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

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:
http://www.tweakit.com/rx7/eshaft1.jpg
RX8 shaft on the right:
http://www.tweakit.com/rx7/eshaftrear.jpg


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:
http://www.tweakit.com/rx7/eshaftholes.jpg

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:
http://www.tweakit.com/rx7/frontstat2.jpg
http://www.tweakit.com/rx7/frontstat3.jpg
http://www.tweakit.com/rx7/frontstat.jpg


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
http://www.tweakit.com/rx7/rearstat1.jpg
http://www.tweakit.com/rx7/rearstat2.jpg
http://www.tweakit.com/rx7/rearstat3.jpg

Picture of the grove machined into RX8 stationary gear to match 13b stationary gear:
http://www.tweakit.com/rx7/rearstatgrove.jpg


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

wow

its nice to have facts this way :)

Great write-up :)

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

Dan,

Thank you SO much! Great INFO!!

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

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:D Just curious, or do the 9.7's overcome any loss of mass due to the e-shaft.
Good info all around.

Dave

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

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.

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

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.

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.

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.

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!

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

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

Dan you build motors for the public? HAHA!

Actually we do, but we don’t advertise for it as we are typically filled up with enough work on our own cars to keep us busy.

Dan

very good write up. still looking into lightweight rotors if possible.

That was an excellent write up.

Well done good sir.... well done.

Excellent information. If anyone knows, does this also apply to FC's?

how about the seals?

I've read elsewhere that:

-rx-8 apex seals can't stand a peripheral exhaust port and warp/wear down; also read that they eat up the non-rx-8 rotor housing chrome
-corner seals with metal plugs eat up non-rx-8 side plates

anyone here with some insight?

Very good info. I never thought about looking up new RX8 parts as replacements for older models.

i'll have an RX-8 E-shaft in my REW when I get it back within the next few weeks...

Nice write up.

I planned on using these parts for the 13B RE build in my FC. Mainly because the 13B RE from the cosmo stationary gears don't support more than 6500RPM (seeing as how they were built for an automatic luxury car) and the price of the MSP gears and e-shaft are much less than that of an REW. I do still plan to keep the RE rotors though.

Very nice write-up! Its nice to have the actual specs now....

Oh - side note. This SHOULD apply to FC 13B engines as well. I don't see why you couldn't use the MSP e-shaft and stat gears in earlier 13Bs... so long as you balance it all accordingly.

This is great info, thanks.

I don't believe anyone has noted the fact that the oil passage on the stationary gears does not line up properly with the passage in the non-Renesis irons. It's an easy matter to open up the passage of the gear to match the iron. The rear gear has about half the passage shadowed.

I'll add another point to this thread. We had a small oil leak on Dan's car toward the end of the year. When we pulled the engine this winter we found that the o-ring on the rear stationary gear was not sealing properly and that was the source of the leak. Upon disassembly, we found that the o-ring had pretty much self destructed on the rough casting of the rear iron and we chose to re-assemble without the o-ring and just use RTV instead. With that being said, the additional machining that I had done to the rear gear is probably unnecessary so long as you use RTV instead of the o-ring.

I've never tired to use an o-ring with the rear gear but I have used RTV (Right Stuff) and Hylomar. Both have worked equally well.

part #s ,pics and very well explained .i wish they were all these clean .great job man

It's far from ideal to have stock renesis apex seals travel over peripheral ports. The metal plugs shouldn't touch the irons.

You also would want to avoid putting non-renesis rotors into a renesis due to seal vs port locations.

All the above have been done but are not smart or suggested.

Paul.

Good right up, I've used rx8 shafts in rew's due to the weight and price difference. Good to see all the info in one place and properly displayed.

Brent

[QUOTE=WE3RX7;47141]I planned on using these parts for the 13B RE build in my FC. Mainly because the 13B RE from the cosmo stationary gears don't support more than 6500RPM (seeing as how they were built for an automatic luxury car) and the price of the MSP gears and e-shaft are much less than that of an REW.

You have any facts to back your statement?

You have any facts to back your statement? Good write up :icon_tup:

I am sure it has been discussed elsewhere, but I was hoping to get it added to this thread:

What about using RX-8 rotors in the 13B-REW?

If one wasn't looking for more than 7-10 lbs of boost, and used water injection, and wasn't looking for peak numbers, but wanted lots more on the low end.... wouldn't these be a good option?

Aside from machining the apex seal groove out deeper (I am unsure of the validity of this) what else would be required?

RX8 rotors are 10:1 aren't they? Isn't that a bit heavy on a turbo car?

There are plenty of turbo-charged RX-8's out there. My question pertains to the compatibility. If someone wanted the most low-end they could get and then run 6-8 lbs of boost... the curiosity becomes whether or not the RX-8 rotors are drop in for an REW.

I don't necessarily plan on doing it, just curious if it is possible. I might build my backup engine this way just for S&Gs if there are no fitment issues.

The msp e-shaft is less than $200. The REW e-shaft is closer to $500 if I recall correctly.

Very true. One of the reasons that I went with the RX8 E-shaft. My memory says closer to $150 for the shaft.

So... my original question still stands