Turbo AVCS/VVT settings

[ztan]

Starting a thread to discuss cam timing and optimization specific to Turbo use. Adjusting the cam overlap is critical to cylinder filling and evacuation. As Td-d has noted previously, small changes in overlap can result in large changes in knock.

I'm still playing with dialing in AVCS timing for the Greddy T518Z kit, but I did have a problematic knock area at 4800-5200 rpm which I think has been cured by reducing overlap by only 5 degrees without significantly affecting volumetric efficiency. My (unverifiable) suspicion is heat from exhaust reversion reduced the knock limit.

It will be interesting to see how forum members are setting up their turbo AVCS and also how our settings compare to the FA20DIT dual AVCS engines. Please share tables, results, and tuning theory. Previous threads in multiple forums by mad_sb, Kodename47, Td-d and arghx7 all noted with thanks.

[SteveIsBored]

How do you verify changes Made to avcs are negative or positive changes without a dyno. What parameters would you log etc

[ztan]

Originally Posted by SteveIsBored

How do you verify changes Made to avcs are negative or positive changes without a dyno. What parameters would you log etc

I don't have unlimited access to a dyno and if I did, probably wouldn't have time to fully dial in dual AVCS at all part and full throttle settings though a full range of AVCS settings - this is why free discussion and information exchange can help everyone learn faster.

I log the following data for a VE calculation (daily driving) which can be sorted by load/rpm or map/rpm:

%calculate VE
%VE=100*((MAF g/s * R L.kPa/K.mol * IAT K)/(MAP kPa * MMA g/mol))/(Disp cycle/L * RPM rev/min / (60 s/min * 2 rev/cycle))
%R = 1.205912 (MAP in psi), R = 0.08314472 (MAP in bar), R= 8.314472 (MAP in kPa), R = 62.363669 (MAP in mmHg).
%MMA = Molar Mass of Air Constant =28.97 g/mol

data.VE=100.*((data.MAF).*8.314472.*(data.IAT+273.15))./((data.MAP).*28.97)./(1.998.*(data.RPM)./(60*2));

How do I know if it is a positive or negative change? If I get less knock with a similar VE, timing, fuelling, I'm sure that the change is positive; if VE decreases much, the engine is not breathing properly. VE too high may mean excess blow-through, though I still do not know when this is likely to occur in my car without being able to monitor pre-turbo pressure in the manifold.

Virtual Dyno can be useful for seeing if you are making more power at full throttle, but is dependant on environmental factors.

At the end of the day, the mixture and ignition conditions inside the cylinder are what actually matter and we make a best guess at this from what we can collect data on.

[steve99]

Alternative

Find someone with a similar turbo setup
Plug in Tactrix logging appropiate AVCS parameters load throttle timing Commanded AFR ect.
Get them to drive arround at the rpm loads you need
Reverse engineer the AVCS tables from log data.

[ztan]

Part throttle AVCS settings at low load before the turbo spools may not need that much tampering with: baseline intake and exhaust manifold pressures will probably not differ wildly from NA at low airflow rates.

Looking at WOT operation on as it is easiest to replicate. Stock NA AVCS settings have 2 ranges of overlap: 2400-2800 and 4400-4800.

Maps from 2015 STI US and JDM (different cams for these engines). We see that overlap decreases progressively to the top end of the rev range with no need for a dual peak in overlap profile - the chamber fill and evacuation parameters are quite different between NA and turbo situations making the second overlap peak unnecessary in a turbo.

[ztan]

So I've had a bit of time to play with AVCS a little bit. I've concentrated on AVCS settings at WOT and largely left the part throttle stuff alone at the moment.

Below are charts of AVCS overlap settings at high load - I have calculated overlap by using:

Overlap = {Exhaust AVCS retard + Start position (-9)} - {Intake AVCS advance - Start position (24)}

I note Kodename47 has used a start position of 14 for intake cam, probably doesn't matter much as we are still looking at changes rather than absolute position, and we don't have detailed cam profile data to make and detailed calculations and really understand the overlap.

I started using Shiv's SBD OFT ROM as a baseline for my T518Z kit and made the following changes: reduced overlap between 2000-3200: this allowed earlier boost on my turbo, reduced overlap over 4000: reduced knock whilst not significantly affecting VE.

Also attached are maps of daily-driven boost logs before/after AVCS tuning with no major changes in EBCS or other settings - earlier spool with reducing overlap earlier.

[Kodename47]

The reason why 14 is used for the intake cam is that the table can contain values down to -10 so effectively 0 isn't as retarded as the cam will go

Which can have you been adjusting for this or is it both?

[cdrazic93]

I think its both cams.

[ztan]

Originally Posted by Kodename47

The reason why 14 is used for the intake cam is that the table can contain values down to -10 so effectively 0 isn't as retarded as the cam will go

Understood, I just happened to use 24 initially and was too lazy to align with the spreadsheet that you shared with me.

Which can have you been adjusting for this or is it both?

Both cams - a little out of each one, more reduction in intake cam around 4000-5000, more out of exhaust cam lower down to try to help spool. No real science. Lots of reading what WRX and STI tuners are doing, checking log data, and a bit of gut feel.

Happy to share my current files with anyone: PM me.