VE dip only in WOT load cells at high RPMs

Hi There,

I've had my car running well for a while now, and got through the whole track season on the tune I built up, but I've been curious about something I learned along the way.

My engine (K20Z3, Drag Cartel 2.2 Cams, RDX Injectors, Skunk2 Ultra Street Manifold, Haltech Elite 1500) seems to have greater volumetric efficiency at slightly less than wide open throttle than it does at wide open throttle from about 7500rpm to redline.

I've evaluated this over probably 100 passes through these cells at this point (all on the street, unfortunately) and this is completely predictable at this point, but the engine seems to flow slightly better with the throttle slightly closed more than it does with it fully open up here. Is this a common phenomenon? I couldn't find anything about it, but I'm positive my fueling is dead on through this whole range and consistently see the -3% fuel correction my LTFT configuration is targeting.

Here's my area of concern:

--Matt

That's possible, and good for you to keep tuning to get the maximum. It's likely intake and exhaust wave effects, or just the swirl of the intake air with a slightly closed throttle that make the difference.

Just because the VE is higher at a given (lower) manifold pressure, does not mean there is any less power at higher manifold pressures.

Why is this a concern?

I guess curiosity is a better word than concern. We're only talking about 2% here, so it's not a big difference.

Thanks for the explanation! It's easy to get lost in chasing numbers and to lose sight of the fact that there's a bigger picture in the hardware 🤣.

--Matt

It is possible, but usually if the throttle body is significantly oversized for the engine. but at 2% difference I would not sweat it. I had this happen to me on an rb26 that we converted from ITBS to Single 90MM TB. We actually lost power and I made most power at 75%. I think this is to dude to loss of air velocity and this engine just wasnt built to take advantage of such a big flow (only 26psi and stock cams).

Have you checked the wide open throttle stop? It may be that the throttle is over traveling and starting to close again. I have seen this occur on DBW throttles that haven't been correctly configured.

I suppose it's possible. I'm redoing the harness and swapping to a smaller 68mm Bosch DBW throttle, so it should open to the proper positions once that's all done.