Help me pick a good turbo once and for all.

[KTZed]

This should help

http://www.not2fast.com/turbo/glossary/turbo_calc.shtml

[subsonic]

Here's the map for the Merc .48  

[jimfoo]

Well, here is that gt20 map with a couple points plotted on it, right on the surge line at cruising speed, but thats probably ok, I assumed you lived at 5000 for the calculations,



  and here are the same couple points on the smallest GT15 map, nice and efficient at cruisiing speed, but out of breath for high RPM and high boost.

So These would be similar to maps for VNT 15's and VNT 20's except a VNT would spool faster than a GT, so you could get the same pressures at lower rpm's.

[jackbombay]

So These would be similar to maps for VNT 15's and VNT 20's except a VNT would spool faster than a GT, so you could get the same pressures at lower rpm's.

  Here is a VNT 15 map note the X scale is in KG/min, divide by 2.2 for lbs.



and a map for the 52mm compressor VNT20,

[jackbombay]

Here are those same 2 points plotted on the Merc map,

[zukgod1]

I can figure some typical operating conditions for you if you like. What RPM do you cruise at on the freeway? How many PIS on the freeway? What altitude do you live at? What max boost do you want to run? What is the highest RPM you run the engine up to?

Thank you sir.
I will look at the maps you posted as well to see if I can make sense of them

I'm @ 4500' so your guess of 5k was probably close enough.

76 mph is about 3000 rpm which is where I cruise most of the time.

Max boost will be ? no idea, I would like to be able to get 25.?

Top RPM is 4200 (ish) I've never taken it past 4500.

[zukgod1]

BTW jackbombay, I REALLY appreciate the guidance here.

I found a Garret T3 today at a local Turbo shop I can get for a song (pays to know people sometimes).
I'm going to get the trim levels so we can see if it will work.

Then I found a good VNT 15 as well.

Do you think a VNT15 will work for me if I can get a control made for the VNT part?

[jimfoo]

VNT 15 won't make anywhere near the pressure you want.

[zukgod1]

VNT 15 won't make anywhere near the pressure you want.

That's what I was concerned about. I could get great spooling but crappy upper RPM performance.

[jackbombay]

With the small displacement of the 1.6 you'll be close to surge with any turbo you choose, and there are only a couple that will push 25 while still staying close to the map. The GT20 map I posted earlier is looking pretty good. your probable highway speed boost and rpm are just on the map, where thats not the case for the merc turbo, or even the VNT 15, or even the stock .35 AR T3 that used to come with the TDs. Which shows that crossing the surge line is not the end of the world, but if you can stay inside of it you'll be better off and have a happier longer lasting turbo.

  The real question is how will the GT 20 spool. Wasn't malone running a GT20 on his franken motor? I'd look through his thread and see if he listed the specific model, there are 5 or so GT20s available on the garrett website, I linked the one I mapped the points for which has the highest PR and will push 25 PSI the best of any of them.

VNT 15 won't make anywhere near the pressure you want.

 Yep, the pressure ratio only goes to ~2.5.

[jimfoo]

But don't the actual VNT points depend on how the vanes are? BTW, I'd bet the GT20 and VNT 20 cold sides can be swapped.

[jackbombay]

But don't the actual VNT points depend on how the vanes are?

   The vanes won't change the characteristics of the compressor map, but they will change how much boost can be produced at what RPM.

  Example, I can plot 30 PSI at 500 (five hundred) RPM, but that does not mean the turbo will make 30 PSI at 500 RPM, but we can be pretty sure that it will make 10 PSI at 3000 on the highway, or close to it, so plotting that point will give us some pretty good info, same with 25 PSI at 4500 RPM.

[jtanguay]

i have a VNT-25 that i bought off Tintin    but i'm saving that for my PD motor.  that thing should be able to move some serious air.

if you want a turbo that is very robust and will outlast your engine, go with a VNT-20, and use Tintin's linkage setup as a guide.  chances are you will wreck the motor before the turbo, just like those TDI guys  :lol:

[zukgod1]

i have a VNT-25 that i bought off Tintin    but i'm saving that for my PD motor.  that thing should be able to move some serious air.

if you want a turbo that is very robust and will outlast your engine, go with a VNT-20, and use Tintin's linkage setup as a guide.  chances are you will wreck the motor before the turbo, just like those TDI guys  :lol:

Trying to find a VNT-20 is the problem.

[jackbombay]

Kerma has them.