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Deciding On Compression Ratio - Supercharged

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I am taking a K20A2 out of a 2002 Civic Type-R and fitting it into my Accord Type-R to replace the H22A7.

I have a TTS Supersport Non AC Supercharger kit on order which uses a centrifugal Rotrex C30-94. They state that at up to 14psi it's perfectly safe on a stock engine and can deliver up to 420bhp without opening the engine up.

I'm slightly dubious about how long my engine will last in a stock state, but looking at other builds there hasn't been any reported failures.

Anyway, I plan to strip the donor engine anyway to inspect all parts and rebuild, but whilst I'm there I am itching to fit forged rods and pistons. That in itself gives me an option to drop the CR.

Stock, the engine runs 11.0:1 but if I forge it I have a wide choice of piston and CR.

Will I gain much by reducing the CR to say 10.0:1, other than some safety in detonation etc?

Would I be gaining any power in doing so?

Or will I just be losing low speed response and efficiency by reducing the CR with no real power benefit?

What fuel?

99 RON Unleaded

Assuming 99RON only, which is about 94 AKI, You've got a few parameters to optimize here, at he very least: boost, spark timing, compression ratio. The lower compression will buy you more safety with similar spark or more power and torque by advancing spark. Trade off is fuel economy and possibly response, but that's less of a concern on a mechanical supercharger. Assuming you don't change the pulley or blower, boost isn't something you can really change.

One option is to tune it with stock compression ratio conservatively, then swap to low compression pistons and see what happens. Try different combinations of timing, AFR, valve timing on both ratios and see what you like best.. it would be a pretty interesting project. Keep everything the same except the pistons and associated parts and study the impact of compression ratio. It's just going to take more time and a bigger budget.

From my own experience with a very similar K20 build, you'll find that the engine will be knock limited at 11.0:1 and 14 psi boost, although not dramatically so. I've found 9.5:1-10.0:1 is a better compromise from the K series engines I've personally dealt with and by dropping the CR you'll be able to optimise the timing under boost and this should give you a gain in power.

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