Changes to the early 12As
03-09-06, 10:53 PM
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Changes to the early 12As
In doing some research on early 12As, I came across a couple of places that said that during the first year of their production (1971, I believe), RX-2 12As made 120 HP, but were subsequently detuned for fuel consumption reasons. If this is indeed the case, what were the differences between the '71 12A and the later revisions? Was it simply port sizes?
03-10-06, 08:46 AM
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I can't give you specifics, but I can tell you that the RX-2 had 3 different carburetors over the years, and they could definitely account for different venturi/jet combinations to change power and mileage over the years. Mikuni-Solex (early builds), Hitachi (most US S1 and S2 cars) and Nikki (lots of RX lines from about 74-on) all supplied carbs, and in roughly that order.
FYI - In 74, Mazda went to what was internally known as the 12B. Because the 12A was widely known as being the big brother to the 10A and started carrying some clout, the "12A" castings didn't change on the housings. The 12B saw the introduction of the thermal reactor (and ACV, I think) and different exhaust ports and stayed on until 85. I've never known what the 6-port or Turbo 12As technically were, though.
FYI - In 74, Mazda went to what was internally known as the 12B. Because the 12A was widely known as being the big brother to the 10A and started carrying some clout, the "12A" castings didn't change on the housings. The 12B saw the introduction of the thermal reactor (and ACV, I think) and different exhaust ports and stayed on until 85. I've never known what the 6-port or Turbo 12As technically were, though.
03-10-06, 11:39 AM
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You're correct on the 1971s having 120Hp. The 1972s had 102HP and the 1973s had 97Hp. It was not fuel consumption reasons, however, but stricter emission controls. Gas mileage was not important until late 1973 when the oil embargo took place. Smaller port sizes were implemented in 1976 for better fuel economy.
03-10-06, 09:05 PM
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so, how was a turbo 12a different? I just installed mine, and noticed some differences:
1. Coolant holes in intake were never drilled (I had mine capped on previous motor anyway)
2. Center plate is very different from my 85 12a plate. Had areas for studs on the bottom.
3. Almost all coolant passages are different. Coolant outlet under the oil metering pump(which was also different), smaller coolant outlet on the opposite side of oil filter on the end plate.
4. Probably plenty of other things
So what else (other than lower compression) is different?
1. Coolant holes in intake were never drilled (I had mine capped on previous motor anyway)
2. Center plate is very different from my 85 12a plate. Had areas for studs on the bottom.
3. Almost all coolant passages are different. Coolant outlet under the oil metering pump(which was also different), smaller coolant outlet on the opposite side of oil filter on the end plate.
4. Probably plenty of other things
So what else (other than lower compression) is different?
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