Interesting...

Marc Wiese (with a 20B)



Myth about the 20B exhaust sleeves

 _____

A lot of people think that the 20B exhaust sleeves are restrictive for noise
reduction. This is not true. They were designed this way for specifically
for spool and low end torque. Mazde specifically engineered them to have a
narrow opening in order to work with the narrow exhaust manifold:

Description:
http://www.rx7club.com/attachment.php?attachmentid=437380&stc=1&d=1311919516

This helped Mazda achieve their targets for boost response and low-end
torque:

Description:
http://www.rx7club.com/attachment.php?attachmentid=437381&stc=1&d=1311919684

That doesn't mean you shouldn't swap sleeves. It just means that Mazda used
the narrow sleeves for a specific performance-oriented reason: they wanted a
torque curve that could compete with the V8's (or twin turbo 6 cylinders) of
the time.