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Discuss all things tuning in this section. News, products, problems and results.
General question here, when tuning boost control, say you do a ramp run on the Dyno in open loop on only spring pressure and look at the boost curve. You can then identify the point the turbo hits full boost...say for example 4000rpm....before that it would be spooling to reach that.
Generally and speaking can we use the boost control solenoid and give more duty during spool up to get the turbo to hit full boost quicker ? Or are we stuck with what we have and can then only increase boost from the same full boost rpm point?
You absolutely can use the boost control solenoid to try and get the turbo up to full boost a little quicker. My experience is that wastegates respond to a pretty narrow pressure range, and once their spring pre-load is overcome and they start to open, its only a few PSI extra later that they are fully open... The final boost pressure you see in your manifold will be somewhere in this window. So if your final boost level is still being determined by the wastegate spring pressure, you've only got a couple of PSI during which the wastegate is opening, but the boost is still increasing, and its only this period you'll be able to improve by keeping the wastegate shut with the solenoid. You'd also need to be very careful of boost spikes, as keeping the wastegate completely closed till the last moment, then snapping it open might compromise the control a bit.
This is all only relevant if you're running wastegate spring pressure to determine the final boost. If you're using the boost control solenoid to determine the final boost, it'll be keeping the wastegate shut till its needed anyhow, and how progressively it opens needs to be determined on a case by case to ensure accurate control :-).
When you do a ramp run, the ramp rate is going to affect spool, just like boosting in different gears.
Agreed Raymond, I am aware of that.
Zac, thanks for the detailed response.
Would your tuning strategy go something like this then? Say you're using the boost control solenoid to achieve a final boost target. Say you need around 50% duty to achieve this. What I'm really wanting to know is should my duty follow the trend I saw with how the boost curve behaved with no duty ? For example of I know the turbo was hitting full spring pressure boost at 4000rpm....would I only use 50% duty from 4000rpm when targeting my new boost target and linearise the duty from 0-4000% rpm? Or should I set the entire rev range to 50% duty and see if it helps the turbo come on boost earlier after which I can re-adjust my linearity to match for the spool up portion?
Not sure if you get what I'm saying.
If you look at the boost response curve, you should be able to mentally extrapolate it out to determine pretty closely the RPM at which the engine will achieve your desired boost level. If you experiment with keeping the wastegate completely shut until you're close to this RPM point, you should be able to get the turbo to spool as quickly as possibly. You'll have to be really careful of boost spikes though, and I think you'll end up finding you need to open the wastegate a little more gradually as you approach your boost target to tune these out.
Basically you can try to run 100% (well, 90-95%, whatever the practical max is) duty up to right before you expect to hit full boost. Then you need to ramp it down, hence Zac's comments about boost spikes. So if you expect to hit full boost at 4000rpm, you could run 95% duty up to 3000rpm, then 75% at 3500, then 50% duty at 4000rpm, or whatever. But you need to do a bunch of iterations to get it right, or you will have boost oscillations or spikes.
I usually start with a flat map and open loop tuning only. Set it to say 20%, then 30, 40, etc until you are just below your target. Then work on the spool area, then work on the high rpm where you want to control how much boost tapers off. However you have to leave the response a little bit lazy running only in the open loop table. Once you add feedback gain it's going to perk up and you risk spikes or oscillations.
It can get to be a multi dimensional exercise in tuning as you start to adjust by gear or throttle angle and air temperature or whatever.