Is my turbo too small? (loosing boost high end)

Breaking news, useful data or technical highlights or vehicles that are not meant to race. You can post commercial vehicle news or developments here.
Please post topics on racing variants in "other racing categories".
djones
djones
18
Joined: Thu Mar 17, 2005 2:01 pm

Is my turbo too small? (loosing boost high end)

Post

Hi,

Can somebody please help with a turbo related question I have.

The car is a Audi TT with the 180 BHP 1.8t engine. It has been 're-mapped' to 210 BHP.

Using some ECU logging software here is a boost graph...

Image

You can pretty much ignore the specified boost line, this is just what the ECU is requesting.

As you can see after peak boost, boost tails off. After about 5500 RPM you can really notice how the engine is tailing off on power after the big surge early on.

The question is, is the problem that the turbo is too small?

The turbo is a KKK K03 Sport, a slightly bigger K03 but not quite a K04. probably about 10% smaller than a k04.

Here is a compressor map for a k04 (I cant find a k03)

Image

It doesn't mean much to me but this is the air mass graph which presumably if you know what you are doing you can see if its a turbo flow issue.

Image


Any help or teaching would be much appretiated!!!


ps. standard boost is about 12 PSI which would leave a very flat boost line until pretty much red line.

manchild
manchild
12
Joined: Fri Jun 03, 2005 9:54 am

Re: Is my turbo too small? (loosing boost high end)

Post

I'd rather say that BOV is set to open at that point.

User avatar
slimjim8201
12
Joined: Sun Jul 30, 2006 5:02 am

Re: Is my turbo too small? (loosing boost high end)

Post

The turbocharger is too small. It cannot move enough air to creat much more power than you are already making.

Boost pressure is merely a byproduct of forcing air into the engine. The engine acts like a big restriction in a pipe, the more fluid you force through it, the higher the pressure differential. The engine is becomes less restrictive as the speed rises. If you push the same amount of air through a restriction and gradually reduce the degree of restriction, the pressure differential will decrease as well. This is what you are seeing with your boost pressure response.

The factory engine designed to provide low-medium turbocharging with quick spool characteristics. This turbocharger is perfectly sized to provide the best response for this engine at about 180+ HP. Boost pressure is limited to about 12 psi throughout the rev range to provide an even torque curve, good fuel economy, and component longevity. By allowing the turbo to move the maximum amount of air possible at all engine speeds, you end up with a boost pressure profile (and torque curve) like you have now. You have essentially maximized the performance everywhere for this specific turbo/engine combination. While your peak HP hasn't jumped significantly, the overall area under your torque curve certainly has.

Jersey Tom
Jersey Tom
166
Joined: Mon May 29, 2006 7:49 pm
Location: Huntersville, NC

Re: Is my turbo too small? (loosing boost high end)

Post

.. and slimjim nails it.

Compare your torque curves, before and after. I bet you gained quite a bit of avg output torque.
Grip is a four letter word. All opinions are my own and not those of current or previous employers.

djones
djones
18
Joined: Thu Mar 17, 2005 2:01 pm

Re: Is my turbo too small? (loosing boost high end)

Post

Thanks for the replys.

It's alomg the same lines as I was thinking.

To add a bit more to it the tuuning company who mapped it to 210 do a "stage 2" where power goes to 235 (you need a few simple modifications apparently like a new exhaust).

How on earth will that work when it looks maxed out as it is!! ?

manchild
manchild
12
Joined: Fri Jun 03, 2005 9:54 am

Re: Is my turbo too small? (loosing boost high end)

Post

How can turbo charger pressure be limited in the sense of what you're suggesting to him? If it is set to make 12 PSI that means that it could make more pressure if BOV wasn't tuned to open above 12 PSI. So, it is not maximum of chargers possibilities when it matters pressure. If it was too small it wouldn't be able to decently feed the engine at low-mid revs. Tighten the BOV and you won't have a drop at high revs.

