Okay guys I know there's alot of threads about the rx8 rotating assembly on an older s4, s5 engine however most of them are about making power. Attached is what I accidentally and stupidly did to one of my NA s4 rotors and instead of trying to send it and straighten it out and build the engine as is I wanted to know if and how I could use the rx8 rotating assembly on my s4 keg. the rx8 rotating assembly is cheaper and easier to find (where im located) than an s4 or s5 rotating assembly or a rotor pair. However I don't want to buy an aftermaket ECU and get the engine tuned if I used the rx8 rotors, i know that the rx8 rotors open up the ports earlier due to some small scalloping but can the stock ecu handle it? I'm rebuilding my NA s4 engine on my fc to be a daily driver and im trying to be on a budget. If the rx8 rotors are indeed an upgrade for my NA setup I will consider getting an aftermarket ECU and getting it tuned, but my main goal is to have the car running under its own power reliably for a while until I can get a real job and be able to pour more money into it (im a college student so my pockets are usually empty).

so TLDR; has anyone tried to use the rx8 rotating assembly with a stock s4 engine and the stock ECU? will it run without tuning or engine management modification? If this will work do I just need the rotors, do I also need the rx8 counterweights and flywheel? should I just try to find another s4 rotor? should i straighten the rotor out and send it (jk)? what to do?

 

My understanding you will have to send your rotors and e shaft to get balanced . I wouldn’t try and fix that rotor . I’ll be going with Rx8 rotor my self and already talk to a builder who said to send rotor assembly to get balance

 

I thought the apex seal groves need to be machined as well. I think they use 1.5mm thick apex seals vs. the 2mm of the 2nd gen rx7.

 

Find another s4 rotor.

 

The apex seal slots will need to be machined deeper too. After machining, balancing, and your ECU concerns, it's probably easier and cheaper to find another S4 rotor

 

You may want to look for new rotors anyway. Those seals will probably not come out due to the rust.

 

not exactly, both apex seals are 2mm but the groove is shallower on the rx8 rotor which is why you need to machine it then re balance the rotating.

also your rx7 rotor shown has side seals rusted in there, toss that boat anchor away or use it for display.

 

That would be strongly recommended in addition to using a matched pair of rotors and dynamically balancing the full rotating assembly.

 

Use either RX8 or S5 NA rotors

 

Use whatever. It'll be fine. For a street build, you are far better off making sure all seal gaps are as tight as you can get them. I shoot for .0015" on the side seals. The difference between .0015" and .004" is incredible at low RPM.