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After looking at one of my spare engines yesterday with Tool, I've been thinking it might be a good basis for a rebuild if the housings are as good inside as they are outside but wondered if anyone has got any experience of running one of these engines, particularly in the US where they were more prevalent. The increased lower end torque does appeal to me.
I'm thinking that I could swap out the NO intermediate plate for a larger port Y plate and then build it back up with maybe lighter s2 rotors and fly which together with the J-spec R5 housings, might form the basis of a decent 12a or do I just swap out the intermediate plate and leave the heavier R5 rotors in place?

A few pics...

top view showing the EGR...




the tell tale blank raised piece above the Mazda logo denoting its a R5, plus the NO casting on the intermediate plate




and the puny NO inlet ports...



anyone run one of these?

I don't run one of these, but certainly the tall port centre would be a good upgrade - they are basically the same size as the end plate ports.

Regarding the internals, food for thought: A lighter reciprocating assembly is aimed toward higher rpm at the expense of some low down torque. Also, running the S2 rotors needs the S2 flywheel obviously but a light flywheel on the heavier s3 rotors would overall give a greater gain than the s2 setup. In other words, I'd do flywheel first, then rotors.

I believe the exhaust ports are the main restriction to power and different series have different sizes - I know S1 are quite small and the s3 are bigger - cant give specifics though, but this was just comparing engines in my shed.

The R5 housings are timed differently to stock FB housings, they open later and close later apparently giving better low down torque ( below 4000rpm)

Here's a R5 housing on the left next to standard housings...



here's the R5 in the centre of 2 standard housings with a line drawn across them for comparison...





From the top, the blank raised piece above the logo identifies the R5 housings and distinguishes them from stock housings...



here's the NO intermediate plate ports compared to the Y plate ports....

From the outside...



and the inside...



Y plate is the way to go.
Regards the rotating assembly, I reckon you're right, keep the R5 rotors and swap out the heavy S1 flywheel and put in a lighter S2 fly.

Very interesting Ian. The housings will be spot on for exhaust opening and timing if they are good!

Regards the flywheels, I was meaning an aftermarket flywheel as opposed to the S2 version as that is matched to the rest of the rotating assembly and can't be swapped in by itself. The lighter flywheel will need that years automatic flex plate counterweight for the flywheel to bolt to. Unless you are willing to balance the lot, which is quite expensive, you can't mix and match the 4 main balance pieces.

Those exhaust ports are a lot bigger than the s1 engine I got, but mine has the tall port centre plate. Who knows why they swapped bits around...? They could have made a sweet factory engine if they combined the best bits!

I've read that the 1981-1982 flywheel and counterweight is the only one that can be swapped in the the R5 setup without affecting the rotating assembly balance adversely although I'm struggling to get my head around flywheel and counterweight swaps.

Oh I see - trick is in the weights which offset each other.

As a general rule you can't, but there will be exceptions of course, and anecdotal evidence to suggest that people have swapped, disregarding the rules, and had no bad come of it.

Certainly flywheel and front counter weight are a good start if they in themselves match.

You could drop in a complete S3 assembly, which gives more chance(maybe) of finding a lightened flywheel setup and larger range of clutch options? Plus S3 stuff is easy to lay your hands on, if you don't already have some already.

First thing though - crack the block open! I'm keen to see what's inside

ian65 wrote:I've read that the 1981-1982 flywheel and counterweight is the only one that can be swapped in the the R5 setup without affecting the rotating assembly balance adversely although I'm struggling to get my head around flywheel and counterweight swaps.

Rear counterweights are 74-83 (all single dizzy up to S2) and S3 (includes 12AT) iirc.

So, in theory a S1 and a S2 flywheel are interchangeable.

Reading all this with interest, can't add anything but I'm soaking all this info up

Nice

I know there is is different 13B housings but didn't knew there was different 12A housings ! What car is it coming from ?

As I'm writting this message I can remember I saw (only in picture) a 12A housing with 3 holes for the exhaust port instead of 1! I don't know if i t was a prototype or an earlier version of the 12A ? :|

l-arsouille wrote:Nice

I know there is is different 13B housings but didn't knew there was different 12A housings ! What car is it coming from ?

I don't know, it's the first J-spec R5 that I've come across....... I have seen other engines with various R5 parts on them..... the gold series 2 in the forum banner had a R5 rear plate and also maybe the housings but not the NO intermediate plate.....

A strange thing with the R5 rear plate is that it isn't tapped for an oil pressure sensor, you have to drill and tap it yourself...... mine has been done fortunately..... I don't fancy drilling into an oil gallery.