I want to know if I should try and get an new engine (maybe trans if I want to go to an S5 engine) or if buying a turbo and associated parts would be better for my S4. I'm not looking for insane horsepower just some more fun like in the 250-300 range

 

Ok so there are a couple thing to think about, you have an NA car. One the powertrain side the NA engines have high compression and 6 intake ports vs the turbo engine with 4 intake ports(no 6pi) and lower compression. The turbo engines also have larger injectors, 2 bar map sensor and VAFs with stronger springs in them as well as the turbo having a bit bigger volume fuel pump. Youre already talking 250-300hp, which is more than you can really safely do on the stock (even turbo)computer.

On the driveline side the turbo engines have larger clutches and flywheels, they have a different starter, different driveshaft, larger differentials and larger axles/stub shafts. When it comes to the flywheels there are 4, turbo flywheels are larger in diameter to accommodate a larger clutch disc. The flywheels are balanced for the engine series, so an S4 engine will need either and S4 turbo or S4 NA flywheel, S5 engines will need either an S5 na or S5 turbo flywheel...assuming you dont swap swapping rotors between series. If you stay under 300whp you can get by for a while on the transmission and differential, clutch dumps and wheel hop will kill the stub shafts.

Given the age of all of these engines, its worth it to pull apart and get the old 3pc apex seals replaces with 2pc apex seals, check the coolant seal grooves, and replace the coolant seals as well at minimum. Really just a full basic rebuild and inspection is a good idea for longevity. With the higher compression you wont wnat to be running as much boost but with more compression are larger port area(wiith the 6port sleeves removed) you wont really be hurting for power you just lose some port velocity at lower revs and off idle. If you choose to rebuild what you have make sure to get the exhaust diffusers that exist for NA engine out to free up exhaust flow.

You havent stated the current condition of what you already have so its hard to say if you should use it. I prefer the S5 stuff as the rotors have higher compression(na vs na and turbo vs turbo), machined compression grooves for more actuate compression, and are lighter(easier to make rev more). If youre engine is in decent mechanical condition(decent or good compression), holds coolant pressure, and isnt burning oil badly it may be worth it to just build up what you have. When it comes to engines there are thin cast and thick cast, ALL S4 are thin cast and a good bit of the S5 are as well which will limit the absolute upper HP you can safely make, but even thin cast has no problem with 300hp.

The last thing to consider is in Iowa is there anyone close who can tune a rotary to make safe/reliable power that has decent drivability?

What would I ultimately do in your position, depends on the condition of your current engine. But what you could do is get whatever standalone the bigger injectors, pump, regulator installed and working so when you make the move that stuff is already installed and proven.

 

Since the car is S4, run S4 electronics unless you want to go find out how to get s5 electronics working. Unless you’re going stand-alone.