[ZuffenHouseRules]

The best discussion I've found on this topic is in a BMW forum. Mystery Solved: Complete correlation between S55 Dyno's, Simulations, and Real World

Despite its excruciating length, detail, intellectually rigorous arguments (mostly) and only occasional nastiness, the 'mystery' is not fully solved.

Personally I find compelling aspects to both the OP's case -- that real-world performance, properly interpreted dyno results and computer simulation (vBox) all correlate in pointing to understated manufacturers turbo ratings -- as well as the counter-arguments presented (see post #29) that higher average-horsepower/area-under-the-curve makes the turbo engine trap as if under-rated going by peak hp figure alone (this was raised by @Black718 in our long thread on the subject).

Only problem I see with the latter theory is that while the 718 has the characteristic broad flat 'torque plateau' of the turbo BMW engine in question, its horsepower curve looks similar in shape to the n/a 981 engine it replaced -- esp. in the critical 2k rpm band below redline where most all the average time is spent during quarter-mile acceleration (see chart*). Could average horsepower and resulting trap performance really behave so differently, based on this curve?

note: chart is just to show similarity between turbo and n/a horsepower (as opposed to torque) curves; I'm not comparing 718 to 981 performance. The real mystery to my mind is how the 718 S traps exact same quarter-mile and 0-100 times as the 400hp 991.1 Carrera S, per C&D track sheets PDK-to-PDK.

Curious as to what other think....

*eta: hard to tell from the scale and 'smoothed' fat plot lines on this chart -- on closer look maybe the turbo does? have higher shoulders and flatter peak, making for higher average-horsepower relative to peak rating. I wonder if the ECU is 'drawing' that shape referencing a stored mapping(s) which might vary with numerous external factors incl. differences between an engine bench-test vs. rolling chassis-dyno/quarter-mile run...

 

[dpcompt]

Just an observation. BMW turbo engines are long stroke engines. 718 engines are short stroke.

 

[ZuffenHouseRules]

Just an observation. BMW turbo engines are long stroke engines. 718 engines are short stroke.

Interesting point.
long stroke > torque
short stroke > high-revving
....is how I think of it. 9A2B4 seems to blend high measures of both, no?

Are you familiar with the S55 turbo in the M3? Sounds like it has a more exaggerated 'plateau' in its horsepower curve, yielding the high average-to-peak-hp ratio described in some posts on the Bimmer forum thread. The 718S curve at peak appears to have a high 'left shoulder' (from about 6200rpm to peak at 6500) so average hp could be higher in that narrow band, at least.

Looking for a general theory how the current generation of German turbos tend to have high average-to-peak-hp ratio (for any number of possible reasons from reduced emissions to exploiting new turbo technology) with the side-effect of appearing underrated...

 

[dpcompt]

Porsche vs BMW and Audi

Both BMW and Audi have long stroke engines. I think the long stroke engines are more efficient at cruising or low output because you can gear them higher. Just my opinion, but I think Porsche is more performance and "hp" oriented and because of this, there is a sacrifice in fuel economy. I had an '09 A4 that was much more efficient than my '17 CS.

 

[Michael1]

You guys can throw out just about everything you have heard on the internet about long vs. short stroke. People think "the crank arm is longer so it must have more torque". Wrong. For the same engine displacement, the piston area is smaller, which 100% counteracts any extra moment created by the longer crank arm.

Here's the bottom line:

Shorter stroke = bigger piston = more room for bigger valves = more valve curtain area = better high RPM breathing = higher horsepower at high RPM

For emissions, you don't necessarily want the largest bore.