MacFever

I have a 996TT tiptronic from 2000 which I bought 1 month ago
The reading of the turbo pressure reads up to 1,2 bar (should normally be 0,8 ???)
I suppose there has been some chiptuning ?
Turbo and exhaust is standard
Does anyone knows how much Hp the engine is putting out with 1,2 bar ?

Thanks

Eric

1AS

A previous poster who dyned his car A LOt suggested each .1 bar = 10 hp This seems to be the difference between an X50 and a GT2 (.1 bar and about 10 hp) AS

Dock

It depends on the RPM.

ebaker

I've read .1 bar = 17hp. on a 996 and 993TT.

ENZO_ETR

depends on turbo size, on K-24s, probably in the 550hp range

Charlie360

Just adding my thoughts here - and they may well be wrong, but I'm interested to know why if they are. Power is basically torque multiplied by radial speed, or RPM. However why does the power/torque depend upon the turbo size, AFAIK the boost pressure reading is for the intake manifold, therefore if the boost reading is 1.2 Bar, surely that is the pressure regardless of the turbo's physical size?! I have heard though that running the k16 turbos above 1 Bar doesn't give them a very long life, just my 2c but am interested in the real reasons. Oh, and a Happy New Year to everyone!

Dale Gribble

happy new year to you too friend,

with regards to the pressure of the turbo depending on size, you need more compressed air to reach the same pressure on a bigger turbo than a smaller one, so when the time to dump all that air back into in the engine comes, you're getting just that much more.

take one of those small plastic coke bottles that you can get froma vending machine. It is filled to capacity with coke and any more and it will break/leak. However, take an empty 2L big bottle of coke and pour the contents from the small bottle into the bigger bottle and you will notice the bottle is no where near full.
its just like with turbos, the amount of air from 1 bar on a small turbo if you were to carry over directly to a bigger turbo, would not read 1 bar (it would read less, lets say 0.5 just for example). thats why bigger turbos take longer to spool up.

having said that, i'm not the most technichally knowlegable member here, if i've mispoken some facts please correct me.

hope this helps.

Charlie360

That's obviously true, however the inlet manifold is AFAIK the same size for X50 or just standard TT isn't it - I guess what it comes down to is where the reading is taken from if it's just down stream of the turbo or right before it enters the cylinder. I also thought that the bigger turbos take longer to spool up because of the extra rotational inertia of the larger diameter internals. Ah well never mind

Zippy

1 bar from a small turbo would produce more heat than 1 bar from a large turbo. The hotter the air, the less dense (less oxygen) it is. Therefore the cooler charge from the 1 bar produced from the large tubo would contain more oxygen and hence be capable of producing more power if the correct amount of fuel is added.

Fred R. C4S

FWIW,

Given a constant volume, (intake manifold), the intake manifold has no idea what size the turbo compressor is that has just charged it with air. The air doesn't care what size the compressor is either. Given a constant volume, air compressed to a given pressure is the same temperature regardless of what was used to compress it. Unless the laws of thermodynamics have changed since I graduated 32 years ago, PV=NRT. N and R are constants for a given gas.

The only difference that I can even imagine would be at what engine speed (rpm) is the small or larger compressor able to achieve the given pressure. This should be negligible. However, how quickly one can develop a given pressure may be different.

Cheers,

Zippy

I was under the impression that a small turbo, spinning at the upper end of its efficiency range would be producing heat in and of itself. This is where the incremental heat is incurred v.s. the larger turbo. Both will produce the same amount of heat from the process of compression. The inefficiency os the smaller turbo opperating above it's efficient range will produce additional heat.

The again, I'm no thermodynamics graduate!

Earlierapex

Fred,
The laws of thermodynamics haven't changed, the formula you describe is just ideal, literally. It is kinda like ignoring the friction in the air when estimating how far a canon ball is going to fly using a formula for distance in physics. The ideal gas law says that a gas will ideally only increase in temperature by the ideal amount when you compress it.

The reality is that the air gets a lot hotter than that because man has not yet invented a 100% efficient compressor. In fact, we measure the realitive efficiency of different compressors based on how much MORE they heat the air than that which would occur ideally. This law is literally called the "ideal gas law." This is why we look at the efficiency ranges of different compressors. Basically, it all boils down to a smaller compressor "chopping" the air more at higher RPMs and heating it more (you gain lower spool times in exchange).

So, a smaller, less efficient compressor pushing 1.2 bar will make the air a lot hotter than a larger compressor at the same 1.2 bar. Hotter air = less power.

Here's a good way to estimate horsepower based on boost. Take the normally aspirated engine and adjust for lower horsepower due to lower compression. For example, the turbo without turbos would probably make about 280hp. Now multiply the N/A engine by the boost and by the efficiency of the system after the intercooler. The small turbos on the 996 are running WAY under 100% efficency (including the intercooler) - probably about 60% or so.

