Boost mapping theories

[Jamo]

I'm looking for some theories on boost mapping if anyone has any link.

The stock ecu and any remap has boost trailing down towards redline and it was noted when at Weston that they couldn't understand why all the Audis were setup like this.

What is the technical basis behind trailing the boost and what happens if you hold boost to redline instead?

[newsh]

At 6k the engine swallows twice the amount of air than it does at 3k so the turb the turbo has to do twice the work to keep the boost pressure the same, I think its just natural that boost will tail off, however, if you have a big enough turbo to supply the top end and you have a big enough wastegate to hold the boost down in the midrange then I would think its possible to hold the same boost to the redline. It may be easier to do mechanically with the right spring and boost controller than with the WGFV, not sure about this, I know my ur holds the same boost to the redline (only about 1 bar) and thats just a spring.

[A80Avant]

i have noticed that with the original WGFV and VEMS, i was able to hold low boost levels to the red line without issues and using the original setup. That is i could hold up to 1.5bar reasonably ok on 4d and 4th gear.On higher gears the problem vanished.

When i tried to keep 2bar boost to the redline on 3d gear it was just not possible. at about 5500rpm the boost would already drop and continue dropping. I would experience similar drop on 4th gear,but everything would be alright on 5th and 6th.In practice boost would trail off back to 1.5bar on 3d and 1.6bar on 4th.

On the turbosmart solenoid and using the top chamber of the wastegate, i am able to hold 2bar longer and it only trails off to 1.7bar @ 6000rpm. Same on 3d gear. On higher gears it stays put.

I think it also has to do with the amount of backpressure you have. I think my car with the small AR and 70mm catback may be causing a more rapid drop. I am still using a 0.5bar spring and i think if i changed to a 1bar spring, things would be much easier to control.

I only keep this setup, because i know the Holset will be more lively and will be able to hold good boost pressures without a drop. If i realise this is happening as well, then the next thing is a stiffer spring(possibly the 034 1.9bar one) and a boost remap calibration.

I do have 3-4 springs of different stiffeness which i had made here locally as well which can be tried. They are about 0.6/0.7 and 0.8bar if i remember correctly

[EspenW]

Small stock turbo can easily be pushed off its map and push hot air (not to mention back pressure) if it is forced to deliver peak boost to redline.
Manufacturer need to be well within the limits in case someone in Dubai decides to pull a trailer on a 50c day up a steep hill. Should be pretty basic stuff for a tuner like Weston?

[GazzaG60]

im with espen.

stock turbo is running out at 5k. itll be a fireball holding by 6k.

saw this many a time with the subaru's where that extra couple of psi gave no gains due to heat.

VAS- On the turbosmart solenoid and using the top chamber of the wastegate, i am able to hold 2bar longer and it only trails off to 1.7bar @ 6000rpm. Same on 3d gear. On higher gears it stays put.

this is gas flow. basically the duty at the gate needs to be increased across those areas. the drop is when the wastegate is open and loosing more than the engine can generate. on eboost there is a setting where you tell it where its tailing off and where you want it to go to and it changes duty to suit accordingly to "hit and hold"

[Jamo]

Weston weren't tuning the cars so too speak, it was at the RR day and I was talking to guys after finishing the day about the various boost profiles and this came up.

[Rusty]

Boost profile is related to the compressor map too, hence why over boosting on a small turbo creates the heat, it's outside of it's higher efficiency zones. The maps are a function of efficiency, pressure ratio and flow rate so to maintain the efficiency of compression you need to alter pressure ratio with changing flow rate. Same with surge line.

[Jamo]

I'm not talking about compressor maps I'm talking about ecu mapping etc that trail the boost down when it is possible to hold higher boost in the high rpm periouds

[GazzaG60]

if its possible to safely hold boost then its not a bad thing. i think intercooling, injector and pump sizing along with turbo and exhaust hare considered.

most tuners are interested in erring on the side of caution in the uk rather than extracting best figures or a compromise.

im a little puzzled by their comments though as most cars ive seen (road going factory cars) are designed this way to give a peak torque around 2-3k then dropping off. its a combination of small turbos and safety measures. a good thump in the back between 2 and 3k is what 90% want. then they change gear at 4k or so having smoked the vecra in the lane next to them.

[Jamo]

Ok for example if you run pure mechanical boost with a big spring you will hold boost all the way to redline, but when using mapping tuners will lower it.

Maybe it is just safety but i'm trying to find the actual theory of why this practice is employed when tuning.

[GazzaG60]

to an extent yes.

the spring is usually one that can be pushed all the way within safety why we got 0.4 spring.

putting a bigger spring in there will help hold on but potentially kill a lil turbo. back to the balance thing.

i understand what you are asking but i think you have answered it yourself now with the safety.

for theories sake what would you think, if it could do it, the standard turbo with the rest of the standard manifold,exhaust holding the boost to x figure do to egt. restrictive system, high compression, lower timing at top comparitively. would it burn the exhaust valves/turbo/exhaust up? i dont know i just thowing a theory in. id expect the average person may use the top end power more and kill it sooner than later.

[Jamo]

I need to know before I kill my stroker

[Rusty]

Having tested various springs, none held boost levels constant. Most dropped off by 30-40%. Careful spring selection could tailor the boost, or removal of the WGFV feed to the WG diaphragm.

[GazzaG60]

your 3076 should hold it fine.

look at air intake temps too. your cooler has impact too.

id start at gate pressure. see how it holds. add bits in. look to ramp in a bar or so first and tail it off. then look at higher end duty to pull up the drop-off. raise gate pressure to ramp a bit more in the middle and repeat the cycle.

im sure you wont kill it with turbo heat. lack of timing/too much timing or fueling issues maybe.

im at 1.8bar now and its a flyer.

[A80Avant]

thanks for the input Gary,
the thing with VEMS is that you can either specify boost level OR solenoid duty cycle to control boost.So i chose to specify MAP and let VEMS deal with the duty cycle. However in case the boost can actually be manipulated with dutycycle on the closed loop graph, although not being sure that it actually plays a role since im MAP based boost, i have put it up to 78% without any change.

If i increace the boost MAP on that area, there might be a chance that i will hit the marks, BUT on higher gears it may go quite higher as well.
eg. If i ask for 310kpa after 5500rpm in order to hit my 290kpa mark on 3d and 4thgear, that will mean that on 5th and 6th it will go to 310kpa, and i dont want that!

On the other hand i can manipulate the offset function a bit and reduce the boost values on spool(early in the rpm) so it doesnt overshoot.

Since this only happens with 3d and 4th gear, im not really bothered to be honest, as the 1.1.6x versions of VEMS also have a gear specific reduction in boost that might come in handy. By that i mean that boost may be able to be reduced as a % in higher gears to compensate.

OR i might go the easier way and just fit me a stiffer spring!

We'll see how the bigger turbo responds...