Miata Mailing List: February 1996, Message #88
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   From: jcurtis@apcc.com (Jeff Curtis)
Subject: Re: Turbo/Super's and pistons..??
   Date: Fri, 2 Feb 1996 13:51:55 -0500

Tell your dad he doesn't know what he is talking about.  To my knowledge none 
of the forced induction modifications available for the Miata require piston 
changes.  Not only that but changing piston diameter will not change the 
compression ratio.

The compression ratio is just that, a ratio.  Specifically it is the ratio of 
the volume of the combustion chamber with the piston at BDC (Bottom Dead 
Center) over the volume of the combustion chamber with the piston at TDC (Top 
Dead Center).  Changing the bore (diameter) has no effect on the ratio.

CR = vol@BDC
           vol@TDC

There are many ways to increase (or decrease) the compression ratio.  Milling 
(removing material from) the head or block or installing taller pistons will 
decrease the cylinder volume at BDC and TDC by the same amount.  Since the 
volume at TDC is much smaller this increases the compression ratio.  Using 
longer connecting rods or a crankshaft with longer throws decreases the volume 
at TDC and increases the volume at BDC and therefore increases the compression 
ratio. 

  If a forced induction system needed a piston change it would be to reduce 
the compression ratio in order to keep the combustion pressures at a safe 
level.  Back in the days of '60s muscle cars engines ran compression ratios as 
high as 12:1.  Adding turbo or a blower required a piston change to shorter 
pistons, which reduced the compression ratio.  The forced induction more than 
made up for the lost compression ratio.

Jeff Curtis
91A blue

>>Anyway, I started talking about that Autorotor turbo installation and all 
those extra horses (visions of Mustnags in my rearview...) and 
supercharging and the like and mentioned that the Sebring Supercharger 
advertises that it's installable by the DIY over the weekend.

My dad looked amazed and said that you can't artificially (inseminate?) 
aspirate a car without putting in bigger pistons becuase the compression 
ratio gets to high. I said, "But Dad, nobody ever says anything about new 
pistions". and he said, "Well then they must not last very long" and I 
said "But there are cars on the list with turbos and stuff and 100k 
miles" and he said "Well they must have new pistions then!" and I said 
"ARGGGG!!"<<

Anyone have bigger pistons?