Rotary Resurrection - Tech Section

I probably see, between posts on the forums and emails direct to me, hundreds of questions per year about swapping engines and wiring between series. Some guys with an s4 car want to put an s5 engine in, and some with an s5 car want to put an s4 engine in, and some just want to swap out the electronics/control system, for some reason or other. This is not really a good idea, because these parts are quite different, although they appear similar at first glance. It’s almost like the 2 series 7’s are 2 different cars in a nearly identical chassis…different wiring, sensors, etc.

In honesty, it is a complete mystery to me why people want to do this sort of thing. The car came with a certain type of control system and engine in place, so why not go back with that plug and play setup? The prices of both series parts is roughly the same, though s4 stuff is usually cheaper by a bit. Availability is roughly the same…there are more s4 parts floating around here in the US where not many s5’s were available, and there are more s5 parts coming in from japan than s4 (as far as turbo parts). As for power, the s5 parts do offer a few hp increase as a package, but it is not worth the headache to swap. Part of this increase would be compression ratio, part of it would be a more efficient intake, part of it would be a faster/smarter ecu with superior programming, but none of these are reasons to perform hours of tracking down parts, poring over wiring diagrams, troubleshooting electrical issues, etc. which is the usual result of such a swap.


We’ll start by looking at a stock s4 ecu. This happens to be a t2 ecu, but the na has the same plug shape and such. Note that there are 3 plugs to the ecu itself. One of them (the largest) comes from under the dash, and goes toward the drivers side. The other 2 (smaller) come from the passengers kick panel, and go out the firewall, these are part of the engine/emissions harness that is part of the engine.
Here are 2 more large plugs, up in the kick panel. One is pulled down for clarity, it is normally stuffed up behind the blower motor. These are junctions where many, many wires between the ENGINE and DASH/MAIN harnesses join. So, not only are there 2 main harnesses in the car, each of which plug into the ecu, but they also plug into each other. There are also a few other small plugs on the engine harness in the kick panel.
A pic of the same engine harness removed from the car. Note the 2 yellow ecu plugs, and the 2 large orange junction plugs which plugged into the dash harness junction plugs. Also a couple of other plugs for the atmospheric pressure sensor and knock control box (both of which are s4 only parts, the s5 does not use these parts).
Here’s a pic of these 4 main important plugs on the engine harness, from a front view.
Here’s a shot of the s5 engine harness out of the car. Note that there are still 2 ecu plugs, and 2 large junction plugs. There are no atmospheric pressure or knock box plugs on the s5. There is an additional ground wire on the s5.
Here’s that same front view of the s5 harness’ 4 main plugs. Note that they’re considerably different from the s4’s above. This means that not only will the harness plugs be different at the ecu, but the junction plugs at the dash harness will be different.
Heres a comparison of s4 ECU plugs and s5 ECU plugs. Totally different…different number and size of pins, shape plugs, etc. They might as well be from 2 different cars.
The same comparison of the large junction plugs that go to the dash wiring harness. Again, totally different.

A comparison of an s4 and s5 ecu. Obviously, totally different, might as well be from 2 different cars.


So what does all this mean? Well, say you have an s4 car, and want to put an s5 engine in (or vice versa). So you get the complete engine with all accessories, sensors, and wiring. Even the ecu. You put the ecu into your car and the s5 engine harness plugs right in to the ecu, but you find out that the dash harness won’t plug into the ecu at all. And, the dash harness junction plugs won’t fit the engine harness junction plugs.

So, you decide to put the s4 ecu in. The s4 ecu plugs back into your dash harness, but now the s5 engine harness won’t plug into the s4 ecu. Bummer.

So you decide to swap back to the s4 wiring harness. But then you find that it won’t plug into the BAC valve, TPS, air flow meter, OMP, injectors, etc. on the s5 engine.

SO then you wind up swapping all the old s4 parts onto the s5 engine block. Meanwhile you’ve spent many hours fooling around with it, and bought parts you didn’t need. You could have simply went with the correct engine and parts to begin with, plugged it in, turned the key, and been done with it.

This is the only really proper way to go about this swap, install the different block but keep all the original wiring/manifolds/control systems that match the chassis.  There is more info elsewhere in the tech section about specific parts to swap.

Rotary Resurrection - Tech Section
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