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Discussion Starter · #1 ·
[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. ;)


Thoughts?


References:
http://www.cargroup.org/assets/speakers/presentations/36/coleman_dave.pdf
http://www.mazdausa.com/MusaWeb/musa2/pdf/specs/2015_Mazda_3_Features_Specs.pdf
http://www.hemmings.com/hcc/stories/2009/06/01/hmn_feature11.html
http://forum.miata.net/vb/showthread.php?t=594456
 

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Maybe the length of the connecting rods are difference between models? I don't know the intricacies of how an engine works so I can't accurately answer this.
With my current knowledge (not a lot), I would assume Toyota has an non compete agreement of sorts with Mazda when they teamed up for the iA.
 

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Discussion Starter · #3 ·
Thanks for the reply. Since this car was designed by Mazda to be a Mazda (in most of the rest of the world), I'm betting there is no difference (mechanically) in the rod length, as that would require a good bit of reengineering and cost. Its just strange because the 1.5L in the new (ND) Miata has a slightly smaller bore and LONGER stroke (meaning a slightly different crank), yet it has a higher redline of 7,500 vs 6,500 (which is contradictory to normal engine design). Just weird.
 

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All skyactiv Engines are unique. The blocks are not shared. They are basically scaled versions of eachother. The 1.5L being the smallest, then the 2.0L scaled larger, 2.5L scaled even larger. If you put the 1.5L next to the 2.5L you would see that everything on the 2.5L is physically bigger. Considering the low piston speed the scion iA should benefit from a 75-7600 RPM rev limiter ^_^.

Now you won't get as much as the ND Miata because they have a more aggressive cam profile, but you'll still see more than 103BHP lol. 103 BHP is just sad on mazdas part. They detuned the heck out of these engines for scion
 

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Discussion Starter · #5 ·
Now you won't get as much as the ND Miata because they have a more aggressive cam profile, but you'll still see more than 103BHP lol. 103 BHP is just sad on mazdas part. They detuned the heck out of these engines for scion
Yeah that's kind of what I was getting at! I'm looking forward to seeing what this little car can do! :)
 

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I did some similar research into our 1.5 L Skyactv engine and found a sentence that answers your original question thus- the connecting rods on the higher rpm versions, e.g. Miata, are made of steel. Our lower rpm engine has them made of cast iron. I think I found this sentence in the Wikipedia article on Mazda 1.5 Skyactv-G.
I'll have to be content to revving up to 6400 I guess :-(
It sure sounds good up to there!
 

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Have you looked more into this? Does the header size of the mada 3 same as our iA? If it is, wondering if corksport header fit our cars. Thanks.
 
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