When to use Alpha N vs Speed Density (Huge cams)

Hey all,

I've been tuning an Evo X on AEM infinity and the car has GSC S3 cams with Magnus intake manifold running strictly E85 and a 7275 next gen PTE turbo with a bosch 82mm TB. I've gone through the mapping of the VE table, but at idle I'm only pulling -1.5 psi of vacuum and cruise is right around 0-2psi. It makes it difficult to map out the part throttle driving. Is this a scenario where I should've been using alpha n the whole time instead of a traditional load axis? Is there a way to do a hybrid between alpha n and speed density or should I just switch over to alpha n?

You should almost always use TPS as the VE load. The AEM fuel model will automatically scale for MAP. If you see high EMAP, you will need to use an additional table to account for the reduction in VE when EMAP becomes excessive. An 82mm throttle body is quite large, and you'll find that the data gathered while tuning with TPS as the load axis will improve overall drivability.

Tuning this is actually quite easy. You'll simply clamp the maximum TPS and let the DBW control force the table rows. With a friend driving, this process goes very quickly.

Do all of this on the wastegate spring. Hopefully, you already have your VVT reasonably dialed in, as it should be mostly set before you fully tune the VE fuel table.

For tuning the TPS-based VE fuel table, start by limiting TPS to 15%. You can even assign the 75%+ pedal rows to the TPS row you want to tune. Begin at the lowest RPM you're comfortable with. Go to wide-open pedal; this will hold TPS at 15%, and the car will accelerate as much as it can. Press Ctrl+L to stop logging, then apply the NewVE values for that single row. Interpolate upward and add a few percent. Lower throttle openings may not reach maximum RPM, which is fine. Just let the 10–25% TPS ranges climb as high as they can.

Next, limit the maximum DBW to 20% throttle. Return to a low RPM, press Ctrl+L, go WOP, and hold 20% TPS. The car will accelerate to whatever RPM it can achieve; let it peak. Press Ctrl+L again and apply the NewVE values. Interpolate up and rinse and repeat all the way up to, in your case, perhaps 40–60% throttle.

You'll eventually find the point where additional throttle opening no longer requires more VE. This is the minimum throttle position required for maximum torque. You should take this value into consideration when configuring your pedal-to-DBW table. For example, if you're achieving maximum horsepower and torque and VE is no longer increasing by 50% throttle, you should map that value to approximately 80–90% of your DBW table.

Start here and get this process dialed in. Once everything is working well, you can begin increasing boost. I know of several 900+ HP EVOs running this VE table strategy. When your EMAP pressure ratio is low, you only need to tune the TPS fuel table. Once you start increasing boost, you'll need to use another table to trim VE. If you've already tuned this setup from mild to high boost levels, you should already be seeing VE taper off at high RPM and MAP values.

Wow Robert, this is an incredible response. I actually did notice at ~49psi, my VE dropped off around 6-7%. I didn't even consider this at first, but it makes sense as the back pressure increases. I'll redo the VE table using TPS instead of MAP now. This car has mivec locked and is just using the magnus cam gears instead so I don't need to worry about that. We'll give this a go!