Which Turbo is Better

[dieselpower]

hi,
which turbo do you think is better:
the K03 off the eco-diesel because its smaller and spools up faster
or the K24 off the TD because its bigger and flows more air

Dr. Diesel, did you end up making that hybrid K24/26 turbo and how did it preform for you with volume and spool up time?

[TDIMeister]

A Garrett GT15 turbo has the same turbine inlet flange as the K03 for TDI applications, which by extension will fit any VW Diesel.  Reading the map, it will suppot more air-flow at a good efficiency point than the 1.6 would ever take, and I have run it at up to 24 PSI boost sustained for quite a while!

Something the size of the K24/K26 would be a waste IMO.  There would be more gained through addition of / improvement in intercooling.

[fspGTD]

Looking at dyno plots of a 1.9l TD (torque peaks low, around 2000rpm - indicating that boost comes on early) vs a 1.6lTD (torque doesn't peak until 2500 or 3000'ish RPM) is indicative of the differences in these two turbos.  The downfall with the smaller 1.9lTD turbo is that it will give more pre-turbine backpressure at high RPMs, causing significantly higher max EGTs.  The larger 1.6lTD turbo will support more peak power at high RPMs but will take longer in RPMs down low before it will start peaking boost pressure.

So, depends what you are going to use it for... overall, I'd say that the smaller turbo would be more "streetable".  But if you are racing or going for peak power the bigger turbo would probably be better.

Or like TDIMeister says, adapt a variable-vane TDI turbo somehow and you'd have the best of both worlds.  I don't know anyone who has adapted one to an earlier car but in theory if you get the vane control mechanism dialed in sans-computer somehow, it could get you peak boost down low and low backpressure up high.

[VWRacer]

Jake, IIRC, the GT15 does not have a variable turbine nozzle. TDIMeister can set us straight, but I think the GT15 is the non-variable turbo used on the MkII TDI's.

TDIMeister wrote

Something the size of the K24/K26 would be a waste IMO.

Why is that? Do you think it's too small a jump over the K03?

[fspGTD]

OK, I guess I should clarify I am talking about the variable vane turbo from an ALH engine code TDI model here.  The ALH TDI (the TDI found in New Beetle and A4 models) came with a Garrett VNT 15 (GT 17V).  It is indeed a variable-vane turbo.  (Here is a reference to it on one of Garrett's web sites:
http://www.egarrett.com/products/gt_15_25.jsp)

It is true that pre-ALH TDI engine models (found on B4 Passat, and A3 models) didn't come with variable-vane turbos, they were wastegated.

[TDIMeister]

TDIMeister can set us straight, but I think the GT15 is the non-variable turbo used on the MkII TDI's.

Yes, you said it correctly, but I think you meant MkIII .  The GT15 turbo I'm referring to is indeed a wastegated turbo.  This type of turbo is the easiest to swap into a 1.6TD or 1.9TD since the VNT versions are vacuum actuated.

As for my choice / assertion for turbo selection, I had posted some missives on both VWVortex and TDIClub, so I'll refer you to them here:
http://forums.vwvortex.com/zerothread?id=1180145
http://forums.tdiclub.com/showflat.php?Cat=&Board=UBB4&Number=653315
http://forums.tdiclub.com/showflat.php?Cat=&Board=UBB4&Number=159807

Looking at dyno plots of a 1.9l TD (torque peaks low, around 2000rpm - indicating that boost comes on early) vs a 1.6lTD (torque doesn't peak until 2500 or 3000'ish RPM) is indicative of the differences in these two turbos. The downfall with the smaller 1.9lTD turbo is that it will give more pre-turbine backpressure at high RPMs, causing significantly higher max EGTs.

Your cause-and-effect statement is bang-on correct, but as I stated elsewhere,

It can be shown from a first law derivation that -- choking flow aside -- backpressure is a function of the total efficiency of the turbochager (i.e. the combined sum of compressor-, turbine- and mechanical efficiencies), not of its physical size.

Also stated elsewhere, somewhat related.:

I've heard a lot of talk (esp from those Vortex guys) about a big turbo heating up the charge air less than a small one. Where does that ridiculous notion come from??  

Post compressor charge temperature is solely a function of the compressor efficiency and nothing else. There is no argument that a larger turbocharger will tend to have higher peak efficiencies than smaller ones. What is not understood by most is why: Blade tips need to be as sharp as possible for best efficiency. However, due to strength and casting considerations, this ideal is not possible. Among other things, on a larger turbo, tip sharpness and blade tip to housing clearances (which are also particularly detrimental to efficiency) are smaller in proportion to overall dimensions of the wheels themselves.

So, while a larger turbo may indeed have a higher peak efficiency island than a small one, it is of absolutely NO benefit if it's so far to the right on the flow map that the engine NEVER makes use of it, and is instead almost always operating on an INefficient part of the map, or worse, straddling at or inside the surge region.

[TDIMeister]

TDIMeister wrote

Something the size of the K24/K26 would be a waste IMO.

Why is that? Do you think it's too small a jump over the K03?

I think it's too BIG!  :lol:

[VWRacer]

Ah yes, I was thinking that the first VNT was on the Mk III, so guessed that the GT15 came from the MkII. I must be off at least a generation... Thanks for setting the record straight!

And thanks too for the clarification on the K03. I am not familiar with the K03, and wondered about its relative size. (Hmm, I just searched on eBay and see a couple of them listed as off of 1.8 turbo Audi/VW's.)

In any case, I am interested in any thoughts you'd like to share about turbo selection for my CSR (1.6TD revving to ~5500 RPMs).

