Project FD RX7 Restomod: Part 12 – Rotor Lightening, WPC and CTP Cryo Treatment

Some engine builders do an additional tip clearance which involves grinding down the tip of the rotor at an angle to provide for even more clearance at the furthest end of the rotor.  This will be a source of a lot of controversy and disagreement, but our engine builder Abel Ibarra does not recommend doing this when the engine is “built correctly” by stabilizing and stiffening the block with proper doweling and studding.  But let’s look into this with some math.

The center of the corner seals on a 13B rotary sit 2.647” from the center of the rotor, which is only 1.191” further out than the radius of the rotor bearing itself.  Since rotor bearings have a clearance of 0.0015-0.0020”, the rotor can only wobble 0.0393515-degrees before the bearing hits the e-shaft. This translates to only 0.0018” of movement at the center of the corner seal, or just under two-thousandth of an inch of additional movement.

With anywhere from 15-20 thousandths of an inch (0.015-0.020”) of static/assembly clearance between the rotor and side housing, there should still be 13-18 thousandths of clearance from the tip of the rotor to the side housing when the rotor is at max deflection.  This means that if there is contact, it is actually caused by the housings twisting and flexing, not from the rotor wobbling.

Abel says the right solution is to address the actual problem by stabilizing the block by studding and pinning it properly.  The stackable nature of the rotary engine does not lend to an inherently stiff block design that has to deal with the extreme forces of combustion and rotation inside the housings.   While this is not a problem at stock power levels; as power increases, the housings and irons start to distort and ‘walk’ as the factory dowels cannot handle these stresses and, causing rotor contact, breaking seals, and leading to engine failures.

According to Abel, tip-clearancing rotors has been a compromised solution that causes a loss of compression and torque, takes away quench, and resulting in less power, efficiency, and a worse running engine.  It is widespread implementation has been a “monkey-see, monkey-do” over the years by people who don’t understand the problem and address the symptom rather than fixing the cause.

Abel Ibarra is one of the world’s fastest rotary-powered drag racers and has a pretty impressive resume.  In the late 90’s and early 2000s, Abel had the fastest RX-7 in the ¼ mile, which stood for over 10 years, he was the first rotary in the 6’s, the first rotary to trap over 200mph, and held titles for the fastest 2 rotor, 3 rotor, Mazda, and even fastest import in the world at one time.  Needless to say, he knows a thing or two about the spinning Doritos and how to make power.

Up close, we can clearly see the lightening of the rotor lobe, this was done to Abel’s specs for reliability on a high-powered turbo application.  With the side clearancing completed, our rotors were finished with machining.