So, your formula for horsepower is going to be 280*0.8bar*60%efficiency = about 420hp. You are *getting* about 140hp with .8bar or about 18hp per tenth of boost. So, at 1.2 bar, you should get 72 more horsepower. HOWEVER, you are running even further beyond the efficiency range of the turbos. each incremental increase in boost is going to result in much less incremental horsepower. In reality, you'll probably get 40 more horsepower and a much hotter and less happy combustion chamber.

Bigger turbos produce more horsepower at the same boost because they heat the air much less and drive up the efficiency factor in the formula. If you increased turbo size and got a more efficient intercooler and increased the efficiency to 80%, even at the stock boost level of 0.8bar, you would be making 33% more incremental power or about 45 more hp at the same boost. The impact is even greater at higher boost levels.

-dc

Kevin

This is not a apples to apples comparison.. There are different designs for compressor wheels.. High Pressure, Low CFM... Low Pressure, High CFM.. If I back up and look at the K16 vs K24 compressor wheel, they basically are Low Pressure, High CFM designs.. However, you will pack more air into your plenum with a K24 compressor wheel per turbo rotation.... (if you keep the same turbine wheel size and A/R's) More volume CFM is being generated...

That's why one can make more torque with a GT1 hybrid vs a K16 or Stock K24 turbocharger..

You can change everything by using a high efficient compressor wheel, or keeping the same compressor wheel, and Zero Clearancing the compressor housing.. More air is compressed and packed into the intercooler with less heat generated..

You will always generate heat when you compress air.. One can pick there solution by increasing compressor efficiency and or installing larger intercoolers..

DC Saying that smaller compressor wheels are less efficient isn't correct.. I really depends on the design and manufacturer of a given compressor wheel.. Borg Warner has TO4E that are smaller in diameter than Garrett TO4's, and they are 8% more effiecient. Bigger turbochargers will not always give you more boost.. per engine RPM.. Bigger turbo's will shift your torque range up the RPM ladder.. For 1/4 mile drags you might be seeking power at >5500 RPM.. If you use a turbo that has a larger turbine wheel, you increase lag.. Yes, you have a larger compressor wheel but you are making less boost/more lag.. (lets say at 2500-3500 RPM). A smaller turbine wheel with the same size compressor wheel as the "big turbo" will make more boost due to the fact that the turbo is producing more CFM per RPM, thus the engine reaches Redline quicker and one shift's to the next gear faster.. A good example is my K27 High Flow vs a K29.. The frame size of the K27 is smaller, but it will out produce the larger K29.. It will make more boost sooner and sustain it past redline.. Why, because I'm using a very high efficient 60mm compressor wheel.. vs a big old 25 year old, low efficient compressor wheel..

Your last statement is correct if you state that keeping the turbine wheel and A/R's the same, and if your larger compressor wheel is more efficient.. Keep in mind the the turbo's RPM's will be slower, but your engine see's the boost faster in the Engine RPM band.. If you start changing A/R and turbine wheel sizes, you can throw out any realm of apples to apples...

Edit.. K16's running at 1.2bars of boost are being run in an overspeed condition, due to the fact that the compressor wheel have run out of air.. The efficiency of the compressor wheel declines at that RPM and more heat is generated... When I tear down K16's, I can gauge the bearing and thrust wear, and guess the programming boost levels by the wear patterns.. (wheel rub is very common in this situation, nice powdered aluminum being sucked into your engine and re-deposited "melted" on your turbine wheel) However, this is cured by installing a larger more efficient compressor wheel (shaft speed is lowered)..

K16 compressor wheel is 76% efficient
K24 compressor wheel is 76% efficient

Earlierapex

Kevin,
I agree completely. I was trying not to get too technical. I probably should've used "more efficient" instead of bigger when describing turbos, but then you may have argued that it's really "efficiency over a certain RPM range and boost level."

I may not have been answering the right question, but the short answer to the question of how a different turbo can produce more power at the same level of boost is directly related to efficiency which can be measured via temperature rise above ideal. In *general* we describe turbos that can produce more cooler boost at higher RPMs as "bigger," and that is not an altogether inaccurate description. It may be incorrect at the margin and in the grey area, but most of the industry uses that broad term. As in "dude, you need a bigger turbo." I also was using the term compressor to describe the entire turbo and not just the cold side (poetic license).

We could iterate ourselves to death with more and more technical discussion about efficiency, and in the end, no single component can be analysed by itself. We are really talking about the efficiency of the system as it relates to RPM ranges, air temps out of the turbo, amount of boost, intercooling, ability of the engine to process various CFMs of air over various RPM ranges and how that matches the rest of the system, etc.

I would like to learn more about various cold side/hot side wheel size and how that changes the compressor map though. Sounds pretty interesting.

As a side note, the formula above relies on the efficiency of the "system" including the intercooler. You can't just pugin the efficiency of the turbo.

-dc

Larry Harris

What they said. Now... how many large coke bottles do you pour into the little one?