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Old 01-19-2008, 11:06 AM   #1
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Default What difference does turbo size make?

Okay guys, humor the SC guy for a minute...

I read lots about 7 psi from turbo x is way different the 7 psi from turbo y. What specifically makes them different? Is it charge temp/density? My limited understanding is that with the heads/valves being the same, the effective opening is still the same therefore 7 psi of manifold pressure is going to move the same amount of air through the opening regardless of how its produced.

I think the same stuff is valid for SC cars as I see some of the same variations blower to blower, it's just that we have fewer options to compare.

Is any of the difference a "tuning" thing? By that I mean do the guys with the bigger turbos, also have better engine management and can therefore extract more power from the 7 psi than someone else?

And maybe 7 psi isn't the best example, does it have to be 15 psi and then compressore efficiency comes into play? But that puts me back to my first question, why does it matter how the pressure was generated?
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Old 01-19-2008, 11:14 AM   #2
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Quote:
Originally Posted by Chris Swearingen View Post
Okay guys, humor the SC guy for a minute...

I read lots about 7 psi from turbo x is way different the 7 psi from turbo y. What specifically makes them different? Is it charge temp/density?

Is any of the difference a "tuning" thing? By that I mean do the guys with the bigger turbos, also have better engine management and can therefore extract more power from the 7 psi than someone else?

And maybe 7 psi isn't the best example, does it have to be 15 psi and then compressore efficiency comes into play? But that puts me back to my first question, why does it matter how the pressure was generated?
1. If all things remain equal 7 psi from a big turbo is the same 7 psi from a small turbo. The difference is temperature of the air, and efficiency at which the turbo compresses it. The other difference is optimal operating range of the turbo. Small turbos work great off the line, large turbos work well toward redline.

2. All things being equal if I run a megasquirt on a t25 and some other guy runs a Hydra with a T25 we should be making the same power if both cars are properly tuned. Tuning plays a role in how much power you make, but it can either be tuned or not.

3. It could be 7 psi and it could be 15, it depends on the turbo itself and its compressor map. It matters how the pressure was generated because you dont want to spin a turbo outside its efficiency range, you get a turbo suited best for your application.

--------

Guys correct me if im wrong somewhere.
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Old 01-19-2008, 11:14 AM   #3
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Think of it as CFM instead of pressure. 7psi through a 2 inch pipe is not the same as 7psi through a 3 inch charge pipe. What you really want to compare is CFM and Charge Temp
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Old 01-19-2008, 12:11 PM   #4
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Quote:
Originally Posted by Chris Swearingen View Post
Okay guys, humor the SC guy for a minute...

I read lots about 7 psi from turbo x is way different the 7 psi from turbo y. What specifically makes them different? Is it charge temp/density?
Yes. Its all about the charge temp. The colder air is, the more dense it is.
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Old 01-19-2008, 01:02 PM   #5
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Old 01-19-2008, 01:34 PM   #6
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Quote:
Originally Posted by Saml01 View Post
1. If all things remain equal 7 psi from a big turbo is the same 7 psi from a small turbo. The difference is temperature of the air, and efficiency at which the turbo compresses it. The other difference is optimal operating range of the turbo. Small turbos work great off the line, large turbos work well toward redline.

--------

Guys correct me if im wrong somewhere.

1 is way wrong. the difference is flow.

small turbo is like blowing through a straw.

big turbo is like blowing through a big pipe.

you can do both at the same pressure, but one will flow a lot more than the other. that's why the bigger turbo works better at higher rpm where the engine requires more mass flow rate of air.

but you also get some of that higher flow benefit at lower rpm if you can get the turbo to spool.

That's how I make >250 rwhp at 9 psi and someone with a 2554 would have to be probably over 15 psi to get the same power.
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Old 01-19-2008, 02:11 PM   #7
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Quote:
Originally Posted by y8s View Post
1 is way wrong. the difference is flow.

small turbo is like blowing through a straw.

big turbo is like blowing through a big pipe.
ditto. if #1 were true, we could run K03's with a huge FMIC and make 300whp at 5psi.
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Old 01-19-2008, 02:46 PM   #8
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Quote:
Originally Posted by y8s View Post
1 is way wrong. the difference is flow.

small turbo is like blowing through a straw.

big turbo is like blowing through a big pipe.

you can do both at the same pressure, but one will flow a lot more than the other. that's why the bigger turbo works better at higher rpm where the engine requires more mass flow rate of air.

but you also get some of that higher flow benefit at lower rpm if you can get the turbo to spool.

That's how I make >250 rwhp at 9 psi and someone with a 2554 would have to be probably over 15 psi to get the same power.
Thats why I love this forum. Thanks
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Old 01-19-2008, 02:58 PM   #9
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it might also help to compare the turbo/SC to an injector. a 230cc and a 650cc both operate at the same pressure, but one sprays a lot more fuel.
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Old 01-19-2008, 03:48 PM   #10
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Quote:
Originally Posted by y8s View Post
1 is way wrong. the difference is flow.

small turbo is like blowing through a straw.

big turbo is like blowing through a big pipe.

you can do both at the same pressure, but one will flow a lot more than the other. that's why the bigger turbo works better at higher rpm where the engine requires more mass flow rate of air.

but you also get some of that higher flow benefit at lower rpm if you can get the turbo to spool.

