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#15
by
saurkraut
on 14 Aug, 2009 12:40
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is it safe to run a K24 with the wastegate set at about 25psi with a wastegate still hooked up and boost controlled?
I seem to have had no problems running one at 25 PSI for a few years. You can't get there by screwing in the set screw in the dash pot on the waste gate. Screwing it in makes very little difference until you get to the point where the spring coil binds and limits the amount the waste gate can open.
There are two solutions: Some sort of a boost controller, or disconecting the boost line to the waste gate and plugging the banjo bolt with RTV. The waste gate majically blows open from exhaust pressure right around 25 PSI and works just fine. The set screw in the dash pot can raise it even further. It seem to adjust better in this range. You can get to 30 PSI by overfulling, but that is boost creep mode and the EGTs get pretty high.
The only failure I've had is loseing a small piece of the trail edge of one of the turbine blades. But that was on a k24 that had over 400,000 miles on it.
http://www.vwdiesel.net/forum/index.php?topic=14895.0&highlight=
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#16
by
arb
on 14 Aug, 2009 13:35
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They both get to 25 PSI, but the k26 is heating the charge air a heck of allot more than the k24.
That is very interesting as the idea gas law says the heat from compression should be the same... maybe the K26 is less efficient at that speed and imparts a lot of energy through more churning of the air (heat from mechanically moving the air around at high speed.)
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#17
by
saurkraut
on 14 Aug, 2009 14:20
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They both get to 25 PSI, but the k26 is heating the charge air a heck of allot more than the k24.
That is very interesting as the idea gas law says the heat from compression should be the same... maybe the K26 is less efficient at that speed and imparts a lot of energy through more churning of the air (heat from mechanically moving the air around at high speed.)
Yup, surprised the bejeepers out of me. Same pump settings, lower compression ratio, and the EGT goes nutz. It has to be the K26 compressor opperating beyond its capabilities.
I should have the K24 back togather and on the motor in a few days.
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#18
by
mean88
on 14 Aug, 2009 21:11
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I started a thread on my small block ford site. there are alot of really smart proffesional guys there. I asked with a quote from here about "15psi is 15psi" My custom cam grinder Jay Allen posted the below:
Quote from: par442k on Today at 01:11:51 am
Wouldnt a smaller turbo need to spin faster to move the same amount of air as the larger one, thus creating more heat.
Posted by Jay Allen:
This was where I was going to go with part #2 of the discussion.
Boost in fact is not boost. 15 psi is not 15 psi.
Indy car back when they had turbos on them used a pop off valve at 25 psi. At first there was 2 cars that seemingly had 100 more HP than everyone else. Hmmm.......Then whatever the name of the sanctioning body was stepped in and imposed a "maximum turbo size" as the fast guys were using a GINORMOUS turbo and it was barely working. The air was being bleed off so no one "really" thought about it.
Do you guys remember who this was? They could not keep engines together at Indy to save their life.
An air charge temperature sensor points this out, instantly. Boost is heat and hence why intercoolers kick ass. A smaller unit with a much higher impeller speed is going to create far more heat. So while "15 psi is 15 psi" in your mind and on the "boost" gauge, there is a significant amount of power to be had with a bigger unit that is not working near as hard.
Where you measure matters as well. Check boost at the turbo, in the intercooler (if there is one), at the blade of the throttle body, and then in the plenum.
Hope This Helps.
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#19
by
burnt_servo
on 15 Aug, 2009 04:03
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maybe things should be reworded .....
15 psi is 15 psi , only if the temp of the compressed air is the same .......
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#20
by
Rabbit on Roids
on 15 Aug, 2009 12:28
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air is air wether its at 0 psi or 4000 psi, its still air. when it gets hot or cold tho, then its not the same, since air expands and contracts quite a bit.
so, witch ever turbo keeps the air the coolest will push more, since it will still be slightly shrunk. but if you have a turbo that heats up the air at the same amount of boost, it will effectively move less air. thats why boost drops when you add an intercooler, the air shrinks and it can compress more to make up for it.
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#21
by
saurkraut
on 18 Aug, 2009 11:14
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The K26 compressor is off, the K24 compressor is back on, and EGTs are back in the ball park.
The boost builds faster too.
The K24 compressor makes 25 PSI boost at a cooler temperature than the K26 compressor.
Go figure.
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#22
by
Rabbit on Roids
on 18 Aug, 2009 15:54
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really? im gonna scratch the idea of a hybrid then, if the 24 performs better, lol.
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#23
by
rabbid79
on 18 Aug, 2009 18:55
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Saurkraut, it's great that you did this research, and had some before and after comparisons. I know I had K24/K26 hybrid on my mind too. Sounds like I'll stick with the K24 and call it a day.
