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..

According to the m.net FAQ, that was only for the 1.6's, 9.4MT vs 9.0AT.

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.

just piston differences. You probably couldn't tell the difference btw the two if you didn't know which one was which.

I just figured I could squeeze a little more out of the lower compression engine.

I'm rebuilding an AT engine, but I'm taking the pistons and shit out anyways and going with Supertechs....

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.

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.

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

there are different part numbers for AT and MT pistons.

B6Z2-11-SADC (MT) & B6Z5-11-SAD (AT)

Cams are different between MT and AT, as well, but that won't have anything to do with compression.

1.6 only, as in post #2 in this thread.
Dude has a 1.8 :slap:

ha whoops. :gay:

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?

possibly, more load=more prone to det.
thats just a guess, no research, ;)

maybe its cause autos suck and they wanted anyone who bought one to suffer :dunno:

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. :crx:

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.

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.

so are the AT cams worse for turbo or no?