oldbmwfan
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- 2017 GT350R
I've seen this with Vanos-delete BMW motors. It can "feel more fun" because what you feel is the inflection point in power gain, and you feel it as a kick in the seat as you accelerate but really what's happening is you're going from "worse" to "same," not "same" to "better." Like you, I'm quite surprised at the drop-off up-top and I hope that can be tuned out. The loss down low is pretty big, and it persists further up the rev band than I would have thought, but if you are mostly spinning the engine at 6K+, the losses below 5500 RPM might be a fair trade-off for reliability and durability.Alright - data is in! It's not great, but it's not terrible!
So to get it out of the way, apologies for the poor format for comparing. The reality is these were done 2 years apart, on 2 different dynos, with 2 different tuners. I just didn't have the ability to control these variables, unfortunately.
So this top image is for the motor with VCT. Runs breakdown as E85(top), VP100 (middle), and 93 tunes (bottom) I had for your average track day. These were quite de-tuned and conservative.
This image is from today with the cams locked out at 120 for the exhaust and 112 for the intakes.
As you can (sort of) see, the big differences are in the drop off at the very top and between 3,500 and 5,500. With VCT, we have about 360ish hp around 4,500. Without VCT, we're only around 320ish. Pretty big loss. And then there's the huge torque loss under 5k. Ha! But 5700-7600 is pretty similar. I'll keep reviewing and digging and share more.
Most of this is not a surprise. The main surprise was the dip up top. Notice how the VCT graph doesn't climb above 7600, but it stays level. Comparatively the VCT-delete motor falls off a decent amount from 7,500-8k. The tuner felt that we could move that up with some adjustment on the cams. I told Tim to optimize power where the stock cams make power and that's what I think we're seeing. Aftermarket cams could help fix this.
Hot takeaway: I am quite pleased how smooth the the pulls are given the lockouts. I expected a much bigger hitch around 5-6k. It's quite smooth. I'll be very curious to see what this motor feels like on track. Objectively it's pretty clearly worse, but my limited driving so far makes it feel pretty fun. We'll see what happens on Saturday on track