TRICKLE69
TRICKLE69
3
Joined: Fri Feb 08, 2008 4:00 am
Location: USA

Re: Is my turbo too small? (loosing boost high end)

Post

That turbo is good to well above 12psi. Probably about 17 I would say safely. Also you need more air out which would require a lager downpipe and exhaust. Also a high flow intake and more fuel and spark which the mapping should take care of, and like was said already the blow off valve needs to be readjusted to hold boost to a higher level. Everything needs to work together.
IT IS WHAT IT IS

User avatar
slimjim8201
12
Joined: Sun Jul 30, 2006 5:02 am

Re: Is my turbo too small? (loosing boost high end)

Post

manchild wrote:How can turbo charger pressure be limited in the sense of what you're suggesting to him? If it is set to make 12 PSI that means that it could make more pressure if BOV wasn't tuned to open above 12 PSI. So, it is not maximum of chargers possibilities when it matters pressure. If it was too small it wouldn't be able to decently feed the engine at low-mid revs. Tighten the BOV and you won't have a drop at high revs.
You may need to do some reading on how turbocharger systems work. BOVs are not used to control boost pressure, especially at full load. If the throttle is wide open, the BOV should be closed.

The wastegate controls how much exhaust gas is pushed through the turbine or around the turbine. The more exhaust through the turbine, the faster the turbo will spin.

I'm not sure what you mean by "if it were too small (the turbo) it wouldnt be able to decently feed the engine at low-mid revs." This is exactly opposite of reality. The problem is that the turbo is not physically capable of push much more air than it already is.

In stock form, and we know this from the relatively flat torque curve, the system allows the turbo to provide a flow rate into the engine in a fairly linear fashion. That is, ~9 lbm/min at 3000 RPM, 18 lbm/min at 6,000 RPM, etc. By tweaking the engine controls, the turbo is allowed to spin to its full potential at all engine speeds, it's no longer limited. This results in MUCH more flow at lower RPM, and thus, the higher boost pressure. The gradual tapering of boost pressure is the turbo running out of steam. Period.

User avatar
slimjim8201
12
Joined: Sun Jul 30, 2006 5:02 am

Re: Is my turbo too small? (loosing boost high end)

Post

TRICKLE69 wrote:That turbo is good to well above 12psi. Probably about 17 I would say safely. Also you need more air out which would require a lager downpipe and exhaust. Also a high flow intake and more fuel and spark which the mapping should take care of, and like was said already the blow off valve needs to be readjusted to hold boost to a higher level. Everything needs to work together.
Rating a turbo on boost pressure is not wise. It doesn't tell anything about the system or the results. The same turbo on two different cars will yield very different results at various "boost levels".

And, of course this turbo is capable of supporting a pressure ratio greater than 12 psi. At 3500 RPM, it's supporting almost 21.

User avatar
slimjim8201
12
Joined: Sun Jul 30, 2006 5:02 am

Re: Is my turbo too small? (loosing boost high end)

Post

A post of mine from S2Ki...

When it comes to forced induction, I think many people have a lack of understanding of boost pressure and what it means for various turbo or supercharger setups. I'm going to attempt to shed some light on the subject. Please feel free to interject any useful information as you see fit.

THE ENGINE AS A RESTRICTION

At full load, our engines act just like a restriction in a pipe. Over time, a specific amount of air flows through the engine, with the engine acting as a bottle neck. If you force more air through the restriction, a higher pressure drop will result. The amount of restriction is dependant on a number of factors: head geometry, valve sizes, valve shapes, valve configuration, port sizing, port finish, etc. The F20/F22 is a VERY free flowing engine. The 4G63 is quite restrictive in comparison.

For a given pressure delta across the engine, a specific amount of air will be able to flow through. Just like a restriction in a pipe, a higher pressure delta will result in a higher flow rate.

BACKPRESSURE

It's important to have an understanding of the pressure delta across the engine, and we'll start with the "low" pressure side, or the exhaust side.

If you push a specific amount of gas through a pipe or collection of pipes, pressure will develop in the system. Small piping, more restrictive designs (lots of elbows and bends or rough interfaces) and flow blockages will cause the magnitude of the pressure to rise.

If you flow an identical amount of air through a tubular turbo manifold and a cast log turbo manifold, the pressure magnitude will be greater in the log manifold. This is due to it's more restrictive, less free flowing design. The same holds true for small turbines versus large turbines. While the flowrate remains the same, the smaller turbine will cause higher pressure development downstream of the engine.

This "backpressure" comes as a result of many factors, but the important players are manifold design (tubular, log, etc), piping diamter (large diameter piping is less restrictive), and turbine size/design (for turbocharged applications)

BOOST PRESSURE

Boost pressure is the air pressure upstream of the engine, the "high" pressure side. A turbine-driven compressor, belt driven compressor, or belt driven positive displacement air pump, pushes air into the engine at a rate that is higher than the engine could normally "suck in". The engine, now a restriction, causes the pressure to rise in the induction system. This pressure rise is what we measure as "boost".