That's how I make >250 rwhp at 9 psi and someone with a 2554 would have to be probably over 15 psi to get the same power.
Not disputing you, but here is where I get lost. If you blow throw the straw or through the pipe and pressurize the intake manifold to 7 or 15 or whatever PSI, you still have the same pressure and the same size opening for that pressure to go out of when the valves open. So what am I missing?
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Old 01-19-2008, 04:22 PM   #11
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They're saying the difference in turbos is not the pressure, it's the volume of air that it can push. It's easiest to just look at the figures. Y8s turbo at 9 psi put him at like 259 HP. My small T25 at 9 psi puts me at more like 190 HP. It's because of the larger turbo pushing more CFM that he gets more power and the temps are lower.
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Old 01-19-2008, 04:46 PM   #12
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if you blowing through that straw at 7psi, and blow through the pipe at 7 psi, more air is moving through the pipe than the straw. But it takes longer to reach that goal whatever psi.
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Old 01-19-2008, 05:10 PM   #13
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Chris, here's another way to say the same thing.

Take your average 1.6litre Miata. Naturally aspirated, the engine pumps 1.6litres of air every other revolution. At 1000rpm, that makes 500x1.6=800litres of air.

But say you're boosted (1bar=14.7psi to make it easy). So, if you're running 1000rpm at 14.7psi, that's 500x3.2litres=1600litres of air per minute. At 7200rpm, that's 3600x3.2=11,520litres of air.

11520litres=407cfm.

If your turbo can push out at 407cfm, then you can make 14.7psi on a 1.6litre motor at 7200rpm.

As others have said, it's not about how much PSI a turbo will make, but how much AIRFLOW it will make. Engine size and RPM are just as critical in this equationas PSI of boost. The biggest problem when making a small turbo overwork itself (such as making a small T25 push 15+psi) is that the compressed air gets inefficiently hot (hot air=thin air=low oxygen).

Here's a great read:
http://forums.nasioc.com/forums/arch.../t-359824.html
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Old 01-19-2008, 05:47 PM   #14
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Quote:
Originally Posted by Chris Swearingen View Post
Not disputing you, but here is where I get lost. If you blow throw the straw or through the pipe and pressurize the intake manifold to 7 or 15 or whatever PSI, you still have the same pressure and the same size opening for that pressure to go out of when the valves open. So what am I missing?
it's more of a dynamic pressure. and there's 8 valves to choose from. so even if some are closed, another pair is open.

and there's different amounts of compression going on in the motor. the psi on your gauge is only at one point--an average in the plenum. as the air enters the ports, the pressure isn't constant.

so the local pressure at the port goes up, the mass flow rate at the port goes up, the cylinder fills with more air than a smaller turbo that can't provide the increased flow through the ports.

I'm sure some form of bernoulli's equation covers exactly how to figure it, but i'm not gonna.
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Old 01-19-2008, 05:54 PM   #15
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Okay some of it is seeping in. Thanks for the patience.

Let me see if I can restate what I think I learned.

The larger turbos have more CFM capacity, and a greater efficiency so they can
a) produce the same pressure but add less heat to the charge
and
b) maintain that pressure more uniformly as the valves open and cause a pressure differential within the intake tract
and
c) that the difference in power output for a given indicated psi is the sum effect of a and b above.

Is that mostly right?
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Old 01-19-2008, 06:24 PM   #16
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That sounds right to me.

So, lets ad another piece of the puzzle... spool. Since the turbo is powered by exhaust gas, and it takes more exhaust to spin the turbine of a larger turbo vice smaller, you're going to spool later on a larger turbo. Assuming WOT, your average T25 will start to spool around 1800rpm and be fully spooled by 3500. Your average T3 will start to spool in the low 3's and be spooled mid-4's. A T4 will start to spool mid-4's and maybe be fully spooled by redline.

So when talking about the POWER a turbo makes, also think about the powerband it produces. I like my T25 because of the great mid-range torque. Somebody who drives on the track a lot might be more interested in a smaller T3 which spools well but has a lot more top-end.
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Old 01-19-2008, 06:41 PM   #17
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im going to repost this for emphasis:

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Old 01-19-2008, 06:58 PM   #18
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Same psi, timing, air fuel ratio?
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Old 01-19-2008, 07:10 PM   #19
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Quote:
Originally Posted by Chris Swearingen View Post
Not disputing you, but here is where I get lost. If you blow throw the straw or through the pipe and pressurize the intake manifold to 7 or 15 or whatever PSI, you still have the same pressure and the same size opening for that pressure to go out of when the valves open. So what am I missing?

Chris, you're absolutely correct. If you have 7 psi in the intake manifold and the air temperature there is the same - you will have the same air flow through the engine regardless of what size turbo, or a supercharger, or even a person blowing through a straw is doing it. Same psi and temperature - you'll get the same flow (CFM).

But, this is if the exhausts were the same - and with the different turbos they are not. Different size turbos, put different restriction in the exhaust and that restriction in the exhaust changes the the air flow ... the same 7psi in the intake manifold will be different air flow depending on the exhaust restriction (the turbo), the least restrictive (highest flow) being with a supercharger (because it is creating zero restriction in the exhaust compared to a turbo). Alas, the supercharger gets the highest flow, but uses some of the produced by the engine power to compress the air (via the belt), so it adds one more variable to the system ...
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Old 01-19-2008, 08:31 PM   #20
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From what I understand, manifold pressure is actually a measurement of the DIFFERENCE in pressure between the intake manifold and the exhaust manifold. When you see a 12psi "difference", or what most people refer to as 12psi of boost, on a small turbo (technically a small turbine/turbine housing), you don't need to flow as much air to maintain that 2psi of difference. With a big turbo that allows pressure to escape from the exhaust manifold faster, you need to flow more air to maintain the same 12psi of difference (boost) in the intake manifold.

Same difference, but more air with the larger turbo, and more power. This is why people like Y8S can make 257whp on 9psi, but it takes me 15psi to get to 219whp with my smaller turbo (GT2876R vs GT2554R).
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