Nozzle selection Vs. Boost pressure

I read posts asking about nozzle upgrades everyday & its always the same answer, this nozzle needs this tune and a turbo upgrade depending on the nozzle in question and the tune option. What I never see (and I know its variable with tune etc) is a relationship beteeen this nozzle & needing to run "x" boost pressure to avoid excessive smoke, high EGT's etc. PP520 - approximatly this boost pressure is recommended, Sprint 502 - around that boost pressure, Race520 - ...

I'm in the m-tdi world so the e-tune part is out for me but the question is still valid as I can still mechanically tune for max pressure. Be handy info for people planning upgrades as they know what boost they would need to sustain for the planned chip/nozzle combo. I'd also be interested in hearing what people think are reasonable boost pressures for the turbos that are available & used in builds around here.
Thanks!

any tuner basically will run the max safe boost for the curent turbo what ever nozzles you have , the cars performance will depend on whether the nozzles are supplying enough fuel for the turbo i.e so you get a light haze to know the fuel is maxing the turbo . or is it clean all the way through in which case you could use more .basically always buy the biggest nozzl;e you can afford as you can always back the fuel off if theres too much but you cant generate any more fuel froma nozzle thats too small .

I can see that but what i'm hoping for is someone who has tuned cars so say, well PP520 typically needs to be boosted to x-x psi range to see its full potential etc...

i dont see how youre going to get that unless you based your parameters on stock vnt . as maxed out pp520s would smoke quite a bit with max safe sustained boost of 18 psi on stock turbo , where as a 17/22 hybrid on same 18 psi boost would clear up all the smoke due to the fact that the bigger turbo can move more air at the same requested boost

taking into consideration the post above, it would be nice to know what, in rough/guideline terms, the usual turbos used for modifications can support in psi.

Don't get me wrong, I am absolutely aware that bigger turbos can flow more for a set manifold/s pressure - I'm not asking for power figures or estimates of suck. I also appreciate that theres hundreds of trim combinations out there for the same family of turbo.

What I am saying, is that it's sometimes hard to know what boost you should run. The math on garretts website seems quite crude to me, and at the moment I'm running a gt2256v on a 2.0tdi @ 20psi. I'm aware this is probably way below it's potential, but I think it would take either an emp gauge (expensive but best option), or second best, some user experience (on here?) with similar setups to help me gauge what I should be aiming for.

Obviously the can of worms here is that somone could run their turbo at an unsafe shaft speed building up an unsafe pressure and just because theirs hasnt blown yet, encourage me into blowing mine.

Most of us know pressure's arn't directly proportional to bhp capacity, before this turbo, I had a GT1552 - yes, a GT15 with a 52m compressor - wastegated - this spooled really nice, but could never get more than 135bhp out of it. At 26psi. I dread to think the strain that was under but she held strong and she's still in working condition, but obviously I have my gt2256v now to combat this.

So - whilst I expect an improvement, just what sort of improvement can be had from running 20psi on two different turbos, is it neglible, 2 or 3bhp, or is it ten or twenty, thirty or fifty?

I don't expect anything too dramatic, but the fact is I don't know. I've no idea of the scale of impact increasing turbine size can be when running the same boost.

Now, I don't know what boost people run on this site, i've personally tried to do searches just to see what is generally accepted for a particular type of turbo. Obviously I remain responsible for the wellbeing of my car, but it is nice info to have, even though anyone saying it isn't proportional to output bhp is absolutely spot on.

Is this post as clear as mud? hope not...

The various nozzles have been around for a while.

Think about it this way: You are at an advantage. People are telling you (for free!) what many learned through countless months and dollars learning the hard way.

At the simplest explaination, you put a bigger nozzle in to get more fuel in the cylinder in the same duration (Sans tune affecting start of injection)

To burn the extra fuel, you need more air in the cylinder. The only way to get more air in is more boost. Without the extra air to burn the extra fuel, you not only get wasted energy but you get more black smoke and potentially higher EGTs due to unburned fuel burning after it has exited the cylinder.

Smoke = Unburned Fuel = Wasted potential.

blackdogvan said:

I read posts asking about nozzle upgrades everyday & its always the same answer, this nozzle needs this tune and a turbo upgrade depending on the nozzle in question and the tune option. What I never see (and I know its variable with tune etc) is a relationship beteeen this nozzle & needing to run "x" boost pressure to avoid excessive smoke, high EGT's etc. PP520 - approximatly this boost pressure is recommended, Sprint 502 - around that boost pressure, Race520 - ...

