Whoa, lots of debate here!
My 0.02:
The way I have always heard turbo lag described is the throttle response description that has already been mentioned above (and so far I've not seen any evidence of this occurring in GT5, but I'm open to other peoples' evidence).
The best way to test this in my opinion is as follows...
You will need:
1) A stopwatch (or the in-game timer)
2) A car with a turbo. Probably best if it's a Stage 3, since in real life a big turbo would be expected to exhibit lag more than a small one.
To avoid any confusion of boost threshold or how boost varies with RPM above the threshold, make the starting point for the test at the max torque rpm. This will be well above the boost threshold.
Get on SSRX, and get up to a reasonably long gear (4th?) so you won't get wheelspin or max possible car speed limiting the test.
Say your peak Tq is at 4000 rpm.
A) Change up to 4th well below this (you can even do a standing start in 4th if you want). Keep the throttle floored, the turbo should eventually be spinning up happily. When you cross 4000 rpm, start the timer. Keeping the throttle floored, stop the timer when the revs reach , say 5000 rpm. Ideally you want the 4000-5000 rpm sweep to take 5-10 seconds to stop human timing error from pressing the stopwatch button being a major influence. Change gear ratios if necessary until you get a decent time period to test.
B) Repeat the test, but instead start at redline in 4th. Close the throttle. You can even start in 5th and then change down to 4th when the speed allows. With the throttle closed, let the car roll, the revs will drop. When the tacho falls to 4000 rpm, stamp the throttle all the way open and start the timer. Stop timing at 5000 rpm as before.
Best to do 3 repeats of each test and take the averages, just to be thorough.
You've just compared the same rev ranges, in the same gear. The only difference is whether the throttle has just been blasted open from being closed (turbo spinning slowly initially, and would have to spin up), or whether it has always been open (turbo initially spinning at full whack)
I've not noticed any difference i.e. lag using this method in GT5.
I think some of the debate above has been caused by the rpm delay between boost threshold and max possible boost. I'm not at all arguing with this observation, it's just not what I've come across when I've heard the term "turbo lag".
Without the "throttle response" turbo lag being in GT5, the different stage turbos (in my opinion) just act like a cam timing change (powerband moves), coupled with an increase in compression ratio (more power overall), i.e. NA tuning!
It's a shame the "actual" GT5 NA tuning doesn't allow reshaping of the powerband :-(
Cheers,
Bread
The dash will show stock turbo boost level, and the HUD will have different boost indication than what the sound of the turbo suggests. This also happens with some other cars which has boost gauge in cockpit - S15, R33 GTR and some others.
