Today, I was browsing the availability and cost of FC rotor housings to see how much has changed since 2010. I was appalled to find that the rear housing for an FC S5 is now $1104.79 with Mazdatrix, and the front housing is NLA, although Atkins has them for $1200. Back in 2010, I purchased two rear turbo FC S5 rotor housings from Atkins at $520 each. This was recommended by one of the sales people because I wanted to build a new motor to swap into a 1975 Porsche 914, where emissions is not an issue. However, on e-bay, there are multiple listings for FD front and rear housings as a package deal for just under $1400.00. Considering these price differences, would it be possible to use these two FD rotor housings of the package deal in a FC N/A engine build? My question is motivated by a response to a question posed by new member of the forum as to whether or not his N/A rotor housings were usable in a rebuild. Based on the photos, there was considerable deep chrome flaking that perhaps extended into the housing by over 2mm. I suggested that, if anything, he should obtain new rotor housings if financially possible in order to build a long-lasting, good performing engine if he plans on keeping the car for a long time.

 

Yes you can use then with no problems .Only a gain of better larger exhaust ports

 

Thank you for your response...I'll spread the word!

 

Was there no change in the spark plug locations?

 

between S4 and S5? yes.

between S5 and S6? no.

 

I have a pair of S4 N/A rotor housings that I bought off e-bay back in 2010. Today I took measurements of the location of the spark plug holes. The leading spark plug hole is about 50mm apart from the trailing spark plug hole from center to center. The center of the leading hole is about 140mm from the bottom of the housing, which makes up the oil pan rail. On the other hand, my brand-new S5 housings, (turbo), have the centers of the leading and trailing holes about 48mm apart, and the center of the leading hole is about 145mm from the bottom of the housing, (oil pan rail). I don't know how the spark plug holes of the S5 N/A housings measure up, as the original engine of my car is still in the car, and looks too difficult to measure with my stainless steel ruler.

 

the only ones that were different were the US market S4 engines, everything else is the same

 

i don't know how you came up with those minor differences, all JDM spec rotor housings/S5/S6 should share the same spark plug timing.

you can remove the exhaust diffusers to swap and use the better flowing Turbo exhaust port sleeves.

the only real difference is the F and R stamps, which are for the different EGR castings for the S4 engines.

 

Ok...So it is possible to use FD rotor housings in an a FC S5 build. But another question that I have is this: Do I need to use both the front and rear FD rotor housings in the S5 build, or can I use two FD front rotor housings for the front and rear in the S5 build? When I was acquiring parts for my S5 build back in 2010, Atkins recommended that I purchase two rear rotor housings for the T2 engine since they were more affordable, ($520 each at the time), if emissions was not going to be an issue. The motor was intended to go into a 1975 Porsche 914, so emissions were definitely not an issue.

 

That is correct, the rear housing costs quite a bit more to boot.

 

if emission were an issue, you can drill the air injection passage to make a front housing work in the rear.
in a 12A*/T2/13B Cosmo/FD engines the air pump air goes in through the rectangular port under the primary intake ports, it then goes down to the bottom where it goes into the rotor housings, and then around the exhaust sleeve.
the 12A* engines had the holes drilled on both sides, so they used the same rotor housings front and rear, the FC and FD engines only have the holes drilled to face the center iron




this doesn't explain the price difference, but the way this is drilled and the R stamp is the only difference i've been able to find between Front and Rear FD housings.

*81-85 12A only, also 84-85 13B, and 89-91 non turbo. 86-88 non turbo is weird, and the air goes through the end irons to make room for the EGR passage

 

Wow! Thank you for the answer and thorough explanation. This is extremely helpful. Nice photos, too!

When I do complete my new engine build and swap it with the original, I want to tear down the latter to primarily go through and replace all the water jacket seals, and secondarily, inspect the rotor housings for wear. I would like to have a new pair of FD rotor housings on hand as I expect there would be noticeable wear on the original housings since the these S5's inject a lot less oil for lubrication than the S4's. Recently, I have begun mixing 2-cycle oil at a 200:1 ratio with non-ethanol fuel to help supplement the lubrication deficiency and stave off as much wear as possible. The original engine starts easily warm or cold, and seems to have excellent power, acceleration, and throttle response. So, I can't imagine that the wear on the housings is drastic. I started using premixed fuel at about 133,300 miles, and wished I had been doing this when I got the car with 128,600 miles.

 

you'd think everything would be equal in the parts world for manufacturers, but they base it on supply and demand. they probably had less demand for rear housings versus fronts on reman engines so the price difference reflects that. it's basically a pencil pushers byproduct.