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Have been asked to tune a rallycar with a 34mm restrictor fitted due to class regulations.
I have no dramas taking it on, just curious if there is any certain strategies those experienced in this area use.
I'm assuming it will be based around low RPM torque with a decent boost spike, then let the boost run down as rpm increases.
I've done minimal, but SOME tuning with this kind of setup. I guess I just took it as "normal" tuning, just the turbo would max out a lot quicker - depending on the turbo you use you'll probably find (if your experience is anything like mine) the "Build boost hard early and then drop off up high" strategy is more by force than intent, the turbo will just choke as soon as you start getting a fair bit of flow through it.
The way I think of it, the restrictor effectively forces the turbo to build a vacuum at the compressor inlet so as the flow goes up the turbo runs at a much higher pressure ratio than conventional thinking would suggest when referring to the boost gauge... so it becomes a bit like you are tuning a car with a maxed out turbo which isn't really actually running that much manifold pressure. IATs can get higher and the car doesn't necessarily make a lot of power, spool can also not be astounding - I guess that's more so that you don't go on a mission to find out why a small turbo is laggier (within reason) than you expect thinking there is an issue, so much as something you can really tune around.
I think the magic is probably before the actual tuning process, so stuff like (if it's an option) choosing a turbo efficient at VERY high pressure ratios (Garrett released a rather expensive rally edition GTX2860R specifically for use with a 34mm restrictor which is fairly efficient up to PR 4.5) and good materials for dealing with the heat resulting from both the HARD effort to build that pressure ratio through a restrictor, not to mention antilag.
Beyond that boost control was pretty easy, it was hard to build more boost than you wanted as the cars I tuned were running turbos which just choked and maxed out well before you'd need to think about easing back on boost and really it was a case of deciding how hard you really wanted to push it - even without antilag the turbos will get a hammering because of the previously mentioned reasons. I set my boost curve slightly softer than "kill" and I'm a pussy, so unless otherwise advised by the person who pays for the parts I oftendrop back boost targets with race cars when IATs and/or ECTs start going far enough north that it's just not worth trying too hard.
That's a quick mind-dump of things which come to mind from the limited playing I've had with that kind of thing - no doubt others will have much more experience etc, but figured I'd get the ball rolling.
My biggest tip is that you will often see the engine produce more power at high rpm with LESS boost. Essentially at high rpm the restrictor is strangling flow into the turbo and the turbo is working way beyond its peak efficiency when you start upping the boost. I've tuned numerous rally cars where dropping the boost targets 1-2 psi above 5000 rpm actually saw a nice net gain in power and torque - Consequently the IAT will also be a little lower - Happy days.
Actually Lith my own experience on a number of rally cars where I'd provided restricted and unrestricted tunes was that often the boost response was better with the restrictor fitted.
True Andre - I should have made that a bit clearer, I mentioned that I used to run less than the max boost the turbo would provide and found that gave overall better happier performance and probably better for general longevity too... I second that, and though in the interest of being scientific about data collection I'm going to stick to my guns and say the ones I tuned were laggier with the restrictor fitted.
The clearest example I have was I tuned an WRX with a Subaru TD05 for a class in Targa NZ where a restrictor was required, then after it completed Targa the restrictor was removed so the car could be used more effectively for other events - and no other changes made, I retuned it and found boost built faster, IATs dropped significantly at higher boost and the car was basically faster in every way.
What I should really add here is that most of that impression of spool/response is actually on the road - dyno wasn't really so noticeable.
I would add, that the turbo might spool faster without the restrictor when it's reasonable sized for the application
/no-shit-mode off :P
Thanks for the tips lads.
Forgot I posted this thread, i'll keep you updated on what the outcome is!