, and V is volume. (higher flow rate = more volume)

You need some certain delta h to produce some amount of work to drive the compressor. You have 3 ways to increase the difference the delta h across the turbine. Increase the internal energy of the working fluid (higher temperature), increase the delta P across the turbine (more restrictive turbine, or a free flowing downpipe, etc), or increase the volume (increase VE efficiency, increase engine RPMs, etc).

From a maximum power standpoint, operating the turbine at a higher mass flow rate and lower delta P to achieve the same work output will reduce back pressure pre-tubine and increase the engines respective VE, increasing HP. Turbine size is critically important.
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