Why does the rx7 fd have a Brappy idle but the rx8 doesn’t?

 

Overlap.

So, in combustion engine cars, you can get a small (or maybe large?) power bump by having the intake ports start to open before the exhaust ports completely close. This is because the exhaust gas has momentum (there's more to it than this but it works for a basic explanation) and it helps to pull in fresh air as it leaves the combustion chamber. This is called overlap.

With pre-Renesis rotaries, you could remove enough material around the ports to result in useful overlap. It also gives the motor that iconic noise. If you get really aggressive with the porting work, you could probably make the engine "lope" like a V8 with a big aggressive cam.

This is impossible to achieve with the Renesis due to the exhaust ports being moved from the periphery of the rotor housings to the side plates. You can't remove enough material to do create overlap without cutting into the coolant jacket.

This is why bridgeporting a Renesis is a complete waste of money.
I mean, you can do it but it won't accomplish anything except costing money while reducing the life of your engine.

So, no brap unless you swap in an older 13B motor.

 

Also from Team's knowledge: RX-8 technically has negative overlap, as there is 6 degrees in the rotation where both intake and exhaust ports are all closed.

This is all done for emissions.

 

I think what many people don't get about the whole bridgeport thing is that it does create plenty of overlap , but it's bad overlap . Exhaust overlap can only be useful if it's AFTER exhaust top dead center . In a Renesis you can only create it before Exhaust top dead center , which has the effect of allowing exhaust gas into the intake , but gives no opportunity for intake air to push back into the exhaust. This is why you lose a lot of power at low rpm with a bridgeport and any top end gains you might expect from the larger intake port area are negated.

 

Car Throttle just did a video on Bridgeporting and Streetporting.


Looks like it's mostly just for the noise. There is power gain but not much.

 

Old video .......... and there is nothing in it that proves anything about what a bridgeport does for power.

 

unfortunately I don't think the rotary shop in those videos has a comparison dyno for a bridge ported renesis,
despite seemingly promoting them