High RPM dip and recover torque

Hi everyone,

Wondering if anyone with experience on Nissan's SR16VE N1 engine can offer some insight.

I have one tuned on Maxxecu and I see this dip then recovery at 7800rpm or so. See attached dyno curve.

Is this something normal with these engines? I do see the VE drop there too which tells me it's likely something with airflow, it's just strange that it dips then goes back up.

Any insight would be appreciated!

If VE ends up matching the torque curve, it's likely real.

I would check if it happens off the dyno in "real world" driving as well, based on the dyno used, just to confirm.

Are you confident valves aren't floating?

I had a similar issue once- turned out to be floating valves indeed despite having stiffer valve springs in place.

Anytime I see a torque dip and recover I think it's probably a resonance problem. Could be intake, exhaust, valve spring harmonic, oil pump cavitation(see below) etc. When a professional engine builder is developing an engine package and sees this, he designs a series of experiments to understand the possible causes. They might run these experiments using engine simulator software, might build different intake or exhaust systems, run high-speed cameras looking through windows in special valve covers.

Related story -- I recently ran a Porsche 944 engine on the dyno to 7000 RPM (expecting the factory rev limiter to cut the torque about 6750 RPM). The engine pulled to 7000 RPM, but there was a larger loss of torque starting about 6600 RPM. A 944 expert told me that that he had discovered (after many failed engines) that a stock wet-sump 944 should never be run over 6600 RPM as the oil pump will begin to cavitate at that RPM, and the #2 rod bearing starves for oil. He claims the loss of torque is from the damage the rod bearing is doing.

Mike, it was confirmed on two different dynos. One dynapack hub and the other dynjojet inertia.

No I cannot rule out valve float, I'm not an expert on the engine building side...how do I decipher if it has anything to do with valve float?

David thanks for the input, we are gonna try changing intakes and exhausts and see if it makes any difference.

Great. With that taken care of, and VE confirmation, you know it's really happening. Now the tough bit is the why.

Valve float can be caused by multiple things, some which you could measure with relatively basic sensors, at relatively low speed, but some you can't so I don't view that as a cheap, simple, viable solution.

The sound an engine makes when it "goes flat" and HP drops like that can be similar whether it's from valve float, or the engine lacking in that area. I know some folks feel otherwise, but I wouldn't tell someone to listen to it and they'll know.

From a non technical standpoint, have you looked around to see if you're the only person with this engine that's experiencing this behavior at this approximate RPM? (keeping in mind sometimes people don't have RPM synched just right on dynos) Sometimes things like this are well documented on a particular engine platform.