by dr20t » Wed Mar 30, 2016 8:19 pm
Good input Dave.
Before sharing my findings i would like to put something out there. I've come to a conclusion and its a big call but I'm willing to back it - the majority of tuners out there have no idea when it comes to optimizing variable valve timing. I'm not just talking about Subaru avcs here, either. Either this is because they don't have the time to spend on each setup, or of greater concern, don't actually understand the full extent of what's going on.
Not saying I have all the answers to avcs, but I think i understand its impacts and application to a fair degree.
Anyway - enough venting. I'm willing to help anyone who has queries relating to avcs, but essentially, there is alot to consider with avcs. Think of it as a Volumetric Efficiency dial, which needs to work in harmony with the rest of the setup. Even a change as slight as different spark plug gap or heat range can change the avcs tuning requirements.
Long story short, I love avcs. Dual is better than single, as it allows complete control over both cam opening and closing points, with many possible outcomes.
My tip - go back to basics of what an engine's function is. Pump air in and out as efficiently as possible. The limitations imposed by physics will be a good guide depending on the setup.
There are two essential pieces of equipment to optimise avcs - a wideband 02 sensor is one. And an ability to measure egt or at least gauge when it goes over a certain threshold. I don't have an egt gauge, but was able to use a rom hack to flash my cel once egt reached a set temperature. This helped alot.
If we're talking Ideal world, a pre turbo exhaust back pressure measurement should be taken too. But in the absence of knowing the preturbo back pressure, it will become pretty clear pretty quickly if your inlet to exhaust pressure ratio is out of whack or exhaust pressure exceeds inlet pressure (absolute).
I believe I have optimized my avcs settings on my engine, after alot of time invested. I have also discovered in this process the flexibility of the dual avcs system in the grb 207. I'm reaching 28psi on my gtx3076 by 3800rpm when hot and 4000rpm cold. That gives me a pretty wide powerband with a 8500rpm redline.
To answer the original question I posted in this thread - e85 absolutely makes a difference to avcs' impact, however this does not preclude a good avcs setup with 98 ron fuel. The knock resistance of e85 makes a difference to the in cylinder pressures able to be reached, and in a turbo car, the ability to therefore spool a turbo more efficiently.
Happy to be critiqued or questioned on anything I've said here
Mick