Quote:
Originally Posted by trey100
Thanks and nice detailed reply.
A couple of things:
1. I always thought the benefit of the 2 turbos was specifically to try and satisfy both low and high rom performance. Meaning rather than provide one larger turbocharger which would create more lag, you could sequence the 2 turbos so that both got spinning faster. Then again my memory tells me my old N54 had one turbo per 3 cylinders which means they weren't sequenced as I described above. 1/2 the turbocharger size for 1/2 the cylinders would seem to be similar to one that is double the size serving all 6 cylinders.
2. Even though with the N55 you could select a turbocharger to enhance low rpm torque over high rpm power, which is what you have eluded to, when I look at dynos of the S55 and N55, they have a very similar drop off n torque at high rpm. Meaning the shape is similar. Of course the S55 goes more rpm but not so much more than the N55. With your logic, I would have expected that the torque curves between the 2 engines have a less similar shape where you could see the benefit of the 2 turbos at high rpm. This is similar to the N54 where it had that same cliff. If I recall the S55 drops off to 300 lb ft of torque at redline while the N54/N55 (335 level) ends at 200 lb ft. Considering the S55 has roughly 100 lb ft of torque more, overall it seems like the same decline.
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In general, you are correct in what you state about the trade-off between single turbos and twin turbos. However, here, it is important to look at the absolute power and torque values, not only the shape of the power/torque curves. This is why I specified that both engines share the same bore-stroke-displacement. From the same basic engine layout, the S55 makes 18% more peak torque than the M2's N55. At 7000RPM, the S55 makes 25% more. That is a substantial difference. To achieve this with a single turbo, you would need a much bigger turbo, which would kill response at both low and high RPM. Going with twin turbos allows to maintain the high RPM flow capabilities of a single LARGER turbo without sacrificing too much response at low RPM. That being said, the two smaller S55 turbos are still sized to favor higher RPM flow compared to the M2 N55 single unit, hence the better response of the M2 N55 at lower RPM.