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After reading about other peoples k04'ed S4s it seems like there is a wide range of boost profiles around. My own car goes well and it runs boost levels that make most people wince.

Now the 'max boost' thread has been done to death and I didn't think anyone would reply to another so instead I grabbed the K04-25 compressor map.




Armed with a pressure ratio of 2.7 (25 psi boost) and an airflow of 364 grams per second at 6000 rpm I set about the map.

First of all the x axis is airflow in m3/s. Now here's a leap of faith, at 25 degrees C and sea level atmospheric pressure air flow in m3/s is 1168 times the mass airflow in g/s.

So at 6000 rpm my engine swallows air at a rate of 364 g/s or 0.311 m3/s. ie each K04 pumps 0.155 m3/s.

Looking at the map I quickly spotted a problem with PR 2.7 and flow of 0.155 m3/s through each K04. It's not on the map!

So I did it again. Doing this for a few rpm / mass airflow figures:




The 380 g/s at 6800 rpm is an extrapolation based on a the graph of mass airflow vs rpm.

Plotting these figures on the compressor map:




That's pretty ugly. Above 5000 rpm I'm taking the K04s over their design max 183,500 rpm. The *only* good thing is that I'm well away from the surge limit of the compressor during spool.

Based on my reasoning you can run 24 psi boost up to 5000rpm then you must fall back to 22 psi as the airflow continues to increase.

Or the other way, you can flow 380 g/s at peak power, but you need to do it at a lower boost of 22 psi and I therefore need less restriction between impeller and the combustion chamber to keep the flow rates.

Yet another way of thinking about it, I must stop my K04s spiking above 24 psi and ease off a bit at higher rpms.

Or I very much shorten the service life of my K04s.



For the second part of my rant. I can run 26 psi to the redline, the K04s will do it, not that I'm anywhere near being on the compressor map in doing so. The engine will struggle to hold 10 degrees of advance through peak volumetric efficiency to redline, it does so with 3-4 degrees of correction.

I can also set up a 22 psi to 21 by redline boost profile. The car takes a bit more timing, but not dramatically so, maybe 3-4 degrees more with little or no CF. The car loses 0.2-0.3 seconds on on a FATS run.

So to the questions! Can anyone spot any flaws in the maths? I am assuming the density of air is 1.168 kg/m3 at 25 degrees C at sea level.

What are other K04 peoples timing figures during a FATS run?

Last question, almost embarrassing. In fact it is embarrassing, but how the hell do I change the time on the dash?!

Any other comments welcome.

disconnect your battery then reconnect it at the time its stopped at

Cheers.

If it's a pre-facelift pull out the left hand knob in the dash and the hands start turning and gradually get quicker.

Pull the button out below it and twist.

spen wrote:For the second part of my rant.

I was so completely lost in equations and figures that I hadn't even realised the thread was a rant!

Mmmm, a passing comment ... if you struggle to change the clock, should you really be messing about with compressor maps?

spen good write up I must admit a little over my head.

my figures show

maf 330 revs 5800-900 advance 12-15 boost 1.3bar at high end, peak 1.5

I can get you some logs if you think it'll help, I run water injection I noticed the timing changes when its on but can't find logs

Very informative post. Been trying to get my head around how you managed to plot it. I can see how you obtained the measurements for the x axis, but how did you work out the boost to plot against it. Where did you get the boost values from?

Whats youve said makes perfect sense.

I actually spoke to a tuner in America today and was told that 22psi is what you should give the turbos at the most if you want them to live long. And have them running about 17psi at red line.

You could get away with running them at 24 psi but id only attempt this with the compression lowered. and then I would run about 19 to 20 psi at red line.

I guess it all depends how long you want the turbos to live.

Just because of this im considering payign the extra and getting some hybrids. Good for 550 brake. But only using them to 470 brake capacity and keeping them well in the confines of the map. That was they get to be ultra reliable.

Do you have a compression map for the K04/RS6 hybrids by any chance?

Guys, about the clock - thanks, I'll pull the 'plungers'! It's a face lift with digital clock.

kies, thanks. good to see a little less boost means generally more timing is tolerated, as would be expected.

Saf, the y axis is the pressure ratio. To calculate it take absolute turbo exit pressure (atmospheric + boost) and divide it by atmoshperic pressure. If we cunningly work in units of bar the calculation becomes (1 + boost in bar) / 1 or just boost+1 ! As it's a ratio the units cancel out so the bar unit is removed.

One word of warning, if you're above 25 degrees impeller inlet temp, or higher than sea level then the shaft speed has to increase to move the same mass of air as the air's density will fall. And my scribblings are based on a few shortcuts which will introduce approximation errors. Having done the exercise I'm no longer happy running 26 psi that's all I was saying. Thanks Saf for introducing the seed of doubt (your k04 boost thread), you may have given my k04s a longer life!


