Hello

I am rebuilding a 1990 FC RX-7 13B engine with suspected blown coolant seals. Can anyone confirm if Series 4 (S4) housings are interchangeable with Series 5 (S5) ones for this rebuild?

Also, which coolant seals should I use with S4 housings?

Any advice on ensuring compatibility and performance would be greatly appreciated.

Thanks!
aanaaacpq

 

first, the coolant seals are the same from the start of the FC in 1985 to the end of the FD, 2002.

two, the center iron for the S4 NA is weird, it has the EGR, and then the ACV uses the end irons, so if you need to pass emissions, its not compatible. the S4 T2 and all the S5's are pretty similar though
the rear iron for the S4 has the full mounting pad for the old top mount starter, with 2 bolts. the S5 only has 1 to make room for the intake manifold.

for the rotor housings you might want to verify that the ACV passages aren't reversed S4 vs S5, if its needed. if they are you can probably just swap them front to rear.

imo its better to build the engine so i could pass emissions if needed. ive seen a few cars where the owner moved here to California, and getting it smogged required an engine rebuild to have the ACV ports functioning

 

Wait, what?

I had no idea there was anything different between S4 and S5 NA irons other than the front cover o-ring boss. I just put together a frankenstein engine out of front and rear S5 NA irons, an S4 NA center iron, and S4 T2 rotor housings. Everything else like the ACV and LIM are S4 NA. You're saying my ACV won't work as a result?

 

if you kept the S4 lower it might, blow some air through the big round passage under the ACV, and it should make it to the exhaust ports "Port Air"

 

OK, I finally started to understand what's going on after poking at my scrap housings. I drew the diagram below for my chimp brain to better understand since it wasn't clicking without pictures. Hopefully this helps someone else as well


I forgot to draw it on the diagram, but all the engines are facing to the right, so left iron is the rear one and right is the front

Some notes - while the irons all seem to universally have those passages underneath the intake ports and the rotor housings all have the 4 holes drilled below the exhaust ports, the respective series/aspiration intake and exhaust manifolds will only have whatever passages they need. I.e. none of the S5 exhaust manifolds have holes for EGR. You also can't put an S5 NA lower intake manifold on an S4 block and expect the ACV to work. Because the rotor and side housings seem to have all the same passages still present, some combinations of different parts can be swapped around as needed and still have both the ACV and EGR (if present) work in your franken engine - the main exception is with the S4 NA engines. The center iron cannot be substituted into any other engine and vice versa, and if you are using S4 T2 or any S5 rotor housings in an S4 NA, you need to swap them front to back for your ACV to work, because the hole connecting to port air is on the opposite side for S4 NA rotor housings

For my specific situation, I need to tear my engine down (again) and just switch the rotor housings so the rear is in the front and the front in the rear. But the S5 NA front and rear irons do have the ACV passages, so that's not a problem

While we're here, might as well note whatever housing differences I ran into while googling for images:

• Getting the obvious out of the way - T2 is 4 port, N/A is 6-port for both series, N/As have those exhaust diffusers in the rotor housing exhaust ports, and the center iron intake ports on NAs are much smaller than the turbos
• T2s have the water passage drilled on the rotor housings between the intake ports to feed water to the turbo. These are left blank on NA engines. Same applies to the turbo oil feed port in the front iron, which is left blocked by a staked plug on NAs
• ACV/EGR passage, port air location differences between series and aspiration noted above
• USDM S4 T2 and I think all S4 NA* have higher leading plug locations than on all other FC engines by about 5mm. S4 T2s everywhere else and all S5s have them lower, which got carried over to the REW
• No S5 engines had EGR, although S5 NA engines still have the pad for it cast in
• The rear S5 NA iron does not have a fully formed top mount starter area as j9fd3s mentioned
• Only T2s had knock sensors - this moved from the center iron below the oil fill in S4s to the rotor housings in S5, where both front and rear housings have the threaded boss, but the sensor is only screwed into the front housing (I read a post saying S5 NA rotor housings also have the threaded bosses, but I haven't been able to find a picture to confirm this, and I only have S4/S6 rotor housings on hand to look at)
• S5 front irons have a larger landing for the front cover o-ring to accommodate a teflon backer ring
• Late production rear irons have a reinforced, thicker rear iron upper dowel pin landing. I think this gets conflated with S5s getting this, because only S5 cars came from the factory with them (IIRC some early S5 cars came with a half measure where they still had the thin casting but got the triangular rib reinforcement added). All new irons (S4 included) you buy now have them as well
• I don't have specifics, but have read S4 and S5 have some small differences in intake port timing/size/shape

*Correct me if I'm wrong here, I'm not aware of a unique S4 NA rotor housing for euro/middle east/wherever else the S4 NA cars were sold, so even though the spark plug hole difference is considered a "USDM vs world" thing, I don't think that applies to S4 NA

Anyways, thanks for the heads up. You definitely saved me from beating my head against a wall for months if it didn't pass emissions. Funny enough, I read a couple forum posts about how other people have managed to pass emissions with their rotor housings swapped, which kind of matches up with my experience of still passing emissions despite having forgotten to put the belt back on the air pump. Might be a different story for California emissions

 

Clarification on the scribbled note about the S4 NA EGR - it passes by the primary intake ports on its way up to the EGR valve and does not connect to them - yet. Exhaust gas does go to those intake ports, but only after doubling back at the valve. Not sure why I worded it the way I did in the diagram. 1AM rabbit-hole brain not English too good