Tuning and peak boost

[Andy_Mac]

So another ramble about stuff I don't fully understand (bear with me guys) like turbo's and what is wrong with the current legacy setup and all that. 
What is it that actually limits how much boost a stock turbo can run?

Say it isn't spiking EGBP at 17psi should the compressor side not be able to spin up to say 19psi, as long as the EGBP doesn't then spike, with a gradual taper to redline, sure it loses efficiency up higher but is EGBP and compressor efficiency the main things giving the limit? Unless its cracking open i'd have thought wastegate issues would give spikes more than drops so don't think that is the culprit.

When it had the tune at DTech they said they could pump the boost right up in the ECU and it'd never go any higher physically, even after switching to ECUEdit since ECUFlash had a messed up def file.

 

Anyone come across any good reading material on the matter? Only really found fluff pieces with bugger all real info on the net so far

 

Still super sure its the ECU causing the issues with mine. Still hits 17psi then drops almost straight back down to 15psi then tapers off slowly. Need to weld a bung on the other header bank to see if both are seeing a similar backpressure figure for a start. 

 

Kinda leaning towards getting a Link ahead of the 257 build since the setup should be identical wiring wise then just have a fiddle with it myself knowing its atleast trying to do what its told rather than the stock one seemingly ignoring inputs.

[Kiwi_Fozze]

Normally the boost comes down to flow vs temp. 

 

I know the vf30 will push 24psi relatively happily, through mid rpms.

 

But. 

 

It isnt a big enough turbo to hold that kinda boost to redline,same with most factory turbos. It simply cannot flow the cfm air volume required to keep the intake pressurised.

 

Think of it as a divetank. You have a small compressor filling the tank, and lets say you are spray painting out the end of it.

 

Sure, short bursts, as you drain the dive tank it'll supply a good level of flow, but only for a short while, and as the tank empties you basically end up running the spray gun on what your compressor can flow. Good for short bursts, no good for really squeezing the most from.

 

 

they sacrifice the top end balls to the wall booooost for something thats more usable. 

 

Also temperature. If your turbo is able to flow the high pressures, you srart to run into cooling issues. Compress air and it heats up. A lot. So sure, you may be able to hit 24 (or 17) but are you running into issues where your tiiiiny turbo (designed for low to mid range response) is huffing out hot air because its outside its efficiency range?

 

The way I underatand it:

 

Factory turbos are small. Sure you may get a relatively high spike, but it will fall off real fast. If it doeant, chances are you are superheating your intercooler, and possibly overspeeding the turbo.

 

 

Tl;Dr

 

Factory turbos are small. Run out of puff, and / or blow really hot air. Get a big twinscroll. You'll choke a 2.5 build with what you currently run

[Andy_Mac]

Oh yea definately getting something alot bigger for the 257, just kinda interested to see if an ecu doing as it should will actually behave any different as there has always been an issue with tuning the boost on this particular ecu

[Guest]

Easiest way to understand boost is that it is a measure of restriction. If you have a 20mm exhaust on your subaru you will have mega boost but no power. You want maximum power from minimum boost. If the turbo is correctly sized you wont see a drop in boost and will have massive area under the curve.

 

Whats egbp telling you so far and whats post turbine reading?

[Andy_Mac]

Yea I get the idea around the same boost on small and large turbo's and the difference in the end result.

Main thing i'm kinda asking is if the back pressure never gets too high to stop the wheel accelerating will the comp wheel

ever reach a point where it can't physically go any faster, within reason of course. And ignoring the fact that in reality the ecu would see the intake temp skyrocket and pull boost and timing. The way Chris worded it made it sound like they could literally get nothing above where it ended up as which to me sounds more like the ecu being finicky more than it seeing a intake temp spike and pulling all the timing out, and I can't play with it myself the public def is completely ballsed up.

 

Can't quite remember what it peaked at. Boost gauge only goes to 2 bar and it didn't get past that. Post turbine was always F all.  Copper capillary tube broke off the bung a while back and haven't got round to fixing it yet.