I'm in the m-tdi world so the e-tune part is out for me but the question is still valid as I can still mechanically tune for max pressure. Be handy info for people planning upgrades as they know what boost they would need to sustain for the planned chip/nozzle combo. I'd also be interested in hearing what people think are reasonable boost pressures for the turbos that are available & used in builds around here.
Thanks!

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Boost pressure is just one of many variables that dictate smoke. It is also important to note that it isn't just how much boost pressure you generate, but when you generate it. The stock turbos can make plenty of boost at low RPM so fuel can be pumped in that much sooner. This has both it's good and bad points.

cog said:

any tuner basically will run the max safe boost for the curent turbo what ever nozzles you have ,

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That isn't always true. I don't know what other tuners do, but I think what you're suggesting will unnecessarily increase pumping losses and over all stress on the engine and it's components.

cog said:

the cars performance will depend on whether the nozzles are supplying enough fuel for the turbo i.e so you get a light haze to know the fuel is maxing the turbo . or is it clean all the way through in which case you could use more .basically always buy the biggest nozzl;e you can afford as you can always back the fuel off if theres too much but you cant generate any more fuel froma nozzle thats too small .

Click to expand...

I might add a caveat to this statement: Some large nozzles can have the effect of reducing atomization so it is important to keep a ballpark power goal in mind. Going too big on the nozzle can have potentially negative affects with regards to fuel economy, emissions, and smoke production. It would be analogous to putting your finger over the end of the garden hose where the more that you restrict it, the further the spray goes. When unobstructed the water flows freely but simply falls to the ground with no spray or atomization. If you need to run big nozzles though, at some point you should definitely consider pump upgrades to get some of the pressure back into the system.

cog said:

i dont see how youre going to get that unless you based your parameters on stock vnt . as maxed out pp520s would smoke quite a bit with max safe sustained boost of 18 psi on stock turbo , where as a 17/22 hybrid on same 18 psi boost would clear up all the smoke due to the fact that the bigger turbo can move more air at the same requested boost

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This concept doesn't make much sense to me. If we are filling a fixed volume with air to 18 psi it doesn't matter what size of turbo got it there, it is there. Now if we are talking about over an rpm range, it make sense. Perhaps a smaller turbo cannot supply the requested boost past a certain rpm where a larger one could.

odinsjetta said:

This concept doesn't make much sense to me. If we are filling a fixed volume with air to 18 psi it doesn't matter what size of turbo got it there, it is there. Now if we are talking about over an rpm range, it make sense. Perhaps a smaller turbo cannot supply the requested boost past a certain rpm where a larger one could.

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First, he's not entirely correct about PP520's smoking with a stock turbo. The smoke can be cleared up quite well with the fueling still pushed to the max. The trick is all in the boost response.

And, you'd be correct to say that this doesn't make much sense, but if you make a few assumptions such as the turbo possibly working outside of it's efficiency range (and introducing excess heat), and the exhaust side being quite restrictive, then there can be improvements made with the same amount of boost pressure on a larger turbo.

Boost pressure is not a simple measure of the air quantity. It is the result of the restriction of the engine, turbine and exhaust. So a larger turbo can flow more air at the same boost pressure because it's better efficiency heats the air less; cooler air is denser.

yeah looking at it now i realise some of that didnt make much sense but it was very late (well in the uk it was )
and i maybe i missread exaclty what he was asking i was just under the impression taht say a vnt 17/22 hybrid with larger compressor wheel will move more air and therefore is able to burn more fuel off than a vnt 17vb at the same boost , or is this not the case

i must admit mikel i wasnt aware you could max out the pp520s without smoke on stock vnt its not what i was told by my last tuner thats all but hey i moved on from first base now and bigger turbo i just need some more fuel and the headache that is 7 hole 764 whtehr they are as bad as the 2520 race or whether the smaller holes reduce excess fuel and heat on the bowl . watch this space

i may still go pp764 annd 11mm as its safer

vwmikel said:

First, he's not entirely correct about PP520's smoking with a stock turbo. The smoke can be cleared up quite well with the fueling still pushed to the max. The trick is all in the boost response.

And, you'd be correct to say that this doesn't make much sense, but if you make a few assumptions such as the turbo possibly working outside of it's efficiency range (and introducing excess heat), and the exhaust side being quite restrictive, then there can be improvements made with the same amount of boost pressure on a larger turbo.

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I understand!