Rotary Resurrection - Tech Section

Many people have the desire/opportunity to use a later engine in an earlier car. Though this is usually good for a couple of upgrades and a few more horses, it is rarely worth the work. It is MUCH easier to simply find the proper engine for your car and bolt/plug it in. Note that some of these swaps have differences in emissions equipment/passages which prevent being able to keep the car emissions legal when doing so. You’d usually wind up removing the whole emissions rack and components to get everything back together properly, though it could be possible to make such a swap pass.

When doing such a swap, it is important to note that the flywheel must be obtained/used that matches the engine block you’re using. For instance, if you’re putting an s5 NA block into an s4 NA car, you must use an s5 NA flywheel with the s5 NA block; using the s4 NA flywheel from your old block will throw the new block out of balance.

These swaps are really pretty simple in theory, but it takes several hours to get everything swapped over. Basically, you’re going to keep everything that used to be on your old block, and transfer it to the newer one. The only times this principle does not apply:

the flywheel must match the s5 block
it’s much easier to leave off the emissions rack, so don’t use either one
3)   if a part on the newer engine is superior to one on the old engine (such as s5 NA intake set, or s5 turbo set)
the temperature probe on the rear iron, will only fit the block it came from (the holes are different sizes for s5 and s4)
Remember that anything electronic, or anything that an electronic part bolts to and would not otherwise bolt to on the newer setup, must be retained from your old engine. Your car still has to think it is an s4, even though it has an s5 block and possibly even s5 manifolds.

The hardest part of the swap, and the part with the most potential for screwup, is the front cover. The oil metering pumps differ between s4 and s5, and the front covers differ to accommodate this. When swapping front covers, it is possible to allow the bearings and spacers to slip on the e-shaft, doing internal damage and creating a lot of endplay and vibration. For more information on this, see the mazdatrix how-to section ( Also note the front cover o-ring (oil passage) which is easily lost/crushed/slipped out which will result in loss of oil pressure, so special care should be taken to ensure that this gets re-sealed well. Mazdatrix also has a nice writeup on this.

Now, here is an overview of how to proceed with the swap.

Strip the s5 engine down to the bare block, leaving the flywheel on, but removing the front cover. Repeat for the s4 engine (your original). Decide which parts you want to use from the s5, if any (intake manifolds or turbo/ex. Manifold are the only parts which were really improved). It is always easiest to simply use all the s4 parts.

Note that when swapping an s4 intake onto an s4 block, the port air passage in the intermediate iron, below both primary intake ports, must be blocked off to prevent a large exhaust (egr) leak. I use a healthy amount of jb weld packed firmly down into the passage.
Things you MUST keep
(originally on the old engine, which plugs into the car) are:
Things you may wish to upgrade to from the s5 engine:
Throttle body with TPS
Oil metering pump and front cover
Wiring harness with fuel injectors
••   airflow meter
water pump housing, water pump, fan
intake manifold set
turbo and manifold
fuel rails(s5 have an integrated fuel pulsation dampner less likely to leak and catch fire)
••   intercooler (s5 has a slightly larger inlet)
Again, the aim of this is to make the car/ecu think it still has an s4 engine, by keeping all the “smart” parts, which are the electronics, from the original engine.

The same principle can be used for s5às4 swaps, however these are not highly recommended because s4 blocks are lower compression, and the ecu is set up to run a higher compression engine, therefore sometimes power and idle quality are compromised when doing such a swap.
Rotary Resurrection - Tech Section
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