Cheers! Stan

[fspGTD]

Someone on the old forum with a modified 1.9lTD posted some exhaust gas temps that were alarmingly high... likely due to the "small" (yes, I know that's an oversimplification, and by small I mean a turbo that is more efficient at lower airflow rates ) turbo that comes on that motor.  They had a large exhaust and intercooler, too.

And the stock TDI (non-variable vane) turbo hits it's design redline at high altitudes, which is why the ECU reduces boost pressure at altitude.  Not much room to spin it any higher than that.  While the 1.6lTD turbo has safe room to spin much higher.

Anyway, I guess it depends what you're going after: good low RPM boost, or wanting the ultimate peak high-rpm horsepower and boost pressure.  It's a simple decision for me: rules in my race class limit my turbo and wastegate settings to the stock 1.6lTD turbo, so I can take my pick of either the 1.6lTD Garrett turbo or the 1.6lTD KKK turbo.  And I get to run them at 10psi. :?

For dad's daily driver TD though, I think a variable-vane turbo would give the ultimate in low-rpm boost with low-backpressure at high RPM.  I haven't analyzed the compressor efficiency plots yet so maybe I should just shut my trap (thanks for the link!)  But I'd assume the variable-vane turbine section would give a huge benefit.  I would think there's gotta be some way to adapt the vacuum vane control to the mechanical IDI motor.

Here is a compressor map of the 1.6lTD turbo - along with a line plot showing where the 1.6lTD motor airflow lines up with it.  Notice that it goes right through the compressor efficiency peak in stock form.  And it looks like it would support 70% compressor efficiency up to about 18psi boost pressure.

[dieselpower]

thanks for all the info
ok...so a small turbo hurts high rpm power, and a large turbo doesn't spool up fast enough for low rpm power.  I dont think im smart enough to make a variable vane turbo work on my diesel.  so what about 2 small turbos running in parrellel. there would be enough air flow so at high rpms there wouldnt be to much pre turbine back-pressure, but what does it do for spool up time having twin turbos.

[fspGTD]

What are you going to be using your car for?  Primarily street driving?  I'd actually recommend one of the small turbos in that case, to extend your boosted RPM range down more towards 2000rpm, where you'll probably do a lot of your driving on the street.

Just be sure to install an exhaust gas temp gauge and watch those temps before you do any fueling level increases...  to melt a piston would not be fun.

As for stan's racer...  I'd take a close look at the compressor efficiency maps posted above and do some airflow calculations for your motor.  You're not even limited to one of the stock VW turbos, so might as well scan Garrett's website (they've got numerous compressor options available aftermarket.)  The book "turbochargers" by hugh macinnes does a great job in explaining how to use these compressor maps in matching a turbo with an engine.

But - don't forget the exhaust side of the turbo needs to be matched too.  And the generally recommended approach of "change turbine housing A/Rs until you find the best one" is not very practical for most of us (or can get pretty expensive quickly.)  So Stan, you might be well off to just stick with a stock 1.6lTD Garrett or KKK...  up the boost.  See how it works, take some post-compressor air temp measurements and calculate it's efficiency.  Then make a decision based on that how you should be changing the compressor sizing.  An aftermarket Garrett GT turbo with a ball-bearing option for quicker spool-up might be worth considering.  But I'll bet you could to pretty well with a stock 1.6lTD Garrett or KKK.  Guess it depends on your budget!

[QuickTD]

Someone on the old forum with a modified 1.9lTD posted some exhaust gas temps that were alarmingly high... likely due to the "small" (yes, I know that's an oversimplification, and by small I mean a turbo that is more efficient at lower airflow rates  ) turbo that comes on that motor. They had a large exhaust and intercooler, too.

That someone would be me. I'm still fighting high EGT's, and I'm still blaming the K03. I did state that it was too small, a gross oversimplification. As TDImeister said, it's running far outside it's peak efficiency island at 15psi and the flow rate that the 1.9TD requires at this boost level. If I crank up the boost it's just making more intake air heat at this point, so more boost does little to cure the EGT problem. Another interesting effect of high boost/flow with a K03 is noise. The hiss suddenly becomes VERY loud beyond 16-17 psi. I'm wondering if the compressor is approaching "choke".

 I'm looking for a K24, or better yet, a K14 from an early 1.9TD. The K03 from the TDI is also a potential solution. It uses different wheels and housings to achieve better efficiency at higher pressure/flow.

I'll agree with FSPGTD, the K03 from the 1.9TD would likely be ideal for a street driven 1.6TD. I can make over 12lbs of boost at 1200rpm! My boost guage needle behaves as though it were directly connected to the pedal, it responds that fast. I'll probably miss that if I change the turbo...

[DieselsRcool]

Wow, this sounds like the perfect turbo for my 1.6TD Samurai. Low torque is king for offroad.

[fspGTD]

Your boost pressure is peaking at 1200RPM!?!?  I didn't know there was any VW Diesel turbo sized to that extreme!  The A/R (size) of the turbine section can also make a huge difference.

Here's some dyno plots from someone else with a 1.9lTD that I think still has the stock turbo.  They show a peak more around 2000rpm or a little above that.
Look down on the first page of this thread:
http://www.hostboard.com/cgi-bin/ultimatebb.cgi/ubb/get_topic/f/4037/t/701

Are you using a different turbo as the "MK3 GTD" is in above thread?

[QuickTD]

Your boost pressure is peaking at 1200RPM!?!?

Not peaking, that's 15psi at about 1800rpm, then it begins to drop as it approaches redline. Same turbo as MK3 GTD but you won't get 12psi @ 1200rpm on a stocker. It requires exhaust, downpipe, timing adjustments, intercooler and fueling mods to get this kind of low end. Gotta love the grunt.