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Planning boost targets

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Just wondering how you guys go about planning and tuning boost targets to keep a turbo running efficiently & reliably?

If its an off-the-shelf unit and you have a compressor map it is easy to do some calcs and plot to see if you are going to be in the right place. What if you don't? Other than going on experience / common sense, how do you guys plan & tune your boost targets?

Would it be reasonable to "map" an unknown turbo by pushing it at rpm break points until it runs off its map and surges or EGTs / IATs rise then back it off to keep within sensible limits? Is there a webinar that covers this?

If the turbo is a complete unknown then I would tread carefully, it's not very often that a turbo isn't identified and if it's not it's likely a cheap one. If you had the option to add sensors I would add EGT and exhaust pressure. If not keep then keep an eye on IATs, duty cycles vs manifold pressure Etc.

Thanks Chris,

Sorry I didn't explain properly, it's not a completely unknown turbo, a 3071/KKK hybrid with a billet comp wheel.

What would the best procedure be to develop a reliable boost map?

It's nice to have the compressor map so you have a reasonable idea whereabouts you're operating, however I usually would be looking at compressor maps when selecting the turbo initially rather than actually tuning. One of the biggest concerns would be driving the turbo into surge, but realistically with a modern, well matched turbo, that shouldn't be a big concern. Beyond that, the dyno is going to give you a lot of feedback as to what sort of boost pressure the turbo is efficient at.

I normally start at the minimum wastegate boost pressure and then begin increasing the boost in 2 psi increments. What I'm looking for is the power increase as I raise the boost. If we are operating somewhere in the middle of the efficiency map you'll typically see decent and consistent gains as the boost is increased. If you are pushing the turbo exceptionally hard you're going to notice smaller/minimal power gains and typically skyrocketing intake air temperatures.

One example that springs to mind is a very high powered Nissan RB26 drag engine I tuned years ago on an HKS T51R SPL turbo. We actually got to a point with that turbo where we could raise the boost an extra 2-3 psi and see zero gain in power on the dyno. Admittedly we were at about 50 psi though when we got to this point. Another example is a restricted rally car where often we see more power at higher rpm with less boost pressure. In both situations though we can see what's happening and make decisions based on a suitable boost target based on the engine power rather than needing to fully instrument the turbocharger.

Thanks Andre for the detailed reply, that all makes good sense.

Is this something you would do at the same time as calibrating the boosted regions of the VE table above 100kpa?

e.g WOT ramp runs increasing boost in small increments, at the same time as calibrating VE if we start to see that we are running the turbo out of its efficiency range then we can extrapolate those values beyond into those areas of the VE map that are outside of the turbo's operating envelope knowing that we can't / wont actually reach them?

Yes absolutely. Tuning the VE table and ignition go hand in hand with changing the boost pressure. As you start getting to the limit of the turbo's capability, you'll see the numbers in the VE/fuel table start to level off and even drop as you raise the boost which is another indicator that you're near the limit.

Thanks again Andre, the procedure makes absolute sense now