Please help me identify my engine
 

The previous owner of my Rx7 FC had an engine swap done. The vehicle is a 1986 NA, and the new engine is also NA (~10 000KM). I am having trouble because I can't find a way to identify whether it is an S4 or S5 engine. I have tried searching for ways to differentiate the two engines, but I always find results such as: "The S5 has slightly optimized apex seals" which is of no use for my purposes. Only internal differences seem to be listed.

The reason I am asking is that my dash is the S4 dash, and the temperature sensor constantly stays exactly at the 1/2 way mark. If I have an S4 engine, this is a very big problem. I am concerned about my cooling system. If it is an S5 engine, I am to understand that this is normal behaviour (assuming of course that the temperature gauge is the same between models and only the sender is different). Before anyone asks, there is no white smoke and there are no bubbles in the coolant. Rad is fine. I am performing a flush, and will see if anything changes.

What I am looking for is a clear and concise explanation of the visual differences between the engines, ideally no disassembly required. Also, if someone happens to have any ideas about the cooling, that advice would also be welcome.

Thank you in advance.

Update
 

I have performed a complete coolant flush and replacement, and the temperature gauge behaviour is the same. The car warms up and reaches 1/2 way on the gauge and stays there.

You should buy an aftermarket water temp gauge. Stock gauge sucks. Glowshift kit with tap and drill bit is around 70 bucks with gauge.


If your OMP looks like this its an S4


https://cimg8.ibsrv.net/gimg/www.rx7...27911190db.jpg

If your OMP looks like this its an S5:


https://cimg1.ibsrv.net/gimg/www.rx7...547a47aa28.jpg

I am not sure how easy this will be but, if you can pop off the inspection plate thing on the top of the bellhousing, it is the flat tin piece, you should be able to see a casting number on the flywheel, from there you can see if it is a S4 or S5 flywheel, which will indicate if you have S4 or S5 rotors.

Unless some doorknob has swapped flywheels and you didn't know.

Aside from the OMP differences.

Intakes are also an easy way, provided the intakes have not been swapped.

S4
https://cimg8.ibsrv.net/gimg/www.rx7...f8d6f8108a.jpg


S5
https://cimg9.ibsrv.net/gimg/www.rx7...85e6ac4f9b.jpg



In all reality, you engine will likely be the S4 engine. Swapping to an S5 is a bit involved, as the wiring is totally different on the engine harness (for the most part)



As far as your cooling, the stock gauge is about as accurate as measuring the exact temperature of the exhaust manifold with your bare hand.

Either way, you get burned and it's not wise to trust that method. Get an aftermarket gauge and save yourself the headache.