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#24
by
barrygti
on 20 Aug, 2009 05:03
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15 pis to 15 psi argument eh???
A little post I read years ago explained it perfectly, this is based on Ford Cosworth YB engine but it is apt to the discussion.
quote:
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Foolishly, sorry, Originally Posted by erm.. someone
Look at it this (simple way so you understand ), imagine a 1 ft diameter balloon inflated to 15psi. Then imagine a 2ft ballon inflated to 15psi - are you trying to tell me that both have the sane volume of air in them?
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That balloon is actually bigger so yes it holds more volume, but this is not applicable to an engine because even if we take all the inlet valves out and pressurise the system with our turbochargers, we are still presenting each turbocharger with the same volume to fill, lets say 2000cc for the cylinders and 1000cc for the intake of the head and the plenum / hoses.
So we have a 3000cc volume to pressurise with air. This does not change unless the engine begins to operate. An engine will only generate more power by shifting more air at the correct AFR. SIMPLE.
The engine will only shift more air if we do one of the following:
1) Improve the airs route into the head.
2) Increases the pressure we push it in with.
3) Improve the mapping.
4) Improve the volumetric efficiency.
So,
We still know a T4 WILL make more power than a T3 at the same boost so why is that?
We don’t appear to have done any of the above mods do we?
The head hasn’t been ported.
We are running the same boost.
So have we changed the engines VE?
Good question, and back to turbo’s.
A T3 50trim with our desired pressure ratio of 2.4 (14.7psi inlet +20psi outlet divided by 14.7psi inlet) will be spinning at 134,000 rpm with a compressor efficiency of 70%.
A T4 60 trim with our desired pressure ratio of 2.4 (14.7psi inlet +20psi outlet divided by 14.7psi inlet) will be spinning at only 97,000 rpm with a compressor efficiency of 82%.
Now lets go to the turbine housing.
The T4 P trim wheel flows a lot more air than the Std T3 trim rear wheel. (They are all the same as std on T3) but it conversely takes more to spin it to speed.
We now have an exhaust backpressure IMPROVEMENT due to a better flowing rear wheel!!!!!!!
Secondly,
We now have a wastegate that will open much sooner and much wider than it would on the T3 as less exhaust volume is required to spin the turbine as we have a 37,000 rpm improvement in efficiency at our 20psi.
HEY, we have another exhaust backpressure IMPROVEMENT.
If our T4 is using a bigger housing, and it IS if compared to a T3 we have another exhaust backpressure IMPROVEMENT!!
So lets go to boost at the intake:
Now our exhaust backpressure is reduced, our cylinders demand for air has increased. We have overlap efficiency gains, we have thermal efficiency gains so we can suck more air and we can suck it with a greater pull because we are actually revving more freely so our peak piston velocity has increased.
So we are CONSUMING more air and this T4 can supply it for fun… But… we aren’t making more power because the T4 pumped more air at 20psi.
We are making more power because this turbo improved the volumetric efficiency of our engine mainly through exhaust backpressure reductions and an improvement in outlet temperatures at the compressor outlet due to Adiabatic Efficiency which we may or may not discuss later. It's ALLLL about back pressure and operating within compressor ranges people.
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#25
by
Rabbit on Roids
on 20 Aug, 2009 13:47
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so i wasnt wrong. not at all. a bigger turbo does flow more air at a certain boost level than a smaller turbo at the same boost. the bigger turbo is working more efficiently. and heating the air alot less too i bet. the bigger turbo moves more air at the same boost because it can also flow more air out the turbine more easily.
so, whoever told me i was wrong, is sadly wrong.
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#26
by
saurkraut
on 20 Aug, 2009 14:44
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The 'bigger' compressor in this instance appears to be the k24.
I was going to do some measurements of the K27 inlet and oulet to just for for s*** and giggles, but I was in a hurry to get the K24 right.
Side by side, the K27 compressor looks skinnier, so maybe that's its downfall.
It flows just dandy at 10 PSI on a bigger engine, but does a hors-it job at higher booste on a smaller engine. Keep in mind the hot side stayed the same, so the turbine flow/restriction was the same.
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#27
by
barrygti
on 21 Aug, 2009 01:33
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You need to think of it more in a relationship to the turbine, it may be bigger and flow more with the K26 front end but the back end is still K24 so it is trying to force more air through the same restrictions....
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#28
by
Rabbit on Roids
on 21 Aug, 2009 11:23
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true, witch is probably why his EGT's were through the roof. those K24 turbine housings are TINY!