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I was wondering where the cut off is when road tuning with regards to increasing boost and retarding the ignition.
I am currently aiming for 1.3 bar on a forged 3.2 Bmw m50 with holset hx40 60/85 76/64 but I haven’t seen in any of the webinars or courses I have watched so far how you determine when to stop adding boost? Is there a cut off say if at 1.3 bar when the engine first makes 1.3bar and timing is 5 degrees because it’s knock limited that you should stop there? Or do you simply carry upping the boost and removing timing until the turbo runs out of puff?
There is a point of diminishing returns. You could run more boost and less timing and make less (or the same) than with less boost and optimal timing (speaking in generalities of course). Where the car makes the most power and has whatever safety margin you deem acceptable is where I would stop. So I would start at your lower boost levels, dial them in so the car is at MBT. Then start dialing the boost up and go from there.
Usually there are rules of thumb depending on your platform. In the Evo on straight 94 octane I don't usually go much past 25psi. Not saying it can't be done, just saying that's where I'm comfortable on pump gas (as its not necessarily always exactly 94). I'm not saying 25psi is ok for your platform, just stating that's where I usually stop in my case.
On the road this gets a little tricky since you don't have feedback on power. If you were on the dyno you'll get to a point where the engine becomes so knock prone that you go around in circles adding boost, retarding timing and basically making the same power with a whole bunch more stress on the engine. In general I tend to find that on 98 octane pump gas this point is normally around 22 psi (1.5 bar) but there are of course exceptions and I've had a handful of engines that have accepted 30 psi on pump gas quite happily.
Ok thanks for the info, I will bear it in mind. So just out of interest is it not unusual for a boosted car 9:1 compression ratio on 98 octane fuel to see ignition timing numbers atdc around when the turbo peak boost / peak torque?
It’s just I’m around 9 degrees btdc currently at 1 bar of boost so I’m expecting there’s a chance to be getting close to zero once running around 1.5bar?