Technical question - KZ400 S2 - How was horsepower lowered?

Apologies if this is a repeated question! I tried searching with a few different terms but couldn't find this having already been asked.
I'm looking at the specs available on kz400.com for the 1976 model KZ400s and there's a note on the D3 image stating max horsepower was increased to 36HP
On the S2 image there's a similar note stating the max horsepower was lowered to 34HP.

Do we know how this change was achieved for the S2? I'm curious how making that change was helpful to Kawasaki at the time as it sounds like all the S2 changes were purely to reduce cost and not to meet a certain performance requirement. I would have expected something more along the lines of them skipping a more expensive machining process or gear size for the S2 and keeping it the same as the previous model over a specific change that reads like it gave the S2 a unique HP value.

Similarly, just because I'm interested - is it known how the D3 engine was changed to allow that small HP increase from the previous model?

Links to the source images I'm looking at below:
KZ400 D3 image

KZ400 S2 image

No, this is not answering to your question, and if you don't like it just ignore it but...

Firstable, the listed HP is meaningless.  It is only for advertisement purposes and different people would read different results, on same vehicle.   

 Secondly, even if real, 2 hp is insignificant.  As if the 400 achieved 100 mph top speed, it may achieve only 99.8 mph.      Who cares.........unless you are racing to a finish line which is an entirely different (an impractical matter).     

  HP means little to engineers.    People like it because it relates to an animal we know...the horse.   But in engineering terms, 1 HP does not equal the force of one horse.  It is not even an expression of force.  It is only a mathematical expression, from the steam engine era, to give an idea of the work an engine can achieve in a specific amount of time.

.................what really counts in everyday driving/riding, and from stop light to stop light is TORQUE.   That is FORCE....rotational force actually., and that what gives the seat of the pant feel of power......if any. 
 

Heya, thanks for taking the time.

I think the tiny, unnoticeable difference is part of what made me curious about those listed specs. When the bikes were developed, either the engineering department  knew there was some reason the power produced by the engine (HP, Torque, kW, whatever measurement you want) was slightly less on the special model and that came out in the advertising specs. If that's the case I'm super curious about what core difference is that causes that.
If that wasn't the case then maybe the data was just fuzzy enough that the marketing department added that piece to upsell people to the Standard model. Entirely possible and less interesting but still a cool thing to know.

Thanks again!

wrote:

Similarly, just because I'm interested - is it known how the D3 engine was changed to allow that small HP increase from the previous model?

The S2 description says 2-1 exhaust, while the D3 seems to have a 2-2 exhaust.  

If that's correct, that could result in a different HP rating. 

But even if the two models made the same power in reality, fitting a different exhaust system could also be used by the marketing folks to make the D3 seems just a bit more tempting by derating the lower cost S2 model.  Anyone who buys the S2 isn't going to complain about a bit of extra power.  

The factory 2 into 1exhaust is restrictive. 


.
 

Thank you, this seems to be the consensus. I didn't realize the exhausts were different but I doubt I'd have considered it as the cause even if I'd known. It does make sense though.
Appreciate the information!

You may find there are also subtle differences to carburation and ignition curves. I suspect if the power decreased on the later model it is likely to be emission related (IMHO)

As mentioned, it could be just marketing and have no relation to reality.
Also, I bet if we had 50 brand new KZ400S2s, and put their engines on a dynamometer, we would see a two horsepower variation from highest to lowest. Maybe more.

Power lowering on motorcycles can be done many ways like, jetting and intake air reduction, reducing the intake rubber boot and/or head to header diameter (welded washer), carb slider or throttle cable limiter, cams with less lift and duration, less compression and de-advancing ignition timing or with rev limiter.