Hello, I'm planning on putting together an engine with a hodgepodge of FC turbo engine internals and I reckoned I'd need a rebalance. Considering the unique considerations of the balancing a rotary (oil in the rotors and such), should I have a rotary specialty shop balance the assembly or is it possible to have any local engine machine shop do the balancing? Or, considering that I live nowhere near rotary specialists, would it be easier to do it myself?

Thanks

 

i had one of mine balanced, and it is very smooth.

however, i think if you're building an engine from at least correct series parts (all S5 or all S4), then you'd be ok up to 8500rpm or so, for short bursts. the next step would be to at least find a pair of rotors that are close in weight, i wouldn't rev it too much higher, but it will be happier.

the rotary engine book actually has all the equations needed to balance a rotary, and any good machine shop has the equipment, so in theory it can be done anywhere... i had my engine done by AZRR, and i know that they took it to a machine shop and had it done, to their instructions/specs so that is a local option. RB and Mazdatrix aren't that far either.

just be careful packing the rotors

 

AZRR shut down a while back, so AFAIK, there's no rotary specialists in town, except for some elusive man I've heard whispers of whose name I still can't pin down. There's a place near where I live, AZ Speed and Marine, which has a pretty extensive engine machine shop. I toured the shop (while they were looking the other way) when I needed my injectors serviced by them, and they don't seem to lack equipment. I'll check tomorrow if they'll do it. Going to California will be a last ditch thing, although I'm scheduled to go there this spring anyways.

For the turbo motors, I was under the impression that it was a dumb idea to rev out the engine to 8.5k in completely stock trim, especially the S4s since they did not have hardened (something - rotor? stationary? both?) gears. I plan on road racing/street driving this motor, so it's going to be up at around 8k for quite a bit, but I'm not shooting for big power. So far, this is what I've decided:
-RX8 eccentric shaft
-RX8 HP stationary gears (with the o-ring groove machined)
-S5/6 rotors
-S6 front stack
-Lightweight flywheel and rear counterweight (ideally S5)

The annoying thing about the last bit is that I already have an S4 counterweight as opposed to the S5. On Atkin's website, the S4 counterweight is supposedly lighter even though the S4 rotors are heavier. I'm hoping it still has more rotating inertia than the S5, so that I can have the S4 counterweight machined down during the balancing rather than having metal added or sourcing an S5.

 

It's fairly common to ship your internals off to have them balanced, since most owners aren't lucky enough to have a shop capable of doing it locally.

The biggest thing is that your rotating assembly is from the same series. You can get away with having an RX-8 e-shaft mixed in with RX-7 rotors. If you have S5 Rotors you need to make sure you also have S5 counterweights (NA or Turbo, they all need to match). You will also need to match your front stack to the counterweights.

For example:

I have 13B-REW irons/housings, an RX-8 E-shaft and stat gears (you don't NEED to have an o-ring groove machined into the rear stat, a line of "right stuff" will seal it up just fine), I have S4 NA rotors (lightened/clearanced), S4 NA counterweights, and an S4 front stack. Everything was balanced by Chip at Chip Motorsports, he's in SE Michigan and I'm in Washington DC. Distance doesn't really matter..

You want to keep the same series for rotor/counterweights because they are already matched. The front counterweight has a slot machined into it to receive the thrust washer so it sits flush. The thrust washer (and all associated bearings) need to match whatever counterweights you use or else it won't fit right. I would NOT mix an S4 counterweight with S6 rotors/stack. Spend the extra money up front to do it right and make sure everything matches.

 

I was under the impression that the S5 Turbo and S6 had the same rotor weights and that when using the RX8 stationary gears, you needed to use the S6 front stack because of the larger thrust washers and bearings? I was going to use the S6 front counterweight anyways, as implied by when I said the whole front stack of the S6. Maybe you got confused when I said the S4 counterweight... I meant to clarify, I have the S4 rear counterweight. I'll see if I can't source an S5/S6 rear counterweight, but it'd save me the hassle if I could just get the S4 rear machined during the balancing rather than get another.

 

the turbo engines stop making power at like 6k, so its sort of pointless to rev it much higher... Mazda did harden the stat gears for 89, and then the FD engines were basically just built to the competition specs.

8500 is a good safe limit for the stock balance

 

I run a mishmash of different S4 bits with unhardened GSL-SE stationary gears and I see a lot of RPM, like the rev buzzer is still buzzing after an upshift RPM, and so far the world hasn't ended.

Maybe I'm lucky? Or maybe the need for hardened stat gears was mostly a thing needed if you use pre-'86 9 roll pin rotors which Mazda has shown are MUCH harder on the stat gears than the '86-up 12 roll pin rotors.

 

its been pointed out, but maybe bears repeating, but during the development of the FD engine the FD engine team ended up going over to the competition; well cubicle, it was only 2 guys. anyways the FD team ended up just building the FD engine to the competition specs, so it has the hardened gears, and fancy bearings, and high oil pressure and all of that nonsense.

so the FD engine, given enough cooling to make an apples to apples comparison, should be able to run a 24 hour race just like the competition engines.

i don't doubt that balancing to a tighter spec, and lightening up the things you can lighten up help, but Mazda's race engine and FD engine really only differ in compression ratio, apex seal material, and port style.

 

All of this sounds lovely, but I'm working with an S4 engine out of a 1987 Turbo-II. This engine doesn't exactly have all of the wonderful later-year revisions the S5 and FD get, so I will be upgrading the engine accordingly. Parts like a thick-cast S5 rear plate, FD corner seal springs, FD rear oil pressure regulator, FD/RX8 stationary gear bearings, and so on and so forth are on the list. So, while I'm at it, I reckoned I'd go the extra mile for a lighter, balanced rotating assembly since it wasn't all that much of a stretch. I'm sure the 8.5 CR rotors that originally came in the engine would be better off in a build it was better suited to anyways.