Justin517

run the turbo before because there was like 20 minutes left on the auction...

no biggie, I hav many many directions I can go in and many resources at my finger tips. I need to get the car home first.

Katman

You've just given me a very useful tidbit:

"The k26 (2664) can support 300 continuous horsepower. The k24 can muster only 270 or so. The efficiency of the k24 in terms of spool up and torque is better."

Confirming my thought that as long as one isn't going for really big power, the K24 is a superior (from a driveability standpoint) turbo.

But the K26 is still way more 'fun' when all the boost comes on in a rush. :-)

boxerfan

When you are looking at running 2.0 to 2.2 bar absolute the k26 compressor is so inefficient that it runs like a flame thrower much more so than the k24.

Rudimentarily drawing a line across the map at 2 and 2.2 demonstrates this does it not?

k24
<img src="http://pictureposter.audiworld.com/67435/k24_map.jpg">

k26
<img src="http://pictureposter.audiworld.com/67435/k26_map.jpg">

RS2 Replica
<img src="http://pictureposter.audiworld.com/67435/rs_map.jpg">

I have a coloured map somewhere that I will try and post as well.

The RS2 replica that I have spools VERY quickly. My understanding is that it is a hybrid. Much faster than the k26 I squished.

boxerfan

<img src="http://pictureposter.audiworld.com/67435/kkk_comparisson.jpg">

The line drawn across at 2.5 bar sets the vertical comparison. Then the thinner diagonal line from there to the horizontal axis completes the map.

When I did my maps I found the diagonal line sat further to the left. when running 2 or 2.2 bar.

Please let me know if I am making a fundamental error.

boxerfan

*edit*

Draw a line across the chart at your desired absolute boost level. 2.0 bar absolute translates to 1 bar of boost or 14.7 PSI.

Next work out your engines flow at redline ad at the desired boost level. Again, I used 2.0bar.

If you follow the line horizontally to the left you will run into the surge line. You need to stay to the right of the surge line. If you plot a line from the surge line to the x axis you will find the minimum CFM to reach your desired boost.

Now look again at your CFM table and see where your motor is capable of that CFM at the desired boost level. With the k24 I work that out to be about 135 CFM. Looking at the table I see that available at about 2200 RPM.

Looking at the k26 it actually indicates about 122 CFM or a little under 2000 RPM.

The difference here is that the k24 is operating at about 65% efficiency and runs right through the sweet spot of the compressor map cresting at 76%+ for a good portion of the time. The k26 starts nearer 60% and likely peaks about 73% for just an instant. At 0.13 m^3/sec it starts to tail off in efficiency again and drops well below 65% efficiency at 0.18m^3/sec.

By contrast the k24 peaks at around 1.4 m^3/sec but to compare apples to apples at 73% efficiency it's running 0.175 m^3/sec or substantially higher into the RPM range. It also stays above 65% until about 0.195 m^3/sec.

In short, from what I see, matchinng the turbo to the engine at 2.0bar of boost the k24 is superior in every way. There is one caution. The k24 seems to run right along the surge line if the system is set for 2.0bar.

So... have I made gross interpretational errors? Please let me know if I have errored.<ul><li><a href="http://www.lovehorsepower.com/CFM1.htm">CFM Calculator</a></li></ul>

Justin517

awesome info! Ive seen the individual maps before, but never an over-lay like that.

Justin517

check it out tommorrow.

boxerfan

Can you email me the compressor map?

Justin517

I think the car is a late '89 so I prob will have the K24 anyway. wether its good now @ 200k miles... thats a different story, lol.

shortyq

These plots are good for 'picking' a turbo, but they are far from real world.... 2 Problems, boost is not constant, Engine Volumetric Efficiency is not a constant. As such, you will not have (or need) 2.5bar immediately, nor will you have a straight line in terms of engine efficiency (VE, intake tuning, etc). IME, when you vary engine efficiency, you actually end up with a bow in the plot of the vertical/diagnal line, and you end up with a bowed ramp to 2.5 bar, less so as you go up in gears, more so as you are taking off in the lower gears.

I'll see if I can find some of the more technical calcs and plots I've done in years past.

Remember whatever you pick for peak VE, you usually get at peak torque rpm, and usually a little less VE at peak HP rpm. Everywhere else, it's a lot less than those two points = less air into the engine required for a given rpm. Enter dual resonance chamber theory or dual intake runner systems, but I digress

Scott J