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Old 10-25-2007, 01:33 AM   #1
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Default 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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Old 10-25-2007, 01:38 AM   #2
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According to the m.net FAQ, that was only for the 1.6's, 9.4MT vs 9.0AT.
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Old 10-25-2007, 01:44 AM   #3
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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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Old 10-25-2007, 01:56 AM   #4
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just piston differences. You probably couldn't tell the difference btw the two if you didn't know which one was which.
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Old 10-25-2007, 02:13 AM   #5
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I just figured I could squeeze a little more out of the lower compression engine.
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Old 10-25-2007, 02:19 AM   #6
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I'm rebuilding an AT engine, but I'm taking the pistons and **** out anyways and going with Supertechs....
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Old 10-25-2007, 02:21 AM   #7
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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.
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Old 10-25-2007, 02:25 AM   #8
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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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Old 10-25-2007, 02:31 AM   #9
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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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Old 10-25-2007, 10:20 AM   #10
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Quote:
Originally Posted by akaryrye View Post
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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Old 10-25-2007, 10:36 AM   #11
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Cams are different between MT and AT, as well, but that won't have anything to do with compression.
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Old 10-25-2007, 11:15 AM   #12
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Quote:
Originally Posted by Atlanta93LE View Post
Cams are different between MT and AT, as well, but that won't have anything to do with compression.
Quote:
Originally Posted by Braineack View Post
there are different part numbers for AT and MT pistons.
B6Z2-11-SADC (MT) & B6Z5-11-SAD (AT)
Quote:
Originally Posted by akaryrye View Post
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.
1.6 only, as in post #2 in this thread.
Dude has a 1.8
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Old 10-25-2007, 11:17 AM   #13
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ha whoops. :gay:
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Old 10-25-2007, 11:23 AM   #14
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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?
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Old 10-25-2007, 11:26 AM   #15
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possibly, more load=more prone to det.
thats just a guess, no research,
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Old 10-25-2007, 01:37 PM   #16
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maybe its cause autos suck and they wanted anyone who bought one to suffer
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Old 10-25-2007, 01:39 PM   #17
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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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Old 10-26-2007, 12:09 AM   #18
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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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Old 10-26-2007, 12:17 AM   #19
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as far as i understand things, cams do make a difference to dynamic compression, but not static compression as pistons are rated, but when the motor is running things are different but i couldn't explain the extent of it.
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Old 10-26-2007, 03:57 AM   #20
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so are the AT cams worse for turbo or no?
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