FWIW I wouldn't automatically go for k04 / rs6 hybrids. The turbine housing and wastegate on the k04 / rs6 might be a restriction. I think it won't be able to flow the exhaust and the wastegate won't have good control over the shaft speed and you are likely to suffer boost creep. Worst of all due to the restriction you will have a high exhaust manifold back pressure, and that leads to poor timing figures and a very hot exhaust valve and manifold. You could clip the turbine to increase flow / reduce rotating mass.... but that leads to other problems.

That's all opinion, and I wouldn't recognise a hybrid RS6 / K04 compressor if you hit me with it. And I can't change a clock (Thanks Nige) so what the hell do I know?!. No doubt someone who sells them will tell you they're great! Maybe they are right? Why not just use RS6 turbos

Saf, I'll see what I can did up on the RS6 turbo's cold side.

Ncie one. When you get a compressor map, the exhaust side of the map doesnt make a difference to the flow characteristics of the cold compressor map do thay? So an RS6 map or a hybrid compressor map would be exactly the same right?

Im partly thinkign thats wrongbecause I know the straight RS6 turbos can flow more than the hybrids. But question is are the maps the same, because of the wheel or are they different because of the exhaust side?

I didnt want to use the RS6 turbos because they just not suited to the heads of the S4. I wanted the power throughout the rev range and not to move the power up up band. That way i get a surge of power ove the whole rev range. thats why I will only go to about 470 brake or so. Any more brake and the power shifts up. This means that the torque will be higher up the range although it will be more powerful. Id rather have the torque usable over the whole rev range and also good delivery. As the hybrids offer more efficiency over the K04's I would have thought they would be mroe reliable.

I guess its a risk. Why do you say the turbone housing would be a restriction because it woont be able to flow the exhaust. i didnt understand that. Why wouldnt it be able to flow it for? Im not sure about the wastegate. That I would have to look into more. Ill ask some questions and get back to you.

thanks

you were running 380g/s at 6800rpm? i thought they would run out of puff there or is that just extrapolation

Did you take account pumping losses? If not you should add about 1.5psi to 2psi and raise your curve accordingly. makes it even worse : )

I've run all my turbos over the map and I've yet to blow a turbo : )

It does shorten the expected life but I think its more in the way you use your engine cold and very hot + the oil + I've always used stock paper filter.

1.6 bar at the redline does not sound good though with kO4s and I'm suprised you feel its faster that way? Maybe its faster until 6500rpms but not after that? There must be a lot of backpressure and hot air with that high boost up top. IIRC I used to run 1.4bar after 6500rpms.

BTW: your table says each when its both (2)

Saf, you are correct. The compressor map is only telling the story of the impeller wheel.

The hot side of a K04 can flow a limit. At full throttle, full boost, at high turbo airflow: The exhaust has to squeeze through the wastegate port (the round hole with a 'door' when viewed from the back) , which is about the size of a 1p coin. Or it has to push the turbine around. The ratio of the two flows controls outlet pressure.

If the wastegate hole becomes a bottleneck for the gas then more will go via the turbine and the shaft will accelerate ie boost will climb in an unwanted fashion. This is boost creep. Taking a die grinder to the wastegate port sorts out mild cases or you could polish the whole turbine housing - they are usually quite rough.

The other problem is that under the same conditions the exhaust gas will be backing up in the exhaust manifold. It has to squeeze through a small wastegate port or push past a turbine wheel so it's not hard to imagine where this pressure is coming from. The pressure can be quite high, in some cases higher than the inlet manifold pressure

Pressure in your exhaust manifold is not your friend generally, unless you run cams with lots of overlap. It means there is pressure in the exhaust port and critically at the exhaust valve. This gas could be 1000 degrees. The pressure just behind the exhaust value means that when the exhaust valve opens it is possible not all the gas escapes the chamber. From the next intake strokes perspective there is some gas in there already (bad, it's taking up volume) and the existing gases are very very hot (really bad, it increases the susceptibility to detonation).

So too much exhaust gas back pressure results in poor tolerance to timing advance and a very very hot manifold and turbo as fuel is still burning in the manifold due to the retarded ignition. EGBP is not the only limiting factor for ignition advance but it does play a role.

The reason I mentioned this is that the K04 was designed with a flow rate in mind. You want more flow than that so you are putting a bigger fan on the front. You are asking the cold side to flow more and this means the turbine has to flow more as a result. Maybe the turbine is up to flowing more? But the impeller and the turbine are connected by more than just a shaft!. I dunno, it could be fine. I'm new to KKK turbos, but that K04 turbine housing looks tiny for the flow you want. The RS6 turbo looks more like it. It's all just gut feel though I'm afraid. Just a thought: KKK matched the RS6's impeller to the sized turbine that was appropriate!

Maybe someone else should advise you? I'm not a tuner, I'm not a mechanic! Who's going to build your hybrid? Ask them what the exhaust gas back pressure is likely to be at your chosen boost pressure.

This post is probably too large for anyone to read. Sorry