As title, ive heard both.

Officially they seem to be said to be 8.5:1, but seen on a few places, places that should know, that they are 8.2:1. Unless its a typo.

Does anyone know for sure?

 

prior to the s5 rotors,, and worse with each earllier generation
there is a fair tolerance up and down due to the finishing techniques
-- several points of compression is normal in the same engine

however the s4 turbo FC will have an intended 8.5:1 static compression
the NA will be 9.4:1

the s5 is 9.0 ( turbo ) and 9.7:1 NA and the production tolerance is much more precise


the GSL-SE ( 83-85 HB cosmo RESI 13b engine ) is 9.4:1

the rx4 1757 13b rotor is 9.2:1

the 12a rotors are 9.4:1 NA and 8.5:1 for the turbo

 

Along those same lines...

The cast rotors (S4 and earlier) have a tolerance of +/- 0.3 CR. So while an S4 TII rotor is officially listed as 8.5:1, it could be somewhere between 8.2:1 and 8.8:1. As mentioned, the machined rotors found in S5s and later were more precise and had a tolerance spec of only +/- 0.06 CR.

 

as RotaryRocket explained, the cast design of the s4 means a wider manufacturing tolerance for compression ratio. But the s4 rotors are also considered to be stronger in high detonation situations or extremely high horsepower setups. I know my s4 rotors have taken some punishment. Occasionally you will read about people having "dents" in the s5 and s6 rotors from detonation... I don't think this difference in strength is a big deal for most applications though.

 

Great info

So, how do you calculate compression ratio on a rotary?

On a piston engine I know how, and once again it will be measuring the dish volume, but how do I known what dish volume equates to what?

Does anyone have the dish volumes for the various standard compression ratios? Should be able to work out most ratios from there.

 

try posting in "rotary car performance" about that

also PM rotarygod , he might know