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Trying to understand VE on Audi 3.0t supercharged

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I took the classes here and read a couple of Greg Banish's books, I think I have a pretty firm grasp of speed density now. But as I look at my cars ECU, I'm not quite clear on the logic. There's no real VE table as far as I can tell. There's a compressor map for the roots blower, and a couple "Offset" and "Slope" tables that look specific to variable cam timing.


If I was to, say, buy a larger crank or make some other significant change, what is my primary VE table to edit? I'm guessing this Supercharger PR map is where, but this map seems to be more of defining specific airflow, rather than VE factor.

I've attached the hpt file, can provide a bin too.

Thanks guys. Really appreciate all of the high quality content too at a great price.

Attached Files

I can't open your tune file (no HPTuners Licences), however, for displacement changes, I would be look under some of those other tabs. Start with the Overall General tab. I would expect you would find base VE tables under the Fuel tab.

I would spend time looking through every tab, reading whatever help / documentation is available about each table you can edit to get a grasp on how the ECM uses that information to determine how much fuel to inject.

Actually, yeah for displacement there's a separate field for Engine Size, that was poor example. Heads or cam changes I guess, really anything that alters VE.

Fuel tab just has Stoich FAR and injector characteristics, then the common O2, Open loop, PE, and Lean modes.

I'm 99% sure the changes would be to the Supercharger map in the above image, but that doesn't seem like a true VE function.

Is here a MAF sensor? If so that handles any increase in efficiency due to head porting, cams, exhaust. So that just leaves tuning the PE for WOT pulls.

Discussion points and thoughts - which may be in error.

As it's a "Roots" type supercharger, and that's a positive displacement design, there should be approximately the same mass of air entering the engine at the same engine-blower rpm. If the blower drive ratio is changed, the fuelling should change in approximately the same ratio. Internal changes to the engine may not be expected to have much affect on fuelling, as the same mass of air should be entering the engine, but improving the ACTUAL volumetric efficiency should reduce pumping losses and improve power/torque - this improved VE can usually be seen as a drop in boost pressure but a gain in torque/power.

As David said, if it uses a MAF, there is a sound expectation that it will self calibrate for small changes, but some tweaking may improve on that. If it uses a manifold boost and/or temperature reference, there could be issues as improving breathing would be expected to lower those while still having the same mass, which could result in a lean condition.

Gord, From the Supercharger tab there are some tables & settings related to bypass, that is probably where excess air mass goes if the throttle plate is closed.

What axis are on the Fuel/Open Loop table?

Yes, there is a secondary throttle body used to bleed off air, monitoring the open angle is a great way to tell if my torque requests are greater than actual engine torque. Fuel tables are all target lambda, with cylinder airmass x rpm axis. Changing over to E85 was simply done by updating the stoich FAR value, real convenient.

Regarding MAF, there are tables relating to a MAF, along with the ability to log the value, however the car doesn't actually have one, just 3 tmap sensors. From what I've come to understand, my SIMOS8.4 ecu has left over logic from earlier SIMOS8.x versions, which did utilize a MAF. The logic was just modified to calculate the MAF value now. This had me real confused for awhile.

So I think the take away regarding VE, is that this ECU doesn't function in a typical speed density manner, where manifold pressure => ideal gas law * VE calculates air mass. Instead this supercharger map *defines* air mass with the VE built into these defined values, and simply performs a lookup of manifold pressure vs rpm to determine the airmass value. So if down the road I was to move to a turbo setup, I would still utilize this supercharger map as the primary way to account for overall changes to pumping efficiency.

Does that sound about right?

Doh, sometimes my head's still stuck in the seventies - you're right, David, most modern supercharger setups, at least as per factory, will use a supercharger bypass/recirculation valve to, IIRC, increase fuel economy by reducing parasitic loads as the rotors are effectively spinning unloaded with no pressure difference across it.