I've been running an interesting experiment recently with my boost levels, standard turbo with gtec chip and a decat.

If I set it to 1.2 bar the midrange is really strong but it quickly tails off to about 0.7-0.8

If I set it to 1 bar the midrange is obviously more progressive but it holds more boost at the top, so by about 5500-6000 rpm it's actually making more boost than it was before. It just holds 1 bar for ages.

Dont know if it's to do with the increased heat from the extra pressure or whether the flow momentum is greater with less boost, but I think I'll be keeping it like this. Less traction issues in the wet, less torque steer, more power!

depending how intelligent the engine management is... could the excessive boost result in the timing being retarded?

i know from experience that many people continue to sing the virtues of high static CR based on the premise of high cr = more power. unfortunately modern fuels mean that too high cr means that retarding the ignition timing is required to obtain the best power figures. however, better power is achieved from lower CR with 'optimum' ignition timing.

high isnt always best !

I dont think it's retarding the timing, it runs perfectly and feels as fast as it should.

Going from memory it's something like this:

From 1.2 bar midrange to 0.85 at 6000rpm to 0.7 at the redline

From 1.0 bar midrange to 0.9 at 6000rpm to 0.75 at the redline



Maybe with my exact setup with the mods I've got I've just found a sweet spot at 1 bar?

Certainly feels noticeably faster than the standard Coupes I've driven which also run 1 bar.

Ditto. On my Greddy Profec the setting on 1 bar, the power delivery feels progressive and holds better than 1.2 bar setting

How are you controlling the boost?

Part of the issue is going to be that you will feel the boost tail off more if you are getting a bigger surge in the mid range...

what gauge are you using?how many times have you done this test on both settings,and have you actually timed the car?im willing to bet you have made the car slower.

There's more to upping the boost on an electronic controller than just setting the max. boost. Attack / gain can cause overboost, which in turn might trigger a reaction from the unit to lower the boost, which might result in the boost backing off in time (and thus revs, in case you're flooring it).

But knock sensors might also have a play in it.

I'm using a simple ball and spring PRV, no placebo effect here, I measured it on the boost gauge.

I think it's to do with heat, it has to be. The turbo can only flow a certain amount of air at a certain RPM, the more heat it makes, the less dense the air...?

are you running an intercooler? i.e. not the stock one?

Even using an electronic boost controller you will never get the stock turbo to hold at the top end - it just can't do it...

I think you get a lot of heat from high initial boost, then retarded ignition hence lower boost.

A sidestory from Nordschleife this summer:
Got high inlet temps efter a few laps in the heat, due to badly covered old air inlet dams. The boost sunk from 1.5bar to 1.0bar. Covered the redundant one with cardboard (like this: http://www.fiatklubben.nu/forum/attachment.php?s=c58a6e40bbe2b4d22803215f489192a1&postid=117589 )
..and soon the boost was up at 1.5bar again.
(no aftermarket boostcontrol)

Yes quite likley, OE ecu's are mapped very carfully aginst knock and or Air temp/timing advance .

Nope, just the stock one, the boost tails off as normal on both settings, the only difference is when it only reached a peak of 1 bar it's a tiny bit higher at the redline.

It's definitely not backing off the timing because if I downshift straight to 5000rpm at 1.2 bar then it holds more boost to the redline than if I go all the way up through the revs at 1.2 bar.

To me this seems to suggest that my setup is just slightly past it's optimal level of efficiency at 1.2 bar, but it's certainly not causing any problems as it's been at 1.2 bar for many years and still has perfect compression.

Maybe at 1.1 bar I'd get the best of both.

less heat more power?

I agree, less heat more power

Also true.

Less heat, more mass.

to get the best from this set up you could probably do with a larger exhaust (downpipe restriction) and a better intercooler,then turn the wick back up to 1.2 bar.

you MAY get more power at the redline,but where is peak power on these engines?it certainly wont be at the redline so the peak power reading on a dyno will in all likelyhood be lower with less area under the curve as its referred to.HOWEVER, it will definitely be more progressive the way you have set it,with less wear on the drivetrain and a reduced intention to wheelspin or torque steer,which could result in it being better in certain circumstances.if this is how you like it,bang on,it will certainly be cheaper to maintain and fuel.

it would be interesting to see results on a dyno with calibrated instruments

Dyno results vary on every single run and are affected by heat soak and and lack of cold air flow so I dont think they would provide an accurate enough picture of what's happening on the road.

Maybe it's got nothing to do with intake temps, maybe the PRV is causing it, when it slams shut as soon as the boost starts to tail off it slows down the flow a little bit with more pressure and less air speed in the exhaust manifold...

At 1.2 bar the PRV would slam shut a lot earlier than it would at 1 bar.

back to back dyno runs would give the best results,with the same conditions on the same day its the closest you are going to get to an accurate result short of going to the 1/4mile and comparing speed over the line(far more consistent than et,it changes very little compared to times)

I dont need a 1/4 mile strip, we got a runway at work, I'll just pretend to be an aircraft.

"Foxtrot Two Zero Victor Tango requesting clearance for take off"

........

"Take off aborted, proceeding to taxi off the dispersal then up the A15 to Asda, have a nice day"

we used to do runway blats at brize norton every year,sure i saw a few coupes down there a few times.

Put it this way - on the stock turbo I tried upping the boost to 1.4 bar on the dyno, and got no more out of it than at 1.2 bar...... this was through a front mount IC as well.

Ah, yes, the value of working towards a low boost engine

I can see some of the advantages in this area now I'm starting to look at turbo sizing calcs.

if the restriction was airflow then you would have found that whilst maximum power may not have increased the torque continued to be delivered lower in the rev range. having huge quantities of torque available at low revs is always beneficial. infact id argue that in non race applications torque at low revs is more valuable than high bhp at top end of the revs.

its all about what is available under the curve, not the peak figure

You always end up thinking "now that I've got great results with low boost, I wonder what I could get by turning it up a bit?"

hmmm, higher fuel consumption, chances of detonation and poor(er) reliability

lol I'm actually green with envy that you have such a cool job!

On subject, with my gtech chip i found 1bar seemed to feel a lot more powerful, certainly a lot smoother, 1.2 with a prv was an animal - my rr results showed 254bhp / 280lb at 1.1 bar, god knows what 1.2 was doing. However after about 2 runs and about 4.5rpm top end seemed non existant so heat in my case was most likely the culprit.

What is the correct method for testing our hypothesis? Checking charge temps? Whats the best way to do this..?

You can bung an air temp sensor in the inlet - I've done this before, so you might be able to find some posts by me on it from ages ago.

I've done all the work already though. At over a bar of boost on the stock IC the stock TB28 shows high charge temps on a long pull in high gears at WOT. at 1.2 bar it's fine in 1st,2nd, and 3rd, but start pushing it through 4th and 5th and you'll get temps of over 60 degrees easily - not only will this sap power, but is considered as the safe limit for detonation.