Damn, all of my text got deleted when I added attachments. I wrote it for more than an hour. I will try to sum it up:

Basically, I think 75 and 90 HP models of Mazda 2 are power restricted. My proofs are factory specs and calculated power curves. I started digging into this when I first revved my Mazda 2 to 6500 RPM and it felt lazy above 4500 RPM, power was not increasing.

1. 75 HP model makes 135 Nm or 100 lb-ft at 3800 RPM, using basic calculations, it turns out that it makes 54 KW or 72.5 HP at 3800 RPM. Therefore 98% of its max power output. It makes no sense for a naturally aspirated petrol engine to output 98% of its power at 3800 RPM and then keep the same value (therefore flat curve) until 6000 RPM where is its 100% power output. Usually NA petrol engines output 30-40% more power at 6000 RPM than at 4000 RPM. That indicates some sort of restriction.

2. 90 HP model has a slightly higher begining of the curve and it outputs 148 Nm or 109 lb-ft of torque at 4000 RPM, that translates into 62 KW or 83 HP. Again, more than 90% of its peak output. So it means the curve is pretty flat in 4000-6000 RPM range. That also indicates some sort of restriction.

3. 115 HP model has the same begining of the curve as 90 HP one. It outputs 149 Nm or 109 lb-ft of torque at 4000 RPM (same value as 90HP engine - within margin of error, so the same 83 HP at the same 4000 RPM) and then it proceeds to output 35% more at 6000 RPM - 115 HP. That means this engine is not restricted.

I fully realize that 75 HP engine lacks much technology compared to 90 and 115 HP ones. It has 4-1 exhaust manifold (instead of 4-2-1 for the more powerful engines), it uses simple piston design (idk about 90HP but 115HP engine has different pistons) and its compression ratio is 12 instead of 14. 90 HP engine can also lack something vs 115 HP one. My theory is that 75 and 90 HP engines due to lack of expensive parts did not fulfill emission regulations and had to be restricted. 75 HP engine seems to be less powerful in the entire range + restricted, while the 90 and 115 HP engines seem to be pretty much the same, but the 90 HP one seems to be restricted from 4000 RPM up.

I think that if there were no restrictions, we would only look at two power variations - 100 HP for the weaker one with 4-1 exhaust and 12:1 compression ratio (if it produced 35% more power at 6000 RPM than it produces at 4000) and 115 HP for both more powerful engines.

Has anyone tried to figure this out? Do you think these restrictions could be removed? I am fully aware that these power curves are estimates, not measurements, but I think they can be pretty accurate, they correspond to both factory specs of the engines and regular power curve shapes of petrol engines with natural aspiration. I think I will definitely make measurements for my 75HP Mazda 2 but probably not this summer.




Power curve calculated for 75 HP Mazda engine

Power curve calculated for 90 HP Mazda engine

Power curve calculated for 115 hp Mazda engine

Power curves overlay

 

According to a couple of 90 HK model owners reports on closed facebook groups, reprogramming the ECU / chip tuning, made a huge difference in HK measured: Up to 151 HK / 185 Nm, which, to me, clearly indicate some sort of restriction. That's a huge jump. One of them had the resonator in the air intake removed + using 100 octane gasoline also, but no separate measurement of that modification was documented.
The general impression I get from reading feedback, is that some, if not all three models have been power restricted from the factory and that modifying the throttle opening code by reprogramming, is the main factor of the gain in performance.

Do you have links or documentation to confirm this? I would really like to read more about it.