[NOTE: This is NOT written by a professional. I have not built or designed engines, this is purely for curiosity. So don’t go preaching any of this as gospel, OK? Thanks!]
So… this is diving a bit more into engine design than I normally have gone, but in the search for data on our Mazda 1.5L SKYACTIV-G Engine, I have ran across some interesting information, questions, and one decision that Mazda/Scion made which has me scratching my head. I really would love for someone with knowledge on these engines (and more on engines in general) to chime in.
First a bit about the engine family that we have:
The SKYACTIV-G engines are Mazda-made and designed gasoline engines that were developed for excellent fuel efficiency, increased performance, and reduced manufacturing cost. The last point (reduced manufacturing cost) I bring up, because it appears that the SKYACTIVE-G line is scalable from 1.3L up to 2.5L. What does this mean? It means that similar combustion characteristics can apply to all SKYACTIV-G engines, “dramatically reduced development and calibration expense”. [WARNING: ASSUMPTION]
To me, this also means that all SKYACTIV-G engines should be designed with relatively similar components, just different sized (crank, pistons, valves, etc.). Maybe not – but this would be my assumption since the engine is scaled.
About our engine:
The Scion iA uses a Mazda-made 1.5L SKYACTIV-G engine.
Compression Ration = 12:1 (the Japanese Mazda2 has 14:1, I assume it was reduced in the U.S. to allow for cheaper fuel usage.
I am also assuming that our cars maintained the 4-2-1 header design to reduce pressure/heat to help minimize detonation (I have not seen the 4-2-1 because our header is wrapped like a mo-fo).
Bore X Stroke: 76mm (2.99in) x 82.6mm (3.25in)
Displacement: 1496cc (91.3 cu in)
HP: 106 @ 6,000 RPMs
TQ: 103 @ 4,000 RPMs
Approx Redline: 6,400 RPMs
The relative low redline of 6,400 RPMs is the big head scratcher to me and this is why I say that: Given what we know about the SKYACTIV-G line, the characteristics SHOULD be generally the same between displacements. However, with such a low redline, the piston speed is quite low compared with other Mazda engines, which says to me that performance could have been improved rather easily.
Let’s first look at the piston speed of the iA.
To calculate general piston speed in Feet per Minute (FPM), you use this formula: FPM = Stroke (in) x 2 x MaxRPM / 12 (inches in a foot). For the iA, that reads: 3.25 x 2 x 6,400 / 12 = 3,468 FPM.
Now, let’s compare that to other Mazda engines:
Scion iA 1.5L– B/S: 2.99 x 3.25; MaxRPM: 6,400. = 3,468 FPM
Miata 1.6L – B/S: 3.07 x 3.29; MaxRPM: 7,000. = 3,840 FPM
Miata 1.8L – B/S: 3.26 x 3.34; MaxRPM: 7,000. = 3,904 FPM
Miata/Mazda3 SKYACTIV-G 2.0L – B/S: 3.28 x 3.59; MaxRPM: 6,800. = 4,069 FPM
Mazda 3 SKYACTIV-G 2.5L – B/S: 3.50x3.93; MaxRPM: 6,500 = 4,265 FPM
Miata SKYACTIV-G 1.5L – B/S: 2.93 x 3.38; MaxRPM: 7,500. = 4,222 FPM
Ok, so now after the barrage of numbers, you might ask, “Why is piston speed so important?”. “The air throughput of an engine is keyed to piston velocity: the faster the piston moves, the more air can be pumped.”.
But that’s not really my question. My question is, given that the SKYACTIV motors are supposedly “scaled”, our engine SHOULD be capable of running a higher redline, assuming the valves/springs, are as capable as these other Mazda Engines. So why didn’t Mazda do that? Maybe they felt this car was encroaching on the Mazda 3 too much. I just find it weird that: Given the 1.5L in the Miata makes 129hp/111 lb tq, why didn’t Mazda give the Mazda2 (Scion iA) a bit more oomph? … In hindsight, I guess I should have started with that question.