S5 Rotors in S4 keg?
 

Rebuild time!

So Im going for an all motor N/A street/track build. Probably street ports, with ITBs and a Haltech Elite setup.

I have a chance to buy some S5 rotors with brand new bearings for $500. I also have a set of mint S4 rotors with new bearings I can sell to help offset the cost. Is it going to be worth it, especially in an all-motor build like mine? Or should I just stick with the S4 rotors I got? Pros? Cons? I want to do this as right as I can my first time without going OVERBOARD (Im not paying the $1750 or so to lighten and clearance my rotors yet, I don't even have the suspension started on this car, the coolant seal let go before I could do anything really in depth). Am I better off putting S4 rotors in now, and potentially switching to lightened and clearance S5 rotors down the line when I start running ITBS and a standalone? Will I see gains on stock ECU using S5 rotors?

Someone else will surely chime in as to whether it's worth the gains (I can't say that myself, although in theory it should provide a bit more power). However, since they have brand new bearings already and you are committed to all-motor power, S5 rotors seem to make sense. Even if you aren't lightening or clearancing them, the S5 rotors are already lighter (stock S5 redline was 8000rpm IIRC vs S4 7000rpm). Keep in mind you'll need an S5 counterweight / flywheel.

Switching to the other rotors down the line is not a trivial task, since it requires another rebuild and replacement of the soft seals. It would be a lot of work to do that, and were I in your position I would probably take the S5 rotors and sell the S4 rotors.

Rebuilds probably a little simpler for me since I got my engine bay so emptied out that pulling the engine is barely any work. I also am using a Aluminum flywheel so that's covered. Ill probably end up getting them.

For N/A, use S5(lighter). Less rotational mass.

the S5 rotors are better, its not night and day, but they are better. basically they went to a machined combustion recess so its consistent.

if you have a factory rotating assembly balancing isn't needed. back when we raced an FC we were spinning it to 9400rpm, and i've seen dyno sheets to 10,500. 9400 was ok, but 10,500 was way way past the power band....

the only mod we did was the FD oil pressure regulator, it bumps pressure to 120psi...

these engines are remarkably tough

use the FD clearances as a start

So you'd say the clearancing is important?

And does the rotating assembly include the e-shaft? because I was planning on reusing mine, but I can see if the guy has the matched E shaft too. Itll have to be rebalanced anyways with the new counterweight and lightweight flywheel right?

so when Mazda was developing the FD, they had it up to 255hp and had plenty of engine failures, but the competition department was making 300+ hp with perfect reliability, so they built the FD with the race car clearances and such, and its fine.

if you're going to spin the nuts off the thing (going past ~8500) you want to have the rotor tip to side housing clearance on the big side

https://cimg6.ibsrv.net/gimg/www.rx7...5d5c73046b.png



e shaft isn't very important in the whole balance thing, if you weigh the rotors and they are close you're good to go. the 10,5000rpm dyno i saw was just two new S5 rotors, and everything else was good used stuff, no balancing.