I have a 20B that I have taken the turbos off and will be using it in an

aircraft, a RV-10.  A friend of mine said that I should use the 9.7

compression rotors instead of the turbo rotors.  What say you all? 

 

I was very disappointed when I learned that Atkins did not switch out the 9.0 rotors for the 9.7 for my NA 20B.  My research turned up a plot of BMEP (same as power other things being equal) vs rpm for different compression ratios done by Mazda.  The effect depends on RPM – basically above about 6000 replacing the 9.0 with the 9.7 will have little effect, and the expected effect is about 4% around 5200 rpm.

 

If you are planning a 2.17 : 1 redrive; then I’d suggest making the change because the 5000 – 5500 rpm is important operating range.  If you go for the 2.85 ratio then it is probably not worth it.

 

Al