S5 engine into S4?

[Kenny Senter]

So I picked up an 88 GXL that has a Atkins Rotary Stg2 streetport S5 engine in it as a parts car and am trying to decide if I should go through the trouble to swap that engine into MY S4. My car has a fresh 20k mile rebuild in it as it is and is running quite well.

The 88 was a friends of mine and the engine started running funny so he just assumed it was blown and parked it. Never compression tested it or tried to diagnose the problem, nothing. So, I'm gonna do some basic testing on it to see how the compression is and hope for the best. At that point, is it worth swapping an S5 engine into my 87, seeing as though its got the higher compression and a streetport job done on it. Obviously the engine ran in his s4 but I dont know at what length he went to to make it run. It never would idle for him, but he said something about a rod or something to the nature as to why it wouldnt idle.

Opinions? and what is involved?
I believe the S5 engine needs the matching S5 flywheel to keep its balance and I would use the majority of my S4 sensors, those that I can, to work with my S4 harness.

[Jarred]

I have an S5 engine swapped into my '86 GXL. I initially bought the car with a blown motor and the only viable option at the time was to buy a freshly rebuilt engine a guy had. The motor was from a '91 and also came with the transmisison.

I wouldn't recommend the swap for beginners or even intermediate mechanics/owners. I didn't know what I was getting to and thought it would be a relatively simple swap. My friend and I had to hack the entire wiring harness apart to get the S5 stuff to talk to the S4 stuff. We basically had to look at schematics of both harness' and splice them together. There were other small differences aswell as you will find out if you go ahead with the swap.

I'm not trying to discourage you from the swap...just trying to be realistic here. Good luck bro!

[TehMonkay]

Or you could modify the S4 throttle body to fit on the S5 manifold (trim a nub off of the bottom for clearance), premix, and use all S4 wiring and use an aftermarket temp gauge (which the OP already has anyway) . It's not difficult at all actually, it's pretty easy. You also could just bolt the S4 NA manifold onto the engine as well.

[FortyfiveD]

The easiest way to do that if your up for it is to use the block from the S5 and the other parts from the S4 (i.e. manifolds, rats nest, etc.) The only thing you have to do to the motor is swap the front cover. The S5 and S4 ones are different for the OMP ones mechanical and ones electric. After that it should be fine, good luck with you project.

I did this to my 86

use the front cover and intake manifolds/tb from an s4 then the stock wire harness will hook right up.

[Jarred]

Everyone here is using S4 stuff on the S5 motor. I actually kept everything S5 on mine...and the swap took 3 months lol

[Kenny Senter]

well the motor already has the S4 front cover on it, so "win" for me. It also already has S4 lower and upper intake manifolds on it, so also "win" for me.

What could the previous owner be talking about when he says that it would never idle correctly, even when fully warm???

The engine is already in an 88 GXL and was running up until "it started running funny" and the previous owner parked it.

[TehMonkay]

Is it surging? Low? High?

if the rpms surge up and down (like stated above) it is a tps issue.
could be a vac leak
did he remove the emissions.. if so possible his plates are leaking, my egr plate was leaking when I first removed and blocked it off.

is the throttle body modded?

could be alot.

[FortyfiveD]

I agree with SpooledupRacing minus the egr plate didn't have that issue(knock on wood)

[Kenny Senter]

its not surging (wasnt surging), he said it would never idle, always had to heel/toe at a stop. Car has sat for a year or so and battery is dead of course. First things first, gonna compression test it.