Equilibria

Before I start off, I'll mention this is largely a feeler/information request. Just interested in what someone would do in this situation...

Seen a few threads regarding turbocharging a 944S/S2, or putting a 16v cylinder head on a turbo car. Now, as my understanding goes, the S/S2 cars have a compression ratio of 10.9:1, while the 951 has a 8.0:1 ratio. The reason you want a lower compression ratio is to allow/handle more boost (correct?) as you can then "shove" more air into the cylinder etc etc. The question I have is what is the "standard" procedure for making this work ie if you have an 16v S motor (simplicity, since its 2.5L), could you just swap in turbo pistons/rods and call it a day? Lets ignore everything else, and say thats all you wanted to achieve (changing compression), being aware that you will be doing performance upgrades in the future (so going standalone is not completely out of the question). So now the questions:

What else is required in order to get an 16v motor working with a lower compression ratio?
Could you order a custom chip for the 944S DME and just get it made to work with a specific ratio?
Would you need to go standalone/piggyback of some sort?
Do internal engine components aside from whats listed need to change?

Thanks for any input (incl. "don't bother", if it comes to that), trying to soak up as much knowledge as possible and figured this is the best place to get it!

V2Rocket

you are correct about low compression being better for boost. the lower the compression ratio of the pistons the more boost pressure you can run without worrying about detonation.

the turbo pistons/rods will work in an S block but im not sure if the cranks are the same; however you can put an "S" head on a turbo shortblock just fine. either way however you will need to accomodate the 2 more valves (cutting extra valve reliefs into the turbo pistons)

the DME doesnt really recognize compression ratio; on the S car it has a knock sensor to retard the timing in case the high compression pistons start detonating fuel prematurely though. if you changed the compression of the pistons the stock DME should be fine.

white924s

so you're looking to lower the compression ratio on an N/A motor? that's not a good idea

lower compression is better for a turbo because, for a given peak pressure, you can cram more air and fuel in. If you don't have a turbo though, all you're doing is just reducing peak pressure in the cylinder, which means you're losing power.

If you want to add the turbo, though, it is a good idea. I would say that you should be able to do it, but you'll need the following:
- turbo pistons with relief cuts or some other custom pistons (check SFR or Lindsey)
- S rods and crank should work fine
- Custom headers, 951 crossover pipe and driver's side engine mount (with turbo mount
- turbo (air cooled is easier to install)
- Turbo oiling provisions - tap for high pressure and return
- custom intake, intercooler

Alternatively, you could turbo charge a stock 944S engine, but you would be limited to fairly low boost so you won't be able to get huge power. But, because you'd have higher compression, your throttle response and low end torque would be better than a lower compression motor (which would be effectively handicapped by the low compression until the turbo gets spooled up).

Van

Look at it this way... higher compression means predetonation (don't forget that raising the octane can help...) All things being equal, a car that's 11:1 takes in 1 bar (atmosphere) of air and compresses it 11 times. A car that's 8:1 compresses 1 bar 8 times.

Now, if that 8:1 car is a turbo, and at full boost it does 1.75 bar, then at full compression, it doing 14x - about the limit for premium gas.

If you start off with a car that is 11:1, cramming an extra 0.25 bar of air (1.25 bar total) would bring you up to almost 14x. 0.25 bar is about 3.5 psi - not in the territory of a turbo... but a perfect range for a super charger!