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I am working towards build a turbocharged car for use in various time attack / time trials series. One of the series (NASA in the US) classes by power to weight so to take full advantage of the rules I’d like to develop and tune the engine to have a flat power curve. For example 220 whp from 5000 rpm to 7500 rpm. Based on some of the examples I have seen in the webinars and boost control classes it would seem like either a DBW throttle or a tightly tuned closed loop boost control could get me the results I want.
My tuning strategy question: Is there an advantage to using the DBW throttle in addition to (or in place of) tightly tuned closed loop boost control for my target WOT power curve and part throttle power modulation?
I’m in the planning stages of the powertrain now and want to make sure I have all the control hardware specified so I buy the right parts the first time. Since you guys probably want to know what I’m working on, it’s an MR2 Spyder and I plan to turbo the 1ZZ that’s in there starting out and then likely swap in a K24 once I’m ready for some crazy power levels.
Both boost control and dbw profiling can be used to achieve the effect you're looking for on your power curve. For what you're looking to do, I'd imagine you're going to want high boost and full throttle from low rpm up to the point you achieve your target power level and then you'll need to taper the boost/dbw away sharply before bringing it back in at higher rpm. It'll provide an interesting experience to the driver but I understand your aims.
Personally I'd use boost primarily and then follow up with dbw targets as required. Understandably the boost can't change instantly so I'd get your power approximately where it needs to be using the boost curve and then use dbw from there. My reasoning is that there's no reason to push the turbo harder than it needs to be pushed as this is just going to add heat and back pressure needlessly.
If your engine/fuel combo is very tolerant and fuel efficiency isn't priority as it would be in endurance racing you can get most of the way there just by sizing the turbo close to its intake choke flow for your highest altitude track. Will give the earliest torque curve for the target power too. Then you can just manage with modest wastegate control effort targeting boost/shaft speed with small potential throttle trimming if required.
You are pretty much asking for people to cheat not using intake restrictors if they don't fit fuel flow meters or similar to power category cars? Even in F1 they are intentionally burning engine oil.
A smaller turbo was my plan, based off the equations I found on Garret’s tech pages I estimated that I would probably run a peak boost of 17 psi and then taper off to 9-10 psi depending on how well the head actually flows.
So would boost control be good enough on its own with a cable throttle for part throttle modulation when exiting corners? Or is this an area where the extra control of DBW would be beneficial. I care more about drivability than extorting the power to weight rules. I have also driven a few turbo diesel cars that have a flat power delivery and prefer them to the typical high rev NA flat torque engines.
Some ecus have torque control and adjusts boost and throttle by itself to hit the desired torquelevel.
Having worked on NASA ST cars where we want to stay within a HP limit, I would suggest that DBW is virtually a necessity. I would say plan for that from the beginning (ie, make sure you have an ECU capable of setting maximum throttle at RPM & gear breakpoints).
Traction control can use DBW but you almost certainly want ignition retard possibly progressing to a fuel or ignition partial cut as there is a lot of time between intake stroke and power stroke for throttle movement to take effect, whereas ignition retard can occur on the next cylinder to fire.
Ok so let me see if I understand the stages of control for what I am trying to do:
Closed Loop Boost Control: course resolution of power production to meet rules limits, need to carefully match the turbo to the engine displacement and power goals to make this work well
DBW Throttle: finer resolution over power curve (ultimately ensure rules compliance), good for part throttle control due to throttle to pedal mapping ability, will likely want/need 3D or 4D tables to best map the pedal to the way the engine makes power
Ignition Retard/Fuel Cut: used for traction control if I set that up down the road, DBW would be slower for this and thus less desirable
Thanks for the help everybody!
This depends a lot on the ECU. What are ECU are you using?
I haven’t picked an ECU yet. I was trying to make sure I knew what features I needed so that I could find the right one.
So far I’ve been looking at either Haltech Elite 1500 or Link G4+ Fury. Besides having good boost control and DBW control for a 4-cylinder turbo; I need the ECU to send data to a MoTec data logger over CAN.