RELATING BOOST PRESSURE AND BACKPRESSURE

It may be confusing to see the hundreds of dyno plots and results from the many F/I members on this and other forums. Some owners are making more power than others but at 5 psi lower boost pressure. It's very easy to jump to conclusions (and usually negative ones) about various turbo setups/kits/cars.

When it all boils down to it, one needs to keep in mind the pressure differential across the engine. Two identical engines will be able to produce approximately the same about of power given the fact that the same amount of air is moving through the engine. That specific amount of moving air corresponds to a particular pressure delta across the engine. Note that a boost pressure/back pressure of 15psi/5psi results in a 10 psi pressure delta. A boost pressure/backpressure of 20psi/10psi results in the same exact 10 psi pressure delta. These two engines will be able to move about the same amount of air, and produce about the same amount of horsepower.

Anytime you reduce the amount of backpressure, you also reduce the amount of boost necessary to move the same amount of air. This is the main reason why a well designed tubular turbo manifold setup will achieve the same power numbers as a more restrictive cast log turbo manifold setup but at a lower boost pressure. It is also the reason why a free flowing exhaust is important for FI applications. Ultimately, a log manifold can achieve the same HP numbers as a tubular manfold, but due to the higher necessary boost pressure to achieve the same pressure delta, it will take longer for a compressor to spool to its operating point. Not only that, but because the pressure delta across the compressor is now higher, a different compressor size/design may be necessary.

OTHER FACTORS

Compression ratio. - A higher compression ratio will enable an engine to make more power for a given pressure delta. Period. However, problems arrise when the effective compression inside the cylinder gets too high and it becomes difficult to control detonation. It may be necessary to reduce the compression ratio if the boost pressure becomes too high. Less restrictive designs can typically get away with higher compression, as the boost pressure does not need to be as great as in a less restrictive setup.

Pressure/temperature related knock. - Higher pressures and/or temperatures in the intake charge are more likely to cause detonation. A compressor will tend to heat up the intake charge much more at 20 psi than at 10 psi. The tuning window will get smaller and smaller as the boost pressure/intake temperature rises. More restrictive setups will result in a smaller tuning threshold.

manchild
manchild
12
Joined: Fri Jun 03, 2005 9:54 am

Re: Is my turbo too small? (loosing boost high end)

Post

slimjim8201 wrote: BOVs are not used to control boost pressure, especially at full load. If the throttle is wide open, the BOV should be closed.The wastegate controls how much exhaust gas is pushed through the turbine or around the turbine. The more exhaust through the turbine, the faster the turbo will spin.
Wastegate isn't by default indispensable part of every turbocharger but just an option. If his turbocharger has a wastegate than it also means that his turbo might not be too small but should work with without the wastegate relying just on BOV (if he seeks performance and disregards how long the turbo and engine will last).

zac510
zac510
21
Joined: Tue Jan 24, 2006 11:58 am

Re: Is my turbo too small? (loosing boost high end)

Post

Manchild every factory petrol turbo car of the last 20 years has a wastegate. For various reasons the BOV is not an efficient way to control boost so nobody does it.

There are various reasons why he could be losing boost but I shan't speculate because usually with a common, close to factory setup like this the problem is usually already known because the path has been well trodden. So best chat to some enthusiasts who have similar modifications so you don't find yourself wasting time and money fixing stuff that isn't broken!
No good turn goes unpunished.

manchild
manchild
12
Joined: Fri Jun 03, 2005 9:54 am

Re: Is my turbo too small? (loosing boost high end)

Post

I don't know what's he's up to with his car. All I tried to say was that if he wants pure power and doesn't care about lifetime of parts than he can "skip" wastegate.

zac510
zac510
21
Joined: Tue Jan 24, 2006 11:58 am

Re: Is my turbo too small? (loosing boost high end)

Post

That's a pointless thing to say because it is clearly mechanically limited by the combination of parts he has.
He's probably already bypassing the wastegate by using a bleed valve to increase the boost level over standard.
No good turn goes unpunished.

bizadfar
bizadfar
0
Joined: Wed Jan 03, 2007 2:51 pm

Re: Is my turbo too small? (loosing boost high end)

Post

typical K03 buddy, same thing happens in my A4 (180hp). Go K04 or bigger. K04 holds boost very well past 5500rpm~