Compression difference between AT/MT?
10-25-2007, 12:33 AM
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Compression difference between AT/MT?
Numerous people have told me that A/T equipped Miatas had lower compression then MT, different pistons, does anyone know if there is truth in this? My spare engine was out of an A/T, I may swap it if that is true..
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10-25-2007, 12:44 AM
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I have always heard this, and assumed it was true since it was based on multiple sources. Anyone know what was changed to achieve this new compression ratio? Is it just pistons, or were the rods and crank throw altered as well? Now that I think about it, sorta wish I rebuilt an automatic block instead of a manual since I live in cali.
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10-25-2007, 01:21 AM
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please buy wiseco pistons, your grandmother will thank you. http://www.raceeng.com/searchadv.asp...x=0&submit.y=0
best price and they stock them so you'll get them in less than a week instead of waiting a month.
best price and they stock them so you'll get them in less than a week instead of waiting a month.
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10-25-2007, 01:25 AM
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9.4 to 9.0 is a significant drop in compression if you are planning to boost the motor. I say if the replacement motor is in good condition, it will be worth it to swap.
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10-25-2007, 01:31 AM
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I want to figure out if there is a difference in the BP engines though. I looked in the Mazda catalog and there isn't any MT/AT designation between the piston sets. Hmm
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10-25-2007, 09:20 AM
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I have always heard this, and assumed it was true since it was based on multiple sources. Anyone know what was changed to achieve this new compression ratio? Is it just pistons, or were the rods and crank throw altered as well? Now that I think about it, sorta wish I rebuilt an automatic block instead of a manual since I live in cali.
there are different part numbers for AT and MT pistons.
B6Z2-11-SADC (MT) & B6Z5-11-SAD (AT)
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10-25-2007, 10:23 AM
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cams:
from the research i've done it seems the 1.6 auto's had different cams than the 1.6 manuals to create more low down torque for that parasitic trans. the 1.8's made enough torque that they didn't need to use different cams in the auto version.
compression: just out of curiousity, why would the compression be lowered on an auto? seems that would reduce the power(which would further explain different cams i guess). are the auto's more prone to detonation?
from the research i've done it seems the 1.6 auto's had different cams than the 1.6 manuals to create more low down torque for that parasitic trans. the 1.8's made enough torque that they didn't need to use different cams in the auto version.
compression: just out of curiousity, why would the compression be lowered on an auto? seems that would reduce the power(which would further explain different cams i guess). are the auto's more prone to detonation?
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10-25-2007, 12:39 PM
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they have different computers, so I'd bet there some neat-o differences in at least the timing map, maybe the fuel map too. i know for a fact, that the a/t has a rev limit of 6k. 
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10-25-2007, 11:09 PM
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Honestly dont know why the compression would be lower.. I would assume it would be to lower power output for longer life out of the AT.. But thats an un-educated guess lol. Its looking like that AT spare engine is identical to my current.. Ill check the cam casting numbers between the two this weekend, just out of curiosity to see if the cams are different like on the B6 engines.
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