Reducing CR by skimming Pistons or new con rods

[MaxRothery]

I've been looking into rallying and getting into the high teirs with a 1.6T and the engine I've got lined up has a CR of 11.1 which is quite high so it would be better with a CR if 8.5-9. I've done the calculations and to reduce it that drastically would require either 2.2mm taking off the piston or the con rod. So which would be easier and stronger to shorten? There is a turbo kit for this engine but it only runs 4 psi of boost and I want to take it up to the 2.5 bar the S2000 regulations allow

[MaxRothery]

The only set I can find involves boring the engine out to 1.9lts and I don't particularly want to spend £730 on Pistons I couldn't use. The stock internals are already strong, but it's just too high of a CR to use

[MaxRothery]

davepoth said:

what engine is it?

. I'm looking into using a jimmy with a 1.6 internals. The M13A with M16a crank, rods and Pistons, creating a 1.6 in a 1.3 block. Only reason for doing this is that the regulations state that it must have a production variant but the engine can be modded to increase engine size. Otherwise I'd be swapping engines over

[MaxRothery]

Huff said:

I'd have thought (as a layman in these matters) if you bolt the thing together as-is at 11:1 and run it in at 2bar+ the rods will very quickly be adjusted to a more suitable length automatically...

. I was more or less aiming to lower the CR to make it more boost friendly. I did a bit of the calculations to work out how much more volume is needed above the cylinder and it comes out a 2.2mm to reduce the CR from 11.1 to 8.5. The engines I'll be using are the M13a block from a Jimmy and the hardened internals from the M16a from a swift sport. Hks do a 4psi kit with stock internals so I'd just be reducing the stress on them. Seeing as other 1.6t are running a CR of 10, a thick gasket and something like half a millimetre off the piston would do that. £700 on some forged Pistons would work as they are stronger and I could take said material off and still be relatively strong

[MaxRothery]

stevieturbo said:

You do realise the difference between 4psi...and 36psi boost ? Like...seriously ?

And whilst you will need more than your calculated 2.2mm to achieve the CR drop you seek, really....have you actually given this project serious thought ?

It seems like a nonsensical question given the end goal you seem to be wantng. Either that or the end goal is nonsensical.

Yeah, I do realise the difference, and I have given it some half decent thought. From what I've found, swift sport internals are relatively strong, and there are forged kits out there for a decent price. There is one or two people running high boost through these engines but the bore it out and make it smaller, doing this makes it s 1.8 with 9.5 CR. And that 2.2mm is to drop it to 8.5 CR, it doesn't have to be as drastic, 1.6mm drops it to around 9.8 which will still be better to turbo than 11.1.

And it's probably both a nonsensical goal and a nonsensical question