Dispelling Myths: Using RX-8 internals in a 13B-REW

[Chadwick]

Rumors, myths and facts…. When the RX8 came out I wonder how many parts from the motor could make the move to a 13B-REW. Unfortunate most of the information found on the internet was more rumor than fact. I am still amazed that 5 years later it is still almost impossible to find accurate information about what can and can not be used.

Challenged with getting the most out of the new motor I’m building for this race season, I started to dig a little further on what could be used to increase performance. The first place I looked was the RX8 rotors and e-shaft. I knew I wanted to increase compression to gain low end grunt and thought 10:1 would be super. It didn’t take long to figure out that the design of the RX8 rotors would be a less than stellar choice so I went with the 9.7:1 rotors out of an 89-91 NA FC.

Now the eccentric shaft was a different story, it was pretty common knowledge that it was lighter but there was a ton of conflicting information about its strength and design and if it would work in a 13B-REW. Also how much lighter was a big question, I saw everything from two pounds to 4 ounces.

The only way to know for sure was to bite the bullet and order one. First thing I did was compare it to a 13B shaft….

RX8 shaft on the bottom:

RX8 shaft on the right:



Well they looked identical, every hole and taper. My friend Brian who is a Mechanical Engineer measured the bearing journals compared to a 13b shaft. The RX8 shaft is .0003 less in diameter then a stock 13B shaft. It also weighs .4 lbs less than a 13B shaft. Well within spec to be used in a 13B motor

Now another rumor is that the RX8 shaft is lighter thus weaker than the 13B shaft. Quite simply this is not true. It is manufacture exactly the same way as a 13b shaft. It is lighter because there is an extra hole drilled into each lobe. As this area is not a high stress point it will not affect the strength of the shaft in the slightest. It may even prove out to be stronger as metal forging and tempering techniques have improved over the last 10 years.

RX8 Shaft on the right:


The next parts I looked at where the stationary gears. Now there was a bit more accurate information to be found on them so it was an easy choice. The front stationary gear is functional identical on both engines. The only difference I notice was the alignment notch is rounded on the RX8 gear and square on the 13B.

Front stationary gear, RX8 gear on the left:





Now the big difference between the rear stationary gears is the O-ring grove. The RX-8 stationary gear does not have one as it uses a grove on the rear housing for an o-ring to seat. In talking to several builders they all suggested using hylomar or silicone to seal it. Although I agree that a sealant would work, I’m a bit anal and wanted to use an o-ring. I had my friend Brian take it to work so one of their machinist could match the 13B grove.

Both stationary gears came with bearings installed and the rear gear has the rear main seal. They are both considerable cheaper to buy than new 13B-REW gears.

RX8 rear stationary gear




Picture of the grove machined into RX8 stationary gear to match 13b stationary gear:



So here are the facts:

You can use a RX8 eccentric shaft in a 13B-REW with no modification. (note: I do suggest having the rotating assemble balanced when ever replacing one of the parts.)
The RX8 eccentric shaft weighs .4 lbs less than a 13B-REW shaft
The RX8 front stationary gear is a direct replacement for a 13B-REW gear
The RX8 rear stationary gear can be used as a replacement for a 13B-REW gear but will require the use of a sealant or have it machined for an o-ring for a proper oil seal.

Part Numbers;

RX8 Eccentric Shaft: N3H3-11-D00
RX8 Front Stationary Gear: N3H3-10-E0YC
RX8 Rear Stationary Gear: N3H3-10-E1YC

Hope this helps,
Dan

[-xlr8planet-]

wow

its nice to have facts this way

[85rx-7gsl-se]

Great write-up

[a7xh7]

do you recall the price difference? and which dealer was it purchased from? ray or trussville?

[Chadwick]

Originally Posted by a7xh7

do you recall the price difference? and which dealer was it purchased from? ray or trussville?

I buy my parts direct from Mazda through Mazda Motorsports.

Rear: $96.75
Front: $75.10
Eccentric Shaft $143.40

Dan

[Herblenny]

Dan,

Thank you SO much! Great INFO!!

I'm going to do one for 99 brakes if anyone interested.

[BigIslandSevens]

Wouldn't using the lighter e-shaft in some sense hurt your search for low end grunt? Only in the sense that recipricating mass = torque in general.( Not sure if top end is the more focused goal range) Now we are only talking about .41 lbs, but you were anal enough to go this far Just curious, or do the 9.7's overcome any loss of mass due to the e-shaft.
Good info all around.

Dave

[felix_is_alive]

nice write-up , would the balancing really be that necesary though ?

[Chadwick]

Originally Posted by BigIslandSevens

Wouldn't using the lighter e-shaft in some sense hurt your search for low end grunt? Only in the sense that recipricating mass = torque in general.( Not sure if top end is the more focused goal range) Now we are only talking about .41 lbs, but you were anal enough to go this far Just curious, or do the 9.7's overcome any loss of mass due to the e-shaft.
Good info all around.

Dave

No, just the opposite, any power used to spin the rotating assembly will be lost to the ground. So decreasing the weight does not decrease usable torque it will actual increase how fast the power is available. Think of it just like a lighten flywheel.

Originally Posted by felix_is_alive

nice write-up , would the balancing really be that necesary though ?

If you are just replacing the E-shaft than no as they are prebalanced. I like to error on the safe side and would still have it balanced.

Dan

[Signal 2]

Dan, Please add my thanks for the write-up.

Just to add...as you'd expect, the thrust washer, plate and bearing are also interchangable. In fact the RX8 part numbers NF01-11-D53 (washer); N390-11-D52 (thrust plate) and N3F1-11-D54 (needle bearing) are identical with the exception that the RX7 bearing part number is NF01-11-D54. The only significant difference I could find was price at MAZDAMOTORSPORTS. The RX7 pieces cost about 3 times more. IIRC, ~ $13 vs. $36.00 respectively.

[Signal 2]

A quick check of main and rotor bearings yield the same part numbers as well. Except the prices appear to be the same from RX8 and RX7. Again, this might be old news to most, but new for me.

[mazpower]

I've been running the RX-8 e-shaft and gears in my T-66 half-bridgeported FD for almost a year now. A lot cheaper and definately capable of making power.

[Rotary RL]

This is really good info! I don't ever plan on interchanging parts, but its always good to know for a future that is more abundant w/ rx-8s and the Renesis engine!

[FreeLance]

Excellent information. I'll be looking into this for my build over next winter

[sm0keyii]

Awesome write-up, nothing better than having the facts.