Tuning for multiple boost levels

[jwmelvin]

For a twin-screw supercharger, is the tune at 6 psi with a "6 psi pulley" different from the tune at 6 psi with a "9 psi pulley"?

I have read on here, and please correct me if I'm wrong, that with a turbo setup, one can tune for the highest boost level and then run anything less than that. As I understand that statement, it is because tuning for a certain pressure, say 10 psig, also requires tuning for pressures below that.

Does this apply to a positive-displacement supercharger? My impression of the characteristics of twin-screw pumps is that they jump very quickly to maximum pressure, so if I have a pulley that makes 9 psi at full boost, there will not be a lot of opportunity to get a steady 6 psi of boost and accurately tune for that pressure such that I could swap the pulley to one that makes 6 psi max and run the same tune. Another factor could be the different operating conditions required to create the 6 psi of boost for each pulley.

So there would be no way to have one tune accommodate both pulleys? (I know that swapping tunes is straightforward with the F/IC, which I will be using, but I'm interested more in the tuning process itself.)

 

[Cody@LoveFab]

The same theory would apply, ie, the higher boost tune should cover the low boost tuning as well. Tune for the 6psi; save the tune. Then tune the 9psi with the same 6psi tune, and you're done. No map switching required unless you're using different fuels, etc.

 

[jwmelvin]

Okay, that makes sense but means I need to switch pulleys during the tuning session. I guess the advantage is better tuning at 6 psi for the 9 psi pulley. Thanks.

 

[NA1MT]

No, you could install a wastegate valve and boost control......I'm pretty sure anyway. Someone will correct me I'm sure if I'm mistaken.

 

[Cody@LoveFab]

Or just run the 9psi and call it done? Lol

 

[DDozier]

Okay, that makes sense but means I need to switch pulleys during the tuning session. I guess the advantage is better tuning at 6 psi for the 9 psi pulley. Thanks.

Keep in mind that changing the pulley will likely require a shorter belt as well, depending on how much of the tensioner adjustment you have left with the 6lb setup. A 9lb pulley on a CTSC or similar system will yield 8-8.5psi and with the F/IC I would take the time to tune with both pulleys and perfect the 6-9psi parts of the tune. As the belt ages and starts to slip you will see boost pressure drop at higher RPMs and you could find yourself in the 5-6 psi range at WOT. All of the fuel offset tables between atmosphere and 5 PSI will be interpolated and should be fine as long as the tune for your highest psi and atmosphere are correct. Tuning for atmosphere can and should be done with the F/IC and new injectors installed before the blower is put in place or after the blower is installed and with the bypass forced open during the run.

Running a true 9PSI without an aftercooler will cause extremely high IATs, you should consider this in your tune.

Dave

 

[jwmelvin]

Running a true 9PSI without an aftercooler will cause extremely high IATs, you should consider this in your tune.

Thank you Dave. That's my main concern with the higher-boost pulley. And why I'd like to make sure I get all the necessary tuning done so I can use the low-boost setup when ambient temps are high, for example.